Prevalence

Leptospira are found worldwide, though different serovars may only be endemic in certain geographic regions. The most common serovars in the United States are Hardjo, Grippotyphosa, and Pomona; however, other serovars can also be found and have been associated with reproductive losses.

Serologic surveys have repeatedly shown that the most common cause of leptospirosis in cattle is infection with serovar Hardjo.^4–7 As mentioned earlier, two genetically distinct types of serovar Hardjo have been identified: serovar Hardjo type hardjo-bovis and serovar Hardjo type hardjoprajitno. Serovar Hardjo type hardjo-bovis is common in cattle populations throughout the world, including North America,⁵ while type hardjoprajitno has been found primarily in the United Kingdom.

Limited studies in the United States estimate herd prevalence of serovar Hardjo to be near 60% in dairies⁶ and 40% in beef herds.⁷ In Ontario, Canada, 8% of dairy herds and 44% of beef herds surveyed that were not vaccinated against leptospirosis were found to have cows serologically positive for serovar Hardjo,⁸ whereas in western Canada seroprevalence appears to be much less in the beef herds surveyed there.⁹ Herd prevalence may vary regionally with higher rates more likely in temperate climates.^7,10 In a large survey of cull cows sampled at slaughter in 49 states, the percentage of cattle serologically positive for a Leptospira serovar was much higher in southern tier and west coast states than other areas of the United States.⁴ In studies of individual adult cows, seroprevalence to serovar Hardjo ranges from 10 to 30% in both the United States and Canada. Higher seroprevalence rates can be expected in individual herds that are endemically infected.

Accurate data for the frequency of abortions attributable to leptospirosis are lacking in North America. In Northern Ireland, leptospirosis was recognized as being responsible for over half of all bovine abortions, with serovar Hardjo being identified as the causative agent in 97% of those in one study.¹¹ In the previously cited study performed in Ontario, Canada, serovar Hardjo was associated with 6% of abortions while Pomona abortions were not recognized.⁸ In the United States, multiple diagnostic laboratory surveys of cases in which there was a definitive diagnosis found that Leptospira was identified in less than 10% of aborted fetuses.¹² Because of the overall difficulty in making a definitive diagnosis in bovine abortions cases in general, it is likely these surveys underestimate the true prevalence of Leptospira-related abortions.