The Role of Predators in the Ecology, Epidemiology, and Surveillance of Plague in the United States

Predators play important roles in the ecology, epidemiology, and surveillance of plague in the United States. Most predators are accidental hosts of plague and, with the possible exception of grasshopper mice (Onychomys spp.), are not important sources of infection for feeding fleas. However, predators undoubtedly do play an important role in the natural cycle of plague by transporting infected fleas between different populations of plague-susceptible rodents. Predators are known to be at least accidental hosts for 40 of the 50 flea species that have been found to be naturally infected with plague in the U.S. Carnivores, including domestic cats, also play an important epidemiological role and have been sources of infection for 24 human plague cases since 1970. Serosurveillance of rodent-consuming carnivores is currently the most cost-effective method of monitoring plague in the western U.S. During the 1990s, these surveys have allowed CDC and other public health agencies to both identify plague risks for humans living in endemic regions and document the spread of plague into areas where it had not been identified previously. Proc. 16th Vertebr. PestConf. (W.S. Halverson& A.C. Crabb, Eds.) Published at Univ. of Calif., Davis. 1994. INTRODUCTION Yersinia pestis, the causative agent of plague, is maintained in nature as a flea-borne disease of rodents. Plague probably was first introduced into the United States around 1900 by rat-infested ships entering the port of San Francisco, California (Barnes 1982). Epizootics occurred in local rat populations as the disease spread from the port area to other regions of the city. These epizootics were the source of infection for 121 human cases between 1900-1904 (Link 1955). Within a few years after its introduction, plague passed from San Francisco's urban rat populations into the wild (sylvatic) rodents of the surrounding countryside. Once plague became established in rodents other than Rattus spp., it spread rapidly across the western U.S. and by 1940 had been identified as far east as the western edge of the Great Plains (Eskey and Haas 1940). Plague continues to exist in scattered foci throughout much of the western U.S., although more than 90 percent of the human cases occur in New Mexico, Arizona, Colorado, and California (Figure 1). The ecological relationships of plague in these established foci are extremely complex and each regional focus typically has its own characteristic flea vectors and highly susceptible mammalian rodent hosts (epizootic hosts) (Pollitzer and Meyer 1961, Barnes 1982, and CDC unpublished data). Epizootics usually occur every few years in major foci and often cause mortality in excess of 80 to 99 % among different epizootic hosts (Barnes 1982). Many of these epizootic hosts are associated with important flea vectors and form regional host/flea epizootic complexes. These complexes are important for regional amplification of plague and increase the chances that Y. pestis will spread to other rodent species or accidental hosts, such as humans, lagomorphs, or carnivores. The most important epizootic hosts (and their fleas) are discussed in Barnes (1982) and include various ground squirrels (Spermophilus spp.), antelope ground squirrels (Ammospermophilus leucurus), prairie dogs (Cynomys spp.) chipmunks Figure 1. Plague surveillance map showing the distribution of human cases and counties where plague-positive mammals or flea pools have been identified. (Tamias spp. and Eutamias spp.), and woodrats (Neotoma spp.). Other rodent species, such as Peromyscus maniculatus (deer mice) and Microtus californicus (California voles), have populations that differ in their resistance to plague-induced mortality. Rodents from resistant deer mouse or vole populations usually survive infection, but become bacteremic and can serve as sources of infection for feeding fleas. These animals are referred to as maintenance, or enzootic, hosts and are believed, by some, to be critical for maintaining plague during