Tuesday, December 17, 2013

Zoonotic pathogens in Crotalus viridis, is there potential for spillover?

Can handling rattlesnakes lead to a spillover of a zoonotic
 organisms to humans?

In his 2012 book Spillover, David Quammen examines zoonotic diseases, pathogens that jump from other animals to humans. The list of diseases that impact humans and have reservoirs in other species is lengthy and sobering. HIV, malaria, SARS, hantavirius, influenza, etc. Quammen does an excellent job tracing the origin of HIV in human populations to the early 20th century in West Africa and discussing the cut-hunter hypothesis. This idea suggests HIV moved from non-human primates (probably chimpanzees) to humans when hunters were butchering bushmeat (chimpanzzees). And, Quammen notes the evidence for at least 12 spillover events for HIV, that is humans were infected a minimum of 12 different times by HIV. While most of the zoonotic diseases that have impacted humans come from bats, rodents, or birds- reptiles seem to have been overlooked. But people certainly kill and butcher rattlesnakes in large numbers during rattlesnake round-ups, an excellent opportunity for a pathogen to jump from Crotalus to humans. A new article suggests the opportunity for a spillover event may be present during such events.

Rattlesnakes make ideal subjects for a variety of different scientific disciplines. The prairie rattlesnake (Crotalus viridis) in Colorado was selected for investigation of its relationship to colonies of black-tailed prairie dogs (Cynomys ludovicianus) to study spatial ecology. A total of 31 snakes were anesthetized and had radiotransmitters surgically implanted.

When captured the snakes underwent the following procedures: (1) they had bacterial culture taken from their mouths for potential isolation of pathogenic bacteria; (2) similarly, they had cloacal bacterial cultures taken to assess potentially harmful bacteria passed in the feces; and (3) they had blood samples drawn to investigate the presence of any zoonotic agents in the serum of the snakes.

The results of the bacterial studies and their implications are discussed in a new article by Kevin Fitzgerald and colleagues. A low incidence of bacterial wound infection has been reported following snakebite. Nevertheless, the oral cavity of snakes has long been known to house a diverse bacterial flora.Fitzgerald and colleagues study, 10 different bacterial species from the mouths of the rattlesnakes, six of which are capable of being zoonotic pathogens and inducing human disease. More studies are necessary to see why more rattlesnake bites do not become infected despite the presence of such pathogenic bacteria. The results of fecal bacteria isolated revealed 13 bacterial species, 12 of which can cause disease in humans. Of the snakes whose samples were cultured, 26% were positive for the presence of the pathogen Salmonella arizonae, one of the causative agents of reptile-related salmonellosis in humans.

It has long been reported that captive reptiles have a much higher incidence than wild, free-ranging species. This study shows the incidence of Salmonella in a wild, free-ranging population of rattlesnakes. In addition, Stenotrophomonas maltophilia was isolated. This is a bacterium associated with wound and soft tissue infections that can lead to sepsis, endocarditis, meningitis, and peritonitis. In addition, this bacterium has been increasingly implicated as an opportunistic pathogen to humans during pregnancies, hospitalizations, malignancies and chemotherapy, chronic respiratory diseases, and presurgical endotracheal intubation. Furthermore, S. maltophilia has an intense resistance to broad-spectrum antibiotics, the results of this study showed the bacterium was resistant to multiple antibiotics.

The results of this work suggest anyone working with snake feces, dead skin, or their carcasses must follow reasonable hygiene protocols. Rattlesnakes tested for West Nile antibodies had positive results but these were invalidated owing to possible cross-reactivity with other unknown viruses, interference with snake serum proteins, and the fact that the test was not calibrated for rattlesnake serum. Still, the interesting implication remains, should we be regularly testing these animals as sentinels against potentially zoonotic diseases. The results of this study clearly show the value of veterinarians in a multidisciplinary study of this sort and the particular skill set they can offer. Veterinarians must get involved in conservation studies if the biodiversity of the planet is to be preserved.

Fitzgerald, Kevin T, Shipley, Bryon K., Newquist, Kristin L.,,Vera, Rebecca, Aryn A.2012. Additional Observations and Notes on the Natural History of the Prairie Rattlesnake (Crotalus viridis) in Colorado. Topics in Companion Animal Medicine. 28:167-176. http://dx.doi.org/10.1053/j.tcam.2013.09.008.

Quammen, D. 2012. Spillover, Animal Infections and the Next Human Pandemic. WW Norton & Co,

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