With annual boosters, the vaccine could keep horses safe from any of the AHS virus serotypes without the risks associated with a live-virus vaccine, which can sometimes produce the disease, said Marina Rodriguez Caveney, DVM, of the Central Veterinary Research Laboratory (CVRL) in Dubai.

The inactivated vaccine effectively protects horses from the disease—or at least from death, CVRL researchers said. It appears to work whether they live in endemic regions of Africa or in zones hit by temporary outbreaks, such as the one that struck Thailand last year, leaving dozens of dead horses in its wake.

Testing Retired Horses in the UAE

Caveney and her fellow researchers tested two types of an inactivated AHS vaccine developed in their facility, focusing on a group of 25 geldings and four mares, ages 20 to 30, living in the UAE.

Eighteen of the horses were divided into nine pairs of horses, and each pair was vaccinated against one of the nine serotypes. The other 11 horses received a vaccine targeting all nine serotypes (in two injections given simultaneously).

All 29 horses received a booster 28 days and 12 months after the first injection.

The researchers took blood samples from each horse every two weeks for the first year of the study to investigate antibody production, Caveney said. They also drew blood at multiple intervals during the two weeks following each vaccination or booster for polymerase chain reaction (PCR) testing.

The team found no virus evident in any of the horses’ systems, they said. However, all the horses developed protective antibodies against AHS.

Those receiving a single-serotype vaccine took longer to acquire a protective effect, Caveney explained. Most of them had effective levels of protection several weeks after the second booster—nearly 100 days after the initial vaccine.

By contrast, the horses receiving all nine serotypes of inactivated virus at the same time only needed one booster one month after the first shot, she said. All but one of these horses—90%—had full protection against all serotypes within five to 14 days of that booster.

The results suggest vaccination with inactivated viruses of all nine serotypes, with a one-month booster followed by annual boosters, could effectively protect most horses from AHS, the researchers reported.

Not That New, But Not Well-Known

The CVRL developed its inactivated vaccine in 2014, and a six-year study on 27 horses in Kenya also showed optimistic results.Its use has yet to be widely adopted either in Africa or in AHS-free countries, said Caveney, who cited “political reasons” for the poor uptake. Although her institution has tried for years to collaborate with various institutions in Africa, “all the answers were negative,” she said, with the exception of Kenya, where many horses have undergone vaccination since 2014, and Sudan, where authorities recently ordered 1,000 doses of CVRL’s AHS inactivated vaccine.

Thai officials did not accept offers for the CVRL vaccine during their 2020 epidemic, said Caveney. “An inactivated vaccine would have been the solution for this outbreak,” she told The Horse. “Instead, Thailand and Malaysia had to fight not only with the field virus but the risk … of reversion to virulence from the attenuated vaccine.”

The Horse’s official sources in Thailand did not respond to requests for comment.

None of the 50 Kenyan horses recently vaccinated with the inactivated vaccine have died from AHS, although six developed a mild form of the disease six months after the initial vaccine, probably from the circulating virus in their endemic region, the researchers reported. They all fully recovered within 72 hours.

Future studies on the team’s inactivated vaccine will include challenge tests—assessing how well the vaccinated UAE horses resist exposure to the virus in real-world conditions, she said.

The report was published in BMC Vet Res in September 2020.