infection in a few colonies of laboratory mice in the United States and Europe. Several clinical cases of leptospirosis have occurred in personnel who worked in these colonies (Alexander, 1984).
Leptospira interrogans serovar ballum is a bacterium, order Spirochaetales, family Leptospiraceae. Members of the genus Leptospira are Gram negative, motile, helicoidal rods, measuring 0.1 x 6.0-12.0 µm. They are visible by dark-field illumination and phase-contrast microscopy. Parasitic members of the genus belong to the species L. interrogans, which is subdivided into approximately 180 serovars by cross-agglutination and agglutinin-adsorption tests. L. interrogans serovar ballum is one of many that occurs naturally in wild rodents (Johnson and Faine, 1984; Alexander, 1985).
Transmission of leptospiras is in part dependent on contamination of and survival in the environment. Conditions of moisture, warmth (25°C), and neutral pH favor survival. Leptospiras are inactivated by dessication, pH below 6.2 or above 8.0, and common disinfectants (Turner, 1967).
The methods of cultivation, identification, and serologic testing for leptospiral organisms are highly specialized and usually are performed by specialty laboratories (Sulzer and Jones, undated; Turner, 1968, 1970). Specimens from human patients are usually submitted through county and state departments of public health to the Centers for Disease Control, Atlanta, Georgia. Specimens from animals can be submitted to state veterinary diagnostic laboratories or to the National Animal Disease Laboratory, Ames, Iowa.
Wild mice are considered the primary reservoir hosts of L. interrogans serovar ballum for laboratory mice and people (Yager et al., 1953; Stoenner and Maclean, 1958; Hathaway et al., 1983; Alexander, 1984). However, wild rodents are considered the most important natural hosts of leptospiral organisms in general and have been found to be infected with numerous serovars of L. interrogans. Wild rats are considered the primary reservoir hosts of L. interrogans serovar icterohemorrhagiae for humans and other species (Turner, 1967; Feigin and Anderson, 1975).
Natural infections of L. interrogans serovar ballum in colonies of laboratory mice have been reported on several occasions (Stoenner and