The Relation Between the Level of Population and the Prevalence of Leptospira, Salmonella, and Capillaria in Norway Rats

Abstract

As a part of a study of the factors affecting the level of Norway rat (Rattus norvegicus) populations (Davis 1951a, 1951b) examinations for some of the internal parasites were made. Three wellknown organisms were selected to serve as examples of the relationships of prevalence to population level. Leptospira icterohaemorrhagiae is a common spirochete that causes Weil's disease in man. Salmonella (several species) is a genus of bacteria known to be pathogenic to rats, humans, and many other vertebrates. Capillaria hepatica is a nematode that has a direct life cycle and is known normally only from rats. The rats were caught in residential blocks in Baltimore in box traps and brought to the laboratory for examination. The size of the rat population in each block was estimated by the methods described by Emlen, Stokes, and Davis (1949). After a sample (usually half the rats) had been removed from a block with a stationary population, a second sample was taken a couple of months later from the now increasing population. In other blocks a decrease of population was due to a gradual deterioration of the environment (from the rat's viewpoint) rather than to trapping or poisoning. The detection of Leptospira in the tissues of the host was accomplished by direct microscopic examination, by animal inoculation, and by cultivation in an artificial medium. Both kidneys were removed aseptically from the animal and were ground with a mortar and pestle. A 30 per cent saline suspension of the finely divided tissue was centrifuged at 1,000 r.p.m. for 5 minutes to throw down the coarse particles, after which the supernatant fluid was examined under the darkfield microscope for the presence of spirochetes. One and one-half to two ml. quantities of suspensions from specimens in which no Leptospira could be demonstrated microscopically were injected intraperitoneally into young guinea pigs, which were examined daily thereafter for evidence of infection. Microscopically negative suspensions were also inoculated into Schiffner's medium and incubated at 30? C. for a period of three weeks, during which time periodic microscopic examinations were made. In some instances, the presence of Leptospira was demonstrated by this method when other methods failed. Serological identification of every fifth positive culture showed Leptospira icterohemorrhagiae to be the species present in every case. Examinations for Salmonella were conducted simultaneously with those for Leptospira. Suspensions of liver, kidneys, and caecal contents were pooled with urine removed from the bladder and streaked directly upon SS Agar plates. The same specimens were inoculated into Tetrathionate Broth, an enrichment medium, which was streaked upon SS Agar plates after an incubation period of 24 hours at 370 C. Final identification of organisms producing characteristic colonies upon SS 1 Contribution from the Vertebrate Ecology Division of the Department of Parasitology, The Johns Hopkins University School of Hygiene and Public Health. This investigation was supported in part by a research grant from the Division of Research Grants and Fellowships of the National Institutes of Health, U. S. Public Health Service. The bacteriological examinations were done in the laboratories of the Department of Bacteriology of the Johns Hopkins University School of Hygiene and Public Health.