Leptospirosis is a bacterial zoonotic disease, which can induce hepatic lesions. Photosensitization (PS) maybe is the result of accumulation of photodynamic agents in unpigmented area of the skin exposed to sunlight which occurred due to hepatic lesions. An outbreak of leptospirosis was occurred in a dairy cattle herd complex with 760 dairy milking cows in March (1990). Totally, 412 cows (54.21%) showed clinical signs including fever, systemic mastitis, haemoglobinuria, jaundice, lameness, abortion and stillbirth. The microscopic agglutination test (MAT) was used for diagnosis of leptospirosis. The herd was infected with Leptospira interrogans grippotyphosa, L. interrogans icterohaemorrhagiae, L. interrogans pomona, L. interrogans canicola and L. interrogans hardjo. Only one (0.13%) cow showed PS in the unpigmented skin dorsum. MAT showed that she was infected with L. interrogans pomona, L. interrogans hardjo and L. interrogans icterohaemorrhagiae with titre of 1 : 3200 of antibody. In spite of the high incidence of leptospirosis, the frequency of PS was too low.

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