animal health consulting
Hendra: why not just go ahead and vaccinate?
Christine King BVSc, MANZCVS (equine), MVetClinStud
Prompted by an enquiry from a client about whether or not she should continue vaccinating her horses against the Hendra virus (HeV), I decided to do a deep dive into the research on this virus and on the equine HeV vaccine. What started out as a short email reply ended up morphing into this multi-part, multi-page treatise.
Jane Austen said it best: "I write so that I'll know what I think." I was living in the US from 1993 until 2018, so I missed the start of the HeV era in Australia (1994) and almost all of the drama since. When I moved back home to Queensland in mid-2019, I had a lot of catching up to do — not just on the science, but also on the 'politics' (for want of a better word) of HeV.
My client's concerns are common ones: she had a young family, so to continue with annual HeV boosters for their three horses was a considerable expense. In her calculations were the following two issues: there hadn't been a case of HeV in her area in ten years, and, in her mind, the effectiveness of the vaccine was still in question, or at least, it had not been firmly established. So, why continue with an expensive product that doesn't appear to be needed and may not work all that well anyway?
This web series delves into those issues, and more. Writing as much for myself as for anyone else, the following pages present my current understanding of these issues, and my current recommendations.
Because there's a lot to discuss, I've placed the following Table of Contents at the top of each page so that you can easily find your way around and find your place again if you leave and come back. I've also included references to the source material as highlighted links, in case you want to read it for yourself (which I highly recommend). Throughout, the 'Read on...' link at the bottom of the page takes you to the next page of the Contents.
Let's begin. On this first page is a summary of the whole thing, listed as 'key points'.
Table of Contents
Summary (key points)
1. The HeV vaccine appears to be very effective.
In an experimental vaccine-challenge study involving a large dose of live virus from the 2008 outbreak in Redlands, QLD, 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 initial 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 27+ years).
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.
Many horse owners still believe that this vaccine has an unacceptably high rate of serious adverse effects, including death. Although it was ultimately unsuccessful, a Class Action lawsuit was filed against Zoetis by a group of horse owners. It basically claimed that Zoetis overstated the risks of the virus and understated the risks of the vaccine, to the detriment of their horses' health and value.
3. The mortality rate for HeV infection in horses is reported to be high (~ 75%).
However, that mortality rate is based on the horses who became seriously ill, which involves a massive systemic inflammatory response ('cytokine storm') that causes more damage than the virus. 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 in the horse population 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 July 2022, there have been 88 confirmed and 20 unconfirmed cases (not outbreaks, individual cases of HeV infection) in horses, in QLD and NSW combined, since 1994. For context, it is estimated that there are a little over half a million horses in QLD and NSW.
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 and early spring, 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 as much as to protect the horses themselves. Some equine vets refuse to treat horses who are unvaccinated or whose vaccination status is "overdue" or unknown, 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 monoclonal antibody product is now available for the treatment of HeV infection in humans. (We have no such treatment available for horses.)
In addition, a Nipah virus (NiV) vaccine containing the same core component as the equine HeV vaccine (the HeV soluble G protein) is in human clinical trials in the US. The Hendra and Nipah viruses are so closely related that the G protein-based vaccines protect against both viruses. It may be only a matter of time before there is a human HeV-NiV vaccine available.
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 initial series of 3 injections.
A lab test which determines the presence/absence of HeV antibodies in the horse's blood using ELISA technology may be used to guide vaccination and other management decisions, as a positive result is proof of immunity. The more expensive HeV serum or virus neutralising titre (SNT or VNT) provides a numerical result which is not only proof of immunity, it also indicates whether or not a booster is advisable in high-risk situations.
© Christine M. King, 2021, 2022. All rights reserved.
Last updated 28 September 2023.