Allan M. Barnes

DYNAMICS OF PLAGUE IN A GUNNISON'S PRAIRIE DOG COLONY COMPLEX FROM NEW MEXICO

A plague (Yersinia pestis) epizootic spread through Gunnisons prairie dogs (Cynomys gunnisoni), and possibly other rodent species, in the Moreno Valley in north-central New Mexico between winter 1984–1985 and autumn 1987. We observed the progress of the epizootic and subsequent population recovery at four prairie dog towns within the valley during this period. At two towns (Midlake and Val Verde) the prairie dogs were marked prior to the epizootic. At two additional towns (Vega and South Entrance) prairie dogs were marked following the epizootic. In 1988, a second epizootic occurred at Vega. One hundred thirty-nine serum samples were collected from prairie dogs and other rodents and 1,750 fleas were collected from animals and burrows. Fleas infected with Y. pestis were collected from prairie dogs, deer mice (Peromyscus maniculatus), and thirteen-lined ground squirrels (Spermophilus tridecemlineatus). Prairie dog fleas included Oropsylla hirsuta, O. labis and O. tuberculata; deermouse associated fleas were Aetheca wagneri and Rhadinopsylla sectilis, and Oropsylla bacchi was associated with thirteen-lined ground squirrels. All of the above flea species were collected from prairie dog burrows. All rodent species shared some flea species. Thirteen-lined ground squirrels disappeared shortly before plague was identified in prairie dogs at Midlake. Meadow voles were rare following the epizootic at Vega in 1986, became abundant in 1987, and disappeared at the time of the second prairie dog epizootic in summer 1988. Although we collected serum from Gunnisons prairie dogs, thirteen-lined ground squirrels, deer mice, and meadow voles (Microtus pennsylvanicus), we identified elevated serum titers against Y. pestis only in Gunnisons prairie dogs. Prairie dog mortality at all towns affected by plague was in excess of 99%. Serum antibody titers indicate that more than 40% of the few prairie dogs left to establish colonies following epizootics survived plague infection.

EXPERIMENTAL PLAGUE IN ROCK SQUIRRELS, SPERMOPHILUS VARIEGATUS (ERXLEBEN)

Experimental infections with Yersinia pestis were followed in groups of rock squirrels. Development of coagulopathy and pneumonia were observed in 2–4% and 11–12% of the test animals, respectively. Susceptibility to experimental infection was heterogeneous with some animals surviving inoculation with large numbers of organisms and others succumbing after inoculation with small numbers. Production and longevity of serum antibody titers, as measured by passive hemagglutination tests, were variable as well, and apparently unrelated to dose. The data presented attest to the need for care in interpreting serologic test results for individual animals.