As African horse sickness (AHS) threatens to spread outside Africa due to climate change and increased international horse movement, the need for an effective vaccine is becoming urgent, experts from around the world agree.

The disease has up to a 90% mortality rate in unvaccinated horses, which often die within days of exposure. Currently the only vaccines licensed for AHS control are attenuated live virus vaccines, which can “revert to virulence” (meaning they can cause active viruses in the horse), said José-Manuel Sanchez-Vizcaino, DVM, PhD, professor at Complutense University, in Madrid, Spain.

The virus exists in nine serotypes (distinguishable strains), and current vaccines are “polyvalent,” meaning they result in antibodies against most or all the serotypes, he said. However, vaccination with an attenuated live virus can cause just enough viremia (activation of the virus in the horse) to become infectious. For instance, if a midge—the natural vector of transmission of AHS—bites a recently vaccinated horse, it can transport that vaccine-introduced virus to other unvaccinated horses. Those horses will then become subclinically (not showing signs) infected, and midges can spread their disease to more horses, maintaining the virus circulation rather than eliminating it.

While the live attenuated vaccines work well in the endemic countries of sub-Saharan Africa, where the virus is prevalent in midges, these vaccines can become problematic in countries where new outbreaks occur