Using occupancy models to understand the distribution of an amphibian pathogen, Batrachochytrium dendrobatidis.

Batrachochytrium dendrobatidis is a fungal pathogen that is receiving attention around the world for its role in amphibian declines. Study of its occurrence patterns is hampered by false negatives: the failure to detect the pathogen when it is present. Occupancy models are a useful but currently underutilized tool for analyzing detection data when the probability of detecting a species is <1. We use occupancy models to evaluate hypotheses concerning the occurrence and prevalence of B. dendrobatidis and discuss how this application differs from a conventional occupancy approach. We found that the probability of detecting the pathogen, conditional on presence of the pathogen in the anuran population, was related to amphibian development stage, day of the year, elevation, and human activities. Batrachochytrium dendrobatidis was found throughout our study area but was only estimated to occur in 53.4% of 78 populations of native amphibians and 66.4% of 40 populations of nonnative Rana catesbeiana tested. We found little evidence to support any spatial hypotheses concerning the probability that the pathogen occurs in a population, but did find evidence of some taxonomic variation. We discuss the interpretation of occupancy model parameters, when, unlike a conventional occupancy application, the number of potential samples or observations is finite.

Reptile infection with Anaplasma phagocytophilum, the causative agent of granulocytic anaplasmosis.

Granulocytic anaplasmosis (GA) is a potentially fatal tick-borne rickettsial disease that occurs sporadically in the far western United States. We evaluated the prevalence of Anaplasma phagocytophilum in multiple species of lizards and snakes from enzootic sites in northern California, described the infestation prevalence of its tick vector Ixodes pacificus on reptiles, and conducted an experimental challenge of western fence lizards (Sceloporus occidentalis) and Pacific gopher snakes (Pituophis catenifer) with A. phagocytophilum delivered via needle inoculation or tick bite. Both serologically and polymerase-chain reaction (PCR)-positive lizards (seroprevalence = 10.8%, PCR prevalence = 10.2%) and snakes (seroprevalence = 5.8%, PCR prevalence = 11.7%) were detected among wild-caught animals. A DNA sequence of the A. phagocytophilum groESL gene from a PCR-positive snake was 100% homologous to that of the human-derived A. phagocytophilum. Experimental attempts to infect naïve animals were unsuccessful for snakes (n = 2), but 1 of 12 lizards became infected for 1 wk only by tick bite. Xenodiagnostic I. pacificus larvae that fed on a PCR-positive lizard did not acquire or transmit rickettsiae. Our findings suggest that lizards and snakes are exposed to A. phagocytophilum by infected ticks, but that they do not serve as primary reservoir hosts of this rickettsia.