Vaccines against African horse sickness using inactivated virus put to the test

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Adult female blood sucking midge, Culicoides imicola.
Adult female blood-sucking midge, Culicoides imicola. © Alan R Walker / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

A study in the United Arab Emirates has investigated the performance of inactivated vaccines against African horse sickness.

The results of the study, carried out by researchers at the Central Veterinary Research Laboratory in Dubai, may lead to the production of safe and effective inactivated African horse sickness vaccines that protect equids against the deadly disease before modern recombinant subunit vaccines targeting the virus become a reality.

African horse sickness is a viral disease, spread by biting midges, which is endemic to sub-Saharan Africa.

The disease has been recognised for centuries. It has a significant economic impact on the horse industry, and hampers horse exports from affected countries.

Currently, prevention and control are based around control of the insects that spread it, and the use of live attenuated vaccines. However, studies have revealed a possible inherent risk associated with this kind of vaccine: The attenuated virus could revert to virulence and subsequently spread the disease.

Inactivated vaccines are considered safe in this context.

For their study, researchers at the Central Veterinary Research Laboratory developed inactivated vaccines from serotypes isolated from horse fatalities in Kenya, where all nine serotypes circulate.

The laboratory’s inactivated vaccines, covering all serotypes, have been made since 2014. These are available as individual serotype vaccines or in two vaccine combinations, termed vaccine 1 (covering serotypes 1, 4, 7, 8, and 9) and vaccine 2 (serotypes 2, 3, 5, and 6).

Marina Rodríguez and her colleagues, writing in the journal BMC Veterinary Research, described an experiment in which 29 horses, divided into various groupings, were vaccinated.

Eighteen horses in the first group were further divided into nine subgroups of two horses each. Each pair were given a vaccine with inactive virus targeting only one of the serotypes, meaning all of the nine serotypes were covered across the nine subgroups. They received a booster shot 42 days later, and another 98 days after the primary vaccination.

The 11 horses in the second group were immunised with all nine serotypes simultaneously with the two different vaccines (covering serotypes 1, 4, 7, 8 and 9; and serotypes 2, 3, 5 and 6). A booster was given 42 days later.

The horses, held at a desert facility in Dubai, were monitored for 12 months. Blood samples were periodically withdrawn for serum antibody tests, and two weeks after each vaccination samples were taken for gene testing and virus isolation.

After receiving their booster vaccinations, 27 of the horses seroconverted. However, the remaining two horses had a poor antibody response.

The study team reported that horses vaccinated with single serotype vaccines needed a booster after six months, given that their antibody levels had declined five to seven months after vaccination.

Those simultaneously immunised with all nine serotypes required a booster after 12 months, they found. In this group, antibodies were positive after the booster given on day 42, and virus neutralisation antibodies had started to appear for some serotypes after primary vaccination. After the booster shot, antibodies against the virus increased in a different pattern for each serotype.

Antibodies remained high for 12 months and increased strongly after the annual booster in 78% of the horses.

Due to the non-availability of a facility in the United Arab Emirates, no challenge infection could be carried out.

The authors said simultaneous vaccination with inactivated vaccines against multiple serotypes seemed to enhance the immune response, which was not observed in other studies when attenuated preparations against multiple serotypes were given.

“This is the first report demonstrating the immune response of horses to inactivated African horse sickness vaccines containing all nine serotypes,” they said.

The study team said the absence of detectable virus neutralisation antibodies to one or more serotypes may not necessarily be suggestive of a lack of protection against the disease, as these animals might appear to be resistant to a challenge that also depends on cell-mediated immunity.

“The regular use of inactivated African horse sickness vaccines should protect against clinical signs and especially death,” they said.

The authors noted that inactivated vaccines are optimal for immunising horse populations against the disease, as an experiment they carried out in Kenya showed, where in 2018/19, no cases of the disease were reported.

Rodríguez, M., Joseph, S., Pfeffer, M. et al. Immune response of horses to inactivated African horse sickness vaccines. BMC Vet Res 16, 322 (2020). https://doi.org/10.1186/s12917-020-02540-y

The study, published under a Creative Commons License, can be read here

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