Mildred M. Galton

The Epidemiology of Leptospirosis in the United States

DURING the past 70 years extensive investigations have shown that leptospirosis occurs in humans and animals in all parts of the world and that it is not a single disease but a group of diseases caused by a variety of leptospiral serotypes. Although considerable attention has been directed to the study of leptospiral infections by European and Asian workers since the early 1930s, much of our knowledge regarding the epidemiology, public health importance, and distribution of these diseases in the United States has been gained only during the past decade. The epidemiology of the leptospiroses has been shown to follow a characteristic pattern based, in part, upon the fact that they are zoonoses, diseases transmitted from animal to animal and from animal to human. The chain of transmission, with rare exception, stops with human infection. For many years rats anld dogs were considered to be the primary animal carriers, but as the search for leptospires continues the host range broadens not only among domestic animals but in a variety of feral mammals (1). Leptospirosis now constitutes a major problem in cattle and swine, and in some areas sheep, goats, and horses become infected. The rat is one of many rodent carriers including mice, voles, and shrews (2-4). In addition, bats, mongooses, bandicoots (5), jackals (6), foxes, opossums, raccoons, skunks, and wildcats (1) have been found infected. In these host animals, leptospires become localized in the kidneys and may be found in the lumina of the convoluted tubules. They may be shed in the urine for long periods. Each leptospiral serotype, usually is thought to have a primary animal host but it may infect other animals, and a so-called primary host for one serotype may become infected with other serotypes or even harbor two types at the same time (7). A classi,c example of this is Leptospira canicola, found principally in dogs; it has also been isolated from cattle (6), swine (8), and jackals (6), and serologic evidence suggests it may infect raccoons (9). Dogs have been found to harbor at least nine other serotype.s (10). Differentiation of the rapidly increiasing number of pathogenic leptospiral strains is highly important from an epidemiological and epizootiological point of view. This became apparent to the Dutch workers in the early 1930s who found that classification of leptospires must be based upon. serologic analysis. During the next 20 years these investigators studied antigenically distinct strains isolated in various parts of the world and developed techniques for serologic identification. Finally, in 1954, Wolff and Broom (11) published a suggested classification scheme based on antigenic analysis which included 34 leptospiral serotypes isola.ted from man and animals. In the 3 years since this report, approxima.tely 25 Mrs. Galton is chief, Leptospira Research Laboratory, Communicable Disease Center, Public Health Service, Chamblee, Ga. This paper was presented at the CDC Conference for Teachers of Veterinary Public Health and Preventive Medicine, and Public Health Workers, Atlanta, Ga., June 12-18, 1958.