Dr. Alyssa Butters gives an update on the status of West Nile Virus and Rabies in Alberta, and gives current vaccine recommendations.
Just as in humans, most vaccinations cause at most very mild outward symptoms after the vaccine is given. Some horses may be a little more lethargic than usual, have some mild tenderness at the injection site, or stiffness in the neck if that is where the vaccine was administered. Occasionally, the horse will develop a mild and transient fever, and this is usually noticed if the horse goes off his feed. These usually resolved uneventfully, although sometimes anti-inflammatories are given.
With several recent outbreaks of neurological herpesvirus infections in the US and Canada, much attention has been turned to this common disease. The virus is not new, and is considered endemic in most regions of the world. Although some aspects of the disease have been understood for decades, there is still much to be learned about the virus and its effects on horses.
Both horses and humans are very susceptible to tetanus, horses even more so than humans. In humans, there are many studies examining the duration of immunity following vaccination, so physicians can feel confident in their recommendations of extending the revaccination interval. In horses, a 6 month study comparing responses to commercial vaccines demonstrated significant antibody response for the duration of the study, but did not measure the response beyond 6 months. There have not been any studies in North America that have challenged vaccinated horses with tetanus toxoid, but antibodies alone can mediate protection. Conclusions about efficacy of the vaccine and duration of protection are based on antibody levels detected in laboratory studies combined with field experience, and the recommendation for yearly vaccination stems from this. There have been rare reports of horses with clinical cases of tetanus that had been vaccinated, however survival of horses with tetanus is strongly associated with previous vaccination.
The theory of an intranasal vaccine for Strangles is that the site of entry and infection with Strangles is via the tonsils located in the nose and mouth. If we stimulate immunity at these sites by introducing a vaccine directly to those tonsils, we can limit the propagation of the bacteria at its site of entry. The Strangles vaccine most commonly used is a modified live bacterial vaccine, which is unable to replicate but mimics the immunity stimulated by a natural infection. However, its efficacy is dependent on an adequate amount of the vaccine reaching the tonsils deep in the head, so it must be administered via the nasal passageways.
Alberta stands out as one of the few jurisdictions in North America in which horses are not routinely vaccinated against rabies. Arguments have always been that the incidence of rabies is much lower than in other regions, and there have been no reported equine cases of rabies in years, so the risk of infection would be so low as not to necessitate vaccination. The rabies vaccine must be administered by a veterinarian, another potential stumbling block to widespread vaccination of equines.
The rate of rabies in potential sources of infection within Alberta is the same whether we are considering a wildlife vector could infect a dog, a cat, or a horse. Yet rabies vaccination of our small animal companions is routine whereas vaccination of horses is almost non-existent. Although relatively rare, rabies is present in wildlife populations in Alberta, clinical disease should it occur is untreatable and (almost) invariably fatal, and a rabid companion animal or even horse can have very significant public health ramifications. Should we be revisiting this policy of non-vaccination of horses in Alberta?
Equine Influenza Virus (EIV) is often cited as the most commonly diagnosed and economically important causes of viral respiratory disease in horses. Therefore, the AAEP states that “all horses should be vaccinated against EIV unless they live in a closed and isolated facility.” Like many other respiratory viruses, it produces fever, nasal discharge, and coughing. So what sets this virus apart from other equine respiratory viruses?
Potomac Horse Fever can cause very serious diarrhea, and other symptoms may include fever, laminitis, and colic. It is caused by Neorickettsia risticii, a bacteria found in freshwater snails that is believed to be transmitted to horses via inadvertent ingestion of infected aquatic insects. It is usually seasonal, seen most commonly in the hot summer months or early fall. If Potomac Horse Fever has occurred in a particular geographic area, it is likely that additional cases will occur in future years. However, vaccination against this disease has been controversial. Evidence of protection against clinical disease is lacking, possibly because the vaccine may not stimulate a protective immune response, or potentially because multiple strains of the bacteria may exist, whereas only one strain is present in the available vaccine.
This is a difficult question to answer, and there is little scientific data to guide recommendations. It is very convenient to have all vaccinations given all at once to save time and money, and it is very commonly done with little adverse effects. There are no studies examining the safety or efficacy of the vaccines when multiple different ones are administered together. This does not mean they are not safe or not effective if given in combination, just that this has not been proven. It does appear that there is a higher rate of adverse reactions if multiple vaccinations are given at one time, however, the overall rate of adverse reactions is still very low. If your horse has experienced an adverse reaction in the past, it may be wise to split up his or her future vaccinations, both to decrease the risk of an adverse event as well as to determine if a particular vaccination is the culprit causing the horse to react. If multiple vaccinations are given on the same date, it must be considered that modified live vaccinations should not be given close to the site of a killed vaccination, as the adjuvant in the killed vaccine may interfere with the activity of the modified live vaccine. Seek your veterinarian’s advice if this is a concern.