By Wendy C. Brooks, DVM, DipABVP
Educational Director, VeterinaryPartner.com
A Northern Mojave (Mojave Green) rattlesnake. Image courtesy of Rachel Olson.
First Some Venomous Snake Terminology
Venomous animals secrete toxic substances that they use to immobilize and potentially kill their prey. There are many venomous animals including spiders, stinging insects, lizards, and, of course, snakes. There are four types of venomous snakes but the type that is responsible for 99% of venomous snake bites in North America are vipers. Vipers can be "old world vipers" or "pit vipers." In North America, we have pit vipers, chiefly rattlesnakes, cottonmouths (also called water moccassins) and copperheads. Rattlesnakes are the most poisonous.
Pit Vipers (Also Known As Crotaline Or Crotalid Vipers)
Pit vipers are named for their facial heat sensing organ, which is called a loreal pit. They have hinged fangs that not only hold their prey but also serve as injection needles for their venom. The prey is rapidly killed through an assortment of complicated mechanisms and the digestion process is actually initiated by the venom. The snake then looks for the dead body of its prey and consumes it.
Snake venoms are highly complicated. At least 26 separate enzymes have been identified with 10 of these enzymes common to all snake venoms (though in different concentrations). All snake bites are not equal. The quality of venom depends not only on the type of snake but on the season, the geographical region, the age of the snake, and how recently it has released venom previously. Treatment depends on the type of venom but more on this later.
Our discussion concerns rattlesnakes native to California. Rattlesnakes are the only venomous snakes native to California and there are six species. The Western rattlesnake has eight subspecies including the Southern Pacific rattlesnake, which is the only one native to the Los Angeles Basin. Ask your veterinarian what venomous species are native to your area.
The physical appearance of each snake species is variable and it is difficult to tell what species of snake you are looking at. Some general principles in distinguishing poisonous snakes are:
- Broad, triangular head with a noticeable neck behind the head.
- Vertical pupils (non-poisonous snakes have round pupils), though hopefully you would not be close enough to evaluate this.
- As mentioned, the Crotalines are also called pit vipers because they have heat-sensing pits on their faces between the eye and nostril as shown above. The pits help them locate prey. If you see a pit, you are looking at a poisonous snake.
Rattlesnakes can be found in rural areas as well as suburban areas where there is sufficient natural habitat. In Northern California snakes will hibernate during cold months and are active March through September. In Southern California they are active all year round.
Dogs Vs. Snakes
Dogs encounter snakes in the snake's natural habitat during play or work. Most bites to dogs occur on the face or extremities. The rattlesnake bite is generally hemotoxic, which means that it exerts its toxin by disrupting the integrity of the blood vessels. The swelling is often dramatic with up to one-third of the total blood circulation being lost into the tissues in a matter of hours. The toxin further disrupts normal blood clotting mechanisms, leading to uncontrolled bleeding. This kind of blood loss induces shock and finally death. Facial bites are often more lethal as the swelling may close the throat or impair ability to breathe.
The bite of this Mojave rattlesnake causes rapid paralysis. This includes paralysis of the respiratory muscles and suffocation. Because the Mojave rattlesnake venom is different, treatment may be different and snake venom vaccine (see below) is not protective against this venom.
The severity of a snake bite depends largely on these factors:
- The species of snake
- The size of the dog
- The location of the bite
- How much venom was involved.
The amount of venom injected (approximately 20-25 percent of bites are dry, meaning no venom has been injected; 30 percent of bites are mild meaning they cause local pain and swelling in the bite area and no systemic symptoms; 40 percent of bites are severe with approximately five percent being fatal.
The faster the bite is recognized, the more effective the treatment is. Do not try to cut the bite wound open or suck out the poison. Seek veterinary care immediately for proper treatment, even if your dog has been vaccinated against snake venom.
Since the most common mechanism of death from rattlesnake bite is circulatory collapse, IV support and monitoring for signs of blood pressure drop are important. Fluids may be started at a relatively slow rate if the patient is stable but if signs of impending trouble occur, circulatory volume replacement is as easy as opening a drip set valve. After a bite, 24 hours of observation is a prudent timeframe while giving IV fluids throughout.
There are numerous misconceptions about antivenin. The first is simply the name of the product. It is not anti-venom. It is not a single injection that provides the antidote to snake bite venom. Antivenin is a biological product consisting of antibodies made by horses or sheep in response to exposure to four common Crotaline venoms. The antibody serum is delivered as an intravenous drip that is run into the patient over at least 30 minutes or so. There are currently four brands: ACP®, Cro-Fab®, VenomVet®, and Rattler Antivenin®. Some involve whole antibodies and some only partial antibody structures. Whole antibody products tend to last longer but the partial structures are able to penetrate deeper into the patient's tissues.
Antivenin is expensive and a large dog with a severe bite is likely to require several vials or IV bags. Because most products are of equine origin, often a scratch test to the ear flap is used to test for immunological sensitivity to predict whether the patient is likely to have an anaphylactic reaction to the antivenin once it is given intravenously. The patient will likely always be sensitive to equine products after being given antivenin, which makes future snake bite treatment problematic.
Antivenin is helpful in the inactivation of snake venom but there is a narrow window during which it must be used. After about four to six hours post-bite, antivenin is usually considered of minimal use. This long-standing idea has been recently challenged and antivenin may have some use beyond this time if it is believed that there is still active venom in the tissues.
- Rattle Antivenin® and Cro-Fab® are helpful against Mojave Green Rattlesnake venom. ACP® and VenomVet® are only helpful against the hemotoxic rattlesnake venoms.
- Mildly affected patients probably do not need antivenin. Scoring systems have been employed to determine if antivenin should be used and if so, how many doses.
Suctioning of the snake bite and using bands or tourniquets to prevent the venom from traveling are urban legends and should not be done. It may be helpful to remove collars or other items that could become constrictive once the swelling starts.
Red Rock Biologics manufactures a vaccine against the venom of the Western Diamondback (Crotalus atrox). This vaccine also protects against the venom of the other California rattlesnakes except the Mojave green rattlesnake, which has a different type of venom. There is reportedly good cross-protection against numerous other venomous snakes native to areas outside California.
Basics About The Vaccine
- Initial vaccination is given in two doses three to six weeks apart. Dogs over 100 pounds and dogs less than 30 pounds in body weight need three doses three to six weeks apart.
- Annual boosters are best given approximately one month before snake season starts in the spring. Dogs who live where snake season is year round or where they hike year round should have boosters every six months. If a vaccine is skipped, the initial vaccination protocol should be re-started.
- Vaccination is safe in pregnancy, lactation, and for puppies four months of age and older.
- Vaccination reactions occur in 0.27 percent of cases (27 per 10,000 doses given) and are largely limited to swelling at the vaccine site seven to 10 days after vaccination. This is particularly true for dogs with a past history of bee stings.
- Vaccinated dogs typically develop protection comparable to two to three vials of antivenin.
A snake bite should always be treated as an emergency even in a vaccinated dog. If your dog is bitten by a rattlesnake, seek veterinary attention immediately.
Snake Avoidance Training
Dogs can be trained to avoid rattlesnakes by scent, sound, and visual appearance. Training classes exist just for this purpose. If your dog hikes in snake territory, consider looking into one of these classes. Call training schools and ask your veterinarian about these specialized classes.