Estimation of Risk of Infection and Contact Frequency with <I>Leptospira</I> During Farmwork

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  • 農作業にともなうレプトスピラ症の感染リスクと接触機会の推定

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Abstract

Risk of infection with leptospirae during farmwork was estimated using information from past leptospirosis outbreaks in both Miyagi Prefecture, Japan (around 1960) and northeastern Thailand (around 2000). Outbreaks of leptospirosis in Miyagi Prefecture were concentrated in October, while it occurred throughout the rainy season, showing a tendency to decentralize, in northeastern Thailand. In 1959, a large leptospirosis outbreak occurred in Miyagi Prefecture. The risk (1,600/100,000) of leptospira infection in the high-risk area during that outbreak was 3.4 times as high as that (470/100,000) in the middle-risk area. The risk in this year was 5.7 (high-risk area), 2.8 (middle-risk area) and 2.0 (low-risk area) times higher than that in the same area from 1960 to 1964 which could be considered as the average risk in the past time. In northeastern Thailand, the risk (50/100,000) of leptospira infection was 30 percent compared with that (170/100,000) in the middle-risk areas in Miyagi Prefecture from 1960 to 1964. Based on the risk, number of leptospirae invading into human body through skin during farmwork was estimated with the dose-response model. In Miyagi Prefecture and northeastern Thailand, the estimated numbers were 65-1,200 and 3.5-42 leptospirae per 100,000 exposures which means frequency of the daily farmwork, respectively. On the other hand, the calculation under possible environmental conditions (e.g. density of rats carrying leptospirae, water depth in paddy field) demonstrated that farmers had been in contact with 4,300 leptospirae only in an hour of their work. The result showing only a small part of leptospirae in contact invaded into human body primarily attributes to the strong structure of unwounded skin composed of stratified squamous epithelium and dermis. And also the result can be explained by a hydraulics theory. Although leptospirae being nearby skin surface can attach to skin since the water flow does not occur there, the spirochetes a little away from skin surface are easily transported with the flow and would be unable to invade into human body.

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