Dynamics of MRSA carriage in veal calves: A longitudinal field study

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Abstract

Colonization of Methicillin-Resistant Staphylococcus aureus (MRSA) in food producing animals has public health implications, but intervention targets have not yet been identified. In this field study occurrence and dynamics of MRSA in veal calves were investigated longitudinally on three farms. Determinants generally associated with MRSA carriage, such as environmental exposure and antimicrobial use, were explored. In addition, the reliability and reproducibility of MRSA detection in nasal samples from veal calves were investigated as well as the additional value of rectal samples to establish MRSA status of an individual animal.

On these three farms, MRSA prevalence and MRSA air loads in stables rapidly increased during the production cycle, especially after releasing calves from their individual houses, but not simultaneously with or directly after treatment with antimicrobials. These observations constitute the hypothesis that antimicrobial use may not necessarily be the only condition for MRSA transmission in veal calves, but indicate that other factors may contribute to transmission as well. MRSA in calves was present both nasally and rectally. The reproducibility and repeatability of the nasal samples were moderate.

The results of this study give a better understanding of the dynamics of MRSA in a field situation.

Introduction

The emerging problem of livestock-associated (LA)-MRSA colonization in food producing animals, such as pigs and veal calves, has put pressure on intensive livestock farming to control this spread. In addition, it has an impact on occupational health among people in regular contact with food producing animals and potential risks of carriage or disease in the general population (Catry et al., 2010, Graveland et al., 2011b, Vanderhaeghen et al., 2010). Therefore, it is of importance to reduce LA-MRSA occurrence in food producing animals and consequently in occupationally exposed humans.

Risk factors for LA-MRSA carriage in humans in close contact with food producing animals have been investigated in cross-sectional studies (Smith et al., 2009, Van den Broek et al., 2009, Graveland et al., 2010). A longitudinal study in veal farmers and their family members showed that LA-MRSA carriage was associated with the intensity and duration of animal contact. LA-MRSA prevalence in people working or living on veal farms decreased during absence of animal exposure, and the detection of LA-MRSA in nasal swabs varied greatly over time (Graveland et al., 2011a). This suggests that both colonization and transient contamination of LA-MRSA occur in humans. This variable status may also occur in animals. Little is known about dynamics, spread and possible changes in carrier status of LA-MRSA in animal populations. Data on dynamics of LA-MRSA in individual animals and at herd-level, and association with risk factors, are relevant to develop control strategies. Knowledge of the reliability and reproducibility of the diagnostic procedure when collecting these data are a prerequisite for correct interpretation of results, but currently lacking.

The aim of this longitudinal study was (i) to describe the dynamics of LA-MRSA in veal calves by analyzing nasal and rectal samples, (ii) to describe the dynamics of environmental contamination, and (iii) to determine the reliability and reproducibility of MRSA detection in nasal samples.

Section snippets

Study design and data collection

The study population consisted of 402 veal calves housed on three veal farms in The Netherlands. No other food producing animals than calves were present on these farms. A positive MRSA history was confirmed by MRSA positive dust samples from calf stables at the end of the previous production cycle. Included calves were housed in closed compartments (i.e. a separate, closed area within the stable). Each compartment has a separate ventilation system. Within each compartment the calves were

Farm and animal characteristics

All farms belonged to the category white veal and applied an all-in-all-out system. They differed slightly in farm structure with regard to the buildings and the total number of animals on the farm. All stables were divided into different compartments. Stable compartments of farm 1 were naturally ventilated; whereas in the stable compartments of both farms 2 and 3 mechanical ventilation was applied. Cleaning, but not disinfection, of the stables before the calves enter the stables was applied

Discussion

This study presents the results of repeated MRSA sampling in veal calves, which give insight in the spread and dynamics of MRSA in a veal calf herd. Approximately 10% of the veal calves were LA-MRSA positive upon arrival on the farm. The origin of the LA-MRSA in these calves is currently unknown, but may be the dairy farm where the calves originate from, or it may be a result of contact with positive calves during transport. In this study only LA-MRSA belonging to ST398 was detected, which is

Competing interest

The authors declare that they have no competing interests.

Authors’ contributions

HG participated in the conception and design of the study, collected the data, carried out the statistical analyses and interpretation of the data, and drafted the manuscript. MB collected the data, carried out the statistical analyses and interpretation of the data and contributed to the critical revision of the manuscript. IO and KV collected data and carried out the laboratory analyses. JW and DH participated in the conception and design of the study, and contributed to the critical revision

Acknowledgments

We sincerely thank all farmers for their participation. We are grateful to Lise van der Burg and Sandra van Kuijk for fieldwork and laboratory assistance. Finally, we would like to thank the Ministry of Economic Affairs, Agriculture and Innovation and the Product Board for Livestock and Meat for their funding. The funders had no role in study design; in the collection, analysis and interpretation of data; in the writing of the manuscript; nor in the decision to publish.

References (18)

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