Leptospirosis in Israel: epidemiological features and laboratory findings in 13 human cases due to Leptospira ballum.

Thirteen sporadic cases of human leptospirosis due to Leptospira ballum were laboratory confirmed during a period of 11 yr. In four cases the diagnosis was proved retrospectively. All but three were residents of the Sharon plain region, living 10 to 15 km apart. All were engaged in some form of agriculture, especially in irrigated fields or groves, or in chicken farming. It was suspected that wet soil contaminated with the urine of rodents, carriers of L. ballum, was the main source of infection. Two types of immunological response were observed: 1) high microscopic agglutination (MA) titers with L. ballum serovar only (nine cases); and 2) considerable coagglutination titers with heterologous serovars as well (four cases). The serological follow-up of these cases for several years revealed that in some, a relatively high MA titer (1:200 to 1:400) against L. ballum serovars persisted for 4 to 9 yr. Sera from 10 patients were tested for IgM and IgG anti-bodies by the MA test. In eight cases the IgM responded to a higher titer and persisted for many years, regardless of whether or not any IgG agglutinins were detected. The importance of blood cultures is emphasized and the possibility of misinterpretation of MA results is discussed.