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.
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.
We get this question quite often, and it would be convenient to combine the treatments to save on an additional call fee. However, we generally recommend against this for a few reasons. One of the drugs we typically use in joint injections is a steroid anti-inflammatory such as triamcinolone or methylprednisolone acetate. These are very potent anti-inflammatory agents, and can cause immune suppression even in the small amounts used in joints. This means that the immune response to a vaccine could be dampened by the steroids injected in the joint, and the horse may not be adequately protected by the vaccine if it is administered at the same time as intra-articular injections. Also, in the case of the intranasal Strangles (Strep equi) vaccine, we are very careful not to have contamination of any injection sites with the snorting of the vaccine after the horse is given this vaccine. If this modified live bacteria is inadvertently injected, it can set up a nasty abscess and this would be particularly devastating if this infection were to occur in a joint.
This is a very interesting and common question, and there is a little bit of research available to help guide recommendations. The first thing to consider is that sometimes, unpredictably, a horse will have a severe, immediate response to the vaccination such as anaphylaxis. Obviously, being ridden when this occurs can worsen the situation, and would put the rider at risk if the horse were to become weak or wobbly, or even fall. If you are going to ride the horse after a vaccination, wait at least 30 minutes to ensure these immediate adverse effects are not seen. Some clinicians feel that if the horse is ridden lightly after a vaccination, they develop less stiffness at the injection site. However, it has been shown that extreme exercise (for example, galloping sets or race training) in close proximity to a vaccination can reduce the horse’s ability to respond to the vaccine appropriately, therefore the vaccine may be less effective. It seems the exercise essentially acts as a form of stress that reduces the horse’s immune response to the vaccine. The bottom line—while it may be safe (or potentially beneficial) to exercise a horse lightly a little while after a vaccination is given, avoid strenuous exercise.
If a human arrives for an influenza vaccination from a vaccine clinic or pharmacy, one of the first questions asked is, “Are you feeling well today?” If the immune response is actively engaged fighting another virus or bacterial process, the response to a vaccination might not be optimal. Therefore, if your horse demonstrates a nasal discharge, has a mild fever, or is recovering from another medical issue, we usually delay vaccination until their immune system is better able to respond to the vaccine reliably.
An upper respiratory tract infection or “cold” in a horse, like in a human, can be caused by many different viruses or bacterial infections—adenovirus, herpesvirus (Type 1, 2, 4, 5), influenza virus, rhinitis A or B virus, Strep equi (“Strangles”)—or even a combination of these agents. Horses can be vaccinated against some respiratory pathogens, namely herpesvirus Type 1 and 4, Influenza, and Strangles, but many other viruses especially are not included in the routine vaccinations (much as humans can be vaccinated against influenza but not the common cold). Additionally, each horse’s immune system will respond differently to a given vaccine, and a small percentage of horses will be “non-responders.” These horses do not mount an adequate immune response following even properly administered vaccinations. If your horse develops an upper respiratory infection even when they have received their yearly vaccinations, they could be infected by one of these other viruses (adenovirus, other herpesvirus types, rhinitis virus), or their immune system simply did not respond to the vaccine as well as some other horses.
With the exception of Strangles, all our commonly used equine vaccines are given intramuscularly. Rabies must be administered by a veterinarian, so it cannot be dispensed for owners to give, however the others can be dispensed. A note of caution, however—any vaccine can produce immediate, unpredictable adverse effects such as hives or anaphylaxis, which can be fatal if not treated promptly. If a veterinarian is administering a vaccination, they are required to have medications on hand that can address these concerns. If administering a vaccine yourself, monitor very closely for signs of these adverse effects and have a plan in place for prompt treatment.
What vaccinations your horse receives should be dependent on their specific risk of exposure to given diseases. For horses that do not travel to other barns, shows, or arenas, and do not mix with other populations of horse, vaccination against respiratory pathogens that are passed from horse to horse such as herpesvirus, influenza, and Strep equi (“Strangles”) is probably not necessary. However, not all diseases that are commonly vaccinated against do not pass from horse to horse. The bacterium that causes tetanus is ubiquitous in the soil, and enters the body via wounds—no contact with other horses is necessary to be afflicted with tetanus. Other diseases can be spread by blood-sucking insects, namely West Nile Virus and Eastern and Western Equine Encephalitis (EEE/WEE). The source of the virus that the mosquitos carry is not other horses, but rather wild birds. Finally, rabies virus is found in the saliva of an infected animal, and is usually spread by bite wounds from infected bats, skunks, raccoons, foxes, or other infected animals. Although a “pasture ornament” may not require all the same vaccines as an actively competing show horse, we still recommend that they are vaccinated against these diseases that are not spread by horse to horse contact—tetanus, West Nile virus, EEE/WEE, and potentially rabies.