Strain distribution of Borrelia burgdorferi and Anaplasma phagocytophilum in sciurids and woodrats in northwestern California

Borrelia burgdorferi (Lyme disease) and Anaplasma phagocytophilum (granulocytic anaplasmosis) are two tick-borne pathogens in the western United States. The ecology of both pathogens varies among areas in northwestern California due to multiple rodent hosts, diverse landscapes, and multiple genotypes within B. burgdorferi sensu lato and A. phagocytophilum sensu lato. I documented prevalence of both pathogens in Del Norte, Humboldt, and Mendocino counties and identified associations among hosts and strain types. Rodents were live-trapped between October 2005 and October 2013. Ear-punch biopsies and whole blood samples were collected to determine the presence of B. burgdorferi sensu lato and A. phagocytophilum sensu lato via polymerase chain reaction (PCR), and positive samples were sequenced to determine genotypes. The program SaTScan was used to evaluate spatial clustering of genotypes. Overall prevalence of B. burgdorferi sensu lato and A. phagocytophilum sensu lato between 2005-2013 was 33.8% and 18.9%, respectively. The top models predicting genotypes of B. burgdorferi were ranked via a corrected Akaike's Information Criterion (AICc) value and included the variables "Host," "Habitat," and the interaction term "Host*Season," and the top model predicting genotypes of A. phagocytophilum included the single variable "Host." Dusky-footed woodrats (Neotoma fuscipes) had a higher prevalence of B. bissettii than B. burgdorferi sensu stricto and both N. fuscipes and sciurids had a higher prevalence of the DU1 strain than A. phagocytophilum sensu stricto. SaTScan detected clusters for both DU1 and B. bissettii at Del Norte study area (P0.001 and P=0.01, respectively). Another cluster was found for DU1 at North Humboldt study area (P=0.001). A greater proportion of captured individuals found within each cluster were N. fuscipes. The results suggest a relationship between strain type and rodent host. Because B. burgdorferi sensu stricto and A. phagocytophilum sensu stricto are more likely to be infective to humans, these results may help identify risks for transmission among humans living and recreating in northwestern California.