Reported cases of human plague infections in the United States, 1970-1991.

From 1970 to 1991, 295 indigenous cases and one imported case of human plague were reported in the United States. Eighty-two percent of the total indigenous cases occurred in New Mexico, Arizona, and Colorado. Ninety-three percent of these cases had onset in the months of April through November. Most cases (89%) presented as bubonic or septicemic plague, or both. Cases were reported more frequently in males (58%), and male mortality exceeded that of females (17 versus 11%). Ground squirrels were the most frequently implicated sources of infection in cases associated with flea bites, and domestic cats were found to play an increasingly important role in transmission of disease to humans during these decades.

Studies on the control of plague in the western United States: laboratory trials of six insecticide dust formulations applied to soil for the control of the plague vector Oropsylla montana (Siphonaptera: Ceratophyllidae).

In this study, six insecticide dust formulations were laboratory-tested in soil mixtures ranging in concentration from 40 to 1,280 parts per million (ppm) against a proven vector of bubonic plague, Oropsylla montana (Baker). Pulicidal effects of the six insecticides, which included three organophosphates, two carbamates, and a pyrethroid, were compared by LD(50) values at both ambient and 75% RH. Permethrin (Pyraperm 455 dust), a pyrethroid with a LD(50) less than 40 ppm, demonstrated the most effective pulicidal action for the 13-wk trial period, followed in order by chlorpyrifos, bendiocarb, propetamphos, diazinon, and carbaryl.

Control of Oropsylla hirsuta, a plague vector, by treatment of prairie dog burrows with 0.5% permethrin dust.

Permethrin, a pyrethroid insecticide, applied on two plots with a pressurized hand-held duster at mean rates of 2.3 and 4.0 g per burrow, was used to determine control levels for Oropsylla hirsuta fleas, a vector of bubonic plague, in black-tail prairie dog, Cynomys ludovicianus, burrows in northern Colorado during the summer of 1988. Burrows were sampled by "flagging" at day 0 and weeks 1, 3, 6, 9, and 12 to determine the mean number of fleas per burrow and percentage of burrows with at least one flea. The 4.0 g per burrow rate was highly effective (P less than 0.001) in controlling fleas for a period of 3 mo, whereas the 2.3 g per burrow rate was not.

Laboratory and field trials of permethrin-treated cotton used as nesting material to control fleas (Insecta: Siphonaptera) on cricetid rodents.

Upholstery cotton treated with four different concentrations (0.25-2.0%) (2,500-20,000 ppm) of an aqueous permethrin solution, used as nesting material by white mice, was laboratory-tested against the potential plague vectors Oropsylla montana (Baker), Thrassis bacchi (Rothschild), and Orchopeas howardi (Baker) and found highly effective (P less than 0.001) for 1 yr. Similarly treated cotton gauze was tested under ambient and 75% RH and was found to be highly effective (P less than 0.001) in both environments for 1 yr. A separate test determined that the LD50 of permethrin-treated cotton was less than 10 ppm. Cotton tested with 0.5% permethrin and distributed under field conditions to cricetid rodents for use as nesting material was found to be highly effective (P less than 0.001 as a pulicide for greater than 4 mo when tested during winter in Larimer County, Colo. Permethrin-treated cotton was less successful in controlling fleas on cricetid rodents during the summer months in a New Mexico hyperendemic plague area.

Plague (Yersinia pestis) in cats: description of experimentally induced disease.

Sixteen healthy cats were fed a 6-wk-old laboratory mouse that had died of experimentally induced Yersinia pestis infection (strain NM77-538), to simulate oral exposure to plague. The cats were closely monitored after ingestion. Physical exams were performed and vital signs were recorded daily. Plague antibody titers and cultures of blood, throat, and oral cavity were performed daily. Complete blood counts and biochemistry panels were performed every 3 d. Complete necropsies were performed on any cats that died. Cats exhibited one of three responses following ingestion of one plague-infected mouse; they either died (6/16 or 38%), developed transient illness and recovered (7/16 or 44%) or showed no signs of illness (3/16 or 19%). A continual fever greater > 40 degrees C was associated with a poor prognosis. The highest antibody titers developed in the group that shed the plague bacillus over an extended period of time. Blood, throat, and oral cavity cultures were positive in 100% of the fatal cases. Throat cultures were positive in 75% of the exposed cats. In contrast to other carnivores, cats infected with Y. pestis exhibit bubo formation and pneumonic lesions similar to those seen in people with plague. Because of the potential transmission of Y. pestis from cats to people, development of a plague vaccine for cats may be warranted.

Experimentally induced plague infection in the northern grasshopper mouse (Onychomys leucogaster) acquired by consumption of infected prey.

In this study, 20 laboratory reared Onychomys leucogaster from a parental population that is naturally exposed to plague were each fed a white mouse that had been inoculated with Yersinia pestis. Three of the 20 O. leucogaster died, four survived with antibody titers against Y. pestis and 13 survived with no titer against Y. pestis. In contrast, when 20 O. leucogaster from a plague naive parental population were fed infected prey, seven died and 13 survived with no antibody titer against Y. pestis. Our results suggest another means by which O. leucogaster from populations that are naturally exposed to plague may acquire the disease.

Susceptibility to Yersinia pestis in the northern grasshopper mouse (Onychomys leucogaster).

The laboratory-born progeny from two geographically distant populations of northern grasshopper mice (Onychomys leucogaster) were challenged with Yersinia pestis to determine their relative susceptibilities to plague. One of the O. leucogaster populations was associated with a known epizootic focus of the disease and was found to be nearly 2,000 times more resistant to mortality than were members of another population from an area historically free of plague. The ecology and omnivorous behavior of O. leucogaster appears to promote strong selection for resistance to plague in areas where they are naturally exposed.