Difficulties in interpreting whole-herd serology with respect to the epidemiology of Leptospira interrogans serovar hardjo infection in dairy cattle

A total of 485 (35.2%) sera out of 1379 whole herd samples from 11 dairy herds, reacted to hardjo genotype Hardjo bovis antigen in the microscopic agglutination test (MAT). All the herds except one had previous evidence of hardjo infection and the serological prevalence at a dilution of 1:10 varied from 2 to 85%. Herds were classified into 'actively' or 'inactively' affected herds depending on their prevalence at a, dilution of 1:100 being higher or lower than 10%. Age-specific prevalence (at a titre of greater than or equal to 1:10) in herds with active infection was high in all lactation groups while herds with inactive infection showed two patterns: either the prevalence increased with age or increased prevalence in lactation's 1 and > 4 i.e., low prevalence in cows of 2(nd) and 3(rd) lactation's. There was no consistent pattern in age-specific seroprevalence at a dilution of 1:100. However, when the proportion of seropositive animals with titres greater than or equal to 1:100 within reactors (greater than or equal to 1:10) were calculated for each lactation group, a pattern emerged which correlated roughly with how old the history of the disease is. In herds with active and recent history of infection, percentage of high titred cows among the reactors decreased with increase in parity. Whilst herds with inactive infection and chronic infection, and a considerable proportion of seropositive animals with titres >1:100 were from higher lactation groups (greater than or equal to 4) followed by lactation 1. To conclude, in a completely susceptible herd, hardjo may appear in active form involving all ages while the occurrence of inactive form might actually be an exhibition of re-infection or chronic infection.