Biologists and physicians are trying to determine why rattlesnake bites in the West have seemed more dangerously toxic in recent years
IN SUMMER 2008, a 29-year-old man was clearing brush in the mountains near Fallbrook, California, when he was bitten by a southern Pacific rattlesnake. He tried to grab the snake and was bitten again. By the time he made it to the Palomar Medical Center, says Dr. Roy Johnson, a private physician who has treated more than 700 rattlesnake bites, “The patient was delirious, having trouble breathing and was in a state of total body contractions with every muscle twitching. Without treatment, he was on a fast track to dying.”
Johnson and his emergency team put the victim on a mechanical ventilator and used medication to paralyze all his muscles. The young man also received 36 vials of antivenom (10 to 12 is normal). He was released after two days.
According to Johnson, severe snakebites of this sort have become increasingly familiar to his practice. University of California-San Diego Medical Center toxicologists reported a rash of unusually powerful snakebites and extreme patient reactions in 2008.
Dr. Steven Curry, director of medical toxicology at the Banner Poison Control Center in Phoenix, Arizona, reports that prior to 2002 his facility saw patients with neurological symptoms like those of the Fallbrook man once every two or three years; “Now we see several of these patients every year,” he says. Dr. Richard Dart, director of the Rocky Mountain Poison and Drug Center, says, “Over the last two or three years, we are having a sustained peak of neurotoxic effects.”
The Centers for Disease Control and Prevention tally about 7,000 venomous snakebites annually in the United States. Venomous copperheads, cottonmouths and even relatively small coral snakes are responsible for a portion of these, but most snakebite emergencies involve rattlesnakes. Only about 15 fatalities result each year—about 0.2 percent of all bites—but snakebites can cause lasting harm without killing. “Fully 25 percent of all snakebite victims incur some permanent damage from a rattlesnake bite,” says Jude McNally, director of the Arizona Poison and Drug Information Center.
According to Dr. Richard Clark, director of medical toxicology at the University of California–San Diego School of Medicine, rattlesnake venom, a complex brew of toxins, “possesses three main components: cytotoxic components that kill cells, hemotoxic components that affect blood and neurotoxic components that attack nerves.” Neurotoxic effects, like those experienced by the Fallbrook victim, are the quickest acting and more life threatening.
Until recently, biologists believed that Mojave rattlesnakes were the only rattlesnakes to produce neurotoxic venom, but lately neurotoxic effects have been showing up in snakebites from eastern diamondbacks, southern Pacific rattlers and timber rattlesnakes as well. Dr. Clark thinks that all snakes produce neurotoxins; “Mojaves may just have more,” he says.
The use of venom is a venomous snake’s primary means for capturing prey. Snake venom immobilizes prey, gives it a distinctive odor so the snake can track it down and helps pre-digest the snake’s meal. That digestive function kills tissue, creating the most long-term problems for people who survive snakebites.
As a defense strategy, biting is not all that useful to a rattlesnake. It is more advantageous for the snake to scare off a potentially dangerous creature, such as a human, than to strike—hence the renowned rattle. According to McNally, if given a chance to escape without biting, a rattlesnake usually will take it. “Snake venom is the snake’s money. It needs that to get its next meal. If it bites us, it’s not going to gain anything.”
Some scientists speculate that the increasing potency of some rattlesnakes may be the product of an arms race between predator and prey. Texas A&M University researcher John C. Perez studied 40 mammal species that are natural prey of rattlesnakes and found 16 had chemicals in their blood that have evolved over time to block the venom effects of western diamondback rattlers. Researchers at the University of California-Davis found substances in the California ground squirrel that did the same for the venom of northern Pacific rattlesnakes. King snakes, which prey on rattlesnakes, have developed immunity to rattlesnake poison that works so well, says Dr. Sean Bush, professor of Emergency Medicine at the Loma Linda University Medical Center, that “rattlesnakes don’t bite or coil when they see a king snake.” They just try to get away.
With animals capable of evolving immunity to rattlesnake venom, is it possible that the snakes have had to adjust their venoms upward to avoid going hungry? Bush admits he sees more neurotoxic envenomations, but he thinks the increase in serious bites has more to do with greater numbers of people pushing into snake territory than with the snakes getting more toxic. He does not rule out the possibility that snake venom could evolve greater toxicity in response to prey resistance, but, he says, “this is something that has happened over the millennia, not in the last few years.”
Dart, however, thinks venom toxicity could evolve more quickly. “If a snake has to switch from rabbits to squirrels, a chemical messenger originating in the cells may turn on a gene that makes an enzyme in the venom that helps the snake digest squirrels,” he says. “The squirrel, on the other hand, defends itself by turning on a gene, which creates antibodies to the snake’s venom.” Squirrels lucky enough to survive a snakebite are the most likely to turn on the gene. According to Dart, long-term evolution may have created the gene, but a changing environment may activate its function.
Johnson thinks that increasingly severe rattlesnake bites in California may be occurring simply because there are more southern Pacific rattlesnakes now than there were in the recent past. The species’ competitors are the red rattlesnake and the speckled rattlesnake, which rattle when people approach; as a result, “a lot of people will go kill them,” he says. The southern Pacific rattles a lot less and is generally left alone. “The reason rattlesnake venom appears to be getting more toxic is that more people are getting bit by southern Pacific Rattlesnakes, and they are one of the most toxic rattlesnakes in the country. The Southern Pacific rattlesnake may be like the coyote. It’s learned to live with us.”
Now Californians may have to learn to live with it.
Michael Tennesen is a frequent contributor to National Wildlife.
A Painful Bite
Rattlesnake venom, injected through hollow fangs, attacks blood cells and also pre-digests tissue around the bite. Even with antivenin treatment, a bite from a rattlesnake is painful, giving a victim the feeling that the wound is being pounded for hours with a red-hot sledgehammer.