Leptospirosis is an occupational disease and currently the most important zoonosis in New Zealand. Vaccination of dairy cattle against leptospirosis in the 1990s coincided with a substantial decline of notified human cases. A new increase of cases in2002 was partially attributed to workers in sheep-only slaughter workers. This paper briefly reviews epidemiological features of the disease and reports about three studies designed to quantify the risk of human exposure associated with slaughtering sheep, to determine prevalence and incidence of human infection of workers at a sheep-only abattoir, and to estimate the duration of antibody titres for informing occupational safety and public health authorities about the risk of workers to multiple infections over time.Abattoir sampling and stochastic risk modelling demonstrated that workers at a sheep-only abattoir were exposed to 5-25 carcasses/day that were kidney-culture positive for Leptospira borgpetersenii serovars Hardjo or L. interrogans serovar Pomona. Another sheep-only abattoir had 9.5% workers with serum antibody ('infected') in the microscopic agglutination test (MAT) for Hardjo or Pomona. The risk of infection was higher for workers at the first stage of slaughter (bleeding, pelting) than at the point of handling kidneys or at meat inspection. Home slaughter was also a significant risk factor for exposure. A preliminary 14 months follow-up of 129 seronegative workers resulted in an adjusted annual cumulative incidence of 7.9% (95%CI 3.1-12.9%) seroconversion to either Hardjo or Pomona. However, none of the individuals experiencing seroconversion reported severe signs of disease. State transition modelling revealed that MAT titres would last about 16 months for Pomona and 8 months for Hardjo. As MAT antibody was reported to correlate with protective capacity of human sera in an animal model, the observed litre duration may be an indication for the time that individuals might return to being susceptible again after experiencing infection. Observing higher prevalence in farm animals and shorter titre duration, both associated with Hardjo, is consistent with more frequent infection with Hardjo than Pomona in notified human cases.We conclude that sheep carcasses constitute a significant leptospire exposure risk for workers, especially those positioned at the slaughter board, that exposure was associated with 8% annual incidence of infection by Hardjo or Pomona, and that MAT titres lasted about 8-16 months and less than 2 years.
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