Rodent-flea-plague relationships at the higher elevations of San Diego County, California.

Rodent-flea-plague relationships were examined at sites located at higher elevations of San Diego County during 1991-2002. The most frequently sampled rodents were the California ground squirrel, Spermophilus beecheyi, and white-footed mice, Peromyscus spp. Higher seasonal mean dependent variables (prevalences of squirrels and indices of O. montana), higher mean amounts of seasonal rainfall, and lower seasonal mean ambient temperatures were found at sites during 1991--1993 compared with 2000--2002. Levels of plague antibody indicate that squirrels seroconverted at most sites during spring/early summer when exposed to infected fleas when cleaning out burrows following hibernation, when interacting more with other squirrels occurred establishing home ranges, and when dispersing juveniles occupied abandoned burrows containing old nests. The majority of squirrels (35/41) were reported seropositive at Heise County Park (elev. 1,281 m) during 1996, 2001, and 2002. This may reflect fewer fleas biting squirrels more often when squirrels spent more time below-ground in order to avoid higher ambient temperatures. In contrast, most squirrels (12/13) were reported seropositive at two higher sites (elev. 1,584 and 1,641 m) on Palomar Mountain during 1993--1996 when there were higher mean dependent variables and amounts of seasonal rainfall compared with 1999--2002. This increased rainfall may have benefited host and flea populations and thus plague transmission. Seasonal indices were usually reduced after dusting squirrel burrows with 2% diazinon dust. Findings also indicated that S. beecheyi is a primary reservoir for plague at the higher elevations of these and other sites in southern California, and that the deer mouse, Peromyscus maniculatus, and other Peromyscus spp. were not major host reservoirs at the sites sampled.