NATURAL HISTORY OF PLAGUE: Perspectives from More than a Century of Research

Abstract

For more than a century, scientists have investigated the natural history of plague, a highly fatal disease caused by infection with the gram-negative bacterium Yersinia pestis. Among their most important discoveries were the zoonotic nature of the disease and that plague exists in natural cycles involving transmission between rodent hosts and flea vectors. Other significant findings include those on the evolution of Y. pestis; geographic variation among plague strains; the dynamics and maintenance of transmission cycles; mechanisms by which fleas transmit Y. pestis; resistance and susceptibility among plague hosts; the structure and typology of natural foci; and how landscape features influence the focality, maintenance, and spread of the disease. The knowledge gained from these studies is essential for the development of effective prevention and control strategies.

Bacterial strategies to overcome insect defences

“…A remarkable case of oral infection occurs when fleas feed on rodents that are infected by Y. pestis; the fleas become plague vectors, as Y. pestis can colonize the flea gut 16 .…”

“…A striking example of bacterial survival in an insect gut is the interaction between the plague bacillus and the flea 16 . Y. pestis is endemic among wild rodents, and fleas …”

Modeling Plague Persistence in Host-Vector Communities in California

“…At least 18 rodents species and 27 or more flea species are involved in ongoing enzootic plague cycles in the western United States, largely independent of the traditional rat-rat flea cycle (Hubbard, 1968;Smith et al, 1998;Gage and Kosoy, 2005). Most cases of Y. pestis infection, including those in humans and rodents, are acquired through flea bites, but infections can occur through direct exposure to infectious respiratory and oropharyngeal secretions; through predation, particularly by cats; and through scavenging by rodents, such as Onychomys leukogaster, on infected hosts and carcasses (Smith et al, 1998).…”

“…Yet, no persistently infected reservoir hosts have been identified in the western United States. In some situations, fleas may function as de facto reservoirs, because they can retain infection for several months or longer, especially in winter climates (Eskey and Haas, 1940;Gage and Kosoy, 2005). It is likely that the dynamic interactions among hosts and fleas determine whether Y. pestis can be maintained in nature; however, it is not known which critical characteristics of these dynamics permit such maintenance.…”

“…Additionally, sylvatic plague threatens human health and several endangered and threatened wildlife species, including the blackfooted ferret (Mustela nigripes) and blacktailed prairie dog (Cynomys ludovicianus) (Carter, and Gage, 2000;Cully et al, 2000;Biggins and Godbey, 2003). Yet, not only is there insufficient ecologic knowledge to accurately describe the mechanisms for persistence and periodic re-emergence of plague but also there is no established predictive framework for anticipating the possible consequences of intentional release of Y. pestis in regions where the bacteria could persist (Gage and Kosoy, 2005).…”