1. Dylan Djani
POP5280
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.

2. 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 Levels
Diagnostic 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 Vaccination
Dynamic 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.

3. 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.