- Author: Robert L Norris, MD; Chief Editor: Joe Alcock, MD, MS more...
There are 2 extant species of lizards known to be venomous, the Gila monster (Heloderma suspectum, with 2 subspecies) and the Mexican beaded lizard (Heloderma horridum, with 4 subspecies).
There is ongoing debate regarding possible venom production by some other lizard species such as the Komodo dragon (Varanus komodoensis), but there is no current evidence to support human envenomations by any other lizards, and these will not be further discussed in this article.
The Gila monster is found in the desert regions of Arizona, western New Mexico, southeastern California, the southern tip of Nevada, extreme southwestern Utah, and northwestern Mexico. The beaded lizard is found only in Mexico and Guatemala, south of the range of the Gila monster.
Both lizards have heavy bodies with large heads and powerful jaws. The beaded lizard is larger than the Gila monster, reaching almost a meter in length, whereas the Gila monster's maximal size is approximately 0.5 m.
The venom apparatus is much less sophisticated than that of most venomous snakes. A pair of multilobed labial venom glands (modified submandibular glands) lie in the anterior portion of the lower jaw. Venom is conducted from each lobe through a single duct (H horridum) or a series of ducts (H suspectum) and is deposited into a labial mucosal pocket adjacent to the anterior teeth.
The teeth (approximately 20 per jaw) are grooved and loosely attached to the jaws. Venom is conducted via capillary action along these grooves into the victim's tissues as the lizard bites and chews. The more irritated the lizard is when it bites, the more it salivates and the greater the venom yield. Effective envenomation in humans probably occurs in less than 70% of bites.
The venoms of these 2 lizards are remarkably similar and contain a number of components, including L-amino acid oxidase, hyaluronidase, phospholipase A, serotonin, and highly active kallikreins that release vasoactive kinins. The venom contains no neurotoxins or any enzymes that significantly affect coagulation. In laboratory animals, the venom is as potent as some rattlesnake venoms. Rare hypersensitivity to helodermatid venom has been reported.[4, 5]
Bites are very infrequent and usually involve captive specimens. A significant number of bites probably go unreported because private keepers of these protected lizards may be reluctant to seek medical attention.
No data regarding current incidence of bites by venomous lizards in Mexico are available.
No data are available to support any racial predilection to lizard bites.
Young males are more likely to intentionally interact with venomous wildlife, and are, therefore at greater risk for bites.
While there are no large studies reporting the age distribution of venomous lizard bites, it is likely that those at greatest risk will be children, adolescents, and young adults. Young children may be at increased risk of bites if they come across a wild Heloderma specimen, as they may not comprehend the risk. Adolescents and young adults may be more prone to intentionally interact with or collect such lizards, despite knowing their venomous nature.
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