Evidence for transmission of bluetongue virus serotype 26 through direct contact

Abstract

The aim of this study was to assess the mechanisms of transmission of bluetongue virus serotype 26 (BTV-26) in goats. A previous study, which investigated the pathogenicity and infection kinetics of BTV-26 in goats, unexpectedly revealed that one control goat may have been infected through a direct contact transmission route. To investigate the transmission mechanisms of BTV-26 in more detail an experimental infection study was carried out in which three goats were infected with BTV-26, three goats were kept uninfected, but were housed in direct contact with the infected goats, and an additional four goats were kept in indirect contact separated from infected goats by metal gates. This barrier allowed the goats to have occasional face-to-face contact in the same airspace, but feeding, watering, sampling and environmental cleaning was carried out separately. The three experimentally infected goats did not show clinical signs of BTV, however high levels of viral RNA were detected and virus was isolated from their blood. At 21 dpi viral RNA was detected in, and virus was isolated from the blood of the three direct contact goats, which also seroconverted. The four indirect barrier contact goats remained uninfected throughout the duration of the experiment. In order to assess replication in a laboratory model species of Culicoides biting midge, more than 300 Culicoides sonorensis were fed a BTV-26 spiked blood meal and incubated for 7 days. The dissemination of BTV-26 in individual C. sonorensis was inferred from the quantity of virus RNA and indicated that none of the insects processed at day 7 possessed transmissible infections. This study shows that BTV-26 is easily transmitted through direct contact transmission between goats, and the strain does not seem to replicate in C. sonorensis midges using standard incubation conditions.

Figure 1. Animal facility layout and attendance management.

The housing box was divided by a metal fence covered with a fine metal mesh. Both sides were accessible through a joint lobby and goats on each side had their own food and water facilities. The designated “dirty side” housed 3 goats which were subcutaneously inoculated with 1 ml of BTV-26 KUW2010/09 BHK₂ as well as 3 uninfected ‘direct-contact’ goats. Four uninfected ‘barrier contact’ goats were housed on the clean side of the box. Animal attendants always entered and exited the clean side through the lobby before entering the dirty side and all sampling of animals was strictly carried out in the order ‘barrier goats’, ‘direct contact’ goats and finally infected goats. Furthermore the cleaning of the box was always carried out from the “clean side “to the “dirty side” towards the floor drains situated on the “dirty side”. Re-entering the clean side required disinfection and showering of waterproof personal protection equipment (PPE) or a change of PPE.

Figure 2. Observed Ct values for bluetongue virus (BTV) serotype 26 in Culicoides sonorensis fed on infected blood via a membrane.

Individual midges were processed either immediately after feeding (day 0; dark grey bars) or after incubation at 25°C for seven days (day 7; light grey bars).