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Australian Veterinary Journal
To the Editor,
The other evening I was called out to yet another horse with colic whose primary veterinarian
had refused to attend because the horse was not currently vaccinated against Hendra virus
(HeV). To be considered current in the manufacturer’s Hendra vaccination registry, annual
boosters are mandated, even though the one study published to date with relevant data on
duration of immunity indicated that annual boosters may be unnecessary in many, and
perhaps even in most, horses.1
The horse had been routinely vaccinated against HeV in the past, but the owner had chosen
not to continue with annual boosters. This decision by horse owners, or refusal to vaccinate at
all, is a common occurrence in my experience and a phenomenon that is well documented in
the veterinary literature.2-4 Disturbingly, there are more studies published on why horse
owners aren’t vaccinating against HeV than there are on the safety and efficacy of the HeV
vaccine. Concerns about vaccine safety, efficacy, cost, and value persist among horse owners
in my practice area.
Rather than continuing to mandate vaccination per the manufacturer’s direction and refuse
service to clients whose horses are unvaccinated or whose HeV vaccination status is lapsed or
unknown, perhaps it’s time we begin taking a more measured approach. I propose we accept
the documentation of HeV-specific antibodies in the horse’s serum as evidence of a HeV-
specific immune response (whether naturally acquired or vaccine induced), with the
likelihood of a rapid anamnestic response in the event of subsequent exposure.
Two separate studies have shown that horses are capable of generating HeV-specific virus
neutralising (VN) antibody titres well above the threshold currently considered protective (≥
16)5 within one week, whether in response to natural infection6 or vaccination.1 The speed
with which horses generated a vigorous VN antibody response in these studies suggests that a
similarly rapid and robust response is possible — indeed, is likely — in the event of natural
exposure in horses who already have HeV-specific antibodies in their serum.
Of course, the point of booster vaccination is to maintain protective immunity throughout the
risk period. However, humoural immunity is only one component of the immune response to
pathogens, and the inevitable decline in titre in the absence of repeated challenge tells us
nothing about the cell-mediated immune capacity, including immunological ‘memory’, of
that individual. Furthermore, we don’t know the serum VN threshold that is protective
against HeV in real-world situations, where the incubation period may be as long as two
weeks7 and the amount of virus encountered by the horse is likely to be much lower than that
administered during experimental challenge: “the experimental horses were exposed to
considerably higher levels of HeV than have been recovered from flying foxes…”.5
As the authors of the HeV vaccine field study stated, “It is possible that protection from field
exposure to virus may also occur in immunised horses with lower (or even undetectable)
titres. The reasons for this include the rapid time-frame over which extensive mucosal
exposure to infective fluid occurs under experimental conditions and the fact that protection
will depend upon the development of an anamnestic response, in addition to pre-existing
antibody levels.”1
As for concerns about human safety, the one vaccine challenge study published to date
showed that a titre as low as 16 prevented meaningful shedding in the face of massive viral
challenge.5 Again, we don’t know the serum VN threshold that is protective against viral
transmission in real-world situations, but it is likely to be lower than that documented in this
single experimental study, in which the lowest pre-challenge titre of the ten vaccinated horses
was 16 and the horses were challenged with 2 million units of HeV via the oronasal route.
The CSIRO Australian Animal Health Laboratory (AAHL) offers two HeV-specific serum
antibody tests: an ELISA and a VN titre.8 Currently, the cost of the ELISA is comparable to
the retail price of the HeV vaccine. Unfortunately, the VN titre is considerably more
expensive. Although the ELISA provides only three possible results (positive, negative,
inconclusive), a positive ELISA is surely evidence of a HeV-specific immune response and
the potential for a rapid anamnestic response in the face of natural challenge that could be
expected to protect against severe illness at least, and probably from viral transmission as
well.
Instead of denying veterinary care to horses in need, let’s start testing horses for HeV-specific
antibodies and use our knowledge of clinical immunology and the available veterinary
literature to tailor HeV vaccination and other risk-mitigation programs. We may even be able
to encourage better uptake of the vaccine with this approach. One thing is certain: it can only
help our tattered reputation among horse owners.
Christine King BVSc, MANZCVS (equine), MVetClinStud
Anima Vet
Park Ridge, Queensland
Submitted 28 August 2020
References
1. Tan RHH, Hodge A, Klein R, et al. Virus-neutralising antibody responses in horses
following vaccination with Equivac® HeV: a field study. Aust Vet J 2018;96:161–166.
https://doi.org/10.1111/avj.12694
2. Manyweathers J, Field H, Longnecker N, et al. "Why won't they just vaccinate?" Horse
owner risk perception and uptake of the Hendra virus vaccine. BMC Vet Res 2017;13(1):103.
https://doi.org/10.1186/s12917-017-1006-7
3. Goyen KA, Wright JD, Cunneen A, et al. Playing with fire — What is influencing horse
owners' decisions to not vaccinate their horses against deadly Hendra virus infection?. PLoS
One 2017;12(6):e0180062. http://doi.org/10.1371/journal.pone.0180062
4. Wiethoelter AK, Sawford K, Schembri N, et al. "We've learned to live with it" — A
qualitative study of Australian horse owners' attitudes, perceptions and practices in response
to Hendra virus. Prev Vet Med 2017;140:67-77. https://doi.org/
10.1016/j.prevetmed.2017.03.003
5. Middleton D, Pallister J, Klein R, et al. Hendra virus vaccine, a One Health approach to
protecting horse, human, and environmental health. Emerg Infect Dis 2014;20(3):372–379.
https://doi.org/10.3201/eid2003.131159
6. Colling A, Lunt R, Bergfeld J, et al. A network approach for provisional assay recognition
of a Hendra virus antibody ELISA: test validation with low sample numbers from infected
horses. J Vet Diagn Invest 2018;30(3):362–369. https://doi.org/10.1177/1040638718760102
7. Broder CC, Weir DL, Reid PA. Hendra virus and Nipah virus animal vaccines. Vaccine
2016;34(30):3525–3534. https://doi.org/10.1016/j.vaccine.2016.03.075
8. https://aahl.csiro.au/info/companion_and_equine_testing.aspx