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.