Salmonella Brandenburg: case-control survey in sheep in New Zealand

To identify environmental, management and animal risk factors associated with the occurrence and severity of disease attributed to Salmonella Brandenburg infection in sheep.

A retrospective case-control study was undertaken, and details of disease prevalence and farm management methods were collected from two affected regions in southern New Zealand. Associations between possible risk factors and disease attributed to S. Brandenburg were evaluated using odds ratios. A case-control approach was used to assess risk factors associated with outbreaks of disease at the farm level, using unaffected farms as controls. A separate analysis was then performed within affected farms only, to assess risk factors associated with increasing severity of disease.

Data were collected from 405 farms containing a total of 1,170,737 ewes. Of the 177 case farms, 172 (97%) had diseased mixed-age ewes, 78 (45%) had diseased two-tooth ewes and eight (5%) had diseased hoggets. Increased odds of farms being affected with S. Brandenburg were reported for farms that strip-grazed sheep, and as total numbers of sheep increased. Reduced odds of being affected were associated with feeding crops, and hilly terrain on those farms not feeding crops. Within affected farms, increased severity of disease was associated with strip-grazing and feeding hay in TT flocks, and later removal of rams in MA flocks. Reduced severity of disease was associated with shearing or crutching after July, and vaccination of sheep against Salmonella spp.

Salmonella Brandenburg occurred in flocks that reported using intensive farming methods and that maintained high numbers of sheep.

Risk factors associated with the occurrence and severity of the disease due to S. Brandenburg have been identified. This information is necessary to identify preventative and control measures that may be effective in reducing the risk of disease.