Salmonella Study

Key results and recommendations for farmers from the national salmonellosis study

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

Background

NZ vet diagnostic labs reported a two- to three-fold increase in confirmed cases of salmonellosis on dairy farms and the emergence of Salmonella serovars previously undetected in NZ in the 10 years since 2011. This pattern concerned the Ministry for Primary Industries (MPI) because of the impact of these cases on the health and welfare of dairy cattle, and the potential risk these zoonotic diseases posed to public health. MPI asked Massey University to undertake a nationwide study starting in 2021 to describe the extent of these outbreaks and to investigate possibly risk factors for them.

How the study was run

One hundred and seventy-three dairy farms were enrolled from 10 NZ regions in the study over the two dairy seasons 2021-22 and 2022-23. Farms were eligible for enrolment if they had spring-calving herds of more than 120 cows and supplied milk to a commercial processor. ‘Case’ farms with naturally occurring salmonellosis outbreaks were identified by vets and were enrolled in the study if they had more than 10 animals affected on the farm and the disease was confirmed by laboratory tests. ‘Control’ farms were randomly selected from Fonterra suppliers and matched with case farms on the date they planned to start calving and their farming region, and were enrolled if salmonellosis had not been diagnosed or suspected on the farm in the current season.

Key study results

• Salmonellosis outbreaks most commonly involved mixed age (MA) milking cows and milk-fed calves
• Abortion outbreaks in dry cows and rising 2-year old heifers (R2’s) were least common
• Calves were most severely impacted in salmonellosis outbreaks- rarely as great as 100% rates of sickness and mortality but more commonly 25-60% of calves were affected and < 20% died
• Commonly 5-10% of R2’s and MA cows were affected in an outbreak and <5% died
• Approximately one quarter of case farms experienced salmonellosis outbreaks in multiple management groups in the same dairy season
• 2% of control farms had experienced a salmonellosis outbreaks in the previous season
• With statistical analyses we identified the following risk factors for salmonellosis outbreaks in
  • Any management group:
    • Increased herd size: 20% increase in risk per 100 cows
    • Presence of wild deer on their properties: four-fold increase if present
  • Calves
    • Increased herd size: 50% increase in risk per 100 cows
    • Increased number of calf rearing staff: two-fold increased risk per person
• We tentatively identified that feeding magnesium chloride to milkers increased the risk of an outbreak in that group four and a half-fold

Recommendations

– Biosecurity measures

• Biosecurity measures should both protect at the farm border and include management plans to control wild animals and spread of infection between management groups
• Farms of increased herd size and number of calf-rearing staff should implement biosecurity measures to prevent salmonellosis outbreaks on their farm, particularly targeting protection of calves
• Because salmonellosis can affect humans, the health of farm staff and families also needs to be protected through best practice hygiene and use of personal protective equipment
  

– Feed management practices

• We recommend farmers check that their magnesium supplementation rates for milking cows are correct, and particularly of magnesium chloride if fed to cows