What is the relationship between reptiles and infectious disease?
Reptiles, including snakes, lizards, crocodilians, turtles, and tortoises, can act as hosts and reservoirs for many infectious disease agents. Some of these agents, particularly bacteria, can be transmitted to humans through direct contact with reptiles or their environments.
Many common bacterial species normally occur in reptiles and generally cause few problems for their hosts. When these bacteria are transmitted to humans, however, through contact with reptiles and their environments, serious illness can result.
Campylobacter jejuni and C. coli are gram-negative, spiral, microaerophilic bacteria that may be present in the feces and contaminated water of pet reptiles. A Campylobacter infection in humans causes vomiting, diarrhea, and other gastroenteritis symptoms. Most cases involving Campylobacter resolve within a week with either no treatment or with a course of antibiotics. A rare but serious complication of this type of bacterial infection is Guillain-Barré syndrome, a neurological disorder in which the body’s immune system attacks the peripheral nervous system.
Edwardsiella tarda, a gram-negative enterobacteria residing in some reptilian species, has occasionally caused gastroenteritis and wound infections in humans who either handled infected reptiles or received bites from pets such as iguanas. Enterobacter spp., frequently part of the normal bacterial flora of reptiles, can cause human genitourinary infections and primary bloodstream infections.
Proteus spp., Staphylococcus spp., Acinetobacter spp., and Shigella spp. are all common bacteria of the oropharyngeal cavities of snakes and can cause multiple health problems for owners of pet snakes or for those working with snakes in laboratory settings.
A number of species of the bacterial genus Pseudomonas are fairly common in the oral cavities of reptiles. These bacteria, perhaps best known for causing what is commonly called hot tub rash and swimmer’s ear, grow well in poorly disinfected water and can also be transmitted to humans through wound contamination from bites and scratches.
Species of the genus Mycobacterium cause a number of diseases in humans, most notably tuberculosis and leprosy. M. marinum is found in salt water and fresh water throughout the world. Although generally found in aquarium fish and their tanks, M. marinum has also been isolated from captive lizards, turtles, snakes, and caimans and thus may be a hazard to reptile hobbyists and zoological facility workers.
The rickettsial bacterium Coxiella burnetii, which causes Q fever, is usually carried by cattle, sheep, goats, and other domesticated livestock and pets. Reptiles, however, can occasionally carry ticks infected by Coxiella; this is a possible source of transmission to humans. Reptiles also serve as a reservoir for another rickettsial bacterium, Rickettsia marmionii, which causes Australian spotted fever.
By far the most common bacterial disease transmitted by reptiles is salmonellosis. Salmonella spp. infections afflict approximately 70,000 people in the United States each year. According to the Centers for Disease Control and Prevention (CDC), the number of cases may actually be thirty times that number because many are unreported. The symptoms of a Salmonella infection include the onset of fever, one to three days after initial infection, and vomiting, diarrhea, stomach pain, and abdominal cramps. Most persons with salmonellosis recover completely, but some develop complications, including sepsis and meningitis. At greatest risk are infants and children younger than five years of age, organ transplant recipients, immunocompromised persons (such as those with human immunodeficiency virus infection), and the elderly.
Among the various reptiles kept as pets, the primary Salmonella hosts are turtles, snakes, and lizards. For many years, most reptile-transmitted salmonellosis cases were contracted from newborn and young turtles. Since 1975, it has been illegal in the United States to sell turtles that have shells less than four inches in length, but enforcement of this law is poor and inadequate. Since 2006, there have been three large multistate outbreaks of Salmonella infections because of the selling of young turtles, primarily at flea markets and tourist shops and by street vendors. In 1996, an outbreak of S. enterica at a Komodo dragon exhibit at a zoo in Colorado led to sixty-five confirmed cases of salmonellosis.
Although rarely documented, fungal zoonotic transmission from reptiles can occur through the inhalation of spores, ingestion of fungal material, or contamination of wounds. The fungi genera most likely to be transferred from reptiles to humans include Mucor, Rhizopus, Candida, Trichosporon, Trichophyton, Aspergillus, Basidoholus, and Geotrichum.
The pentastomids, or tongue-worms, are parasites of reptilian respiratory systems. As adults, most pentastomids, of which there are approximately sixty species, live in the lungs of snakes, lizards, and crocodilians. Visceral pentastomiasis occurs in humans when pentastomid eggs are consumed with the meat of reptiles or by accidental ingestion of feces or body secretions. Nymphs develop in various internal organs, causing damage to the spleen, liver, lungs, eyes, and mesentery. Pentastomiasis cases have been reported in many parts of the world, including Africa, Malaysia, the Philippines, Java, and China.
The most straightforward way to avoid bacterial and fungal infections from reptiles is good handwashing technique. Children and adults should wash hands and any other body parts exposed to reptiles immediately after any contact. Sensible precautions for those who keep reptiles include not allowing the animals to have free range of living quarters, keeping water dishes and aquariums clean and disinfected, and not washing animals and their artificial habitats in the kitchen sink, bathtubs, or showers, unless these areas are completely disinfected after use.
In countries where pentastomids are a problem, good hygiene is again the primary preventive measure, although this may be difficult in areas in which people do not have access to soap, disinfectants, and clean water. In those cultures in which the eating of reptiles is common, one should thoroughly cook the meat at a temperature high enough to kill parasitic organisms.
Austin, C. C., and M. J. Wilkins. “Reptile-Associated Salmonellosis.” Journal of the American Veterinary Medical Association 212, no. 6 (1998): 866-867. Discusses the transmission of Salmonella by various reptile species.
Centers for Disease Control and Prevention. “Turtle-Associated Salmonellosis in Humans: United States, 2006-2007.” Morbidity and Mortality Weekly Report 56, no. 26 (July 6, 2007): 649-652. Provides a summary of Salmonella cases transmitted by turtles in several state outbreaks.
Jacobson, Eliot R. Infectious Diseases and Pathology of Reptiles: Color Atlas and Text. Boca Raton, Fla.: CRC Press, 2009. A complete treatise on infectious diseases that affect reptiles. Includes more than one thousand color photographs of reptile species with an emphasis on anatomy and histology.
Roberts, Larry S., and John Janovy, Jr. Gerald D. Schmidt and Larry S. Roberts’ Foundations of Parasitology. 8thed. Boston: McGraw-Hill, 2009. A classic work focusing on the parasites of humans and domestic animals.
Romich, Janet A. Understanding Zoonotic Diseases. Clifton Park, N.Y.: Thomson Delmar, 2008. A good introduction to zoonotic diseases, including those caused by reptiles, in humans.