Comparative host-pathogen associations of Snake Fungal Disease in sympatric species of water snakes (Nerodia)

Abstract

The ascomycete fungus Ophidiomyces ophiodiicola (Oo) is the causative agent of ophidiomycosis (Snake Fungal Disease), which has been detected globally. However, surveillance efforts in the central U.S., specifically Texas, have been minimal. The threatened and rare Brazos water snake (Nerodia harteri harteri) is one of the most range restricted snakes in the U.S. and is sympatric with two wide-ranging congeners, Nerodia erythrogaster transversa and Nerodia rhombifer, in north central Texas; thus, providing an opportunity to test comparative host-pathogen associations in this system. To accomplish this, we surveyed a portion of the Brazos river drainage (~ 400 river km) over 29 months and tested 150 Nerodia individuals for the presence of Oo via quantitative PCR and recorded any potential signs of Oo infection. We found Oo was distributed across the entire range of N. h. harteri, Oo prevalence was 46% overall, and there was a significant association between Oo occurrence and signs of infection in our sample. Models indicated adults had a higher probability of Oo infection than juveniles and subadults, and adult N. h. harteri had a higher probability of infection than adult N. rhombifer but not higher than adult N. e. transversa. High Oo prevalence estimates (94.4%) in adult N. h. harteri has implications for their conservation and management owing to their patchy distribution, comparatively low genetic diversity, and threats from anthropogenic habitat modification.

Figure 1

Varying signs of Ophidiomyces ophiodiicola infections. Lesions in Nerodia harteri harteri (panels a and b) and N. rhombifer (panels c, d, and e) were observed during a 29-month surveillance effort in the upper Brazos River basin (Texas, USA).

Figure 2

Number of wild-caught adults and total sample size (all) for three Nerodia species tested for the presence of Ophidiomyces ophiodiicola via qPCR indicated by gray bars. Closed circles represent estimates of prevalence for each species and a subset of adults, with black lines showing the 95% confidence intervals.

Figure 3

Histogram of the total number of snakes captured per month, the number of snakes that tested positive for Ophidiomyces ophiodiicola via qPCR, and the number of snakes showing signs of infection. Surveillance efforts were performed from 16 April 2018 through 6 October 2020.

Figure 4

Map of sampling sites demarcated with open circles. Cross-hatching indicates the historical range of Nerodia harteri harteri at the county level within the Brazos River drainage designated by the shaded area. Each pie chart represents the relative proportion of Nerodia that tested positive or negative for Ophidiomyces ophiodiicola via qPCR. The size of the pie chart represents the relative number of snakes collected per site. QGIS version 3.16.3 (https://www.qgis.org/en/site/) was used to generate the map.