Plague Studies in California — the Roles of Various Species of Sylvatic Rodents in Plague Ecology in California

The status of our knowledge of the roles of various sylvatic rodents in plague ecology in California is reviewed. Two theories, Pavlovsky's doctrine of focality of zoonotic diseases and Baltazard's proposal that plague is maintained in nature in resistant rodent species, form the framework for our understanding of the occurrence and persistence of plague. The concepts of resistance, reservoir species, susceptibility, and recipient species are defined and discussed. The ecological attributes that appear to enhance the role of certain rodent species as reservoirs are proposed, and the ecological features that appear to produce epizootics are briefly outlined. Based on current information, the roles of individual species of rodents, rabbits, and some insectivores are presented and discussed in relation to epizootic potential and the epidemiology of human infection. Man's role in plague ecology leading to greater exposure to sylvatic plague is emphasized. Plague, caused by the bacterium Yersinia pestis, is an infectious disease found primarily in wild (sylvatic) rodents. Other mammals which become infected secondarily with plague include those (e.g., carnivores) that feed on sick or dead infected rodents, and those (e.g., rabbits, shrews, and moles) that share common habitats with sylvatic rodents and are more likely to encounter infected rodent fleas. Humans may become infected when they enter or live in an area where an outbreak (epizootic) is occurring in sylvatic rodents or rabbits. Even more serious to humans is an outbreak of plague in commensal rodents in urbanized areas, where humans and rodents are closely associated and where this association may lead to epidemics. The fear of epidemic situations has led the international health community to mandate the continuous surveillance and suppression of plague wherever it is indigenous. Plague is indigenous in California; it is firmly entrenched in populations of sylvatic rodents. The major routes of transmission among rodents and other mammals are: (1) the vector route (bite of infective fleas), (.2) the oral route (ingestion of infected specimens by cannibalism or predation), and (3) the pneumonic route (droplet inhalation, such as during stereotyped greeting behavior). Transmission to humans by fleas and among humans by the pneumonic route is well-known. It is not widely-known that humans can become infected by the oral route (ingestion of undercooked, infected meat, such as rabbit) and by direct contamination (inoculation of bacteria into wounds and abrasions on the hands during such activities as skinning infected carcasses or preparing them as food). Plague activity has increased and become widespread in California in the last decade. Since 1970, there have been thirteen human cases with five deaths (Table 1). At least three of the fatalities developed secondary pneumonia. During the same period, epizootics occurred in a variety of locales: wilderness areas, ranches and other private properties, recreational areas (including national parks, campgrounds, and vacation communities), and urban-suburban fringes of the expanding cities in southern California. These outbreaks have caused concern among public health personnel and the people who live in and frequent these areas. A comprehension of plague ecology is necessary to develop effective educational and control programs that reduce the potential for human infection. This report reviews the status of our knowledge of plague ecology in California. The report is based on current data and is interpreted within the framework of established theories in ecology and the epidemiology of zoonotic diseases (diseases of animals transmissible to man). The topics discussed are (1) theories of how plague is maintained in nature, (2) roles of various species of sylvatic rodents in plague ecology, and (3) man's role in plague ecology. THEORIES OF PLAGUE OCCURRENCE AND PERSISTENCE Two theories are presented to account for the persistence and occurrence of plague. The first is the doctrine of natural nidality of transmissible diseases (Pavlovsky 1966) or, as it is also known, the landscape theory of epidemiology in reference to arthropod-borne diseases, the theory of natural foci of diseases, or the localization of diseases (Audy 1958). Pavlovsky first formalized this doctrine in 1939, although the concepts underlying his doctrine were recognized earlier by several workers. Use of the doctrine has led to greater understanding of zoonotic diseases by reducing the bewildering complexity of these diseases to a common denominator (Olsen 1970) and by directing research to unravel this complexity (Audy 1958). Pavlovsky proposed that zoonotic diseases exist in nature in discrete foci. The natural focus is that portion of a territory with a definite geographic character in which there has evolved a definite interspecies set of relationships between (l) the disease agent, its vectors, the animal donors (reservoirs) and (2) the animal recipients of the agents and its vectors, where the external environment favors the circulation of the disease agent. Natural foci continue to exist as long as the interspecies set of relationships, called biocenoses or communities, remain intact. Foci may become extinct through natural or man-made changes, and foci may be created through human activity. The biocenose within the focus is viewed as a superorganism; therefore, the concept is holistic as well as dynamic. Kartman (1960) declared that the focal theory of disease constitutes an ecological precept having fruitful application both to academic and practical investigation.