Christine King BVSc, MANZCVS (equine), MVetClinStud
Hendra: why not just go ahead and vaccinate?
Prompted by an enquiry from a client about whether or not to continue vaccinating her horses against the Hendra virus, I decided to do a deep dive into the research on this virus and the equine vaccine. What started out as a short email reply ended up being this multi-part, multi-page treatise. Jane Austen said it best: "I write so that I'll know what I think."
1. The Hendra virus (HeV) vaccine appears to be very effective.
In an experimental vaccine-challenge study involving a large dose of live virus from the 2008 Redlands outbreak, the HeV vaccine protected all 10 vaccinated horses against illness and prevented significant viral shedding in their body fluids.
To date, no fully vaccinated horses (completed the primary series of 3 injections, followed by annual booster) have shown signs of HeV infection. However, this fact must be weighed against the extremely low incidence of HeV infection in horses (less than 100 confirmed cases in 26 years — that's less than 4 confirmed cases per year, on average).
2. Questions remain about the safety of the HeV vaccine.
Adverse effects of the HeV vaccine (Equivac® HeV, made by Zoetis) are reported to be mild and uncommon, but as with all pharmaceuticals, not all adverse effects are recognised as such or reported.
Rumours of severe adverse effects persist among horse owners.
A Class Action lawsuit, filed against Zoetis by a group of horse owners, is making its way through the federal court. It alleges that Zoetis understated the risks of the vaccine and overstated the risks of the virus.
3. The mortality rate for HeV infection in horses is reported to be high (~75%).
However, the mortality rate is based on the horses who became seriously ill, which involves a massive systemic inflammatory response that causes more damage than the virus does.
Not all HeV infected horses become ill, and not all who do will die.
We don’t know the current incidence of HeV exposure and naturally acquired immunity because no large-scale surveys have been done since HeV first appeared in Queensland in 1994.
4. The overall incidence of HeV infection in horses is low.
Outbreaks of HeV infection in horses are uncommon, patchy, and difficult to accurately predict. As of June 2020, there have been 85 confirmed and 20 probable cases (not outbreaks, individual cases of HeV infection) in Qld and NSW combined since 1994.
Flying fox habitat and health are important components; ‘spillover’ events occurred where flying fox secretions (saliva, nasal discharge) and excretions (urine, manure) contaminated areas where horses feed.
The period of greatest risk appears to be in the winter, when young flying foxes in particular may be nutritionally stressed.
5. The HeV vaccine is promoted to protect human health as much as horse health.
Vaccination of horses against HeV is widely encouraged to protect humans from infected horses.
Some equine vets refuse to treat unvaccinated horses, in part for their own safety but also because some have faced legal action for not taking "adequate" steps to protect human health.
However, the factors that determine a person’s risk of disease and death from HeV are unknown. Since the virus first appeared in Australia in 1994, there have been only 7 confirmed cases of HeV infection in humans; 4 died and 3 survived.
A HeV-specific antibody product is available for the treatment of HeV infection in humans in Australia, and a human HeV vaccine is in phase I clinical trial in the US.
6. A blood test for the presence of HeV antibodies is available for healthy horses.
Based on available data, annual boosters do not appear to be necessary for continued protection in most horses who have received the primary series of 3 injections.
A lab test which determines the presence/absence of HeV antibodies in the horse's blood may be used to guide vaccination and other management decisions. The more expensive HeV antibody titre provides even more information for guiding vaccination/management decisions.
There are several reasons why horse owners are choosing not to vaccinate their horses against the Hendra virus (HeV). Chief among them are cost, concerns about the safety of the vaccine, cost, questions about the effectiveness of the vaccine, cost, low incidence of HeV cases and therefore perception of low risk, and cost.
I don’t subscribe to the notion that if there’s a vaccine available, it should be used. Instead, I base whether to vaccinate on a simple formula: risk vs. benefit, and the outcome of weighing all factors involved is unique to each horse.
But before getting into the science of HeV infection in horses and the safety and effectiveness of the HeV vaccine, I want to talk about the cost of the vaccine, because it’s not part of the risk vs. benefit calculation, yet it’s an important consideration for many horse owners and a breakpoint for some, particularly now that we're in a COVID-triggered recession.
Currently, there is only one HeV vaccine registered for use in horses in Australia: Equivac® HeV, made by Zoetis (formerly Pfizer Animal Health). That means Zoetis has a monopoly and can charge whatever it thinks the market will bear.
Furthermore, Zoetis manages the Hendra registry, or HeV vaccination database, through which equine vets determine whether or not a particular horse is currently vaccinated against HeV. Admission to the database requires that the horse be microchipped for fraud-proof identification and vaccinated by a registered veterinarian. The horse is not considered properly vaccinated unless the vaccine was administered by a veterinarian approved by Zoetis and following Zoetis’ recommendations (annual boosters after the primary vaccination series of three injections).
It’s worth noting here that there is little published data on the duration of immunity for this vaccine, and what we do know suggests that ongoing annual boosters may not be needed in most horses. More on that later.
For now, Zoetis has us over a barrel. However, there is at least one other HeV vaccine in development. For example, in 2016 Merial published the results of a study involving 9 ponies. Their HeV vaccine (which uses their canarypox viral vector technology) induced HeV-specific antibody titres in all 9 ponies that were above the level found to be protective in the viral challenge study of the Zoetis vaccine (which involved 10 horses in all, and is discussed at length later). Let’s hope that Merial is soon giving Zoetis a run for its money in Australia.
It's important to note that the cost of not vaccinating is much higher than the cost of vaccinating if the horse is exposed to HeV and becomes seriously ill — and it's the horse who pays the most, as the mortality rate in such cases is high.
Even so, for horse owners with more than one horse, the cost of the three vaccines required to complete the primary series, combined with the cost of the vet’s fees for each of those three visits, is considerable. If the horses are not already microchipped, there is that additional expense if the horses are to be entered into the Hendra registry. Thereafter, it's annual boosters for life. It all adds up, and for many horse owners it's a big, fat thumb on the scale of risk vs. benefit.
It would be very good if HeV antibody testing became commonplace and the standard of care for horses in coastal Queensland and New South Wales.
Armed with the results of either test, horse owners can make better vaccination and other management decisions, and equine vets concerned about treating horses who are not fully vaccinated according to Zoetis can have more confidence that they are at low risk of contracting or spreading HeV from these horses.