More hosts, more problems: Factors related to the distribution and abundance of a four-host trematode parasite in the San Francisco Bay Area of California, USA

Background/Question/Methods

A flatworm trematode macroparasite genus, Alaria spp., is a poorly understood parasite found in an aquatic system well suited to study the distribution, effects, and implications of infectious diseases. This parasite has a complex, multi-host lifecycle involving Helisoma trivolvis snails, Pseudacris regilla and Anaxyrus boreas amphibians, and canids. Using a survey of 67 ponds in Northern California in 2009, four aspects of Alaria spp. infection in amphibian hosts were studied: (i) natural occurrence, (ii) effects of host biology, (iii) spatial distribution, and (iv) environmental determinants of occurrence. A major question in ecology is whether dispersal or local environmental determinents structure ecological communities. Field data of local site level abiotic and biotic variables as well as calculated dispersal variables were analyzed in three separate generalized linear models of (i) local variables, (ii) dispersal variables, and (iii) combined local and dispersal variables were compared for best fit of average abundance and presence of Alaria spp. amphibian infection. A thorough review of past reports of Alaria spp. as well as analysis involving paired t-tests, linear models, and Moran’s I were also carried out.

Results/Conclusions

The present study identifies Anaxyrus boreas, the Western toad, as a novel host of Alaria spp. As for frequency, Alaria spp. was found at 27% of sites and in 25.5% of amphibians (range:5% to 72% hosts per site), and the average abundance across hosts was 1.21 parasites per frog (range:0.02 to 5.33). There was no significant difference in Alaria spp. infection between Pseudacris regilla and A. boreas as well as between sexes of hosts, however host body size was significantly correlated with Alaria spp. infection. Alaria spp. infection in amphibian hosts was randomly spatially distributed in the study system (p>0.2). Finally, no environmental factor was significantly correlated for the presence of Alaria spp. and neither model was significant. The local model was the best fit (AICcWeight=1.0) for the average abundance of Alaria spp. with vegetated surface area, pond surface area, and pH as the significant environmental factors. The negative correlation of vegetated surface area and abundance as well as the negative correlation of pH and abundance are not well supported or understood. Experimental investigation of mechanisms and further surveys of Alaria spp. across years and geographical regions are necessary to fully understand the ecology of Alaria spp.