Mohave Rattle Snake Envenomation 

Envenomation by some rattlesnakes, such as the Mohave rattlesnake (formerly Mojave rattlesnake) (Crotalus scutulatus), may cause a different clinical presentation than that generally encountered after most rattlesnake bites. In addition, other species, such as the Southern Pacific rattlesnake Crotalus oreganus helleri, (formerly Crotalus viridis helleri), may cause signs and symptoms consistent with typical rattlesnake envenomation combined with signs and symptoms similar to Mohave rattlesnake envenomation.^[1] (See Snake Envenomations, Rattle for a more complete discussion of typical rattlesnake envenomation.)

Mohave rattlesnakes inhabit desert areas of the southwestern United States and central Mexico. Specimens with type A venom, which cause a different pattern of injury than other rattlesnakes, have been reported in southern California, Nevada, Utah, Arizona, Texas, and New Mexico.^{[2, 3, 4, 5]} Populations with venom B and intergrades of types A and B venom have been found in south-central Arizona, around Phoenix and Tucson.^[6]

The Mohave rattlesnake may be difficult to distinguish from the western diamondback rattlesnake (Crotalus atrox), which inhabits an overlapping geographical range. Some Mohave rattlesnakes are greenish, but they may have a similar color as western diamondbacks. In the Mohave rattlesnake, the diamond pattern fades into bands along the caudal third of the back, whereas the diamonds continue to the tail in the western diamondback.

The Mohave rattlesnake's white rings encircling the tail are much wider than the narrow black rings, whereas western diamondbacks have much more predominant black rings. The postocular stripe extends posteriorly above the mouth in the Mohave but intersects the corner of the mouth in the western diamondback. In Mohave rattlesnakes, supraocular scales are separated by fewer than 4 scales at their closest point. In western diamondbacks, at least 4 scales (usually >4) separate the supraocular scales.

See the image below.

Other rattlesnakes in the Mohave rattlesnake's range and niche are distinguishable by the absence of a dorsal diamond pattern with light margins, black and white tail rings, facial stripes, or by the same criteria used to distinguish Mohave rattlesnakes from western diamondbacks.

Venom A populations of Mohave rattlesnakes possess Mohave toxin, which has been experimentally shown to induce neurotoxic effects. Mohave toxin or a similar toxin has been detected in the venom of other rattlesnake species. This toxin impairs presynaptic acetylcholine release. Mohave toxin may cause severe neurologic effects clinically, although this presentation has been reported only a few times in the literature. Envenomation by several other species of rattlesnakes has been reported to cause serious neurologic signs and symptoms (eg, severe motor weakness, respiratory difficulty).

Venom A Mohave rattlesnakes cause less local injury and less hemorrhagic/proteolytic effects than other rattlesnakes. In contrast, venom B specimens cause local, proteolytic, and hemorrhagic effects typical of other rattlesnakes. Severe rhabdomyolysis with myoglobinuric renal failure has been reported with Mohave rattlesnake envenomation.^{[7, 8, 9]} This article focuses mainly on envenomation by venom A populations of Mohave rattlesnakes.

Mortality/Morbidity

Mohave toxin is one of the most lethal venom components found in US snakes.

• Venom B populations are less lethal than venom A populations.
• At least one death has been attributed to a Mohave rattlesnake in the Annual Report of the American Association of Poison Control Centers, although a number of deaths have been documented.^[10]
• Most documented deaths are associated with bites in which the bitten individual was intentionally interacting with the snake and when a delay occurred in seeking medical care.

Sex

Males are bitten more commonly than females.

Age

Young adults are most commonly bitten.