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 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. The dsRNA genome is then transcribed conservatively, producing positive-sense RNA strands which are both translated and used as templates for produing the negative-sense strand of the genome. Complete virus particles can then assemble and be released from the cell.

The 24 distinct serotypes of BTV are distinguished by epitopes on the outer capsid protein VP2, although VP5 also can influence neutralization through its conformational influence on VP2 [11]. The L2 gene, which encodes VP2, is the only serotype-specific BTV gene and there is considerable variation amongst all 10 genome segments of field strains of BTV within endemic areas such as California [15,25]. This variation of BTV genes in field strains of the virus has arisen as a consequence of both drift and reassortment of individual viral genes. Reassortment of BTV genes has been demonstrated after infection of either the ruminant host or insect vector with different strains or serotypes of BTV [29,30]. Individual BTV gene segments evolve and reassort independently of serotype in the field. Genetic drift of individual BTV genes occurs by the selective acquisition and amplification in vector insects of specific variants from the quasispecies virus population that arises in the blood of infected ruminants (founder effect; 6,7).

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.

Thus far 24 serotypes are recognised. There are numerous strains - each isolate is a different strain based on molecular analysis, regardless of serotype. The virulence of BTV strains varies considerably. However, other factors also influence the severity of the disease in sheep, including breed, age, exposure of animals to sunlight, walking on rough ground and stress. 2.5 The serotypes are differentiated by serum neutralisation tests, but there are cross-reactions between some serotypes. All BTV’s share group antigens, which can be demonstrated by agar gel diffusion tests, fluorescent antibody tests and the group reactive ELISA. 2.6 Several other Orbiviruses have been loosely termed ‘bluetongue-related’ viruses because of serological and other relationships to BTV. The only such viruses known to be pathogenic for livestock are some members of the epizootic haemorrhagic disease of deer (EHD) serogroup and the Palyam serogroup of Reoviridae.

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.

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. Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition), Merial.
6. 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.
7. MacLachlan, N J et al (2009) The Pathology and Pathogenesis of Bluetongue. Journal of Comparative Pathology, 141(1), 1-16.
8. 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.
9. Afshar, A (2004) Bluetongue: Laboratory Diagnosis. Comparative Immunology, Microbiology and Infectious Diseases, 17(3-4), 221-242.
10. 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.
11. 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.