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Although granulocytic anaplasmosis, caused by infection of Anaplasma phagocytophilum, is an emerging human and domestic animal disease, the ecology and natural history of the parasite is not well understood. Gray foxes (Urocyon cinereoargenteus) are relatively common, occasionally peri-urban mesocarnivores whose geographic distribution overlaps the reported distribution of granulocytic anaplasmosis in humans and domestic animals in North America. We evaluated the potential of foxes as hosts and reservoirs of A. phagocytophilum in both urban and backcountry habitats of the Hoopa Valley Indian Reservation, Humboldt County, California, USA. We trapped 54 individual foxes and had 16 recaptures for a total of 70 fox samples between June 2003 and October 2004 in delineated urban and backcountry zones. We collected 296 adult and 145 nymphal ticks from the 70 captured foxes including 193 Ixodes pacificus, 149 Ixodes texanus, 98 Dermacentor variabilis, and one Dermacentor occidentalis. There were seasonal differences in tick intensities, with most I. pacificus adults occurring in winter and spring (P < 0.001), most I. texanus nymphs in spring (P = 0.03), and most D. variabilis adults in spring and summer (P = 0.01). Thirty-six (51%) of the 70 fox sera had antibodies against A. phagocytophilum, with a higher (P = 0.24) prevalence in backcountry foxes (16 of 23) than in urban-zone foxes (12 of 31). Six (9%) of 70 fox samples were polymerase chain reaction-positive for A. phagocytophilum. Twenty-eight (31%) of 90 domestic dogs sampled from vaccine clinics within the study area were seropositive for A. phagocytophilum. There was a significant difference in prevalence between dogs and backcountry foxes (70%), but no differences were found between dogs and urban foxes (39%). We propose that gray foxes are a good sentinel species for A. phagocytophilum infections in northwestern California.

A total of 2,121 small mammals in California were assessed for Anaplasma phagocytophilum from 2006 through 2008. Odds ratios were >1 for 4 sciurids species and dusky-footed woodrats. High seroprevalence was observed in northern sites. Ten tick species were identified. Heavily infested rodent species included meadow voles, woodrats, deer mice, and redwood chipmunks.

Anaplasma phagocytophilum is a zoonotic tick-borne rickettsial pathogen that causes granulocytic anaplasmosis (GA) in humans, horses, and dogs. In California, dusky-footed woodrats (Neotoma fuscipes) are a putative reservoir host, and Ixodes pacificus is a vector for transmission from rodents to humans, dogs, and horses. Cases are clustered in coastal and Sierra Nevada foothill regions, but not necessarily in proximity to infected woodrats. This study was designed to compare exposures and active infections of A. phagocytophilum in multiple rodents at a fine spatial scale in a hyperenzootic area and to evaluate the spatial clustering of infections. Of 331 rodents, the seroprevalence was 14.5%, with 60% in tree squirrels (Sciurus griseus and Tamiasciurus douglasii), 29% in woodrats, 14% in flying squirrels (Glaucomys sabrinus), and 5% in chipmunks (Tamias senex). No seropositive ground squirrels (Spermophilus beecheyi) were detected. The seroprevalence was significantly higher west of the Trinity River (23.1%) than east (11.8%) of the river. One Douglas squirrel and one western gray squirrel were polymerase chain reaction (PCR) positive. There was more spatial clustering among seropositive animals compared with all animals tested across the spatial scales evaluated, and this clustering was significantly greater than expected by chance alone. A significant cluster of 24 seropositive animals was found west of the Trinity River, with a population of 56 animals considered within the 50% population-at-risk, and a radius of 362.8 meters. The diversity of cricetine and sciurid rodents infected suggests that squirrels and chipmunks may be underappreciated contributors to A. phagocytophilum ecology in the western United States. The spatial clustering of exposed animals suggests interesting underlying spatially heterogeneous environmental variables that could facilitate the persistence of A. phagocytophilum in nature.

Epidemic typhus, caused by Rickettsia prowazekii, is maintained in a southern flying squirrel (Glaucomys volans) sylvatic cycle in the southeastern United States. The northern flying squirrel (Glaucomys sabrinus) has not been previously associated with R. prowazekii transmission. A second rickettsial pathogen, Anaplasma phagocytophilum, infects dusky-footed woodrats (Neotoma fuscipes) and tree squirrels in northern California. Because northern flying squirrels or their ectoparasites have not been tested for these rickettsial pathogens, serology and polymerase chain reaction (PCR) were used to test 24 northern flying squirrels for R. prowazekii and A. phagocytophilum infection or antibodies. Although there was no evidence of exposure to R. prowazekii, we provide molecular evidence of A. phagocytophilum infection in one flying squirrel; two flying squirrels also were seropositive for this pathogen. Fleas and ticks removed from the squirrels included Ceratophyllus ciliatus mononis, Opisodasys vesperalis, Ixodes hearlei, Ixodes pacificus, and Dermacentor paramapertus.

Anaplasma phagocytophilum is an emerging pathogen throughout much of the Holarctic, where Ixodes spp. tick vectors occur. This organism was expected to be present at study sites in Humboldt County, north-western California, based on the presence of appropriate tick vectors, seropositivity in sentinel hosts, and previously reported human infections. However, despite high seroprevalence suggesting circulating A. phagocytophilum, active infections in dogs and wildlife (including suspected reservoir species) were rare using published polymerase chain reaction (PCR) protocols. This finding was possible if the published PCR protocol lacked sensitivity for strains in the study areas. We report a new TaqMan-PCR (TM-PCR) assay targeting the msp2 gene that has greater sensitivity and specificity for diverse A. phagocytophilum strains from this region. The new assay detected as few as one plasmid copy and a range of genetically diverse strains of A. phagocytophilum. Specificity was confirmed by failure to amplify targets of closely related bacteria. Application of the TM-PCR to samples from northern California confirmed PCR-positivity in 94 woodrats (71%; n=134), three (4%; n=80) bears, and seven (7%; n=97) domestic dogs. The msp2 TM-PCR protocol appears to be more sensitive for use in assays of samples from parts of western North America and possibly in other regions where populations are genetically diverse or divergent from eastern United States strains of A. phagocytophilum.