Ophidiomycosis Prevalence and Disease Ecology in a Natrix tessellata (Laurenti, 1768) Population From Northern Italy

Fungal pathogens pose a growing threat to vertebrate biodiversity. In snakes, Ophidiomyces ophidiicola (Oo) has garnered particular concern, although its impact in Europe remains poorly understood. We conducted a season-long, standardized survey of dice snakes (Natrix tessellata) along the northern shore of Lake Como (Italy) to quantify Oo and ophidiomycosis prevalence, identify the circulating strain, and explore the association with environmental, morphological and behavioral traits. Between March and October 2024, we collected 96 N. tessellata samples (23 sheds and swabs from 73 live individuals; scale clips were also collected from 60 out of the 73 live individuals). These samples were analyzed through qPCR, histopathology, and direct field observations. After excluding four recaptures, the dataset comprised 92 N. tessellata samples (23 sheds and swabs from 69 individuals), of which 49 tested positive for Oo (53.3%). Among live individuals, 26 tested positive (37.7%). Of these, 21 showed clinical signs (i.e., skin lesions; 80.8%), and histology confirmed ophidiomycosis in 10 of 20 tested Oo-positive samples (47.6%). Among the five Oo-positive snakes without skin lesions, only one showed histological evidence of ophidiomycosis. This resulted in "at least apparent" ophidiomycosis (i.e., pooling the case-classification categories "Apparent ophidiomycosis", "Ophidiomycosis" and "Ophidiomycosis and Oo shedder") being confirmed in 22 out of 69 live snakes (31.9%), corresponding to an overall disease prevalence of 23.9% (22 out of 92) across the full sample set. All sequenced samples belonged to clade II. Bayesian models revealed that skin lesions predicted both Oo detection and ophidiomycosis, while snout-vent length was inversely related to both pathogen presence and disease, suggesting age-linked susceptibility. Both Oo-positive and diseased snakes had lower body temperatures but showed no clear preference for warmer substrates, suggesting limited or absent behavioral fever. Body-condition index (BCI) did not differ between Oo/disease-positive and Oo/disease-negative snakes, suggesting possible host tolerance. An assessment of antipredator behavior revealed a marked reduction in musking among Oo-positive snakes, potentially compromising antipredator defenses. Our findings identify N. tessellata as a possible model for European ophidiomycosis research and highlight the need for multi-season capture-recapture studies.