Objectives: To determine the relative prevalence of human infections attributable to Streptobacillus moniliformis in California over the past 3 decades. Methods: A retrospective analysis of all the data collected was conducted on S. moniliformis cultures identified by the Microbial Diseases Laboratory (MDL) from January 1970 to December 1998. Results: Information on a total of 45 S. moniliformis isolates was analyzed. Overall, 91% of the isolates were from human sources; 58% were received since

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... 1990. These strains were divided almost equally between males and females, with 50% of the isolates from patients 9 years old or younger. In 75% of the cases of human infections where a diagnosis was given, rat-bite fever (RBF) was suspected; 83% of these suspected cases involved either a known rat bite or exposure to rodents. Conclusions: As crowding becomes an increasing environmental reality, humans are more frequently being exposed to zoonotic diseases as a result of encounters with "wild" animals. Domesticated animals also are exposed more frequently to wild animals; thus, increasing human exposure to once rare zoonotic illnesses. Rat-bite fever is a disease that seems to be easily recognizable by clinicians, easily identified in the clinical laboratory (if suspected), and successfully treated when the appropriate therapy is administered. Physicians should consider RBF as a possible diagnosis when fever, rash, and exposure to rats are part of the patient's history.