Q. I have questions about a vaccine for snakebites: My own veterinarian has not mentioned this but my neighbor’s vet recommended that they vaccinate their horses against rattlesnake venom. Last summer six horses in two-mile radius of our farm sustained snakebites, and I am trying to anticipate the upcoming season. I was told by my neighbors the vaccine is a series of three injections, and often horses develop a temporary swelling or irritation at the injection site. Can you tell me more about this vaccine? Is it effective against all rattlesnake species in the United States? Is a horse that previously has been bitten by a rattlesnake more or less likely to have a severe reaction if bitten again?

Betty Fey, via e-mail

A. There is, indeed, a rattlesnake vaccine labeled for horses. And I must disclose that I have done collaborative research work with the company that produces the vaccine, Red Rock Biologics. The rattlesnake vaccine is made with venom from the Western Diamondback Rattlesnake, and in vitro studies showed it to be effective in neutralizing this snake’s venom. We know that rattlesnake venoms, while different in many ways, are also similar in many ways and contain many similar toxins. This knowledge would lead one to believe that antibodies made against one rattlesnake venom may be at least partially effective at neutralizing other rattlesnake venoms. Scientific studies have shown that some rattlesnake venoms are more similar than others; however, a vaccine against any one venom would not likely provide protection against all rattlesnake venoms in the United States or elsewhere. So, the answer to your second question is that it is unlikely that antibodies produced from vaccination with the (Western Diamondback) rattlesnake vaccine would be fully protective against every species of rattlesnake in the United States. To my knowledge, studies of this type have not been done.

The initial vaccine series is a series of three injections given in the muscle 30 days apart. There are not specific instructions for administration location of the vaccine, but any of the usual sites for an intramuscular injection would be acceptable, including the neck or low in the semimembranosus/semitendinosus muscles (hamstring area).

I participated in the safety trial for the vaccine, and we only had a couple of horses develop injection site reactions that were very mild and resolved without any treatment.

The timing of the vaccine, in my opinion, should be such that the horse will have the highest antibody titer during the peak rattlesnake season, which may vary depending on the area of the country where you reside. This means that horses would need to receive all three vaccines prior to the beginning of rattlesnake season, with the last one being at least 10 days prior to rattlesnake season.

Your question about previously bitten horses is very good and one that does not have an exact answer. I can only provide an answer in relation to what we know in other species. From my research we know that horses do develop antibody titers against rattlesnake venom after being bitten by a rattlesnake, but we do not know how long these antibodies last or if they are protective if the horse is bitten again. People that are bitten by poisonous snakes multiple times tend to have weaker reactions each subsequent time they are bitten. When I was in private practice I treated dogs and horses bitten by rattlesnakes, and animals that were bitten more than once seemed to have weaker reactions with each subsequent bite; however, I do not have enough data currently to prove this.

The rattlesnake vaccine is rather new in the horse, and I do not believe there are any published studies as of this moment; however, we have submitted a paper on the comparison of antibody titers in naturally bitten horses with vaccinated horses.