Why this matters: Diseases of amphibians and reptiles have important implications for trying to preserve healthy populations of these animals in the wild. Many consequential diseases affecting amphibians and reptiles are caused by pathogens that were introduced outside their native ranges – likely through the movement of animals in the pet trade. The fungus Paranannizziopsis is causing skin disease in wild snakes in the U.S. and Canada, and more work is needed to understand the distribution, origins, and impacts of the pathogen.

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Infectious disease is increasingly recognized as a threat to the conservation of wild reptiles, yet disease has been poorly studied in most reptile species in North America. The emergence of snake fungal disease (SFD; ophidiomycosis), caused by the fungus Ophidiomyces ophidiicola, in the United States has drawn attention to the potential impacts of such diseases. Several other fungi are known to cause outbreaks of fatal infections in captive reptiles, but these fungi have rarely been reported from wild reptiles. As part of the National Wildlife Health Center’s (NWHC) investigation into SFD, Lorch et al. (2023) discovered that fungi in the genera Paranannizziopsis (which is closely related to O. ophidiicola) are found on wild snakes in the United States and Canada. Although much rarer than O. ophidiicola, Paranannizziopsis is widespread (detected in Florida, Minnesota, Louisiana, and Washington, USA; and British Columbia, Canada), causes skin infections similar to SFD, and infects a variety of snake species. At least three species of Paranannizziopsis were detected on wild snakes, and these species may vary in the severity of disease they cause.

The origin of Paranannizziopsis in North America is unknown. However, given the frequency with which this fungus occurs in captive reptiles, Paranannizziopsis may represent an introduced pathogen. A cluster of severe cases of Paranannizziopsis infection in gartersnakes (Thamnophis spp.) in British Columbia and Washington is especially concerning and in need of further monitoring and investigation. Detection of Paranannizziopsis is challenging because clinical signs of infection mirror those of SFD and coinfections of Paranannizziopsis and Ophidiomyces can occur. The NWHC has developed a rapid PCR-based detection method for Paranannizziopsis to assist in pathogen screening and accurate diagnosis of the disease.