Effect of Culicoides sonorensis salivary proteins on clinical disease outcome in experimental bluetongue virus serotype 8 infection of Dorset sheep.

The severity of bluetongue clinical disease in ruminants varies greatly depending on the outbreak serotype/strain, animal species/breed, and immune status of the herd. To predict disease risk from any of the 26 bluetongue virus (BTV) serotypes identified to date, experimental animal susceptibility studies are often conducted. Although sheep are the most susceptible livestock species in the US, infection of domestic breeds by injection of field isolates rarely produces the level of clinical disease observed in natural Culicoides midge­transmitted outbreaks. Thus, outbreak risk assessments based on experimental animal infections can underestimate the severity posed by a potential outbreak with a given virus serotype or strain. The aim of this study was to determine whether secreted Culicoides salivary proteins injected simultaneously with virus, to more closely mimic midge­delivered virus, would affect clinical disease outcome in a BTV­8 sheep susceptibility study. Eight sheep were intradermally inoculated with BTV­8; 4 received virus mixed with secreted Culicoides salivary proteins (BTV­8 + Cu SP), 4 received virus alone. Clinical signs were monitored daily for type, severity and duration. In sheep receiving the BTV­8 + Cu SP inoculum, clinical signs were more varied, more severe, and duration was three times longer compared to sheep receiving virus alone. These results suggest that Culicoides salivary proteins may play a contributing role in BTV pathology and that use of these proteins in experimental animal infections may allow development of a more robust target­host animal model.

Evaluation of lamb and calf responses to Rift Valley fever MP-12 vaccination.

Rift Valley fever (RVF) is an important viral disease of animals and humans in Africa and the Middle East that is transmitted by mosquitoes. The disease is of concern to international agricultural and public health communities. The RVFV MP-12 strain has been the most safety tested attenuated vaccine strain; thus it is being considered as a potential vaccine for the US national veterinary stockpile. This study was designed to establish safety protocols for large animal research with virulent RVF viruses, establish a target host immune response baseline using RVF MP-12 strain, and independently evaluate this strain as a potential US emergency response vaccine. Ten, approximately four month-old lambs and calves were vaccinated with RVF MP-12 strain; two additional animals per species provided negative control specimens. The animals were monitored for clinical and immune response, fever, and viremia. Two animals per species were sacrificed on 2, 3, 4, 10 and 28 days post infection and full necropsies were performed for histopathological examination. No clinical or febrile responses were observed in this study. The onset and titer of the immune response is discussed. There was no significant histopathology in the lambs; however, 6 out of 10 vaccinated calves had multifocal, random areas of hepatocellular degeneration and necrosis. RVF MP12 antigen was detected in these areas of necrosis by immunohistochemistry in one calf. This study provides independent and baseline information on the RVF MP-12 attenuated vaccination in vaccine relevant age target species and indicates the importance of performing safety testing on vaccine relevant aged target animals.

Experimental infection of white-tailed deer (Odocoileus virginianus) with Northern European bluetongue virus serotype 8.

Bluetongue (BT) is an insect-transmitted, economically important disease of domestic and wild ruminants. Although only five of the 26 reported bluetongue virus (BTV) serotypes are considered endemic to the USA, 10 exotic serotypes have been isolated primarily in the southeastern region of the country since 1999. For an exotic BTV serotype to become endemic there must be susceptible animal species and competent vectors. In the USA, sheep and white-tailed deer (WTD) are the primary sentinel livestock and wildlife species, respectively. In 2006, BTV-8 was introduced into Northern Europe and subsequently overwintered, causing unprecedented livestock disease and mortality during the 2006-2007 vector seasons. To assess the risk of the European strain of BTV-8 to North American WTD, and understand the role they could play after a similar introduction, eight bluetongue-seronegative WTD were inoculated with BTV-8. Body temperatures and clinical signs were recorded daily. Blood samples were analyzed for BTV RNA with quantitative real time reverse transcriptase polymerase chain reaction (qRT-PCR), serum analyzed for BTV antibodies by cELISA, and tissues taken for histopathology and qRT-PCR. All eight deer became infected and developed moderate to severe clinical disease from days 8 to 15. Peak viremia was from day 7 to 10 with detectable titers through the end of the study (28 days) in most deer. Serum antibody was detected by day 6, peaked by day 10 and continued through day 28. We conclude that North American WTD are highly susceptible to BTV-8 and would act as clinical disease sentinels and amplifying hosts during an outbreak.