Dylan DjaniPOP5280

Bluetongue Virus in Cattle

Nature of the Agent – Impact on Population and Health Belonging to the family Reoviridae, this is an arbovirus with a naked, icosahedral

capsid, a double-stranded RNA genome, and the potential to cause hemorrhagic fever of ruminants with vascular damage (hence bluetongue), mucosal sloughing and abortion in some cases.

26 serotypes currently estimated to exist (8). Impact on economics if spread is greater than impact on health, since the disease

is frequently asymptomatic in cattle in endemic regions (5). Study on a Dutch dairy herd measuring the impact of bluetongue virus serotype 8

disease showed that there is a slight increased mortality attributable to the bluetongue virus in clinical and subclinical conditions within the region of possible infection (9).

Infectiosity and Contagiosity Being an arbovirus, it must be transmitted through insects; in this case it spreads

indirectly via certain species of Culicoides biting midges (5). The virus is thus infectious, being that viruses are infectious agents; however, the

virus is not contagious because the Culicoides vector is required for spreading to occur.

Transmission is highly dependent on vector abundance, so year-round transmission is seen in the tropics, whereas seasonal transmission is seen in temperate zones mostly in the late summer and early fall.

Ro Value and Conclusions in Literature The Ro value of the virus must equal 1 in endemic areas, which is further

supported by the phenomenon of cattle being asymptomatic in these regions. Factors that would cause the Ro value to increase are increases in vector density

or host density (3). Recently a two-host, two-vector model for calculating Ro has been put forth to

illustrate the complexity of transmission when many species of Culicoides are involved in the transmission in a given region (11).

The Ro value is less than 1 for areas where the Culicoides vector cannot reach and/or areas that are not endemic; however, this does not preclude the entry of the virus into a farm via semen from certain serotypes or by transporting cattle out of endemic regions.

Duration of Shedding Viremia allows vectors to pick up and spread disease (5). The duration of viremia that is actually infective to vectors is important and was

shown to be about 21 days in cattle in one study (2). The degree of shedding of the virus in bull semen directly correlates with degree

of viremia.

Transmission Routes The main transmission between animals is via the Culicoides vector, but only

specific species of this all-encompassing genus for biting midges. Transplacental transmission is possible, and consequences are related to fetal age

of infection, ranging from abortion to immune tolerance. Not all serotypes of the virus cause abortion to the same degree (5)

Venereal transmission is possible with some serotypes, including serotype 8 in Europe, making the shedding of the virus in bull semen important.

Sampling Recommendations to Characterize Disease LevelsDiagnostic Tests – Sensitivity and Specificity

Spread can be established using serology and vector abundance (6). Cross-sectional sampling is appropriate in endemic areas, whereas longitudinal sampling would be better suited when concerned about transmission between herds.

One study demonstrated the application of robust surveillance on bluetongue to evaluate alternative surveillance strategies (7).

Diagnostic tests include serology (ELISA) and real-time PCR (1). Sensitivity of the ELISA in the European study (1) was shown to be adequate,

which may indicate an important trade off with specificity, especially because of the potential for seropositive asymptomatic animals.

Specificity of ELISA and RT-PCR is ensured via targeting the product of the VP2 gene: the VP2 protein on the viral capsid (5, 1).

Potential for Vaccine Strategies – Rationale for Repeat VaccinationDynamic of Maternal Antibody and Impact on Vaccination Strategy

Vaccines are available for bluetongue and offer homologous protection, so that multiple effective vaccines are required for the most protection in endemic areas with multiple serotypes (10). This means repeat vaccination may be necessary.

Available vaccines are mostly either attenuated or inactivated; however, some mention of virus-like particles and recombinant vectors is seen in the literature.

Serology titers may be used to demonstrate whether a need for further or repeat vaccination exists (4). Vaccination of pregnant animals must be avoided.

Maternal antibody is an important consideration when vaccinating calves, and one study showed that calves become seronegative between 84 and 112 days, indicating an ideal time to vaccinate (12).

Flow Strategies to Control and/or Eliminate Disease and Biosecurity to Control Spread Limiting the flow of animals, particularly within or from endemic areas, will help

reduce the chance of the virus spreading between herds (7). Effective biosecurity measures include vector control and appropriate vaccination

in order to minimize the impact of the disease on the cattle populations. Mass medication does not lend itself to this disease because there is no treatment

available other than supportive care throughout the duration of the infection. Disinfectant strategies are not helpful because the virus is maintained in the

bloodstream of viremic animals instead of in the environment.Bibliography

(1) Batten et. al. (2008) Blue tongue virus: European Community inter-laboratory comparison tests to evaluate ELISA and RT-PCR detection methods. Veterinary Microbiology. 129, 80-88.

(2) Bonneau et. al. (2002) Duration of viraemia infectious to Culicoides sonorensis in bluetongue virus-infected cattle and sheep. Veterinary Microbiology. 88(2), 115-125.

(3) Bruger, K., and Rubel, F. (2013) Bluetongue disease risk assessment based on observed and projected Culicoides obsoletus spp. vector densities. Institute for Veterinary Public Health, University of Veterinary Medicine. Vienna, Australia.

(4) Hund et. al. (2012) A two year BTV-8 vaccination follow up: molecular diagnostics and assessment of humoral and cellular immune reactions. Veterinary Microbiology. 154 (247-256).

(5) Maclachlan et. al. (2009) Pathology and pathogenesis of bluetongue. Journal of Comparative Pathology. 141, 1-16.

(6) Meroc et. al. (2008) Establishing the spread of bluetongue virus at the end of the 2006 epidemic in Belgium. Veterinary Microbiology. 131, 133-144.

(7) Montiero et. al. (2012) Robust surveillance of animal diseases: an application to the detection of bluetongue disease. Preventive Veterinary Medicine. 105, 17-24.

(8) Mulholland et. al. (2014) The development of an accelerated reverse-transcription loop mediated isothermal amplification for the serotype specific detection of bluetongue virus 8 in clinical samples. Journal of Virological Methods. 202, 95-100.

(9) Santman-Berends et. al. (2011) Mortality attributable to bluetongue virus serotype 8 infection in Dutch dairy cows. Veterinary Microbiology. 148, 183-188.

(10) Schwartz-Cornil et. al. (2008) Bluetongue virus: virology, pathogenesis and immunity. Veterinary Research. 39, 46.

(11) Turner et. al. (2013) Two-host, two-vector basic reproduction ratio (Ro) for bluetongue. Department of Epidemiology and Population Health Institute of Infection and Global Health, Department of Mathematical and Physical Science, University of Liverpool, United Kingdom.

(12) Vitour et. al. (2011) Colostral antibody induced interference of inactivated bluetongue serotype-8 vaccines in calves. Veterinary Research. 42, 18.