Effect of vaccination with an inactivated vaccine on transplacental transmission of BTV-8 in mid term pregnant ewes and heifers.

The effect of vaccination with a commercial inactivated Bluetongue virus serotype 8 (BTV-8) vaccine on the ability of BTV-8 to cross the ruminant placenta was investigated in two experiments. Ten pregnant ewes (Experiment 1) or heifers (Experiment 2) were vaccinated according to the manufacturer's instructions. Three weeks after the completion of the vaccination schedule, all vaccinated animals were infected with BTV-8 together with ten non-vaccinated pregnant animals that served as challenged controls. Four additional pregnant animals received a mock challenge at the same time point. Three weeks after the challenge, the foetuses were collected. In the sheep experiment, the lambs of the vaccinated ewes and the mock infected ewes were negative in the virus isolation, whereas BTV-8 could be isolated from 11/23 lambs of 6/10 ewes in the BTV-8 challenged control group. The incidence and severity of BTV associated lesions, such as haemorrhages, meningitis/encephalitis and necrosis in the placentomes was significantly higher in the BTV-8 challenged control group. The rate of transplacental transmission was less in the cattle experiment: BTV-8 could be detected in 2/10 calves in the BTV-8 challenged control group. All other calves were negative. Vaccination clearly reduced transplacental transmission of BTV-8 in the sheep experiment, whereas in the cattle experiment, the incidence of transmission was too low to demonstrate a significant reduction of transmission by vaccination. However, the vaccine very effectively blocked viraemia, which suggests that the vaccine might prevent transmission in cattle as well. Transplacental transmission of BTV has serious economical consequences, due to the loss of progeny to the livestock industry. Vaccination can be an important aid in the reduction of these economic losses.

Transplacental transmission of Bluetongue virus serotype 8 in ewes in early and mid gestation.

The ability of Bluetongue virus serotype 8 (BTV-8) originating from the 2006 European outbreak to cross the ovine placenta during early and mid gestation was investigated in two separate experiments. In the first experiment, 16 ewes were infected with BTV-8 at 70-75 days gestation. The foetuses were collected at 18-19 days after infection (dpi). BTV-8 could be isolated from at least two organs of 19 out of 40 lambs and from 11 out of 16 infected ewes. In the second experiment, 20 BTV-8 infected ewes in early gestation (day 40-45) were euthanized at 10 days (10 ewes) or 30 days (10 ewes) after infection. The presence of BTV could be demonstrated in two foetuses from two ewes at 10 dpi and in 4 foetuses from four ewes at 30 dpi. The main pathological findings in the foetuses in mid gestation were meningo-encephalitis and vacuolation of the cerebrum. In the foetuses early at gestation, haemorrhages in various foetal tissues and necrosis and haemorrhages in the placentomes were found. These experiments demonstrate for the first time the presence of infectious BTV in lamb foetuses at different stages of gestation, combined with a difference in transmission rate depending on the gestation stage. The high transmission rate found at mid term gestation (69%) makes our model very suitable for further research into the mechanisms of transplacental transmission and for research into the reduction of this route of transmission through vaccination.