Methicillin resistant Staphylococcus aureus ST398 in veal calf farming: human MRSA carriage related with animal antimicrobial usage and farm hygiene

Abstract

Introduction: Recently a specific MRSA sequence type, ST398, emerged in food production animals and farmers. Risk factors for carrying MRSA ST398 in both animals and humans have not been fully evaluated. In this cross-sectional study, we investigated factors associated with MRSA colonization in veal calves and humans working and living on these farms.

Methods: A sample of 102 veal calf farms were randomly selected and visited from March 2007-February 2008. Participating farmers were asked to fill in a questionnaire (n = 390) to identify potential risk factors. A nasal swab was taken from each participant. Furthermore, nasal swabs were taken from calves (n = 2151). Swabs were analysed for MRSA by selective enrichment and suspected colonies were confirmed as MRSA by using slide coagulase test and PCR for presence of the mecA-gene. Spa types were identified and a random selection of each spa type was tested with ST398 specific PCR. The Sequence Type of non ST398 strains was determined. Data were analyzed using logistic regression analysis.

Results: Human MRSA carriage was strongly associated with intensity of animal contact and with the number of MRSA positive animals on the farm. Calves were more often carrier when treated with antibiotics, while farm hygiene was associated with a lower prevalence of MRSA.

Conclusion: This is the first study showing direct associations between animal and human carriage of ST398. The direct associations between animal and human MRSA carriage and the association between MRSA and antimicrobial use in calves implicate prudent use of antibiotics in farm animals.

Figure 1. Human MRSA prevalence, working hours in calf stables per week and MRSA positive calves on the farm.

Figure 1 A shows the relationship between MRSA carrier prevalence in humans and the number of working hours / week in the calf stable, adjusted for gender, age, smoking habits and percentage of positive calves on the farm. Figure 1 B shows the relationship between MRSA carrier prevalence in humans and the percentage of positive calves on the farm, adjusted for gender, age, smoking habits and number of hours working/week in calf stables. In both figures smoothed plots are given with 95% confidence bands.

Figure 2. MRSA carriage in veal calves, age and antibiotic treatment.

The relationship between MRSA carriage in veal calves and age for calves treated and not treated with antibiotics as group treatment (adjusted for age, number of calves per farm, farm hygiene, calf category (white versus rose veal), number of stables on farm, number of calves per pen, presence of other animals on the farm and rodent control). Smoothed plots are presented with 95% confidence bands.