Bluetongue Virus

Description

Bluetongue is a non-contagious, 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 oedema¹. Although cattle are the main reservoir of infection, sheep are more severely affected and often suffer a cyanotic tongue, lending the disease its name. The virus has been isolated from hosts and vectors on all continents(excluding Antartica)², despite being initially recognised in Africa in the late 19th and early 20th centuries³. 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 diseases⁴.

Aetiology

Bluetongue virus particle. Source: Wikimedia Commons; Author: CDC (2007)

Bluetongue virus is a species of the genus Orbivirus, within the 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 proteins². 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 enzymes⁵. 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.

All BTV’s share group antigens, which can be demonstrated by agar gel diffusion tests, fluorescent antibody tests and the group reactive ELISA¹. There are 24 distinct serotypes, which are are distinguished by epitopes on the outer capsid protein VP2⁴, 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 serotypes¹. Numerous strains of bluetongue virus also exist, and these are characterised by molecular analysis.

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 bluetongue¹.

Pigs and horses do not become infected with BTV, but may act as a food source for the Culicoides midges that transmit bluetongue virus to ruminants. Their habitats may also provide areas suitable for vector breeding.

Transmission and Epidemiology

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.

Vectors

The arthropod vector for bluetongue virus is the Culicoides biting midge. These are biological vectors of BTV, so the virus replicates in insect tissue after feeding on an infected host⁶. It takes 10-14 days for the virus to dissminated 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 higher², and midges maintaine a life-long infection.

These insects take blood meals from vertebrate hosts and breed in damp, dung-enriched soil enriched, and so are abdunant in the vicinity of domestic livestock. Once eggs are laid in soil, Culicoides progresses through four larval stages and pupates before becoming an adult midge. The lifecycle is greatly influenced by temperature: in temperate regions such as Britain, the adult midge population declines in October and is absent by December. The fourth larval stage overwinters, and adults re-appear the following April. The environment affects the activity of adult midges in a variety of ways. Culicoides survive around 10 days in warm weather but up to one month when conditions are cooler and are most active at night, from an hour before sunset to an hour after sunrise. Activity is decreased by windy conditions, and increased during the day when the weather is dull. Bluetongue virus replicates more rapidly in vector species when temperatures are warmer.

Classically, the major vector for BTV is Culicoides imicola. This midge is found throughout Africa, the Middle East, southern Asia, Portugal, Greece, Corsica, Sardinia, Sicily and areas of Italy¹, and its distribution appears to be extending northwards. However, C. imicola has not yet been demonstrated in the United Kingdom. The Culicoides species found in the British Isles are C. pulicais and C. obsoletus, which are also common across central and northern Europe. Knowledge of the distribution of these species in the UK is incomplete but the insects tend to gather where breeding sites and hosts occur in tandem, with the highest midge concentrations in areas containing cattle, horses and pigs. Removal of livestock decreases populations of Culicoides by a factor of 10 to 20¹, but some persist by feeding on wild animals and man. Hill sites have fewer midges as climatic conditions are less favourable, and sheep have a less positive influence on distribution.

In Britain, studies are ongoing to determine midge distribution, seasonal incidence and the competency of the various Culicoides species to act as BTV vectors.

Vector Competence

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 virus (another Orbivirus) in Spain. C. obsoletus and C. pulicaris were also the most abundant Culicoides species detrected 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.

A British population of C. obsoletus has been shown to have and oral susceptibility rate of less than 2%¹, 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.

Two other Culicoides species, C. nuberculous and C. impunctatus, exist in mainland Europe and the UK, and have been experimentally infected with bluetongue virus.

Epidemiology

Althbough 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 density¹.

Although vertical and venereal transmission of bluetongue is possible, only to the presence of competent insect vectors influences the epidemiology of BTV². 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 favourably for vector activity¹. In Britain, transmission occurs mainly in late summer and autumn. Once bluetongue virus is transmitted to a vertebrate host, there are two possible outcome: either the host may die, or an immune response is mounted against the virus and renders the host 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 obtascle. This means that even in zones where bluetongue virus is endemic, persistence dynamic and comprises perpetually shifting "hot spot" of infection¹. 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 vectores 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 availble for onwards transmission.

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.

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.

Bluetongue virus infection was first confirmed in the UK in September 2007 when a veterinarian spotted suspicious clinical signs on a cattle holding near Ipswich⁷. 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.

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^{8, 9, 10}. However, the severity of disease varies greatly with species and cattle in particular express very few signs.

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 spleen¹¹. 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 necrosis¹⁰ which can increase vascular permeability to cause oedema. Endothelial damage can also give thrombosis, leading to tissue infarction. In sheep and deer a consumptive coagulopathy may occur².

Several factors can influence the presentation of disease. Firstly, each virus strain is associated with its own particular virulence and thus clinical manifestation¹⁰. Host factors are also important: breed^{3, 12}, stress, nutritional status and age^{12, 13} can all affect bluetongue presentation.

Diagnosis

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.

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^{1, 14}. 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 froms a crust around the nostrils. Facial oedema affects the nose, lips, tongue and submandibular areas. Severe swelling of the tongue results in protrusion and cyanosis, lending the disease its name. The oral mucosa becomes congested, petechiated and ulcerated which leads to inappetance, dysphagia and frothing at the mouth. A serous nasal discharge is later seen to become mucopurulent, and the conjuntiva are often injected. Inflammation of the coronet causes lameness, and the junction of the skin and hoof is purple-red in colour. Skeletal muscle damage in advanced disease may also contribute to lameness. The course of ovine bluetongue can vary from peracute to chronic, and mortality is between 2% and 30%. In peracute cases, severe pulmonary oedema leads to death by asphyxiation around one week after infection. Acute, mild cases normally have a rapid and complete recovery. In chronic cases, death occurs 3-5 weeks after infection due to exhaustion and bacterial complications such as pasteurellosis.

In acute cases, the lips and tongue become very swollen and oedema may extend over the face to include the ears and intermandibular space. The hyperaemia becomes more intense and tiny, flat, red or purple (petechial) haemorrhages appear on the mouth, nose and conjunctival linings. The clinical feature that gives the disease its name, a deeply cyanotic (blue) tongue, occurs in only a small percentage of cases. Necrotic lesions develop on the gums, cheeks and tongue 5–8 days after the onset of fever. These heal slowly under a membrane of pus and serum (diphtheritic membrane). Breathing becomes difficult. Profuse bloody diarrhoea occurs in some cases. Vomiting may also occur and lead to inhalation pneumonia. Foot lesions, on one to four feet, may appear towards the end of the fever period. There is acute reddening and petechial haemorrhages on the coronary band at the top of the hoof. Affected sheep stand with arched backs and are reluctant to move. There is rapid weight loss and weakness due to loss of appetite and specific muscular necrosis. Spasmodic twisting of the head and neck to one side (torticollis) is sometimes a late sign. 6.3 The mortality rate is variable: in highly susceptible sheep it can be up to 70%. Deaths may occur at any stage up to a month or more after the onset of signs. Convalescence in surviving sheep is prolonged. Breaks occur in wool, which add to the production losses. 6.4 Infection of pregnant ewes may lead to abortions, mummified foetuses, or the birth of stillborn or weak lambs, which may have congenital defects. 6.5 Goats are less commonly, and less severely, affected than sheep. The pathogenesis is similar and the clinical signs are milder. 6.6 Infection in cattle, although of great epidemiological significance, is generally sub-clinical. A report from the United States suggested only 0.01% of cattle infected with BTV show clinical signs. These include inflammation and mucosal erosions in the mouth and nose, mild laminitis and a stiff gait. Infection of early pregnant animals may lead to embryonic death and resorption. Technical Review - Bluetongue: The Virus, Hosts and Vectors ___________________________________________________________________________ 15. Version 1.5; 21 November 2002 6.7 Severe disease and mortalities occur in white-tailed deer in the United States where the pathogenesis and clinical signs are indistinguishable from the closely related EHD virus. 6.8 Other species of farmed or wild feral deer may have BTV antibodies, but usually no disease is observed.

Clinical signs occur more rarely in cattle, but when seen are similar to those in sheep. Signs can include pyrexia, tachypnoea, lacrimation, salivation, stiffness, oral vesicles or ulcers, hyperesthesia, and a vesicular, ulcerative dermatitis.

When cattle or sheep are infected during pregnancy, abortions or foetal malformations can arise. Malformations can include hydranencephaly or porencephaly, which results in ataxia and blindness at birth.

Laboratory Tests

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.

Pathology

Necrosis of the left ventricular papillary muscle and petechiation, ecchymoses or haemorrhages in the wall of the pulmonary artery have been described as "pathognomic" lesions for bluetongue. However, pulpy kidney disease and Rift Valley fever may occasionally cause similar lesions in sheep, and the lesions may be difficult to visualise in mild or recovering cases. Haemorrhage and necrosis may be found in other locations where fragile capillaries are damaged by mechanical abrasion. These can include the buccal oral mucosa, the mucosa of the oesophageal groove and the omasal folds as well as the skeletal muscle, myocardium and the intestines. Post-mortem examination may also reveal subcutaneous and intermuscular oedema, hydrothorax or hydropericardium, pericarditis, and pneumonia.

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

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