Snakes are threatened with extinction in many places around the globe. Disease, often caused by parasites or fungi, is thought to be one of the key factors alongside habitat destruction. Prominent among fungal diseases is ophidiomycosis – also known as snake fungal disease – which is caused by the fungus Ophidiomyces ophidiicola (Oo). In the 20 years since its discovery, it has been found in many snake species.
Now, to achieve a grounded overview of snake health, researchers in the southeastern US surveyed seven pathogens in wild native snakes to compare pathogen prevalence over time, space, and among snake species. The results were published in Frontiers in Veterinary Science.
“The fungus Oo and the parasite Raillietiella orientalis (Ro), appear to be the most important infectious agents in free-ranging snakes in the southeastern US,” said first author Dr Corinna Mishin (formerly Corinna Hazelrig), a researcher at the University of Georgia. “We also show that increased risks of ophidiomycosis was strongly associated with coinfections in free-ranging native snakes.”
No picture of health
For the study, which was funded by the Morris Animal Foundation, the researchers captured over 500 snakes at two wildlife refuges in South Carolina and Florida, representing 29 species. They also sampled opportunistically across the southeastern US. Most snakes were captured alive, swabbed, and had blood samples taken. A few snake carcasses found on roads or in the field underwent full postmortem examination, including tissue sampling.
“For around 10 years, free-ranging snake research almost solely focused on Oo. We wanted to evaluate snake health on a more comprehensive level and really establish a broader view,” Mishin said.
In just under 20% of all snakes, no pathogens were detected. The most commonly detected pathogens were the bacterium Salmonella enterica which was found in 63% of snakes, and Hepatozoon spp., a tick-born parasite, found in 53%. Another bacterium, detected in 18% of snakes, was the antibiotic-resistant Mycoplasma spp., which can cause upper respiratory disease and has never before been reported in wild snakes in the US.
44% of snakes were infected with more than one pathogen. In around 29%, 11%, and 3% of snakes the team respectively found two, three, or four pathogens simultaneously.
“When an animal has become sick from an infection their immune system is compromised which increases the risk of further disease exacerbation from other infectious agents that may have once been subclinical,” Mishin explained.
Vulnerable rattlers
The work also showed that snake species was a predictor of likely pathogens with which they were infected. For example, pygmy rattlesnakes were most likely to be sick with snake fungal disease. 12 out of 34 rattlers tested positive for Oo, with many having concurrent signs of disease, but just one in 55 eastern ribbon snakes and three out of 36 ring-necked snakes did.
Rattlesnakes were also most likely to host Ro, an invasive, obligate crustacean parasite commonly known as snake lungworm. 14 out of 34 rattlesnakes were infected. In other snake species, the same parasite was found less often or not at all, as was the case among Florida green watersnakes.
“We hypothesize that certain species with poorer general population health, specifically rattlesnakes with historic and current increased risks of human persecution, are likely more susceptible to infection with subsequent disease,” said Mishin. “In addition, pygmy rattlesnakes were expected to have an increased prevalence of Ro, as they primarily consume lizards and frogs which are known to transmit the parasite.”
Other factors also influenced snakes’ likelihood of hosting pathogens. For example, snakes sampled in Georgia were much more likely to have Oo, but Ro was found only in Florida snakes. Skin lesions favored Oo infection, too. Snake fungal disease was detected in over 30% of snakes with skin lesions, but just in 2% of snakes without.
Moving pathogens
The results are limited by the fact that sampling was done in just a few counties which could mean the results reflect these habitats specifically. Difficulties in Ro detection, which in live snakes is done by analyzing feces, presents another limitation. As snakes generally have extended periods between meals, consistent fecal availability is not always given. Therefore, Ro rates are likely underestimated.
Nevertheless, the results are important for the conservation of native and control of invasive species, the team said. For example, Burmese pythons and brown anoles – both invasive in the US – are known to be competent hosts of Ro.
“Our data provide important information of which pathogens native snakes may likely have but also which they are likely naïve to. This can inform actions needed to prevent pathogen spillover from captive snakes,” Mishin concluded. “When translocating wildlife, it’s essential to consider which pathogens may be translocated with them – and the potential downstream effects.”
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