The Latest Buzz: Rattlesnake vaccine available for horses |

The Latest Buzz: Rattlesnake vaccine available for horses

A vaccine is now available to protect horses against rattlesnake venom.
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Rumor Has It Dr. Dale M. Wallis, Senior Staff Veterinarian at Red Rock Biologics, responds to claims of adverse reactions to canine and equine rattlesnake vaccines. The equine rattlesnake vaccine was licensed about eight years ago.  To date, there have been zero significant reactions and no deaths.  I believe there may have been a handful (less than ten) total reactions reported in horses over this entire time, all of these being swelling at the injection site.  In all cases, they resolved without treatment. The internet is not a source of truthful reporting; the primary report against the canine rattlesnake vaccine is a false narrative about a dog named Blossom, who developed autoimmune hemolytic anemia three days after her first dose of rattlesnake vaccine.  What is never explained is that Blossom went to the vet that day (2.5 hour drive to vet) because she was "off" according to her owner.  In other words, she was already ill.  The vet did a physical exam, but no bloodwork, and did not find anything wrong.  Blossom's owner saw a poster for the rattlesnake vaccine and decided to have her dog vaccinated.  Three days later, Blossom was in hemolytic crisis, due her body's immune system attacking her red blood cells.  Her owner is convinced that the rattlesnake vaccine is to blame, however, this is not biologically possible. It takes several weeks to generate antibodies to anything, much longer than the three days elapsing between vaccination and when Blossom became ill. Antibodies are very specific; each one only reacts with the exact antigen which caused it to be formed in the first place.  Antibodies to red blood cells don't even react with closely related white blood cells in the same animal, let alone venom antigens from a snake. There are no canine antigens in the rattlesnake vaccine, thus, vaccination could not possibly have stimulated existing antibodies to attack, nor generated new antibodies against canine red blood cells. Blossom's owner is not a scientist, and does not want to believe her dog was ill before vaccination, even though that is the reason she travelled 2.5 hours to the vet that day.  It is easier for her to put the blame on the vaccine. Most of the rest of the negative reports posted on the internet are pure (and largely false) anecdote - we have traced several of them and they run along the lines of:  "My wife's cousin's friend said he heard from his neighbor that the vaccine was involved in xyz"  - when you locate this person, and try to trace the rumor back to the source, everything dries up or it turns out the afflicted animal was never even vaccinated with the rattlesnake vaccine. Snake bitten horses die at a much higher frequency than dogs.  Literature reports indicate the mortality rate is over 25 percent.  Bites to the legs can result in swelling with loss of circulation to the foot, and consequent sloughing of the hoof.  Bites to the face can result in nasal swelling which occludes the airway, resulting in suffocation. The only specific treatment for snakebite is antivenin (anti-venom antibodies).  Vaccination creates anti-venom antibodies in the vaccinated horse which are circulating at the time of envenomation.  By being already on-board, these antibodies begin neutralizing the venom immediately, and significantly decrease the rate and extent of swelling; vaccinated horses have a much better chance of withstanding envenomation than non-vaccinates.

The buzz of a rattlesnake is all too familiar to many in the western United States. Though the warning is still as ominous as ever, it may not have the same dire consequences for horses that it once had.

The rattlesnake is the most common venomous snake in the western U.S.  The potency of a bite depends on the amount of venom injected by the snake, the type of toxins in the venom, as well as the size of the animal bitten.  Cattle and dogs may recover without treatment, even when bitten on the nose or face, which is often where animals are bitten, because they are curious and approach the snake.  Swelling around the nose can shut off the airways, but cattle and dogs are able to breathe through their mouths, unlike a horse.  Horses are more likely to suffocate. The other common location for snakebite is the leg, which may be less serious, but still should be treated as an emergency, and may result in lameness. A vaccine may make all the difference.

“When an unvaccinated animal gets bitten, the swelling comes on quickly and continues for several days before it starts to go down,” says Dr. Jennifer Taylor, senior research scientist at Hygieia Biologics. “What I’ve heard from veterinarians about animals that were vaccinated and then bitten, is that the swelling is almost gone within 12 to 24 hours.  There is much less swelling and pain, and less tissue damage at the site of the bite and less necrosis.”  There is also less risk for systemic infection (septicemia from infection that often follows snakebite, because it’s a dirty wound with lots of tissue damage which makes ideal environment for bacteria) and potential injury.

“We recommend that horse owners still treat snakebite as an emergency, even if their horses were vaccinated,” Taylor says. “You never know how much venom they received from the bite, and it could have been enough to overwhelm the immunity obtained from vaccine.  They should request that their veterinarian still treat it as a snakebite.  We know that a portion of the venom was neutralized, however, so the severity of the reaction should be much less.”  It may make the difference between survival or not.

Taylor worked on getting the horse vaccine licensed for Hygieia Biologics.  “We already had the canine vaccine on the market–licensed now for 16 years,” she says.  This vaccine is marketed by Redrock Biologics.

After the canine vaccine became available, people asked about a vaccine for horses. The composition of the equine vaccine is slightly different from the canine vaccine. “The first thing we tried was to test the dog vaccine on horses but we had to tweak the horse vaccine a little; the concentrations of the components differ slightly,” Taylor says.

Taylor worked on efficacy studies, vaccinating horses in different timelines, with different doses and tracked the immune responses. “We followed the animals long-term to see how long they were protected.  We collaborated with Dr. Lyndi Gilliam at Oklahoma State University on a couple of studies and published a paper about the results,” says Taylor.

Gilliam participated in several studies testing the equine vaccine, including a study in mice. “My work was mainly looking at titers (antibody levels) in horses that were vaccinated, to see if the horses actually developed antibody titers,” Gilliam said. “I compared those titers with those of naturally bitten horses.”

Horses that have been previously bitten have natural immunity for a variable length of time.  “From my research we know that horses do develop antibody titers against rattlesnake venom after being bitten, but we don’t know how long these antibodies last or if they are protective if the horse is bitten again,” Gilliam says. People bitten by venomous snakes multiple times usually have weaker adverse reactions to subsequent bites, a phenomenon reflected in Gilliam’s observations of horses and dogs, though she says she doesn’t have the data to prove it.

Before releasing the vaccine, the group did extensive studies on horses in all phases of life, including pregnant mares and suckling foals.

“Prior to putting the vaccine on the market we vaccinated 600 horses for the safety study—which was in addition to the 200 horses we’d vaccinated experimentally over the previous 4 years,” Taylor says.

The safety study utilized horses in several different regions of the country.  “We had participants from southern California, northern California, Texas, Oklahoma and several other states. Throughout that entire study we only had had a few minor local reactions—small, temporary lumps—out of 600 horses,” says Taylor.  In that study, each animal was observed daily for several days after being vaccinated.  They also looked at immune response, longevity of immunity and protection in older horses.

For the highest efficiency, Taylor recommends giving three doses a few weeks apart the first year, then giving a booster every year, about a month before snakes get active, in northern areas, where snakes are dormant for part of every year. That would give horses a chance to build up immunity before they come into contact with rattlesnakes. In Southern areas, like Texas and California, where rattlesnakes are out year-round, she recommends giving a booster every six months.

The equine vaccine is specifically against venom from the western diamondback rattlesnake but it also covers all of the western species (prairie rattler, northern and southern pacific rattler, Great Basin, etc.).  “We even see some protection against copperheads as well—which we didn’t expect.  Antibodies from vaccinated animals bind to the venom.  I’ve had reports from veterinarians who said their clients’ dogs were bitten by copperheads and did very well.  We were pleased to know that the vaccine worked even better than we expected, and gave that much coverage,” Taylor says.


HORSE OWNER’S COMMENTS – Jan Elliot, a horse owner (Fritch, Texas) who works at Lee Veterinary Hospital has been vaccinating her horses ever since the equine vaccine came out in 2010.  “The vaccine works well and is very safe.  I’ve only had one incident where there was a little localized swelling from the injection, when I gave it in the chest muscle, but the swelling didn’t last long.  I will continue to give it to my horses twice a year because our area has a lot of rattlesnakes almost year round.  Most years they come out early; we’ve seen them in February on days it gets warm.  I vaccinate my horses in the spring and in the fall, just as a precautionary measure,” she says.

“I have worked for Dr. Lee for 29 years.  We often use the canine vaccine here at Dr. Lee’s clinic and have done that for at least 15 years.  I was eager to know when the equine vaccine would be available, and have been using it ever since it came out.  I am an advocate; I tell everyone who owns horses that they need to vaccinate,” says Elliot.


Published in 2016 Horse Roundup by Tri-State Livestock News

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