Zoetis Canada has stopped production of the Equine Arteritis Vaccine “Arvac”. We have been advised that the item is “discontinued” and that there is no intention to recommence production.
Given that there are EAV-positive and shedding stallions in Canada, this is a bad situation for North American breeders. Canada has a low incidence of EVA, but – as with any disease – if controls are not in place, there can always be an outbreak. To give you an idea of the effect of such a outbreak, one can look at the results of the 2006 outbreak which originated in New Mexico, USA. Two stallions were identified as “shedders” on the index farm – and this identification occurred only by accident when the farm’s own broodmare band exhibited a low pregnancy rate at their 60 day check. Both stallions were shipping semen. By the time the outbreak was identified and brought under control, an estimated 2,022 horses housed in 50 different facilities in 19 different states were affected – note “affected”, not “infected”, meaning that they had been exposed and consequently were placed under observation and quarantine. Some of those animals did indeed become EAV-seropositive (although we do not know if stallions were affected), and new regulations for shipping semen were introduced into Montana, Washington, Idaho and Oregon states. This was from a single incident, so the implications are significant.
For mare owners breeding to an EAV-positive and shedding stallion, this presents an additional complication to the breeding process if the mare is not current on vaccination. An unvaccinated (or low-titre) mare will be highly likely to contract the disease if bred to an EAV-positive and shedding stallion. While she herself will have no reduction in pregnancy establishment or maintenance rates, if she is exposed to other pregnant mares or a stallion within the 4 weeks or so of contracting the disease, she herself is in a “shedding” state through oronasal secretions, and this route of infection carries a high rate of infection for other pregnant mares (with a high likelihood of subsequent abortion for those mares) or stallions. It is therefore essential that mares bred to an EAV-positive and shedding stallion be kept isolated from susceptible stock following the breeding process, and for an additional 28 days. Neonatal foals at foot for the mare being bred also face increased risk of infection, and although in the case of colt foals development of “shedding” status is not a concern at that age (it requires elevated testosterone levels in order to develop shedding status, which colts prior to puberty will not have), in rare instances – particularly with already sick or debilitated foals – it could prove fatal.
For owners of unvaccinated stallions or those with low titres, it is essential that they do not allow exposure to a mare in the active (acute) stages of the disease, as a stallion in that category could become infected and subsequently a permanent shedder. The stallion is the natural reservoir for the virus. This situation could in particular occur with a mare which has been previously bred to a shedder stallion, failed to become pregnant, and then on the next cycle is presented for breeding to the naive stallion.
From a scientific perspective, although mares bred to shedder stallions will develop naturally elevated antibodies (and therefore immunity), there is an increased risk of mutation of the virus. Currently with Arvac, we in North America (now only the USA) have a good vaccine which protects against the strains present. If a mutation occurs however, this protection may no longer be present.
We have several articles and a presentation regarding Equine Viral Arteritis on the website in the articles section – they are towards the foot of the page.
