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 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

When Culicoides feed on a bluetongue-infected host, they become persistently infected with the virus for the duration of their lives⁴. The global distribution of BTV is related only to these competent insect vectors²: although vertical and venereal transmission of bluetongue is possible, these routes do not influence the epidemiology of BTV. The vector species varies with location and is poorly characterised in some areas, including northern Europe. However, as described above, it has been demonstratated that ambient temperature has profound effects on vector survival and feeding activity, and the replication of BTV in the insect⁶. Therefore, the relationship between temperature and effective transmission of BTV is likely to be a limiting factor to the spread of BTV, which may potentially be overcome by global warming.

It is appears that in different regions of the world, specific serotypes of bluetongue virus have co-evolved with vector species of insect vector⁷. For example, different serotypes of BTV are prevalent in the USA to those in those the Caribbean and Central America although there is no real geographic barrier separating these areas, and animals move between the regions. This is thought to be due to the variation in the species of vector insects: Culicoides sonorensis is the primary vector in the USA and is most competent for serotypes 10, 11, 13 and 17, but in the Caribbean and Central America C. insignis carries serotypes 1, 2, 3, 4, 6, 8, 11, 12, 13, 14 and 17⁴.

Bluetongue virus infection was first confirmed in the UK in September 2007 when a veterinarian spotted suspicious clinical signs on a cattle holding near Ipswich⁸. There are several possibilities for how the virus spread to the British Isles: a) through transport of infected animals; b) by local vectors acquring virus from infected animals (unlikely given the island geography of the UK), or c) via movement of vectors carrying BTV. 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.

BTV is non-contagious. BTV is transmitted biologically by Culicoides insects (biting midges), but only a limited number of species are efficient vectors. Cattle are the main amplifying hosts for BTV. They are also probably important maintenance hosts. The competent Culicoides vector species feed more abundantly on cattle. 5.2 BTV is limited to those geographical areas where competent Culicoides vectors are present. Its transmission is limited to those times of the year when the climatic conditions are favourable for adult vector activity. Peak populations of vector Culicoides occur in the late summer and autumn and therefore this is the time when BT is most commonly seen. 5.3 Persistence of BTV within a particular geographical area does not mean “static”. Once a vertebrate host is infected with BTV it either dies or mounts an enduring antibody response and so becomes resistant to further infection. This means that within any small geographical area (a farm or village) most or all of the initially susceptible hosts are likely to be infected and thus become “unavailable” to the virus within a fairly short space of time. BTV can only survive under such constraints by continually moving to new locations occupied by naïve vertebrate hosts. These movements are via the agency of viraemic hosts or infected vectors. BTV is therefore a peripatetic virus and even within its enzootic zones its activity may be envisaged as a pattern of endlessly shifting viral “hot spots”. 5.4 The incidence and geographical distribution of BTV infections are determined largely by the distribution of insect vectors and this can vary from year to year. Infection in sheep will usually be preceded by widespread infection of cattle and an increase in vector density. 5.5 Where annual bouts of BT occur, they may represent new introductions (from adjacent infected areas) or may be the visible evidence of low-level persistence from year to year. Annual re-introduction is possible if enzootic foci of the virus are geographically close by, as infected Culicoides can reputedly be transported on the wind over distances of 100 kilometres or more. Culicoides do not normally fly far if there is a source of food (large mammals) and breeding sites. Since BTV is not transmitted transovarially through its vectors (and is rarely transmitted directly from vertebrate to vertebrate) long term persistence (i.e. an enzootic zone) is currently thought to be possible only in areas where active adult vectors are present throughout the year. In such situations if vector-free periods do occur then they must be of shorter duration than the maximum period of viraemia in the local susceptible vertebrate population (up to about 50 days in sheep and 100 days in cattle). Otherwise the last infected vertebrate host will have died or recovered before new vectors arrive on the scene.

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.

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

• Defra - Bluetongue
• BTV Control in Cattle and Sheep (Intervet)
• Institute for Animal Health - Bluetongue
• Bluetonguevirus.org - BTV information and resource portal

References

1. DEFRA (2002) Technical Review - Bluetongue : The Virus, Hosts and Vectors.
2. Gibbs, E P J and Geiner, E C (1994) The Epidemiology of Bluetongue. Comparative Immunology, Microbiology and Infectious Diseases, 17(3-4), 207-220.
3. Spreull, J (1905) Malarial catarrhal fever (bluetongue) of sheep in South Africa. Journal of Comparative Pathology and Therapeutics, 18, 321-337.
4. 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.
5. Carter, G R and Wise, D J (2005) A Concise Review of Veterinary Virology, IVIS.
6. 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.
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8. IAH (2008) Institute for Animal Health - Bluetongue Research Programme
9. Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition), Merial.
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11. MacLachlan, N J et al (2009) The Pathology and Pathogenesis of Bluetongue. Journal of Comparative Pathology, 141(1), 1-16.
12. 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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14. 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.
15. 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.