Survival of a pathogenic Leptospira serovar in response to combined in vitro pH and temperature stresses

Leptospira interrogans is a zoonotic pathogen hosted commensally by many mammalian species. On the Western Pacific island of American Samoa a seroprevalence survey conducted in 2004 indicated that 17% of subjects tested had been exposed to several Leptospira serovars, including L. interogans icterohaemorrhagiae. Resource management agencies promoted improved animal waste management practices, including composting, to reduce the risk of water contamination and human exposure from small pig husbandry facilities. This study simulated temperature and pH conditions expected in composting. We subjected L. interrogans serogroup icterohaemorrhagiae serovar Copenhageni strain M-20 to temperatures (25-50°C) and pHs (5.2-7.9) for varying durations of exposure (4-96h) to determine survival, indicated by culturing. Temperatures ≥45°C were lethal at the shortest duration of exposure (4h). The effects of temperature were enhanced by pH for temperatures <45°C. The results are summarized in a logistic model of the proportions (p(est)) of leptospires surviving the combined stresses of pH and temperature (p(est)=0.071e((1.091 pH-0.183 T(°C)))/1+0.071e((1.091 pH-0.183 T(°C)))). A graph of this function, with results of other studies carried out with serovar icterohaemorrhagiae, indicates that composting has the potential to kill leptospires, provided that all wastes are fully exposed to temperatures ≥45°C for at least 4h.