animal health consulting

Hendra: why not just go ahead and vaccinate?

Christine King  BVSc, MANZCVS (equine), MVetClinStud

Table of Contents

Summary (key points)



   the virus

   the 'vector' (flying foxes)

   the vaccine

   vaccination status


   the vaccine

   experimental study

   vaccine field study

   vaccination status

HeV antibody testing

Letter to the Editor (AVJ)

Final thoughts


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 (including whether to booster) on a simple formula: risk vs. benefit; and the outcome of weighing all factors involved is unique to each horse, because the set of factors involved is unique to each horse.

Vaccination is a deliberately provocative action i.e., vaccines are deliberately designed to provoke a vigorous immune response — so adverse effects are common, even if mild and self-limiting in most cases. A small but significant percentage of individuals will experience more serious adverse effects; and in some the result may be chronic ill health, permanent disability, or even death. So, for every vaccine in every horse, my ethos is this: the benefit must outweigh the risk.

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 staring down the barrel of a COVID-triggered recession.

Vaccine cost

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.

Zoetis reported gross revenues of US$7.8 billion in 2021, for a net income of US$2 billion for the year. At the current exchange rate, that's a profit of AU$2.85 billion in 2021. For the first quarter of 2022, Zoetis reported gross revenues of US$2 billion and a net income of US$595 million — for just the first 3 months of the year.

Zoetis is a publicly-traded company, so its first responsibility is to its shareholders, not to its customers nor to the animals in our care. That fact is patently clear.

Furthermore, Zoetis owns and administers the "Horse Health" registry, or HeV vaccination database, through which equine vets determine whether or not a particular horse is currently vaccinated against HeV.

"This tool is designed to help you easily look up the Hendra vaccination status of horses across Australia" using the horse's microchip number.

Admission to the registry requires that the horse be microchipped for fraud-proof identification and vaccinated by a registered veterinarian. The horse is not considered currently vaccinated unless the vaccine was administered by a veterinarian approved by Zoetis and following Zoetis’ recommendations: booster every 12 months after the initial series of 3 doses; no exceptions, no delays, no deviations from the schedule.

It’s worth noting here that the published data on duration of immunity for the HeV vaccine is incomplete, and what we do know suggests that ongoing annual boosters may be excessive in most horses. I'll discuss this particular issue in detail, with data and graphs, in a later section.

For now, Zoetis has us over a barrel. However, there may be at least one other HeV vaccine in development. For example, in 2016 Merial published the results of their 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 ... I discuss it at length later in this series). Now that Merial is part of the pharmaceutical behemoth, Boehringer-Ingelheim, let's hope it is soon giving Zoetis a run for its money in Australia with a HeV vaccine of its own.

* In the Merial vaccine, the canarypox viral genome was modified to express two HeV surface proteins: the attachment glycoprotein or G protein and the fusion protein or F protein. The Zoetis equine HeV vaccine is based on a synthetic (recombinant) version of the HeV G glycoprotein.

Incidentally, the potential market for these vaccines is not as small as it may appear, given the limited geographic distribution of HeV infections in horses to date. Both of these HeV vaccines (Zoetis and Merial) also induce antibodies against the closely-related Nipah virus (NiV), which causes disease in humans, pigs, and other animals that is similar to HeV infection in horses and humans. There is a huge market for a safe and effective HeV-NiV vaccine, and that’s good news for horse owners, because healthy competition tends to drive prices down.

Cost of not vaccinating

It's important to note that the cost of not vaccinating is much higher than the cost of vaccinating if the horse is infected with HeV and becomes seriously ill — and it's the horse who pays the most, as the mortality rate in such cases is high (estimated to be ~ 75%, but potentially much lower with effective antiviral treatment).

The other costs to not vaccinating are perhaps less tangible or direct but are no less problematic: refusal of service or entry by veterinarians, owners/managers of boarding (agistment) or training facilities, organisers of equestrian events, etc. to unvaccinated horses and even to those who are simply "overdue" according to the fox guarding the henhouse.

For many horse owners, this may not be a problem — until it is, and then it can be a huge and pressing problem, such as being unable to find a vet who will come see your colicking horse! (I have an entire section on this troubling issue.)

Even so, for horse owners with more than one horse, the cost of the 3 doses required to complete the initial vaccination series, combined with the cost of the vet’s fees for each of those 3 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 vaccination registry. Thereafter, it's annual boosters for life, and they too must be administered by a registered and Zoetis-approved veterinarian if the horse is to remain in good standing in the Hendra registry.

It all adds up, and for many horse owners it's a big, fat thumb on the scale of risk vs. benefit.

Proof of immunity instead

I encourage vets, farm/barn managers, and event organisers to accept proof of immunity as an alternative to proof of vaccination.

It would be great if annual HeV antibody testing became commonplace and the standard of care for horses in Queensland and New South Wales at the very least. Then there would be no need to deny service or entry to any horse with proof of immunity to HeV.

Currently, the HeV ELISA costs about the same as a dose of the HeV vaccine, although this test just provides a positive/negative result. (It simply tests for presence/absence of HeV antibodies in the horse's blood.)

The HeV serum or virus neutralisation (SN or VN) test is much more expensive but it provides a numerical result, or titre.

I discuss these tests and their interpretation in more detail later.

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 currently vaccinated according to Zoetis can have more confidence that they are at low risk of contracting or spreading HeV from these horses.

Read on...

© Christine M. King, 2021, 2022. All rights reserved.

Last updated 26 May 2022.


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