Rapid muscle twitching, as if a person had snakes crawling under the skin, is the telltale sign to Roy Johnson that the Southern Pacific rattlesnake has struck. On occasion, this symptom can progress to difficult breathing, coma and death. This snake's bite is one of the few to induce neurological symptoms, in contrast to most other rattlesnake bites, which initially produce swelling and bruising around the wound, notes Johnson, a physician in Palomar, Calif., who has treated some 700 snakebite cases. Increasingly, the proportion of rattlesnake bites in southern California are skewing to those like the more deadly Southern Pacific species, and scientists are not sure why.

Every year the Centers for Disease Control and Prevention logs 7,000 reports of snakebites in the U.S., which lead to about 15 deaths. Roughly 25 percent of the survivors incur some permanent damage. In southern California, reports from area hospitals and medical centers show a spike in serious bites the facilities say that, where they formerly saw patients with severe neurological symptoms once every two to three years, they now see several of these types of envenomations every year.

Johnson, for one, suspects that humans themselves are to blame for the increase. Most rattlesnakes warn off potential predators by shaking their noisy tails that is what the red rattlesnake and the speckled rattlesnake, southern California's other two dominant coastal species, tend to do. But the noise also makes the reptiles more likely to end up on the killing end of a shovel if the threat is human. In contrast, Southern Pacific rattlesnakes are more apt to lay low or move away than hiss and rattle when confronted, a strategy that may boost their chances of surviving, Johnson says. He speculates that by clubbing its competitors, humans have paved the way for the Southern Pacific to move into new areas. That animal "is adapting to human habitats much like the coyote whether we like it or not," John son remarks.

The rising incidence of supertoxic bite cases could also reflect a change in the species' venom. To predigest their prey, most rattlers produce so-called cytotoxins and hemotoxins, which damage tissue and disrupt blood clotting. But the Southern Pacific also produces a neurotoxin, which is more serious because it quickly affects breathing and muscle control. Anecdotal reports suggest that the snake's venom contains more neurotoxin than it did a few years ago. Richard Dart, director of the Rocky Mountain Poison and Drug Center in Denver, does not rule out that the species could have made its venom more toxic, perhaps by crossbreeding with the more deadly, desert-dwelling Mojave green rattlesnake or by turning on dormant genes developed over time in response to more resistant prey.

The amount of neurotoxin is indeed dramatic compared with the creature's close cousin, the Northern Pacific rattler. This species preys on ground and rock squirrels, which by six weeks of age develop a natural resistance to withstand a full envenomation, suggests research at the University of California, Davis. The Southern variety, however, has enough neurotoxin to overcome any such natural resistance, says biologist William Hayes of Loma Linda University. "Southern Pacifics have no problem getting lots of squirrels."

Still, Hayes does not believe that the rattler's venom has become more toxic. Rather he thinks that people are becoming less tolerant of snake venom, perhaps because of "pollution weakening human lungs and the immune system."

Sean Bush, a treating physician at Loma Linda University Medical Center, says that the Southern Pacific is definitely the "people-biting snake in California" and attributes the increase in incidents to humans encroaching on the animal's coastal and mountain habitat. We are, Bush says, "only now learning how potent and varied rattlesnake venom can be."

Editor's Note: This story was originally published with the title "Snakebit"

   
Already a Digital subscriber? Sign-in Now
If your institution has site license access, enter here.

ABOUT THE AUTHOR(S)
Michael Tennesen is a freelance science writer based near Los Angeles.