Bulk tank milk samples were collected from 47 dairy herds in Mashhad suburb. From 19 herds, one sample was collected at each season in 2009 and from the rest of the 28 dairy herds one sample was collected only at autumn, 2009 and samples was tested by ELISA method for detecting antibodies to Leptospira interrogans serovar hardjo. These herds, which had not been vaccinated against leptospirosis within the previous five years, were categorised according to the herds’ regions. Nine herds (19.15%) out of 47 had a positive ELISA result. The antibody level in the herd milk samples were affected by the herd size (P<0.05). Region and season had no significant effect on antibody level (P>0.05). Larger herds were significantly more likely to have higher mean concentrations of antibody. It was concluded that a number of unvaccinated dairy herds in Mashhad suburb are exposed to infection with Leptospira hardjo.

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