Dynamics of a Plague Outbreak in Gunnison's Prairie Dog

In the third year of my study of social behavior and population dynamics in the Gunnison's prairie dog (Cynomys gunnisoni), one colony was eliminated by sylvatic plague (Yersinia pestis). The passage of plague through prairie dog colonies has been reported previously (Eskey and Haas, 1940; Ecke and Johnson, 1952; Lechleitner et al., 1962, 1968), but only one other colony has been observed prior to the onset of a plague outbreak: Clark's (1977) study of white-tailed prairie dogs (Cynomys leucurus). The present report provides detailed population data prior to and during a plague outbreak in the Gunnison's prairie dog. I monitored a 3.4-ha study site west of Willow Creek, near Blue Mesa Reservoir in Curecanti National Recreation Area, Gunnison Co., Colorado (38'28'N, 107'06'W). The study site was within a 60-ha prairie dog colony, which was bounded by the reservoir, hills, and U.S. Highway 50. No burrows were found more than three meters up the hillside. Small subpopulations existed across the highway but expansion was limited by hills and ranches. The predominant plants in the colony were borages (Cryptantha fendleri, Lappula redowski), mustards (Lepidium virginicum, Cardaria draba, Descurainia sophia, Sisymbrium altissimum), and sagebrush (Artemisia spp.). I live-trapped prairie dogs almost daily in June through August during 1979 and 1980, and from 25 March to 16 April 1981. At the end of 1980, 194 animals were residents of the study area and an estimated 1000-1500 animals constituted the entire colony (Table 1). In 1981 prairie dogs began emerging from hibernation in early March, a month or two earlier than usual for the Gunnison River Valley (D. Hill, U.S. National Park Service Naturalist, pers. comm.). When I arrived at Willow Creek on 25 March, about 40 animals were active in the study site; this was fewer than anticipated three weeks after emergence. Within the Tribe Marmotini, males typically emerge before females (Michener, 1984), and male prairie dogs typically are more trappable than females; however, in 1981 I trapped disproportionately few males (Table 1). On 7 April 1981, I found a dead yearling female that had had an unusually large number of fleas on her body when trapped three days earlier. During normal handling procedures it is common to see several fleas on an animal, but hundreds of fleas had been hopping on this animal and on myself. On 11 and 12 April three more animals were found dead at their burrow entrances. Two had been trapped within the preceding two weeks. All three animals were sent to the Center for Disease Control (CDC), Plague Branch, in Fort Collins, Colorado, where they were diagnosed as having died of sylvatic plague caused by Yersinia pestis bacterium transmitted by the fleas Opisocrostis hirsutis and 0. tuberculatus cynomyris. Furthermore, every flea examined was highly infectious, making this one of the "hottest" plague outbreaks the CDC had seen (CDC, pers. comm.). As spring progressed, prairie dogs continued to die of plague shortly after emerging from hibernation. Known individuals were observed repeatedly, then disappeared. Newly emerged animals were active for about a week before disappearing. The scarcity of the earlier emerging males might have been due to the sequential effect of plague on emerging animals or to greater susceptibility of males to infection (as with rats, Kartman et al., 1962b). Although mortality rates of females and young were higher in Clark's (1977) study, his mortality data were confounded by the midsummer immergence of adult males. By 21 April only a single individual remained alive in the study site and only isolated, scattered groups were found elsewhere in the colony. By 25 April seven solitary individuals were distributed across the entire colony; four days later no survivors remained. Plague had exterminated the entire population of 1000-1500 individuals in less than two months. The rapid progress and high mortality of this plague outbreak are similar to the 85 percent mortality over 5 months in Clark's population of 62 white-tailed prairie dogs (Clark, 1977) and with the near 100 percent mortality reported by others (Ecke and Johnson, 1952; Lechleitner et al., 1962, 1968; Fitzgerald and Lechleitner, 1974). Because of the paucity of data on prairie dog or other ground squirrel populations prior to a plague outbreak, I have provided complete population data for the record. Population data from the study site are assumed to be representative of population growth in the colony overall (Table 1). In the 1979 field season all individuals of unknown age were considered to be adults; residents could not be distinguished from 194 Vol. 66, No. 1