Assessment of the Public Health significance of meticillin resistant Staphylococcus aureus (MRSA) in animals and foods1 Scientific Opinion of the Panel on Biological Hazards

The European Food Safety Authority (EFSA) asked its Panel on Biological Hazards to deliver a scientific opinion on: The assessment of the Public Health significance of meticillin resistant Staphylococcus aureus (MRSA). There are different states of interaction between S. aureus and its host: infections, carriage or colonisation, and contamination. Meticillin resistant S. aureus (MRSA) can be persistently or intermittently carried by healthy humans, and colonisation is the major risk factor for infection. Infection can be mild to severe and, in some instances, fatal. MRSA are now a major cause of hospital acquired infection in many European countries, with large differences in prevalence and control policies. A limited number of lineages of MRSA tend to predominate in specific geographical locations. CC398 is the MRSA lineage most often associated with asymptomatic carriage in intensively reared food-producing animals. MRSA commonly carry enterotoxin genes but there has been only one report of food intoxication due to MRSA. On the question on what is the risk to human health posed by MRSA associated with food-producing animals, the Panel concluded that: Livestock-associated MRSA (LA-MRSA) represent only a small proportion of the total number of reports of MRSA infections in the EU. However, this proportion differs between Member States. In some countries with low prevalence of human MRSA infection, CC398 is a major contributor to the overall MRSA burden. In countries with high overall human MRSA prevalence, CC398 is considered of less significance for the public health. CC398 has, albeit rarely, been associated with deep-seated infections of skin and soft tissue, pneumonia and septicaemia in humans. Where CC398 prevalence is high in food-producing animals, people in contact with these live animals (especially farmers and veterinarians, and their families) are at greater risk of colonisation and infection than the general population. The risk to human health from different levels (dose response) of MRSA during carriage in animals (and in the environment) is not known. On the question of what is the importance of food, food-producing animals, and companion animals in the risk of human infection and/or food-borne disease caused by MRSA in both the community and hospital settings, the Panel concluded that: Food may be contaminated by MRSA (including CC398): eating and handling contaminated food is a potential vehicle for transmission. There is currently no evidence for increased risk of human colonisation or infection following contact or consumption of food contaminated by CC398 both in the community and in hospital. MRSA (including CC398) can enter the slaughterhouse in or on animals and occurs on raw meat. Although it may become part of the endemic flora of the slaughterhouse, the risk of infection to slaughterhouse workers and persons handling meat appears to be low based on currently available data. Where CC398 prevalence is high in food-producing animals, people in direct contact with these live animals (especially farmers and veterinarians, and their families) are at risk of colonisation and subsequent infection. The potential for CC398-colonised humans to contribute to the spread of MRSA in hospitals currently seem to be less than for hospital associated MRSA strains. MRSA infections in companion animals are increasingly reported and in almost all cases, the strains causing infection in animals are the same as those commonly occurring in hospitals in the same geographical region. Humans are likely to spread MRSA to companion animals, and these can then be a reservoir for humans both in the community and in health care facilities. Horses can become colonised and/or infected with MRSA from humans or from other animal sources in their environment. There are sporadic reports of human disease, usually minor skin infections, attributable to an equine source. On the question of which animal species (and if appropriate, foods derived there from) represent the greatest risk to humans, the Panel concluded that: The primary reservoirs of CC398 in affected countries are pigs, veal calves, and broilers. CC398 has also been found in companion animals and horses on farms with colonised livestock. MRSA has now been reported from dogs, cats and horses with sporadic reports of isolation from wide range of other companion animals. There are no specific studies which examined the relative risk of different small animals and horses as sources of infection or colonisation in humans. On the question of which methods are best suited for the isolation and molecular typing of MRSA of animal origin, the Panel concluded that: There is a wide variety of methods available for the isolation of MRSA. MRSA can be identified using phenotypic (antimicrobial susceptibility testing) or genotypic methods. For diagnosis of infection, samples taken directly from a lesion, biopsy specimens or blood cultures are cultured onto non-selective and selective media. For detection of carriage or contamination, swabbing of noses (for individuals), dust (for herds or flocks), and sampling of food are used. Increased sensitivity is obtained when using selective liquid enrichment methods. spa typing is applicable for lineage detection in first line typing because of wide congruence with results of MLST and other typing methods. The Panel recommended that further work should be performed on harmonising methods for sampling, detection and quantification of MRSA during carriage in both humans and animals, as well as for detection of MRSA as a contaminant of food, and in the environment including from dust both in air and on surfaces. On the question of what control options (pre- and post-harvest) can be considered to minimize the risk of transfer of food-associated and animal-associated MRSA to humans, the Panel concluded that: Monitoring and surveillance are not control options as such, however these processes are essential for determining control strategies and for the evaluation of their effectiveness. Surveillance of MRSA in humans, including spa typing of a representative number of isolates is necessary in order to monitor the occurrence of different strains of MRSA including CC398 in people. The Panel indicated that periodic monitoring of intensively reared animals in all Member States would provide trends in the development of this epidemic, and recommended carrying out systematic surveillance and monitoring of MRSA in humans and food producing animals in order to identify trends in the spread and evolution of zoonotically acquired MRSA. Animal movement and contact between animals is likely to be an important factor for transmission of MRSA. In the absence of specific studies on the spread and persistence of MRSA, general control options on farms, in slaughterhouses and in food production areas are likely to be the same for MSSA as well as MRSA, and include good husbandry practices, HACCP, GHP, and GMP. Monitoring and subsequent restrictions on movement may reduce transmission. Since the most important routes of transmission to humans are through direct contact with live animals and their environments, the most effective control options will be at pre-harvest. LA-MRSA carriers in hospital and other healthcare settings can be managed in the same way as HA-and CA-MRSA carriers in both staff and patients by screening and infection control measures. Strategies for screening (together with actions taken following their results) vary considerably between different MS's. Search and destroy policy seems to be the most effective, however its implementation is difficult when MRSA is already prevalent. The panel recommended that protocols for screening at admission to hospitals should be expanded to include humans exposed to intensively reared livestock. Transfer of MRSA to humans from companion animals and horses is difficult to control. Basic hygiene measures are key, especially hand washing before and after contact, and if possible, avoiding direct contact with nasal secretions, saliva and wounds. Decolonisation of these animals is a potential control option but controlled studies are lacking. The Panel recommended that intervention studies should be carried out in order to evaluate the effectiveness of control measures to reduce the carriage of CC398 in livestock. Such studies should be longitudinal over consecutive production cycles. In addition, the factors responsible for host specificity, persistence in different environments, transmission routes (including airborne transmission) and vectors, should be investigated. The panel also recommended that intervention studies should be carried out in order to evaluate the effectiveness of control measures to reduce the carriage of MRSA in companion animals and horses and their human contacts.