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Livestock-associated MRSA: A Foodborne Illness?

Amesh A. Adalja MD FACP FACEP FIDSA September 23, 2016

One of the main fronts in the battle against antimicrobial resistance involves the presence of antimicrobial resistant bacteria in the food supply. Agricultural use of antibiotics for growth-promotion, in contrast to treatment of actual animal bacterial diseases, is increasingly being recognized as a driver of resistance. Furthermore, it has been shown that those in direct contact with or proximity to livestock can contract drug-resistant infections from animals and that drug-resistant bacteria can be found in food meant for consumption. However, it has been unclear to what extent transmission occurs to consumers of food harboring such bacteria. A new Danish study, published in Clinical Infectious Diseases, establishes that the risk of transmission extends away from the immediate livestock area to the general population.

 

National Surveillance Data Analyzed

To determine the burden of livestock-associated methicillin-resistant Staphylococcus aureus (LA-MRSA) infections, Larsen and colleagues queried that State Serums Institute database for human infections between 2009 and 2015 caused by the specific subtype of MRSA that originates in livestock (spa type t899). The search resulted in 12 cases, 10 of which were urban in nature and 2 in mink farmers. Only one urban isolate came from an individual exposed to poultry meat as a professional food service worker. These isolates were also compared to other European isolates obtained from humans, animals, and foods. 

The 12 cases formed a distinct clade within the larger LA-MRSA group. This clade were unique in that the genome contained an integrated phage in a specific gene. This phage is not an element of livestock isolates ever previously identified in Denmark and confers the ability to evade human host defenses and, as such, is evidence of human adaptation. Some of the urban isolates formed a distinct poultry-associated subclade and some were epidemiologically linked as determined by investigation of the case patients’ histories and contacts. Interestingly, these types were also seen in poultry isolates from other parts of Europe. 

 

Food-borne Origin

Because the isolates in this clade were not native to Danish livestock, the origin of the infections is likely to be from outside Denmark. Because of the number of cases uncovered, food-borne transmission was thought to be more likely than human-to-human transmission. 

 

Epidemiological Importance

The study is important in that it provides further evidence of transmission of MRSA isolates from livestock animals to humans. While the proportion of the overall MRSA burden that is derived from such transmission events is likely small, it is nonetheless important to study, survey, and track.

 

Reference

Larsen J, Stegger M, Andersen PS, et al. Evidence for human adaptation and food borne transmission of livestock-associated methicillin-resistant Staphylococcus aureus. Clin Infect Dis 2016; http://cid.oxfordjournals.org/content/early/2016/09/01/cid.ciw532.full#ref-2. Accessed September 22, 2016.