Conditions or ailments that are the cause of a problem that you see - your observation.

Your vet may diagnose

Poisonous Snake Bite, Rattlesnake or Pit Viper

Synonyms: Pit Viper Envenomation


Rattlesnakes are the most widespread pit vipers in the United States. Copperheads and Cottonmouths are other pit vipers found in North America. There are many species of pit viper elsewhere- found almost worldwide, mostly in tropical, subtropical arid and warmer semi-arid areas. They are absent from the poles.

In our local region, we have the Prairie Rattler (Crotalus viridus) and the Western Diamondback Rattler (Crotalus atrox). Most of the bites I have seen have been by the Prairie Rattler, which tend to be more common in our region. How likely a given snake is to bite a horse depends on many factors. Different snake species are known for different temperaments. How aggressive a given snake is depends on species, size, temperature, time of year, and that snake's particular personality. Pit vipers are usually only aggressive when cornered, but there is great variation in temperament among species and among individuals. The larger pit viper species (mostly tropical) tend to deliver large quantities of venom, and the bites are more likely to be fatal.

Most bites I have seen have been on the horse's face, although I have seen some on the legs and body (in horses that were lying down when bitten).

Snake venom is destructive to local tissues. it is loaded with a variety of toxic proteins that destroy tissue and cells. The poison from a bite is also taken up into the blood stream, where it can have a variety of toxic effects.

In most cases, a rattlesnake bite does not kill a horse. Usually, massive swelling at the bite site results. In some cases, the facial swelling is severe enough that it interferes with breathing through the nose. Even horses that experience difficulty breathing can usually breathe well enough to survive, unless they become excited or are exercised. The swelling peaks in a day or two and then begins to gradually decrease over time.

DIAGNOSIS is usually pretty straightforward. Sometimes the bite is witnessed. More commonly, a caretaker notices a severe swelling somewhere on the horse, often accompanied by two tiny blood spots. a few cm apart.

TREATMENT almost always involves anti-inflammatories (usually NSAIDS) to reduce swelling and pain, and antibiotics, to control bacterial infection of the bite site.

In the past, the use of specific antivenin (anti-serum) was impractical in horses because of cost (a great volume was needed and it was not commercially available). Now, however, there is at least one commercial, FDA approved product that is reasonably priced and may be helpful in some cases.

my vet's role



Other conditions or ailments that might also need to be ruled out by a vet.

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The prognosis for most pit viper bites is usually fair to good. Factors that affect prognosis include the particular species of pit viper, the amount of venom injected, the size of the horse, the general health of the horse, and the promptness of treatment. The appropriate use of antivenin, when available, may improve course of treatment and help prognosis.

The larger pit viper species (like the Diamondbacks and some of the South American species) are more likely to inject a fatal amount of venom.

Prairie Rattlesnakes and some other species of Pit Vipers have elements in their venom that damage heart muscle cells, potentially causing long-term heart damage and heart failure.

Horses bitten in the lower limbs can suffer from severe lameness, particularly if the bite enters a joint or tendon sheath.

my role


I might observe

You might make these observations when a horse has this condition.

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Questions To Ask Your Vet:
  • What follow-up do you recommend to ensure that the horse does not have any chronic problems from the bite?
  • Do you have access to and recommend the use of the FDA-approved antivenin?
  • Is the rattlesnake vaccine effective?

Remove venomous snake species from your barn and turnout areas. In areas where there are large populations of poisonous snakes, stabling reduces the likelihood of snake bite.

A commercial rattlesnake vaccine may reduce the severity of signs in the case of bite by the Western Diamondback. I do not know if it is helpful in cases of envenomation by other species.

further reading & resources

Related References:

Smith, BP ed. Large Animal Internal Medicine 4th ed. St. Louis: Mosby Elsevier, 2009.

Author: Doug Thal DVM Dipl. ABVP