Dusky-Footed Wood Rats (Neotoma fuscipes) as Reservoirs of Granulocytic Ehrlichiae (Rickettsiales:Ehrlichieae) in Northern California

Abstract

Dusky-footed wood rats (Neotoma fuscipes) andPeromyscus sp. mice (P. maniculatus andP. truei) were collected from one site in Placer County, one site in Santa Cruz County, and two sites in Sonoma County in northern California. Serum or plasma samples from 260 rodents were tested for antibodies to the agent of human granulocytic ehrlichiosis. Of these, samples from 25 wood rats (34% of those tested) and 10 (8%)Peromyscus sp. mice were found to be seropositive, but only those from one site. PCR assays targeting the groESL heat shock operon were conducted on all seropositive specimens and a subset of seronegative blood specimens. Ehrlichial DNA was identified in 17 (68%) of the 25 seropositive wood rat blood samples and in 1 of the 10 (10%) Peromyscus sp. specimens. None of 40 seronegative blood samples was PCR positive. Both seropositive and PCR-positive animals were collected during each trapping period. One male tick out of 84 Ixodes pacificus adults collected was PCR positive; samples of Dermacentor occidentalis nymphs and adults were negative. Nucleotide sequences of amplicons from three wood rat blood specimens and from the single PCR-positive tick differed by one and two bases, respectively, from a sequence previously obtained fromEhrlichia equi. At one site in Sonoma County, wood rats had a concurrent high prevalence of seropositivity and PCR positivity, while other sigmodontine rodents collected at the site were only occasionally infected. We suggest that dusky-footed wood rats serve as reservoirs of granulocytic ehrlichial agents in certain areas of northern California. The tick species involved in the transmission of granulocytic ehrlichiae among wood rats remains unknown.

Update on Human Granulocytic Anaplasmosis

“…The prevalence of A. phagocytophilum in ticks ranges from 0-51% in the eastern U.S. (Magnarelli et al 1995, Pancholi et al 1995, Daniels et al 1997, Varde et al 1998), but typically is much lower in the western U.S., from 0.8-11% (Barlough et al 1997, Nicholson et al 1999, Lane et al 2001. The prevalence in I. pacificus nymphs is also low, with 0/465 nymphs positive even where the adult prevalence was 11% (Lane et al 2001) and only 1/47 pools positive from Sonoma County (Barlough et al 1997).…”

“…The spatial distribution of human and equine cases of GA in the western U.S. corresponds with the range of Ixodes pacificus, the western black-legged tick, a common tick throughout much of low mountain California (Furman and Loomis 1984, Madigan and Gribble 1987, Richter et al 1996. I. pacificus has been recorded from 55 of 58 counties in California, and adult I. pacificus containing A. phagocytophilum DNA have been detected in Alameda, El Dorado, Humboldt, Napa, Orange, Sacramento, Santa Cruz, Sonoma, and Yolo Counties (Barlough et al 1997Nicholson et al 1999;Lane et al 2001), although tick surveillance has been limited. The vector competence of I. pacificus for A. phagocytophilum has been evaluated.…”

“…Reservoir competence was documented by experimental transmission of A. phagocytophilum by feeding larval I. scapularis on infected P. leucopus, allowing these larvae to molt, and transmitting the infection to naïve P. leucopus individuals via nymphs (Telford et al 1996). There is not much evidence of anaplasmosis in other Peromyscus spp., with 11% seropositive in P. boylei, 6% in P. maniculatus, and 8% in P. gossypinus (Nicholson et al 1998(Nicholson et al , 1999. PCR-positivity in P. maniculatus ranged from 0-21% (Nicholson et al , 1999Zeidner et al 2000).…”

“…There is not much evidence of anaplasmosis in other Peromyscus spp., with 11% seropositive in P. boylei, 6% in P. maniculatus, and 8% in P. gossypinus (Nicholson et al 1998(Nicholson et al , 1999. PCR-positivity in P. maniculatus ranged from 0-21% (Nicholson et al , 1999Zeidner et al 2000).…”

“…Woodrats are likely reservoirs for A. phagocytophilum in California. Serologic evidence of exposure was found in Neotoma fuscipes (17.4-26.0%), N. lepida (25%), N. albigula (1%), and N. mexicana (4.7%) (Nicholson et al , 1999, and A. phagocytophilum DNA was recovered from seropositive woodrats (Nicholson et al 1999). Titers in woodrats averaged 194 (geometric mean titer) compared with 410 in all seropositive Peromyscus (Nicholson 1998).…”