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 VP2², 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

Vectors

The arthropd 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. Culicoides generally avoid entering buildings and other closed spaces.

3.4 Culicoides imicola is the major vector of BTV in the Old World. It is one of the most widely distributed of Culicoides species. It occurs throughout most of Africa, the Middle East, southern Asia, much of Portugal, south-west Spain and the Balearics, many Greek Islands, substantial parts of the Greek mainland, Corsica, Sardinia, Sicily and wide areas of southern and central mainland Italy. C. imicola appears to be expanding its range both northwards and westwards but is still restricted in Europe to southern parts. C. imicola has not been recorded in Great Britain. 3.5 Culicoides obsoletus is probably one of the commonest Culicoides species across the whole of central and northern Europe. Similarly, C. pulicaris is also common throughout central and northern Europe. Both of these species are widespread throughout most of the British Isles.

3.7 The distributions of C. obsoletus and C. pulicaris group midges in UK are not well understood. Observations of both have been made in many parts of the British Isles. When observations have not been recorded in certain areas it usually means that efforts have not been made to collect rather than the species is absent there. Generally, the insects congregate where there are breeding sites and hosts upon which to feed.

Thus, the highest concentrations of C. obsoletus and/or C. pulicaris group midges are found where cattle, horses, pigs and, to a lesser extent, sheep populations are highest. If domestic animals are removed from a site, over several months the midge population reduces significantly, by a factor of ten to twenty times, but will usually persist at the lower level if other ecological factors are favourable, by feeding on wild hosts and/or humans.Vector numbers are likely to be low in hill sites where sheep are at low densities and where the climatic conditions are likely to be more extreme. 3.8 Studies of Culicoides spp. in Britain are being expanded under a DEFRAfunded project out of the Institute of Animal Health, Pirbright. Monitoring is being expanded to twenty-five or more sites, one of the aims being to determine the species list, species distribution, seasonal incidence and vector competency of the various species.

Vector Competency

3.9 The C. obsoletus group has long been suspected of being a vector, mainly on the basis of BTV isolations from this species made in Cyprus, and African horse sickness virus (AHSV) isolations made from mixed pools of C. obsoletus and C. pulicaris in Spain. In this context it should be borne in mind that BTV and AHSV tend to utilise the same Culicoides species as vectors. 3.10 It is strongly suspected that C. obsoletus and/or C. pulicaris group midges acted as BTV vectors in northern Greece and southern Bulgaria during the 1999 BTV epizootic, as they were by far the most abundant and most prevalent detected. It is similarly suspected that these species may also have mediated the BT outbreaks in Serbia, western and southern Bulgaria, FYR Macedonia, Croatia and Bosnia during the period 2001-2002. C. imicola has not been recorded in these regions. 3.11 Vector competence studies on a British population of C. obsoletus have recorded oral susceptibility rates of less than 2% in comparison with a known major vector C. sonorensis (19.5%). This initially suggested that C. obsoletus is likely to be only a minor or inefficient vector of BTV. Nevertheless, the high abundance and survival rates of C. obsoletus as exhibited in Bulgaria in 1999, and as seen on farms and around stables in South East England, could compensate for its low levels of vector competence. Observations of cattle exposed to midges have shown up to ten thousand bites per hour. It should be noted that C. brevitarsis, the major vector of BTV in Australia, has an experimental competency of only 0.3 percent when feeding on sheep although it is quite an effective vector in the field. 3.12 Vector competence for a particular virus is a hereditary trait and populations of a vector species with high, low or intermediate levels of competence can be derived by selective breeding. Technical Review - Bluetongue: The Virus, Hosts and Vectors ___________________________________________________________________________ 6. Version 1.5; 21 November 2002

Impact of Global Warming

3.13 Vector competence of Culicoides vectors for Orbiviruses is partly influenced by temperature. Orbivirus development in Culicoides vectors is unable to occur at temperatures below about 10°C to 15°C depending on the Orbivirus species and serotype. Furthermore, there needs to be a minimum amount of time at suitable temperatures (expressed as “day degrees or hour degrees”) for completion of the development cycle in the Culicoides vector before virus transmission can occur. This “physiological” time is the cumulative product of virus development time multiplied by the temperature in degrees above the threshold for virus replication. Increasing environmental temperature (climate change) will also extend the vector season. Combined, these conditions may result in Orbivirus development within Culicoides being able to take place over a greater proportion of the year and over a wider geographical area. In addition, within the range of temperatures over which Orbivirus development can occur, the levels of vector competence of a Culicoides vector population for some Orbivirus serotypes increases linearly with temperature and so the impact of warmer temperatures may be even greater. 3.14 Temperature can also affect the competence of ‘non-vector’ Culicoides species. For example, C. nubeculosus generally is considered to be incapable of transmitting BTV due to a midgut infection barrier. However, exposure of the immatures to rearing temperatures close to their upper lethal limit (33-35°C) can result in >10% of adults becoming competent to transmit BTV. It is likely that the integrity of the gut wall of some adults is damaged by the extreme rearing temperatures, thereby allowing virus particles to bypass the midgut barriers, enter the haemocoel and develop as in a normal vector. The increase in frequency and intensity of extremely warm days predicted to occur with climate change will enhance the chances of this phenomenon occurring in non-vector Culicoides species and hence could increase the number of BTV competent adults within populations. 3.15 The vectorial capacity of a Culicoides population (and hence the potential for virus transmission) is affected by (a) the number of adult midges in the population and (b) the proportion of adults capable of transmitting the virus, and is greatest when these factors are at a peak. 3.16 Within favourable limits, the development rate of Culicoides from egg to adult is directly related to temperature. Thus increasing temperatures coupled with an extension in the developmental season may result in a greater number of generations (and therefore adults) per year. In addition, the overwintering ability of adult Culicoides is likely to improve, as winters become both warmer and shorter. Improved overwintering success is also likely to increase the spring population input, which in turn could result in even larger populations during the summer Technical Review - Bluetongue: The Virus, Hosts and Vectors ___________________________________________________________________________ 7. Version 1.5; 21 November 2002 3.17 The proportion of adult Culicoides capable of transmitting virus is dependent on (a) vector competence (the capacity for the virus to develop in and be transmitted by the vector), (b) adult survival, (c) the blood-feeding interval and (d) the extrinsic incubation period (EIP; development time of the virus in the vector). In order to transmit virus Culicoides must not only be vector competent, but also survive long enough to blood-feed after the completion of the viral EIP. Culicoides vectors are more likely to satisfy these criteria at high temperatures (e.g. 27-30°C), because, although adult survival is reduced at high temperatures, this is more than compensated for by the accompanying decrease in duration of the EIP and blood-feeding interval. Consequently, it is likely that warmer temperatures as a result of climate change will increase the likelihood that Culicoides will survive long enough to transmit virus. 3.18 Changes in weather (temperature, precipitation, humidity and wind) and climate from global warming could produce both wider distribution of vectors towards the poles or upwards in elevation and increased vectorial capacity (the ability of a vector population to transmit virus to a vertebrate population) of Culicoides vector populations, resulting in increased prevalence of BTV in Europe. The present BT outbreak in the Mediterranean Basin is already the most serious epizootic on record. 3.19 An expansion in the range of C. imicola will increase the areas of Europe at risk from BTV. Also, the extended distribution of C. imicola could bring BTV into the range of C. obsoletus group and C. pulicaris group midges much more frequently and this could result in even greater areas of Europe being affected by BTV. 3.20 The impact of climate change on the vectorial capacity of Culicoides populations will have three main effects on BTV transmission in the Mediterranean basin: · the greater abundance of adult Culicoides combined with the increased proportion of adults capable of transmitting the virus will increase the likelihood and severity of an epizootic, following the introduction of BTV into an area. The greatest risk will be at times of the year when temperatures reach approximately 25-30°C (i.e. when conditions are optimal for Culicoides development and virus transmission · as temperatures will be conducive for both viral and Culicoides development for a greater proportion of the year, the length of the viral transmission season will increase. · the enhanced overwintering success of adult Culicoides combined with the extension in the Culicoides development season will prolong the seasonal occurrence of adult midges and hence improve the overwintering chances of BTV. 3.21. Studies are needed to correlate the day degrees required for BTV development in the potential vectors against British climate data to establish the risk of establishment of a BTV infection under present climatic conditions and with global warming.

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. Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition), Merial.
7. 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.
8. MacLachlan, N J et al (2009) The Pathology and Pathogenesis of Bluetongue. Journal of Comparative Pathology, 141(1), 1-16.
9. 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.
10. Afshar, A (2004) Bluetongue: Laboratory Diagnosis. Comparative Immunology, Microbiology and Infectious Diseases, 17(3-4), 221-242.
11. 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.
12. 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.