Malayan Pit Vipers

The Malayan pit viper is a medium-sized venomous ground snake, Calloselasma rhodostoma, of tropical Southeast Asia. It inhabits forest edges from Vietnam to Myanmar (Burma) and southward through peninsular Malaysia to the islands of Sumatra and Java. Adults are just under 3 feet (1 meter) long.

The body is pinkish gray with a series of large brown triangles, bordered in white, along the sides. A thin black line extends along the back from neck to tail. The head is markedly triangular, with large plates on the crown. The eyes have vertical pupils. A pit or depression between each eye and nostril is a heat sensor that locates prey by their difference from the surrounding temperature.

The snake shelters in leaf piles, thickets, or fallen logs during the day. It becomes active at dusk, prowling for small mammals, birds, and frogs. When disturbed, it coils its body and vibrates its tail, making a rattling sound among the leaves and debris. In defense against predators, it can spread its ribs and flatten its body against the ground. Bites to humans from accidental encounters are frequent and can be very damaging, as the snake has long fangs and powerful venom, but there are relatively few fatalities. The Malayan pit viper is bred as a laboratory animal for the production of antivenin and to study an anticoagulant in its venom that can be useful medicinally.

Malayan Pit Vipers has a reputation for being bad-tempered and quick to strike. In northern Malaysia it is responsible for some 700 incidents of snakebite annually with a mortality rate of about 2 percent. Remarkably sedentary, it has often been found in the same spot several hours after an incident involving humans. Its venom causes severe pain and local swelling and sometimes tissue necrosis, but deaths are not common. Many victims are left with dysfunctional or amputated limbs due to the lack of antivenom and early treatment. In a 2005 study of 225 Malayan pit viper (Calloselasma rhodostoma) bites in Thailand, most victims had mild to moderate symptoms, but 27 of 145 patients (18.6%) developed permanently swollen limbs. There were only two deaths (related to intracerebral hemorrhage) and no amputations. The antivenin manufactured in Thailand seemed effective in reversing the blood clotting caused by the venom. Most patients remained stable and did not require antivenin. The authors suggested that victims not use traditional healers and avoid overuse of tourniquets. In a prospective phase of the study, bites occurred throughout the year but mostly early in the monsoon season (May and June).

The Malayan pit viper’s brooding behavior is unusual for a snake. The female lays one to two dozen eggs in August or September and coils her body around them until they hatch six weeks later. During incubation she defends the eggs against intruders. The hatchlings are 6 inches (15 centimeters) long and are identical to adults in coloration.

The Malayan pit viper resembles the moccasin and copperhead pit vipers of North America, and it was formerly placed with them in the genus Agkistrodon. The Malayan pit viper is sometimes called the Malayan moccasin. Pit vipers are classified in the viper family, Viperidae, subfamily Crotalinae. Some authorities place them in a separate crotalid family, Crotalidae.