Salmonella Study

Key results and recommendations for farmers from a National Study of Salmonellosis on NZ Dairy Farms

Chris Compton, EpiCentre, School of Veterinary Science, Massey University

Background

NZ veterinary diagnostic laboratories reported a two- to three-fold increase in confirmed cases of salmonellosis on dairy farms between 2011 and 2021, and the emergence of Salmonella serovars previously undetected in NZ. These patterns concerned the Ministry for Primary Industries (MPI) because of the impact of these cases on the welfare and productivity of dairy cattle, and the potential increased risk to public health from this zoonotic disease. MPI requested Massey University to undertake a nationwide study in the dairy seasons starting in 2021 and 2022 to describe the extent of salmonellosis outbreaks on dairy farms and to recommend control measures against them to farmers.

How the study was run

Dairy vets and vet diagnostic labs helped us register 107 farmers on-line for the study whose animals had experienced one or more lab-confirmed cases of salmonellosis that were not Brandenburg serovar during the study. From these, we enrolled 53 ‘case’ farms with 10 or more animals affected and, with the help of the Fonterra On Farm Excellence (OFE) Operations Team, a further 118 unaffected ‘control’ farms were enrolled as a comparison group. All farms had spring- or split autumn-spring calving patterns and supplied a milk processor. Each eligible farmer provided further information about their farms and management practices by telephone interview with the researchers.

Key study results

• Dairy farmers that experienced salmonellosis outbreaks most frequently reported diarrhoea (scours) among groups of mixed age (MA) milking cows (60% of farms), milk-fed calves (36% of farms) and scours or abortion in dry cows (34 % of farms) and less frequently among rising 2-year-old (R2) heifers (17 % of farms)
• On farms where scours outbreaks involved mixed age milking cows, typically 4-10% of cows were affected and 0.4-3% of cows died, whereas when scours affected calves, typically 20-50% of calves were affected and 2-20% died
• Although calf-rearing farms were not eligible for the study and not included in other analyses, several self-registered calf-rearers reported that typically 30-100% of the calves were affected and 2-20% died
• Approximately one quarter of case farms experienced salmonellosis outbreaks in multiple management groups in the same dairy season
• 13% of farmers on control farms reported a diagnosis of salmonellosis in the previous 5-years, compared with 21% of case farmers in the same period
• Half of the case farm farmers used a Salmonella vaccine on their R2 heifers and MA cows in the event of an outbreak and 6% of the control farms had a preventive vaccination programme

We identified by use of statistical analyses the following risk factors for salmonellosis outbreaks in different management groups:

• In any cattle management group:
  • Greater herd size- 30% increased risk per 100 cow greater herd size
  • Dairy effluent application rate- increased risk 3.3 times if rate > 15mm compared with =< 15mm
  • Presence of feral deer on their farm- increased risk 5.1 times if feral deer reported seen on the farm
• In milking cows only:
  • Use of feeds including pre-mixed magnesium- increased risk 5 times compared with no feeds including pre-mixed magnesium
• In pre-weaned calves only:
  • Greater herd size- 50% increased risk per 100 cow greater herd size
  • Greater number of calf rearing staff- 2 times increased risk per additional person

Recommendations to control salmonellosis on NZ dairy farms

• Biosecurity practices
  • Farms of above average herd size and multiple calf-rearing staff should implement strict biosecurity measures to prevent salmonellosis outbreaks on their farm, particularly measures that target protection of calves, and when salmonellosis is first diagnosed in a season
  • Greater number of calf rearing staff- 2 times increased risk per additional person
• Feed and effluent management practices
  • Check the management of magnesium supplementation for milking cows to ensure that dosage rates are not excessive, particularly if using a feed blend that includes magnesium
  • Check dairy effluent management is best-practice, and if application rates exceed 15 mm, consider ways to reduce this rate
• Public health measures
  • Because salmonellosis can affect humans, the health of farm staff and families need to be protected when salmonellosis is diagnosed on the farm, apply good hygiene practices, use clean personal protective equipment and switch to shop-bought milk if currently drinking raw milk from the herd