Dr. Paul Auerbach is the world's leading outdoor health expert. His blog offers tips on outdoor safety and advice on how to handle wilderness emergencies.See all posts »
A New Method for Recognizing Dangerous Snakes in the United States and Canada
In addition to recognition of definitive clinical signs and symptoms of venomous snakebite, the way to determine whether or not a biting snake is venomous is to identify it properly. In the United States and Canada, that is usually determined for pit vipers by identifying the typical triangular head, elliptical pupils, and undivided (e.g., single row) subcaudal (under the tail) scales. However, these three features are subjective, shared with some harmless species, or both. The most definitive characteristic of a pit viper is the heat-sensitive facial pit, but this must be observed with close examination of the snake face, which is potentially dangerous from a bite perspective. For coral snakes, the order of colored rings is suggested to determine the venomous species. Recall, “Red on yellow, kill a fellow. Red on black, venom lack.”
In a wonderfully unique article (22, 304-308, 2011) in a recent issue of Wilderness & Environmental Medicine, entitled “Recognizing Dangerous Snakes in the United States and Canada: A Novel 3-Step Identification Method,” Michael Cardwell proposes a new way to make these identifications. Cardwell is a snake expert of worldwide notoriety from the Department of Biological Sciences at California State University in Sacramento, California. Cardwell notes that the mid-dorsal scales of native pit vipers (viperids) are invariably with a longitudinal ridge akin to a keel, and that these ridged scales can be easily distinguished from smooth scales without ridges. Furthermore, he notes that all native viperids have a single row of subcaudal scales, while harmless native snakes have a double row of subcaudal scales. The three snakes that are exceptions to this rule, in that they are harmless but have a single row of subcaudal scales, have smooth dorsal scales (indicating that they are nonvenomous). So, to determine a venomous species, one first asks, “Are scales near the dorsal (top surface) midline keeled?” If the answer is no, then the snake is not dangerous. If the answer is “yes,” then the question is asked, “Are most of the subcaudal scales in a single row?” If the answer is no, then the snake is not dangerous. If the answer is yes, then the snake is dangerous.
Coral snakes in the United States are ringed in red, black, and yellow (or white). With the venomous species, the rings completely encircle the body, and every other mid-body ring is yellow (or white). So, to determine a venomous species, one first asks, “Is every other mid-body ring yellow or white?” If the answer is no, then the snake is not dangerous. If the answer is “yes,” then the question is asked, “Do all three colors completely encircle the body, including across the belly?” If the answer is no, then the snake is not dangerous. If the answer is yes, then the snake is dangerous.
Rattlesnakes have both keeled (ridged) dorsal scales and a single row of subcaudal scales, but it is entirely reasonable to forego close inspection when a rattle is obviously present.
The article concludes with important safety warnings and denotes the geographic limitation of this new identification technique. I highly recommend reading this article.
Recent Blog Posts
Feb 11, 2013
Topical Ivermectin Lotion for Treating Head Lice
Feb 04, 2013
Public Health Interventions and Snowmobile Fatality Rates
Jan 28, 2013
Physician Warnings for Persons with Impairments