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| − | Also known as: '''''BTV — Bluetongue
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| − | ==Introduction== | + | ==Description== |
| − | Bluetongue is a non-contagious, [[:Category:Arthropods|arthropod]]-borne disease of ruminants, caused by bluetongue virus (BTV). The clinical severity of disease is variable, but is characterised by inflammation of mucous membranes, haemorrhages and oedema<sup>1</sup>. Although cattle are the main reservoir of infection, sheep are more severely affected and can suffer a cyanotic tongue, lending the disease name. The virus has been isolated from hosts and vectors on all continents (excluding Antartica)<sup>2</sup>, despite being initially recognised in Africa in the late 19th and early 20th centuries<sup>3</sup>. Originally thought to be a disease of tropical and sub-tropical regions, bluetongue has shown a propensity to become established in temperate areas, and in recent years has spread North, through the Mediterranean Basin, to become endemic in many European countries including the UK. Although BTV's transmission and epidemiology is dependent on insect vectors, bluetongue greatly influences the global trade of ruminants as it is included on the Office International des Epizooties List A of animal diseases<sup>4</sup>.
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| − | ==Virus Characteristics==
| + | Bluetongue is an infectious, noncontagious arthropodborne viral disease primarily of domestic and wild ruminants. Infection with bluetongue virus is common worldwide but is usually subclinical or mild in most infected ruminants. Bluetongue is almost exclusively a disease of sheep, particularly the fine-wool and mutton breeds, although white-tailed deer ( Odocoileus virginianus ), and pronghorn ( Antilocapra americana ) and desert bighorn sheep ( Ovis canadensis ) may develop severe clinical disease in North America. |
| − | [[Image:Bluetongue Virus.gif|thumb|right|150px|Bluetongue virus particle. Source: Wikimedia Commons; Author: CDC (2007)]]
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| − | Bluetongue virus is a species of the genus Orbivirus, within the [[:Category:Reoviridae|Reoviridae]] family. The Reoviridae are non-enveloped and possess a double-stranded RNA genome contained in an outer-shelled icosohedral capsid. The BTV genome is arranged into 10 segments and encodes 7 structural and 4 non-structural viral proteins<sup>2</sup>. The BTV receptor is currently unknown, but is proposed to included sialic acid and junctional adhesion molecules. After interaction with this receptor, the virus enters an endolysosome where the capsid is partially digested to allow the genome into the cell. Replication begins at this partially uncoated stage since the virus particles contain all the necessary enzymes<sup>5</sup>. First, the dsRNA is transcribed to form positive sense RNA, of which some is delivered to cytoplasm for ribosomal translation and the remainder is packaged into partially assembled virions. Complementary negative sense RNA is then formed in the virions, to give a dsRNA genome. Complete virus particles are released from the cell. | |
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| − | All BTVs share group antigens, which can be demonstrated by agar gel diffusion tests, fluorescent antibody tests and the group
| + | ==Aetiology== |
| − | reactive [[ELISA testing|ELISA]]<sup>1</sup>. There are 24 distinct serotypes, which are distinguished by epitopes on the outer capsid protein VP2<sup>4</sup>, encoded by L2, the only serotype-specific BTV gene. Serotypes are differentiated using serum neutralisation tests, although there is some degree of cross-reactivity between serotypes<sup>1</sup>. Numerous strains of bluetongue virus also exist, and these are characterised by molecular analysis.
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| | + | Bluetongue virus is the type-species of the genus Orbivirus in the family Reoviridae. There are 24 serotypes worldwide, although not all serotypes exist in any one geographic area, eg, only 5 serotypes (2, 10, 11, 13, and 17) have been reported in the USA. Distribution throughout the world parallels the spatial and temporal distribution of vector species of Culicoides biting midges, which are the only significant natural transmitters of the virus. Of more than 1,400 Culicoides species worldwide, fewer than 20 are actual or possible vectors of bluetongue virus. Continued cycling of the virus among competent Culicoides vectors and susceptible ruminants is critical to viral ecology. In the USA, the principal biologic vector is C variipennis sonorensis , which limits distribution of the virus to southern and western regions. In Australia the principal vector is C brevitarsis , while in Africa, Europe, and the Middle East it is C imicola . In each geographic region, secondary vector species may attain local importance. Vectors become infected with bluetongue virus by imbibing blood from infected vertebrates; transovarial transmission has not been reported. High affinity of the virus to blood cells, especially the sequestering of viral particles in invaginations of RBC membranes, contributes to prolonged viremia in the presence of neutralizing antibody. The extended viremia in cattle (up to 9 wk), and the host preference of most vector species of Culicoides for cattle, provides a mechanism for year-round transmission in domestic ruminants. Mechanical transmission by other bloodsucking insects is of minor significance. Bluetongue virus is not contagious, and concentrations in secretions and excretions are minimal, making oral or aerosol transmission unlikely. However, semen from viremic bulls can serve as a source of infection for cows through natural service or artificial insemination. Embryo transfer is regarded as safe, provided that donors are not viremic and an appropriate washing procedure for embryos is used. Accidental infection has been reported in dogs in the USA following administration of a modified live virus vaccine that was contaminated with the virus. Serologic evidence of infection with bluetongue virus has been found in large carnivores in Africa, perhaps as a result of ingesting virus-infected viscera. |
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| | ==Hosts== | | ==Hosts== |
| − | All ruminants are susceptible to bluetongue virus infection, including sheep, goats, cattle, deer, buffaloes, camels and antelopes. Sheep are most severely affected, and disease is occasionally seen in goats. Although cattle BTV infection is significant in the epidemiology of disease, the condition is generally subclinical in this host. Mortalities in white-tailed deer due to bluetongue have been reported in the USA<sup>1</sup>.
| + | *Ruminants, including sheep, cattle, deer, goats, and camelids |
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| − | Pigs and horses do not become infected with BTV, but may act as a food source for the [[Ceratopogonidae|''Culicoides'']] midges that transmit bluetongue virus to ruminants. Their habitats may also provide areas suitable for vector breeding.
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| − | ==Transmission and Epidemiology==
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| − | BTV is transmitted by biting insects. Although vertical and venereal transmission between ruminant hosts can occur, it is insignificant in the overall epidemiology of bluetongue.
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| − | ===Vectors===
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| − | The arthropod vector for bluetongue virus is the [[Ceratopogonidae|''Culicoides'']] biting midge. These are biological vectors of BTV, so the virus replicates in insect tissue after feeding on an infected host<sup>6</sup>. It takes 10-14 days for the virus to disseminated from the insect's gut to its salivary glands, after which bluetongue virus may be transmitted to a new, susceptible ruminant host. This incubation period may be reduced when ambient temperatures are higher<sup>2</sup> and once infected, midges maintain BTV infection for life.
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| − | Classically, the major vector for BTV is ''Culicoides imicola''. This midge is found throughout Africa, the
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| − | Middle East, southern Asia, Portugal, Greece, Corsica, Sardinia, Sicily and areas of Italy<sup>1</sup>, and its distribution appears to be extending northwards. However, ''C. imicola'' has not yet been demonstrated in the United Kingdom.
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| − | ===Vector Competence===
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| − | Certain information can help inferences be made regarding BTV vectors in Britain. Both ''C. obsoletus'' and ''C. pulicaris'' have been implicated in transmission before. Previously, BTV has been isolated from ''C. obsoletus'' in Cyprus, and [[African Horse Sickness|African horse sickness virus]] (another Orbivirus) in Spain. ''C. obsoletus'' and ''C. pulicaris'' were also the most abundant ''Culicoides'' species detected in the 1999 BTV epizootic in Greece and Bulgaria, and so are strongly suspected of acting as vectors in this case. They may also have mediated outbreaks in Serbia, FYR Macedonia, Croatia and Bosnia in 2001-2002, where ''C. imicola'' has not been recorded. Both species are therefore contenders to transmit bluetongue virus in the UK<sup>1</sup>.
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| − | A British population of ''C. obsoletus'' has been shown to have and oral susceptibility rate of less than 2%<sup>1</sup>, suggesting that ''C. obsoletus'' is likely to be an inefficient or minor vector of BTV in the UK. However, it is possible that a high abundance or survival rate may compensate for this low vector competence. Indeed, ''C. brevitarsis'', the major Australian vector of BTV, has an extremely low experimental competency yet is an effective vector in the field.
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| − | Two other ''Culicoides'' species, ''C. nuberculous'' and ''C. impunctatus'', exist in mainland Europe and the UK, and have been experimentally infected with bluetongue virus.
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| − | ===Epidemiology===
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| − | Although bluetongue virus is capable of infecting any ruminant, cattle are the main amplifying and maintenance hosts and are most abundantly fed on by ''Culicoides'' vectors. Infection of sheep with BTV is therefore usually preceded by widespread infection of cattle and an increase in vector density<sup>1</sup>.
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| − | Although vertical and venereal transmission of bluetongue is possible, only to the presence of competent insect vectors influences the epidemiology of BTV<sup>2</sup>. This is illustrated by the fact that bluetongue virus is limited to geographical areas where competent vectors are present and that transmission only occurs at times of the year when conditions are favourable for vector activity<sup>1</sup>. In Britain, transmission occurs mainly in late summer and autumn. Once bluetongue virus is transmitted to a vertebrate host, there are two possible outcomes: either the host dies, or an immune response is mounted against the virus and the host is rendered resistant to re-infection. Either way, animals quickly become "unavailable" for BTV infection as the virus spreads, particularly where livestock populations are small. This presents a hurdle that must be surmounted if bluetongue virus is to persist in an area. By movement of infected vectors or viraemic animals, BTV can become established in new locations with naive hosts in order to overcome this obstacle. This means that even in zones where bluetongue virus is endemic, persistence is dynamic and comprises perpetually shifting "hot spots" of infection<sup>1</sup>. Creation of an enzootic zone is only possible in locations where adult midges are present throughout the year since bluetongue cannot be maintained through vertebrate-vertebrate or vector transovarial transmission. Any points where vectors are absent from the system must not exceed the maximum duration of viraemia in the ruminant host, otherwise the last infected vertebrate will have died or recovered by the time new vectors are available for onwards transmission.
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| − | In some areas, bluetongue can occur in annual bouts. This may be due to new introduction of virus each year from adjacent areas where the disease is endemic, via the transportation of ''Culicoides'' on the wind for up to 100 kilometres. Alternatively, this could be the manifestation of low-level persistence.
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| − | Introduction of bluetongue virus to a new area has the potential to occur in several ways. Firstly, infected animals may be transported to the region, and local insect vectors could spread and perpetuate BTV infection within naive animals. It is also possible that local vectors could acquire BTV from infected animals in neighbouring areas, where there is a cross-over in the distribution of ''Culicoides'' species. Finally, infected vectors can be acquired from areas where bluetongue infection exists. ''Culicoides'' can be transported considerable distances on the wind, and it is also conceivable that the distribution of competent vectors may expand to colonise previously unpopulated areas under the influence of climate change.
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| − | Bluetongue virus infection was first confirmed in the UK in September 2007 when a veterinarian spotted suspicious clinical signs on a cattle holding near Ipswich<sup>7</sup>. It is believed that BTV-laden vectors were dispersed to the UK on the wind, since meteorological conditions on 4th August 2007 were capable of carrying midges from northern Europe to East Anglia. This would be expected to produce disease at a point that would coincide with the first case, given the time necessary for clinical detection to occur.
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| | ==Pathogenesis== | | ==Pathogenesis== |
| − | The pathogenesis of BTV infection has been shown to be similar in sheep and cattle, and is assumed to be similar in other species of ruminants<sup>8, 9, 10</sup>. However, the severity of disease varies greatly with species and cattle in particular express very few signs.
| + | *Transfer occurs through blood from viremic animals via biting midges ('''Culicoides spp.''') |
| | + | *Replication in haematopoietic and endothelial cells of blood vessels |
| | + | *Clinical signs vary between species, with sheep most severely affected |
| | + | **Pyrexia |
| | + | **Ocular and nasal discharge |
| | + | **Drooling from mouth uclers |
| | + | **Swelling of the mouth, head and neck |
| | + | **Lameness |
| | + | **Subdural hemorrhages |
| | + | **Inflammation of the coronary band |
| | + | *Cattle as the main reservoir |
| | + | *A blue tongue is rarely seen as as a clinical sign of infection |
| | + | *Resulting loss of condition, reduction in wool an meat production, which can be followed by death |
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| − | When a BTV-infected midge takes a blood meal from a ruminant host, innoculated virus spreads from the skin to replicate in the regional lymph nodes, tonsils and spleen<sup>11</sup>. A secondary cell-associated viraemia then carries the virus to many tissues where further replication occurs in macrophages and endothelial cells. In the process of reproducing, bluetongue virus causes endothelial cell injury and necrosis<sup>10</sup> which can increase vascular permeability to cause oedema. Endothelial damage can also give thrombosis, leading to tissue infarction. In sheep and deer a [[Disseminated Intravascular Coagulation|consumptive coagulopathy]] may occur<sup>2</sup>.
| + | ==Diagnosis== |
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| − | Several factors can influence the presentation of disease. Firstly, each virus strain is associated with its own particular virulence and thus clinical manifestation<sup>10</sup>. Host factors are also important: breed<sup>3, 12</sup>, stress, nutritional status and age<sup>12, 13</sup> can all affect bluetongue presentation.
| + | The typical clinical signs of bluetongue enable a presumptive diagnosis, especially in areas where the disease is endemic. Suspicion is confirmed by the presence of petechiae, ecchymoses, or hemorrhages in the wall of the base of the pulmonary artery and focal necrosis of the papillary muscle of the left ventricle. These highly characteristic lesions are usually obvious in severe clinical infections but may be barely visible in mild or convalescent cases. These lesions are often described as pathognomonic for bluetongue, but they have also been observed occasionally in other ovine diseases such as heartwater, pulpy kidney disease, and Rift Valley fever. Hemorrhages and necrosis are usually found where mechanical abrasion damages fragile capillaries, such as on the buccal surface of the cheek opposite the molar teeth and the mucosa of the esophageal groove and omasal folds. Other autopsy findings include subcutaneous and intermuscular edema, skeletal myonecrosis, myocardial and intestinal hemorrhages, hydrothorax, hydropericardium, pericarditis, and pneumonia. In many areas of the world, bluetongue in sheep, and especially in other ruminants, is subclinical and, therefore, laboratory confirmation based on virus isolation in embryonated chicken eggs, susceptible sheep, or cell cultures, or the identification of viral RNA by PCR is necessary. The identity of isolates may be confirmed by the group-specific antigen-capture ELISA, immunofluorescence, immunoperoxidase, serotype-specific virus neutralization tests, or hybridization with complementary gene sequences of group- or serotype-specific genes. For virus isolation, blood (10-20 mL) is collected as early as possible from febrile animals into an anticoagulant such as heparin, sodium citrate, or EDTA and transported at 4°C to the laboratory. For longterm storage where refrigeration is not possible, blood is collected in oxalate-phenol-glycerin (OPG). Blood to be frozen should be collected in buffered lactose peptone and stored at or below -70°C. Blood collected at later times during the viremic period should not be frozen, as lysing of the RBC or thawing releases the cell-associated virus, which may then be neutralized by early humoral antibody. The virus does not remain stable for long at -20°C. In fatal cases, specimens of spleen, lymph nodes, or red bone marrow are collected and transported to the laboratory at 4°C as soon as possible after death. A serologic response in ruminants can be detected 7-14 days after infection and is generally lifelong. Current recommended serologic techniques for the detection of bluetongue virus antibody include agar gel immunodiffusion and competitive ELISA. The latter is the test of choice and does not detect cross-reacting antibody to other orbiviruses, especially anti-EHDV (epizootic hemorrhagic disease virus) antibody. Various forms of virus neutralization test, including plaque reduction, plaque inhibition, and microtiter neutralization can be used to detect type-specific antibody. |
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| − | ==Diagnosis==
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| − | Where animals present with clinical signs of bluetongue a presumptive diagnosis may be made, especially in regions where bluetongue is endemic. Post-mortem examination can be used to confirm the diagnosis. However, many cases of bluetongue are mild or subclinical and so laboratory confirmation of disease is required. Cattle in particular show few clinical signs.
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| | ===Clinical Signs=== | | ===Clinical Signs=== |
| − | Bluetongue is primarily a disease of '''sheep''', and in the face of infection these animals can display clinical signs ranging from acute to subclinical<sup>1, 14</sup>. Acute disease follows an incubation period of about one week, which may depend on the infectious dose of virus received. Signs begin with pyrexia of around 40.5-42°C, and hyperaemia of the oral and nasal mucosa is seen 24-36 hours later accompanied by hypersalivation and a serous nasal discharge. The nasal discharge quickly becomes mucopurulent and potentially blood-tinged, and dries to form a crust around the nostrils. Oedema of the head occurs, which particularly affects the lips and tongue but may also spread to include the ears and submandibular areas. In a few cases the tongue becomes severely swollen and cyanotic, lending the disease its name. Petechial haemorrhages appear on the still-hyperaemic mucosae, and areas of necrosis appear on the gums, cheeks and tongue 5-8 days after the onset of fever. Covered by a diphtheritic membrane, these necrotic lesions heal slowly and contribute to inappetance, dysphagia and hypersalivation. In some cases, profuse bloody diarrhoea is seen. Erythema and petechiation of the coronary band can cause lameness, and sheep stand with an arched back, reluctant to move. In advanced disease, skeletal muscle is necrosed and contributes to rapid weight loss, along with inappetance. Animals in the late stage may also suffer torticollis. In pregnant ewes, infection with BTV may lead to abortions, foetal mummification, or the birth of stillborn or weak lambs, which may suffer congenital defects.
| + | The course of the disease in sheep can vary from peracute to chronic, with a mortality rate of 2-30%. Peracute cases die within 7-9 days of infection, mostly as a result of severe pulmonary edema leading to dyspnea, frothing from the nostrils, and death by asphyxiation. In chronic cases, sheep may die 3-5 wk after infection, mainly as a result of bacterial complications, especially pasteurellosis, and exhaustion. Mild cases usually recover rapidly and completely. The major production losses include deaths, unthriftiness during prolonged convalescence, wool breaks, and possibly reproductive loss. In sheep, bluetongue virus causes vascular endothelial damage, resulting in changes to capillary permeability and subsequent intravascular coagulation. This results in edema, congestion, hemorrhage, inflammation, and necrosis. The clinical signs in sheep are typical. After an incubation period of 4-6 days, a fever of 105-107.5°F (40.5-42°C) develops. The animals are listless and reluctant to move. Clinical signs in young lambs are more apparent, and the mortality rate is higher (up to 30%). About 2 days after onset of fever, additional clinical signs such as edema of lips, nose, face, submandibular area, eyelids, and sometimes ears; congestion of mouth, nose, nasal cavity, conjunctiva, and coronary bands; and lameness and depression may be seen. A serous nasal discharge is common, later becoming mucopurulent. The congestion of nose and nasal cavity produces a “sore muzzle” effect, the term used to describe the disease in sheep in the USA. Sheep eat less because of oral soreness and will hold food in their mouths to soften before chewing. They may champ to produce a frothy oral discharge at the corners of the lips. On close examination, small hemorrhages can be seen on the mucous membranes of the nose and mouth. Ulceration develops where the teeth come in contact with lips and tongue, especially in areas of most friction. Some affected sheep have severe swelling of the tongue, which may become cyanotic (‘blue tongue”) and even protrude from the mouth. Animals walk with difficulty as a result of inflammation of the hoof coronets. A purple-red color is easily seen as a band at the junction of the skin and the hoof. Later in the course of disease, lameness or torticollis is due to skeletal muscle damage. In most affected animals, abnormal wool growth resulting from dermatitis may be observed. |
| − | | + | The pathogenesis of bluetongue in cattle seems to differ from that in sheep and is based on immediate IgE hypersensitivity reactions. Clinical signs in cattle are rare but may be similar to those seen in sheep. They are usually limited to fever, increased respiratory rate, lacrimation, salivation, stiffness, oral vesicles and ulcers, hyperesthesia, and a vesicular and ulcerative dermatitis. Susceptible cattle and sheep infected during pregnancy may abort or deliver malformed calves or lambs. The malformations include hydranencephaly or porencephaly, which results in ataxia and blindness at birth. White-tailed deer and pronghorn antelope develop severe hemorrhagic disease leading to sudden death. Pregnant dogs abort or give birth to stillborn pups and then die in 3-7 days. |
| − | Considering all cases, including those which are subclinical, mortality due to bluetongue in sheep ranges between 2% and 30%<sup>14</sup>. Death can occur up to a month after the onset of clinical signs, or a protracted recovery may follow acute infection. Recovery in mild cases is often much more rapid.
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| − | Disease is most often sub-clinical in '''cattle''' despite its epidemiological significance in this species: it has been reported that only 0.01% of infected cattle show signs of bluetongue<sup>1</sup>. Clinical signs, when seen, can include inflammation or erosions of the oral and nasal mucosae and a stiff gait. Similar signs to sheep may also occur: pyrexia, tachypnoea, lacrimation, salivation and an ulcerative dermatitis are all possibilities. In cattle in early gestation, embryonic death and resorption can result from BTV infection.
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| | ===Laboratory Tests=== | | ===Laboratory Tests=== |
| − | In the United Kingdom, bluetongue is a notifiable disease and so samples from suspected cases should be submitted to the Institute for Animal Health (Pirbright) for laboratory diagnosis. Samples are collected from sheep with raised temperatures and include jugular blood collected into a plain tube to provide serum for an antibody test, and a heparinised blood sample to be used for PCR. Serum from in-contact ruminants is also submitted, as well as spleen and lymph node from all post-mortem cases. All samples should be stored at 4°C and never frozen. Paired serology may be necessary.
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| − | There is an array of laboratory tests available for the diagnosis of bluetongue, and a table published by DEFRA<sup>1</sup> summarises these:
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| − | {| class="wikitable collapsible" width="75%" align="center"
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| − | | <center> <u> '''Test''' </u> </center>
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| − | | <center><u>'''Specimen Required'''</u> </center>
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| − | |<center><u>'''Test Detects'''</u> </center>
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| − | |<center><u>'''Timescale'''</u> </center>
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| − | |-
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| − | |<u>'''Virus Isolation'''</u>
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| − | |<center>Whole EDTA blood</center>
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| − | |<center>Virus</center>
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| − | |<center>1 - 3 weeks</center>
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| − | |-
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| − | |<U>'''Antigen Detection'''</U> <BR>Sandwich ELISA <BR>PCR <BR>Serum Neutralisation<BR>
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| − | |<center><BR>Whole heparin or EDTA blood; tissues <BR>Whole EDTA blood; tissues <BR>Whole heparin or EDTA blood; tissues<BR> </center>
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| − | |<center><BR>Antigen - group specific <BR>Viral RNA - group specific<BR>Serotype<BR> </center>
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| − | |<center><BR>Blood: 5 - 14 days; Tissues: 4 hours <BR>2 days <BR>2 - 4 weeks<BR> </center>
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| − | |-
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| − | |<U>'''Antibody Detection'''</U> <BR> Competition ELISA <BR> Serum Neutralisation <BR> Pathogenicity Testing in Sheep <BR>
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| − | |<center><BR>Serum <BR>Serum <BR> Virus isolate <BR> </center>
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| − | |<center><BR>Antibody - group specific <BR>Antibody - serotype specific <BR>Virulence<BR> </center>
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| − | |<center><BR>3 hours <BR>2 - 4 weeks <BR>2 weeks<BR> </center>
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| − | |-
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| − | |}
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| | ===Pathology=== | | ===Pathology=== |
| − | A haemorrhagic gross pathology of BTV infection reflects the endothelial damage responsible for disease pathogenesis<sup>1, 4, 10, 12</sup>. Certain lesions have been described as "pathognomic" for bluetongue: these include necrosis of the papillary muscle in the left ventricle, and haemorrhage in pulmonary arterial wall. However, these lesions may be difficult to visualise in mild or recovering cases and may occasionally occur in other diseases such as [[Pulpy Kidney|pulpy kidney]] disease or [[Rift Valley Fever]].
| + | Complete loss of integrity of epithelium. Uncommon. |
| − | | + | *Characteristic of Bluetongue Virus, |
| − | In addition to these characteristic lesions, the oral mucosa is found to be hyperaemic and oedematous and occasionally cyanotic on post-mortem examination, and petechial or ecchymotic haemorrhages may be present. The ruminal pillars and omasal folds can also appear hyperaemic, and abrasions may be seen on the lips, dental pad, tongue and cheeks. These are sometimes covered by grey necrotic material. Moderate lymphomegaly and splenomegaly are apparent, and there are areas of necrosis in the skeletal musculature. Pulmonary oedema and catarrhal inflammation of the upper respiratory tract is seen in some cases.
| + | *Epithelium lost and haemorrhage produces blue / black discoloration of the [[Oral Cavity - Tongue - Anatomy & Physiology|tongue]], hence the name. |
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| − | Histologically, endothelial damage in capillaries and minor arterioles causes thrombus formation and vascular occlusion, leading to tissue infarction. Haemorrhage, necrosis and mononuclear cell infiltration may be seen in the myocardium.
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| − | ==Control==
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| − | There is no efficient treatment for bluetongue, and so the emphasis is on prophylaxis and control.
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| − | ===Vaccination===
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| − | Vaccines against BTV are available, and in different situations may be used to prevent or control outbreaks of bluetongue. Initially, modified live BTV vaccines were used, particularly in Africa, the United States and southern Europe<sup>4</sup>. Although useful for the control of disease, these vaccines have the potential to introduce novel strains of virus into the environment, which could lead to vector infection and reversion to virulence by evolution or genome reassortment with wild-type viruses. Foetal infection and teratogenesis are also possible. Killed, adjuvanted vaccines for some serotypes are now available, which are much safer. It is this type of product that has been used in recent years for control of BTV-8 in the UK.
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| − | ===Vector Control===
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| − | As the cycle of bluetongue transmission involves a ''Culicoides'' vector, elements of disease control can be targetted at controlling midges. This can involve the use of insecticides on ''Culicoides'' breeding grounds to reduce midge numbers, and insect repellants on livestock to limit feeding on potential BTV hosts. Animals can also be housed indoors at dawn and dusk, when midges are most active. So far, little evidence has supported the value of these control measures, and the practicalities of implementing these strategies may preclude them from widespread use.
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| − | ===Surveillance===
| + | *Grossly: |
| − | Surveillance is a key component to controlling the spread of bluetongue. As well as vigilance in the face of a bluetongue outbreak to determine the extent of disease spread, ongoing surveillance is required to limit the possibility of BTV entering areas where the virus is not already circulating. The UK currently achieves ongoing surveillance through two mechanisms<sup>15</sup>. Firstly, bluetongue is a notifiable disease, and so it is a legal requirement for livestock holders to report all new cases of bluetongue on their premises. This allows new midge-transmission from the continent and any re-emergence of disease to be assessed. Secondly, every susceptible animal imported from Continental Europe, where bluetongue is currently circulating, is post-import tested for all serotypes of bluetongue virus. This ensures infected animals are not introduced to the country.
| + | **Infarctions -> necrosis |
| | + | **Haemorrhage |
| | + | *Histologically: |
| | + | **Necrosis -> calcification or regeneration (depends on age of lesion) |
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| − | ===Controlling Spread of Bluetongue in Europe=== | + | ==Treatment== |
| − | Although some factors influencing the introduction of bluetongue to an area are outwith human control, such as transportation of ''Culicoides'' on the wind, certain measures can be taken to help avoid BTV becoming established in an area. This involves designation of areas where bluetongue is circulating as restricted (or protection) zones for the specific serotype, and imposing movement limitations within and between these areas. Animals in protection zones may be moved within the zone, and to confluent protection zones for the same serotype. However, they may not be moved to free zones (BTV-free areas), although animals from free zones may be moved without restriction. A map of the current restriction/protection zones in Europe is available from the [http://ec.europa.eu/food/animal/diseases/controlmeasures/bt_restrictedzones-map.jpg European Commission Website].
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| − | Since animals may be moved freely between protection zones for the same serotype, a protection zone which does not currently have circulating BTV is at risk of becoming re-infected. This fact has lead to the designation of lower risk zones. Vaccination is only normally permitted within protection zones, but livestock holders in lower risk zones are able to vaccinate their animals. There are also more stringent regulations on bringing in animals from confluent protection zones. Therefore, the risk of re-introducing bluetongue is reduced and the area can move towards BTV freedom more confidently. A further advantage of lower risk zones is that fewer limitations apply to the export of livestock to free zones or areas with other BTV serotypes.
| + | *BTV is '''NOTIFIABLE''' |
| | + | *Vigilance in recognizing clinical signs |
| | + | *Restriction of movement: |
| | + | **Protection Zone: 100km radius around infected premises, movement within zone allowed but not in or out |
| | + | ***Vaccination within PZ using appropriate serotype is encouraged but still voluntary |
| | + | **Surveillance Zone: 50km radius beyond PZ |
| | + | *Vector control: ectoparasiticides, etc. |
| | | | |
| − | ===Control in the UK===
| + | Prophylactic immunization of sheep remains the most effective and practical control measure against bluetongue in endemic regions. Three polyvalent vaccines, each comprising 5 different bluetongue virus serotypes attenuated by serial passage in embryonated hens’ eggs followed by growth and plaque selection in cell culture, are widely used in southern Africa and elsewhere, should epizootics of bluetongue occur. A monovalent modified live virus vaccine propagated in cell culture is available for use in sheep in the USA. Live-attenuated vaccines should not be used during Culicoides vector seasons because they may transmit the vaccine virus(es) from vaccinated to nonvaccinated animals, eg, other ruminant species. This may result in reassortment of genetic material and give rise to new viral strains. Abortion or malformation, particularly of the CNS, of fetuses may follow vaccination of ewes and cows with attenuated live vaccines during the first half and the first trimester of pregnancy, respectively. Passive immunity in lambs usually lasts 4-6 mo. The control of bluetongue is different in areas where the disease is not endemic. During an outbreak, when one or a limited number of serotypes may be involved, vaccination strategy depends on the serotype(s) that are causing infection. Use of vaccine strains other than the one(s) causing infection affords little or no protection. The vector status, potential risk from vaccine virus reassortment with wild-type viral strains, virus spread by the vectors to other susceptible ruminants, and reversion to virulence of vaccine virus strains or even the production of new serotypes also should be considered. Although a number of noninfectious vaccines are in development, they are not yet commercially available. Control of vectors by using insecticides or protection from vectors by moving animals into barns during the evening hours lowers the number of Culicoides bites and subsequently the risk of exposure to bluetongue virus infection. |
| − | In 2007, BTV-8 was confirmed in the UK and control measures implemented. However, the last BTV-8 infected premise was confirmed in the UK in November 2008, and since this date the situation has remained unchanged. No imported ruminants have tested positive for BTV-8 and there is no evidence that the disease has been circulating since this point. On 12th June 2010 the UK's status was changed from a protection zone for BTV-8 to a lower risk zone (LRZ). This status continues to allow vaccination against BTV, and imposes stricter restrictions on importing animals from zones with the same BTV serotype. The aim of this is to help prevent bluetongue re-entering the country. Up-to-date information on the current UK bluetongue situation can be found on the [http://www.defra.gov.uk/foodfarm/farmanimal/diseases/atoz/bluetongue/latest/index.htm DEFRA Bluetongue: Latest situation] webpage. At this point in time (Autumn 2010), vaccination is voluntary within the UK.
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| − | | |
| − | In the UK, Bluetongue is a notifiable disease and so cases must be reported to the local AHO. There are also further obligations for notification: within 24 hours of confirmation of bluetongue in the UK, the Chief Veterinary Officer must inform the European Commission and the OIE Central Bureau. There are several broad principles of disease control when a bluetongue outbreak occurs in the UK<sup>16</sup>. Firstly, premises suspected of having the disease are inspected by a veterinary surgeon, and a ban is placed on moving animals on and off the site. Once it has been confirmed that bluetongue is circulating (i.e. there is not a single, isolated case of disease, for example due to importation), a restricted zone is imposed around the infected premises. A restricted zone is the overall area where restrictions apply and is composed of a protection zone (100km radius) surrounded by a surveillance zone (50km radius). The sizes of these zones are dictated by EU legislation for bluetongue control. The restricted zone may also contain control zones of tighter restrictions in the immediate vicinity (20km) of infected premises. Movements are permitted within protection and suveillance zones, and from the surveillance zone to the protection zone. Animals from neighbouring BTV-free areas may move into either the surveillance or protected zones. However, the converse of any of these movements is not permitted unless animals are travelling to slaughter. No movements are permitted within, to, or from the control zone.
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| − | | |
| − | In addition to movement restrictions, surveillance for disease and vectors is implemented as necessary in an outbreak, and relevant communications are made to livestock owners and veterinary surgeons to advise of the measures in place. The aim at all times is to tightly control disease, with an aim to eradication, and vaccination will normally play a large part in this according to plans laid out by DEFRA. A test and slaugher policy is not used, because BTV is not transmitted directly between susceptible animals.
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| − | | |
| − | {{Learning
| |
| − | |flashcards = [[Cattle Medicine Q&A 06]]
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| − | |literature search = [http://www.cabdirect.org/search.html?rowId=1&options1=AND&q1=title:(Bluetongue)+OR+(%22Blue+tongue%22)&occuring1=freetext&rowId=2&options2=AND&q2=&occuring2=freetext&rowId=3&options3=AND&q3=&occuring3=freetext&publishedstart=2000&publishedend=yyyy&calendarInput=yyyy-mm-dd&la=any&it=any&show=all&x=42&y=4 Bluetongue publications since 2000]
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| − | }}
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| | | | |
| | ==Links== | | ==Links== |
| | | | |
| | *[http://www.defra.gov.uk/foodfarm/farmanimal/diseases/atoz/bluetongue/index.htm Defra - Bluetongue] | | *[http://www.defra.gov.uk/foodfarm/farmanimal/diseases/atoz/bluetongue/index.htm Defra - Bluetongue] |
| − | *[http://www.oie.int/eng/maladies/fiches/a_A090.htm Office International des Epizooties (World Organisation for Animal Health) - Bluetongue]
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| − | *[http://www.bluetongue-info.co.uk BTV Control in Cattle and Sheep (Intervet)]
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| − | *[http://www.iah.ac.uk/disease/bt_aw.shtml Institute for Animal Health - Bluetongue]
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| − | *[http://www.bluetonguevirus.org/ Bluetonguevirus.org - BTV information and resource portal]
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| | | | |
| | ==References== | | ==References== |
| | | | |
| − | #DEFRA (2002) [http://www.defra.gov.uk/foodfarm/farmanimal/diseases/atoz/documents/bluetongue_technical.PDF Technical Review - Bluetongue : The Virus, Hosts and Vectors.]
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| − | #Gibbs, E P J and Geiner, E C (1994) The Epidemiology of Bluetongue. ''Comparative Immunology, Microbiology and Infectious Diseases'', '''17(3-4)''', 207-220.
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| − | #Spreull, J (1905) Malarial catarrhal fever (bluetongue) of sheep in South Africa. ''Journal of Comparative Pathology and Therapeutics'', '''18''', 321-337.
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| − | #MacLachlan, N J (2004) Bluetongue: A Review and Global Overview of the Only OIE List a Disease that is Endemic in North America. ''Proceedings of the 55th Annual Meeting of the American College of Veterinary Pathologists (ACVP) and 39th Annual Meeting of the American Society of Clinical Pathology (ASVCP)'', p1237.
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| − | #Carter, G R and Wise, D J (2005) '''A Concise Review of Veterinary Virology''', ''IVIS''.
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| − | #Mellor, P S (2000) Replication of arboviruses in insect vectors. ''Journal of Comparative Pathology'', '''123''', 231-247.
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| − | #IAH (2008) [http://www.iah.ac.uk/disease/bt_aw.shtml Institute for Animal Health - Bluetongue Research Programme]
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| − | #Barratt-Boyes, S M and MacLachlan, N J (1995) Pathogenesis of bluetongue virus infection of cattle. ''Journal of the American Veterinary Medical Association'', '''206(9)''', 1322-1329.
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| − | #MacLachlan, N J (1994) The pathogenesis and immunology of bluetongue virus infection of ruminants. ''Comparative Immunology, Microbiology and Infectious Diseases'', '''17(3-4)''', 197-206.
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| − | #Mahrt, C R and Osburn, B I (1986) Experimental bluetongue virus infection of sheep; effect of vaccination: pathologic, immunofluorescent, and ultrastructural studies. ''American Journal of Veterinary Research'', '''47''', 1198-1203.
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| − | # Pini, A (1976) Study on the pathogenesis of bluetongue: replication of the virus in the organs of infected sheep. ''Onderstepoort Journal of Veterinary Researh'', '''43''', 159-164.
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| − | #Parsonson, I M (1991) Overview of bluetongue virus infection of sheep. '''Bluetongue, African Horse Sickness and Related Orbiviruses''', ''CRC Press''.
| |
| − | #Thomas, A D and Neitz, W O (1947) Further observations on the pathology of bluetongue in sheep. ''Onderstepoort Journal of Veterinary Science and Animal Industry'', '''22''', 27-40.
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| − | #Mullens, B A et al (1995) Effects of temperature on virogenesis of bluetongue virus serotype 11 in Culicoides variipennis sonorensis. ''Medical and Veterinary Entomology'', '''9''', 71-76.
| |
| − | #[http://www.defra.gov.uk/foodfarm/farmanimal/diseases/atoz/bluetongue/control/index.htm www.defra.gov.uk - Bluetongue: Surveillance and control.]
| |
| − | #DEFRA (2008) [http://www.defra.gov.uk/foodfarm/farmanimal/diseases/atoz/documents/bluetongue-control-strategy081201.pdf UK Bluetongue Control Strategy.]
| |
| − | #Gould, A R and Hyatt, A D (1994) The orbivirus genus: diversity, structure, replication and phylogenetic relationships. ''Comparative Immunology, Microbiology and Infectious Diseases'', '''1''', 163-188.
| |
| | #Merck & Co (2008) '''The Merck Veterinary Manual (Eighth Edition)''', ''Merial''. | | #Merck & Co (2008) '''The Merck Veterinary Manual (Eighth Edition)''', ''Merial''. |
| − | #Dal Pozzo, F et al (2009) Bovine infection with bluetongue virus with special emphasis on European serotype 8. ''The Veterinary Journal'', '''182(2)''', 142-151.
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| − | #MacLachlan, N J et al (2009) The Pathology and Pathogenesis of Bluetongue. ''Journal of Comparative Pathology'', '''141(1)''', 1-16.
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| − | #Afshar, A (2004) Bluetongue: Laboratory Diagnosis. ''Comparative Immunology, Microbiology and Infectious Diseases'', '''17(3-4)''', 221-242.
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| − | #Gould, E A and Higgs, S (2009) Impact of climate change and other factors on emerging arbovirus diseases. ''Transactions of the Royal Society of Tropical Medicine and Hygiene'', '''103(2)''', 109-121.
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| − | {{review}}
| + | [[Category:Orbiviruses]][[Category:Cattle]][[Category:Sheep]][[Category:Pig]] |
| − | [[Category:Orbiviruses]][[Category:Cattle Viruses]][[Category:Sheep Viruses]][[Category:Goat Viruses]][[Category:Camelids]] | + | [[Category:Tongue_-_Pathology]][[Category:To_Do_-_Lizzie]] |
| − | [[Category:Tongue_-_Pathology]] [[Category:Brian Aldridge reviewing]] | |
| − | [[Category:Oral Diseases - Cattle]][[Category:Oral Diseases - Sheep]][[Category:Oral Diseases - Goat]]
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| − | [[Category:Respiratory Diseases - Sheep]][[Category:Viral Myositis]]
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Description
Bluetongue is an infectious, noncontagious arthropodborne viral disease primarily of domestic and wild ruminants. Infection with bluetongue virus is common worldwide but is usually subclinical or mild in most infected ruminants. Bluetongue is almost exclusively a disease of sheep, particularly the fine-wool and mutton breeds, although white-tailed deer ( Odocoileus virginianus ), and pronghorn ( Antilocapra americana ) and desert bighorn sheep ( Ovis canadensis ) may develop severe clinical disease in North America.
Aetiology
Bluetongue virus is the type-species of the genus Orbivirus in the family Reoviridae. There are 24 serotypes worldwide, although not all serotypes exist in any one geographic area, eg, only 5 serotypes (2, 10, 11, 13, and 17) have been reported in the USA. Distribution throughout the world parallels the spatial and temporal distribution of vector species of Culicoides biting midges, which are the only significant natural transmitters of the virus. Of more than 1,400 Culicoides species worldwide, fewer than 20 are actual or possible vectors of bluetongue virus. Continued cycling of the virus among competent Culicoides vectors and susceptible ruminants is critical to viral ecology. In the USA, the principal biologic vector is C variipennis sonorensis , which limits distribution of the virus to southern and western regions. In Australia the principal vector is C brevitarsis , while in Africa, Europe, and the Middle East it is C imicola . In each geographic region, secondary vector species may attain local importance. Vectors become infected with bluetongue virus by imbibing blood from infected vertebrates; transovarial transmission has not been reported. High affinity of the virus to blood cells, especially the sequestering of viral particles in invaginations of RBC membranes, contributes to prolonged viremia in the presence of neutralizing antibody. The extended viremia in cattle (up to 9 wk), and the host preference of most vector species of Culicoides for cattle, provides a mechanism for year-round transmission in domestic ruminants. Mechanical transmission by other bloodsucking insects is of minor significance. Bluetongue virus is not contagious, and concentrations in secretions and excretions are minimal, making oral or aerosol transmission unlikely. However, semen from viremic bulls can serve as a source of infection for cows through natural service or artificial insemination. Embryo transfer is regarded as safe, provided that donors are not viremic and an appropriate washing procedure for embryos is used. Accidental infection has been reported in dogs in the USA following administration of a modified live virus vaccine that was contaminated with the virus. Serologic evidence of infection with bluetongue virus has been found in large carnivores in Africa, perhaps as a result of ingesting virus-infected viscera.
Hosts
- Ruminants, including sheep, cattle, deer, goats, and camelids
Pathogenesis
- Transfer occurs through blood from viremic animals via biting midges (Culicoides spp.)
- Replication in haematopoietic and endothelial cells of blood vessels
- Clinical signs vary between species, with sheep most severely affected
- Pyrexia
- Ocular and nasal discharge
- Drooling from mouth uclers
- Swelling of the mouth, head and neck
- Lameness
- Subdural hemorrhages
- Inflammation of the coronary band
- Cattle as the main reservoir
- A blue tongue is rarely seen as as a clinical sign of infection
- Resulting loss of condition, reduction in wool an meat production, which can be followed by death
Diagnosis
The typical clinical signs of bluetongue enable a presumptive diagnosis, especially in areas where the disease is endemic. Suspicion is confirmed by the presence of petechiae, ecchymoses, or hemorrhages in the wall of the base of the pulmonary artery and focal necrosis of the papillary muscle of the left ventricle. These highly characteristic lesions are usually obvious in severe clinical infections but may be barely visible in mild or convalescent cases. These lesions are often described as pathognomonic for bluetongue, but they have also been observed occasionally in other ovine diseases such as heartwater, pulpy kidney disease, and Rift Valley fever. Hemorrhages and necrosis are usually found where mechanical abrasion damages fragile capillaries, such as on the buccal surface of the cheek opposite the molar teeth and the mucosa of the esophageal groove and omasal folds. Other autopsy findings include subcutaneous and intermuscular edema, skeletal myonecrosis, myocardial and intestinal hemorrhages, hydrothorax, hydropericardium, pericarditis, and pneumonia. In many areas of the world, bluetongue in sheep, and especially in other ruminants, is subclinical and, therefore, laboratory confirmation based on virus isolation in embryonated chicken eggs, susceptible sheep, or cell cultures, or the identification of viral RNA by PCR is necessary. The identity of isolates may be confirmed by the group-specific antigen-capture ELISA, immunofluorescence, immunoperoxidase, serotype-specific virus neutralization tests, or hybridization with complementary gene sequences of group- or serotype-specific genes. For virus isolation, blood (10-20 mL) is collected as early as possible from febrile animals into an anticoagulant such as heparin, sodium citrate, or EDTA and transported at 4°C to the laboratory. For longterm storage where refrigeration is not possible, blood is collected in oxalate-phenol-glycerin (OPG). Blood to be frozen should be collected in buffered lactose peptone and stored at or below -70°C. Blood collected at later times during the viremic period should not be frozen, as lysing of the RBC or thawing releases the cell-associated virus, which may then be neutralized by early humoral antibody. The virus does not remain stable for long at -20°C. In fatal cases, specimens of spleen, lymph nodes, or red bone marrow are collected and transported to the laboratory at 4°C as soon as possible after death. A serologic response in ruminants can be detected 7-14 days after infection and is generally lifelong. Current recommended serologic techniques for the detection of bluetongue virus antibody include agar gel immunodiffusion and competitive ELISA. The latter is the test of choice and does not detect cross-reacting antibody to other orbiviruses, especially anti-EHDV (epizootic hemorrhagic disease virus) antibody. Various forms of virus neutralization test, including plaque reduction, plaque inhibition, and microtiter neutralization can be used to detect type-specific antibody.
Clinical Signs
The course of the disease in sheep can vary from peracute to chronic, with a mortality rate of 2-30%. Peracute cases die within 7-9 days of infection, mostly as a result of severe pulmonary edema leading to dyspnea, frothing from the nostrils, and death by asphyxiation. In chronic cases, sheep may die 3-5 wk after infection, mainly as a result of bacterial complications, especially pasteurellosis, and exhaustion. Mild cases usually recover rapidly and completely. The major production losses include deaths, unthriftiness during prolonged convalescence, wool breaks, and possibly reproductive loss. In sheep, bluetongue virus causes vascular endothelial damage, resulting in changes to capillary permeability and subsequent intravascular coagulation. This results in edema, congestion, hemorrhage, inflammation, and necrosis. The clinical signs in sheep are typical. After an incubation period of 4-6 days, a fever of 105-107.5°F (40.5-42°C) develops. The animals are listless and reluctant to move. Clinical signs in young lambs are more apparent, and the mortality rate is higher (up to 30%). About 2 days after onset of fever, additional clinical signs such as edema of lips, nose, face, submandibular area, eyelids, and sometimes ears; congestion of mouth, nose, nasal cavity, conjunctiva, and coronary bands; and lameness and depression may be seen. A serous nasal discharge is common, later becoming mucopurulent. The congestion of nose and nasal cavity produces a “sore muzzle” effect, the term used to describe the disease in sheep in the USA. Sheep eat less because of oral soreness and will hold food in their mouths to soften before chewing. They may champ to produce a frothy oral discharge at the corners of the lips. On close examination, small hemorrhages can be seen on the mucous membranes of the nose and mouth. Ulceration develops where the teeth come in contact with lips and tongue, especially in areas of most friction. Some affected sheep have severe swelling of the tongue, which may become cyanotic (‘blue tongue”) and even protrude from the mouth. Animals walk with difficulty as a result of inflammation of the hoof coronets. A purple-red color is easily seen as a band at the junction of the skin and the hoof. Later in the course of disease, lameness or torticollis is due to skeletal muscle damage. In most affected animals, abnormal wool growth resulting from dermatitis may be observed.
The pathogenesis of bluetongue in cattle seems to differ from that in sheep and is based on immediate IgE hypersensitivity reactions. Clinical signs in cattle are rare but may be similar to those seen in sheep. They are usually limited to fever, increased respiratory rate, lacrimation, salivation, stiffness, oral vesicles and ulcers, hyperesthesia, and a vesicular and ulcerative dermatitis. Susceptible cattle and sheep infected during pregnancy may abort or deliver malformed calves or lambs. The malformations include hydranencephaly or porencephaly, which results in ataxia and blindness at birth. White-tailed deer and pronghorn antelope develop severe hemorrhagic disease leading to sudden death. Pregnant dogs abort or give birth to stillborn pups and then die in 3-7 days.
Laboratory Tests
Pathology
Complete loss of integrity of epithelium. Uncommon.
- Characteristic of Bluetongue Virus,
- Epithelium lost and haemorrhage produces blue / black discoloration of the tongue, hence the name.
- Grossly:
- Infarctions -> necrosis
- Haemorrhage
- Histologically:
- Necrosis -> calcification or regeneration (depends on age of lesion)
Treatment
- BTV is NOTIFIABLE
- Vigilance in recognizing clinical signs
- Restriction of movement:
- Protection Zone: 100km radius around infected premises, movement within zone allowed but not in or out
- Vaccination within PZ using appropriate serotype is encouraged but still voluntary
- Surveillance Zone: 50km radius beyond PZ
- Vector control: ectoparasiticides, etc.
Prophylactic immunization of sheep remains the most effective and practical control measure against bluetongue in endemic regions. Three polyvalent vaccines, each comprising 5 different bluetongue virus serotypes attenuated by serial passage in embryonated hens’ eggs followed by growth and plaque selection in cell culture, are widely used in southern Africa and elsewhere, should epizootics of bluetongue occur. A monovalent modified live virus vaccine propagated in cell culture is available for use in sheep in the USA. Live-attenuated vaccines should not be used during Culicoides vector seasons because they may transmit the vaccine virus(es) from vaccinated to nonvaccinated animals, eg, other ruminant species. This may result in reassortment of genetic material and give rise to new viral strains. Abortion or malformation, particularly of the CNS, of fetuses may follow vaccination of ewes and cows with attenuated live vaccines during the first half and the first trimester of pregnancy, respectively. Passive immunity in lambs usually lasts 4-6 mo. The control of bluetongue is different in areas where the disease is not endemic. During an outbreak, when one or a limited number of serotypes may be involved, vaccination strategy depends on the serotype(s) that are causing infection. Use of vaccine strains other than the one(s) causing infection affords little or no protection. The vector status, potential risk from vaccine virus reassortment with wild-type viral strains, virus spread by the vectors to other susceptible ruminants, and reversion to virulence of vaccine virus strains or even the production of new serotypes also should be considered. Although a number of noninfectious vaccines are in development, they are not yet commercially available. Control of vectors by using insecticides or protection from vectors by moving animals into barns during the evening hours lowers the number of Culicoides bites and subsequently the risk of exposure to bluetongue virus infection.
Links
References
- Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition), Merial.
