Horses that receive the recommended vaccine shots against the deadly Hendra virus appear to be well protected, the results of Australian field research shows.
The Hendra virus, found only in Australia, occasionally spills over from bats into horses, causing disease that results in euthanasia. Seven people have caught the virus through contact with the bodily fluids of infected horses, with four of those cases proving fatal.
Hendra was first identified in 1994 in an outbreak at a racing stable in the Brisbane suburb of Hendra, where several thoroughbred horses developed a mysterious illness. Thirteen horses died, along with their trainer, Vic Rail.
Five further outbreaks occurred between 1994 and 2004, and there have been annual spillover events consistently since 2006, with most occurring in the cooler months. Most involved single horses infected in an outdoor grazing environment.
The Hendra virus vaccine was developed for use in horses and provides a One Health solution to the prevention of human disease. By protecting horses from infection, the vaccine indirectly protects humans as well, as horses are the only known source of infection for humans.
The vaccine was released by Pfizer Animal Health (now Zoetis) for use in Australia at the end of 2012.
Researchers Kim Halpin, Kerryne Graham and Peter Durr, all with the Australian Centre for Disease Preparedness, part of the science agency CSIRO, set out to determine how the vaccine is performing in the field.
They analysed serum samples from 332 horses, spread across three states, for which details were available on age, dates of vaccinations, dates of sampling, and location.
The serological response to the vaccine is measured using a serum neutralisation test, also known as a virus neutralisation test.
The study team found that provided the horses received at least three vaccinations (consisting of two doses three to six weeks apart, and a third dose six months later), horses had high neutralising titres.
Only three horses did not register a titre, but they had received only two vaccinations, and the time between the last vaccination and sampling was more than three and a half years.
“Providing horses had three or more vaccinations, neutralising antibody titres were all in the presumed protective range. There were only two poor responders in this group (three or more vaccinations), but neither had followed the prescribed vaccination schedule.”
Since 2016, the Hendra vaccination schedule was altered from six-monthly boosters to annual booster shots.
“The Hendra vaccination schedule is now no more arduous than the schedules for the most commonly administered equine vaccines in Australia, which are those for tetanus and strangles.”
The researchers noted that there is a degree of “vaccine hesitancy” among Australian horse owners to use the vaccine, with the frequency of boosters being one of the identified influencing factors.
“The finding from our study that protective titres can last up to 24 months indicates a biennial booster vaccination regime is possible, which might also reduce this vaccine hesitancy.
“Nevertheless, the rigour of Australia’s vaccine regulatory procedures will require that there be strong supporting evidence, which in turn might require a formal double-blind, randomised clinical trial in a susceptible population.”
In conclusion, they said the vaccine induces significant neutralising antibody titres following the completion of the priming schedule and annual boosters.
“Furthermore, if horses receive seven or more annual boosters, they are likely to have even higher titres. However, further research is needed to evaluate neutralising antibody decline.”
Halpin, K.; Graham, K.; Durr, P.A. Sero-Monitoring of Horses Demonstrates the Equivac® HeV Hendra Virus Vaccine to Be Highly Effective in Inducing Neu-Tralising Antibody Titres. Vaccines 2021, 9, 731. https://doi.org/10.3390/vaccines90707 31. The study, published under a Creative Commons License, can be read here.