A case-control study to identify risk factors for acute salmonellosis in New Zealand dairy herds, 2011-2012

In late 2011 the New Zealand Ministry for Primary Industries reported an increase in confirmed laboratory diagnoses of salmonellosis in dairy herds. To identify risk factors for herd-level outbreaks of salmonellosis we conducted a case-control study of New Zealand dairy herds in 2011-2012. In a multivariable analysis, use of continuous feed troughs [adjusted odds ratio (aOR) 6·2, 95% confidence interval (CI) 2·0-20], use of pelletized magnesium supplements (aOR 10, 95% CI 3·3-33) and use of palm kernel meal as a supplementary feed (aOR 8·7, 95% CI 2·5-30) were positively associated with a herd-level outbreak of salmonellosis between 1 July 2011 and 31 January 2012. We conclude that supplementary feeds used on dairy farms (regardless of type) need to be stored and handled appropriately to reduce the likelihood of bacterial contamination, particularly from birds and rodents. Magnesium supplementation in the pelletized form played a role in triggering outbreaks of acute salmonellosis in New Zealand dairy herds in 2011-2012.