Rattlesnake

A Northern Mojave (Mojave Green) rattlesnake. Image courtesy of Rachel Olson.

First, Some Venomous Snake Terminology

Venomous Snakes
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 snakes. There are four types of venomous snakes, but the type responsible for 99% of venomous snake bites in North America is the vipers. Vipers can be "old world vipers" or "pit vipers." In North America, we have pit vipers, chiefly rattlesnakes, cottonmouths (also called water moccasins), 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. Snakes with hinged fangs, such as rattlers, strike, deliver their poison, release the prey, and then look for the dead body. The prey is rapidly killed through an assortment of complicated mechanisms, and the digestion process is actually initiated by the venom.

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.

The only venomous snakes in California are the rattlesnakes, and there are eight species: the Southern Pacific rattlesnake, the Northern Pacific rattlesnake, the Great Basin rattlesnake, the Western diamondback, the red diamondback, the sidewinder, the speckled rattlesnake, and the Mojave green rattlesnake. The Western rattlesnake has eight subspecies, including the Southern Pacific rattlesnake, 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 snake species 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 and suburban areas where there is sufficient natural habitat. In Northern California, snakes hibernate during cold months and are active from 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 the Mojave rattlesnake causes rapid paralysis, including paralysis of the respiratory muscles and suffocation. Because the Mojave rattlesnake's venom is different, treatment may be different, and the snake venom vaccine (see below) is not protective against this venom.

The severity of a snake bite depends mainly on these factors:

• The species of snake
• The size of the dog
• The location of the bite
• The amount of venom 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.

Treatment

The faster the bite is recognized, the more effective the treatment. Do not try to cut the bite wound open or suck out the poison. Pressure bandages have been advocated, but while these will help keep the poison from moving centrally in the circulation, they can also increase the local tissue damage by concentrating the poison near the bite wound. It is best to seek veterinary care immediately for proper treatment and not try anything on your own.

It is not unusual for large areas of skin or muscle to die off, and open wound management, possibly followed by surgery, to become necessary. Further, the blood clotting disruption, as well as damage to the kidneys from the toxic shock, can take up to three weeks to become fully evident, so expect monitoring tests to be needed.

Antibiotics may be necessary if there is significant dead tissue, and, of course, pain management will be important, considering all the inflammation. Hyperbaric oxygen has been very helpful in treating snakebite damage, but this type of treatment is not readily available. It may be worthwhile to locate the nearest hyperbaric chamber that is available for animal use.

IV Fluids
Since the most common mechanism of death from a 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 IV fluids are given throughout.

Antivenin
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 snakebite venom. Antivenin is a biological product consisting of antibodies made by horses in response to exposure to four common Crotaline venoms. The antibody serum is reconstituted into an intravenous drip run into the patient for at least 30 minutes.

Antivenin is expensive (at least $400 per vial) and a large dog with a severe bite is likely to require several vials. Because the product is of horse origin, often a scratch test to the ear flap is used to test for immunological sensitivity (i.e. to predict whether the patient is likely to have anaphylactic reaction to the antivenin once it is given by IV). The patient will likely always be sensitive to equine products after being given antivenin, which makes future snake bite treatment problematic.

A more purified antivenin of sheep origin has recently been marketed (Cro-Fab antivenin), but it is even more expensive, at approximately $700 per vial. This product is made for human use, but also works in dogs.

Other products include: Rattler Antivenin® (which includes antivenin against the Mojave green rattlesnake), VenomVet®, Antivipmyn Tri® (a Mexican product), and Polivet-ICP® (a Costa Rican product).

Antivenin is very helpful in inactivating snake venom, but there is a narrow window during which it must be used. After about 4 hours post-bite, antivenin is of minimal use.

A separate antivenin is available for coral snake venom, but this is not an issue for California.

Vaccination

Red Rock Biologics has released a vaccination against the venom of the Western Diamondback (Crotalus atrox). This vaccine also has activity against the venom of six out of seven of the other California rattlesnakes (the Mojave Rattlesnake has such significantly different venom that it is not covered), and there is at least partial cross-protection against numerous other venomous snakes native to areas outside California. Hiking dogs and dogs that live in rattlesnake areas are good candidates for this product. That said, there is controversy about how well it works as the manufacturer does not wish to do efficacy testing that would involve a control group of unvaccinated dogs. Furthermore, a history of vaccination does not preclude the need for emergency treatment after a snake bite; the goal of the vaccine is to reduce the effect of the snake venom. Patients still need professional assessment after a snake bite even if vaccinated. Efficacy studies thus far have involved mice so it is not surprising that there is controversy surrounding how well it might work in dogs.

Basics about the Vaccine

• The initial vaccination is given in two doses three to six weeks apart. Dogs over 100 pounds and 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 that 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 restarted.
  
  
• 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.
  
  
• According to the manufacturer, vaccinated dogs typically develop protection comparable to two to three vials of antivenin.

Snake Avoidance Training

Dogs can be trained to avoid rattlesnakes by scent, sound, and visual appearance. Training classes exist just for this purpose. Consider looking into one of these classes if your dog hikes in snake territory. Call training schools or ask your veterinarian about these specialized classes.