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Questions and answers about methicillin-resistant Staphylococcus aureus (MRSA)
Updated BfR FAQ, 12 July 2012
Methicillin-resistant Staphylococcus aureus (MRSA) are bacteria resistant to certain antimicrobials which can cause, among other diseases, wound infections and inflammation of the respiratory system in humans. In the past, these bacteria were mainly found in hospitals where they were transmitted via contact from person to person. In recent years, there have been increased numbers of cases of infections which people have contracted outside hospital.
MRSA have also been detected in production animals and in food which can therefore be a source of infection for humans. The Federal Institute for Risk Assessment (BfR) has compiled Frequently Asked Questions about MRSA and their presence in the food supply chain.
- What are MRSA?
- What are the consequences of the methicillin resistance of Staphylococcus aureus?
- What groups of MRSA are there?
- How can a person become infected with MRSA?
- What is the difference between infection and colonisation with MRSA?
- How frequently do colonisations with MRSA occur in healthy individuals?
- Can consumers notice MRSA on their skin?
- To what extent is MRSA prevalent in different food-producing animal populations?
- Since when has MRSA been detected in agricultural production animals?
- Are the same types of MRSA types detected in different livestock production animals?
- Are organic farms also affected by MRSA?
- How frequently are MRSA prevalent in production animals the cause of infections in humans?
- Can people become infected with MRSA through direct contact with production animals?
- Can people become infected with MRSA during a farm visit?
- Do persons who live in the vicinity of animal farms run a higher risk of colonisation with MRSA?
- Are livestock-associated MRSA also transmitted from person to person?
- Are livestock associated MRSA causing infections in hospitals?
- How can livestock farmers reduce the prevalence of MRSA in their herds?
- How common are MRSA in food?
- Can people become infected with MRSA from food?
- Can consumers tell whether a food is contaminated with MRSA?
- Are organically produced foods free from MRSA?
- What can consumers do to protect themselves from MRSA found in foods and production animals?
- Is MRSA widespread in Germany compared to other countries?
- In what areas does the BfR conduct research on MRSA?
- What are public authorities doing to combat MRSA in animal populations?
- What are public authorities doing to combat MRSA in hospitals?
The abbreviation “MRSA” stands for the bacterial group “methicillin-resistant Staphylococcus aureus”. Staphylococcus aureus is a widespread bacterium which colonises the skin and mucous membranes of humans and animals. According to the European Centre for Disease Prevention and Control (ECDC), about 30 % of people carry Staphylococcus aureus on their skin. Normally this colonisation goes unnoticed. As S. aureus is found on the skin, it is frequently involved in infections of the skin and mucous membranes.
The methicillin-resistant variant of S. aureus is resistant to beta-lactam antimicrobials, i.e. to penicillins and cephalosporins. These antimicrobials are no longer effective when it comes to treating an MRSA infection, i.e. they no longer kill the infectious pathogen.
The reason why MRSA infections pose a challenge for attending physicians is that standard antimicrobials which are used against this group of pathogens are not effective. Due to the necessary switch to less well tolerated or less effective alternative therapies, MRSA infections can lead to longer hospital stays and increased death rates.
On the basis of their presence and the most important location of their transmission, MRSA are categorised into three main groups:
- MRSA that are transmitted predominantly in hospitals (hospital-acquired MRSA, haMRSA)
- MRSA that are transmitted outside the hospital setting from person to person (community-acquired MRSA, caMRSA)
- MRSA that are prevalent among production animals and are mainly found in humans who are in regular contact with farm animals due to their profession (livestock-associated MRSA, laMRSA).
Infections with MRSA most frequently occur in hospitals, especially in intensive care units. Chronic diseases, a weakened immune system, and the use of antimicrobials, especially if incorrectly administered, pose a specific risk of contracting an MRSA infection. Direct contact with MRSA-carrying persons and indirect contact via shared objects such as towels or a lack of hygiene constitute the main MRSA transmission paths.
Since 2005, more and more reports have appeared on MRSA detected in production animals and in persons who are in regular contact with production animals.
A distinction must be made between infection and colonisation with MRSA. Transmission of the bacteria followed by colonisation does not lead to disease symptoms. However, the person remains a carrier. Individuals colonised with MRSA have a greater risk of contracting MRSA infections, for example after surgical operations. Consumers should discuss with their GP or local health authorities what measures need to be taken in case of MRSA colonisation.
Under all circumstances, affected individuals or individuals belonging to a risk group should state this when they are admitted to a hospital or health care centre. This will then allow doctors to take precautions to avoid an MRSA infection before any medical procedures are carried out.
For Germany, only isolated figures are available on the frequency of MRSA colonisation in healthy individuals. However, it is estimated that approximately one to two percent of the population are carriers of MRSA. The National Reference Centre for Staphylococci of the Robert Koch-Institute collects and characterises MRSA and analyses the available information.
No. Colonisation does not normally lead to any disease symptoms. S. aureus is part of the skin’s normal bacterial flora, with roughly 30 % of the population carrying this bacterium on their skin. Colonisation with MRSA can only be determined by a laboratory examination. This is normally done by taking a swab of the nasal mucosa.
As a coloniser, Staphylococcus aureus belongs to the normal skin and mucosal flora of both humans and animals. S. aureus has been a recognised pathogen in inflammations of the mammary gland (mastitis), particularly in cattle for some time now. In production animals, a specific MRSA type is frequently detected which is prevalent within animal populations. It has been found in more than 50 % of farms with fattening pigs, but also in 20 % of veal and turkey holdings. This laMRSA usually belongs to a specific clonal complex (CC) 398. Colonisation of animals is usually not associated with illness. However, in cattle, this MRSA-strain - as is the case with other S. aureus - can lead to inflammation of the mammary gland.
As part of zoonosis monitoring, representative data on the prevalence of MRSA in different types of food-producing animals were collected in the years 2008 to 2010. Detailed results can be found on the Internet (in German only):
The first MRSA findings in animals were observed in the 1970s in mastitis isolates of S. aureus from cattle. Until 2005, reports on MRSA in production animals only appeared sporadically. In Germany, MRSA have been more frequently been isolated from production animals since 2004. They were identified as part of a retrospective investigation into S. aureus which were isolated during diagnostic post mortem examinations of pigs. Older isolates were not available for that study. It is not known, therefore, whether livestock associated MRSA (laMRSA) was already present in Germany’s pig population before 2004.
The first isolates from the poultry food chain (turkey and fattening hens) were made available to the BfR in the year 2008 from a voluntary monitoring programme conducted at slaughter houses and at retail. Here too, the possibility of the pathogen being present in the population even earlier cannot be excluded.
Detailed reports from Belgium and Germany about MRSA in dairy cattle are available from the years 2007 and 2008, respectively. Reports on MRSA in fattening calves appeared in the Netherlands in 2008. It was possible to confirm these findings for Germany as part of the zoonosis monitoring programme 2009.
In all tested samples, types of bacteria of the clonal complex CC398 accounted for the majority. To varying extents, isolates of other clonal complexes are found as well. At present, this is most often the case with poultry where isolates of the clonal complexes CC9 (broilers) and CC5 (turkey) are found.
The number of studies conducted on the various production animals is still limited. There are positive test results from organic pig farms. Overall, however, MRSA is found less frequently in such farms. In addition, the number of colonised animals within such facilities is usually lower.
In July 2009, it became compulsory in Germany in accordance with infection protection laws to report detected MRSA from blood cultures. In the years 2010 and 2011, 3341 and 4125 cases respectively were reported to the Robert Koch Institute. However, those cases only account for a small proportion of the overall findings of MRSA. Since the isolates are not required to be analysed in more detail, it is not known what proportion of these infections is attributable to laMRSA. In an overview by the National Reference Centre for Staphylococci for the year 2010, the strain was not implicated in such cases of illness. However, there are also reports from individual laboratories which suggest that such infections may be possible in particular cases.
Contact with colonised animals can lead to human colonisation with MRSA. So far, this has been described most frequently after direct contact with pigs, calves and poultry. However, since there have been reports of colonisations and infections in connection with almost all types of production animals, each of these animal species is a potential source of MRSA colonisation of humans under certain circumstances.
Persons who are frequently in contact with pigs and other production animals through their profession are more frequently colonised with MRSA than the general population. A study conducted in Lower Saxony showed that about 25 % of persons who had professional contact with farm animals were colonised with MRSA. In contrast, MRSA was found in only 1.5 % of tested persons without any occupational contact with production animals. Some of these persons colonised with MRSA had indirect contact with animals, however, or contact with such animals through household members who were exposed or regularly visited agricultural facilities.
Cases of disease involving skin and wound infections or respiratory tract infections with livestock associated MRSA have only been observed rarely compared to the high rates of colonisation of such persons. It is likely that the pathogen can be transmitted to humans both through direct contact with the animals and through inhalation of highly contaminated stable dust.
As is true for all zoonotic pathogens, this is possible in principle, especially if the animals are touched or their stables are entered. Studies from the Northwest of Germany have shown that persons who visit agricultural establishments on a regular basis, for example in order to buy eggs or milk there, are more frequently colonised with livestock-associated MRSA compared to the general population. However, the risk is much lower compared to persons exposed to the pathogens through their work. A study from the Netherlands has shown that in many persons who only had occasional contact with animals the colonisation disappeared spontaneously after a short time.
MRSA are contained in the stable air and are therefore also released with the exhaust air from stables. In the surroundings of stables, the concentration then decreases very quickly, so that MRSA can only occasionally be detected in the air in the immediate vicinity of the stables. On the ground, these bacteria can still be detected in somewhat greater distances from the stables. So far, there are no indications that the proportion of colonised persons in the vicinity of production animal farms or in areas with intensive animal production is higher than in the rest of the population. Little is known as yet about the resilience of livestock-associated MRSA in the environment. More studies are needed here.
In principle, MRSA can be transmitted between humans. As far as is currently known, livestock-associated laMRSA is less often transmitted between humans than has been described for other MRSA (ha and caMRSA) in hospitals.
The clonal complex CC398 accounted for roughly 3 % of MRSA detected in hospitals, according to information provided by the Robert Koch Institute. In most cases, the bacteria were isolated from colonised persons, rarely from infections. In the literature, the spreading tendency within hospitals is described as lower for CC398 than for other MRSA. Nevertheless, these MRSA can in principle be found in most types of illness associated with S. aureus, i.e. in infected wounds and pneumonia as well as septicaemia.
The presence of MRSA in animal production facilities is determined by the introduction of the bacteria into the populations and spreading of the bacteria within the population. It is to be assumed that cautious use of antimicrobials can reduce the selection pressure in the direction of resistant bacteria. In addition, controlling the animals before they are put in the stables, thorough cleaning and disinfection between fattening groupss and preventing the introduction from the environment of the stables (e.g. from neighbouring stables) are relevant measures.
Based on the current state of knowledge, raw meat of all animal species may contain MRSA, although the bacterial concentrations are often extremely low. As part of zoonosis monitoring, the federal and regional authorities have tested meat of various animal species for MRSA. The results from 2009 and 2010 show that fresh meat was contaminated with MRSA. The detection rates were between 11.7 % and 43.4 %. Meat from calves, pigs, broilers and turkeys was tested. At 43.4 %, turkey meat had the highest MRSA detection rate, followed by broiler meat with 23.7 %. Data on turkey meat from 2010 with 32 % of fresh meat samples positive for MRSA confirm these results.
In all tested samples, types of bacteria of the clonal complex CC398 accounted for the majority. To varying extents, isolates of other clonal complexes are found as well. At present, this is most often the case with poultry where isolates of the clonal complexes CC9 (broilers) and CC5 (turkey) are found. These results are consistent with reports from the Netherlands which also described contamination of food. The detailed findings of these studies are available online (in German only):
So far, there have only been few reports of cases in which an MRSA infection in humans could be attributed to food. In these few known cases, the foods had been contaminated with MRSA by infected persons. Many foods do not offer favourable conditions for S. aureus which means that the bacteria cannot multiply readily. In principle, all heat-treated foods such as pasteurised milk, roasted or cooked meat are safe. However, care must be taken to ensure that the food is not re-contaminated after heat treatment. Based on the current state of knowledge, raw meat of all animal species may contain MRSA, although the bacterial concentrations are often extremely low. In individual cases, for example in thawing water of broiler meat, the bacterial count may be higher.
Provided that the rules of kitchen hygiene are observed, the risk of an infection or colonisation with MRSA via food is low.
No, contamination of foods with MRSA can only be established through extensive laboratory testing.
MRSA can be detected in organically produced food as well, since animals from organic farms too can be colonised with the bacteria. The same standards of hygiene that are applied to conventionally produced meat must therefore also be applied to organically produced meat. To date, no comparative studies on the frequency of MRSA in organically produced meat are available.
The normal hygiene recommendations for handling food and animals apply when it comes to protection against MRSA colonisation: after contact with animals as well as before and after preparing raw meat, consumers should wash their hands thoroughly with soap and water. Furthermore, efforts should be made to avoid any direct contact between the mouth and animals or raw meat. These hygiene measures also provide protection against other pathogens such as Salmonella, Campylobacter and verotoxin-producing Escherichia coli (VTEC).
The BfR has summarised further tips on how consumers can protect themselves from food-borne infections in the leaflet Verbrauchertipps: Schutz vor Lebensmittelinfektionen im Privathaushalt (423.3 KB) (in German only).
Data on MRSA infection in humans are collected and published Europe-wide in the “European Antimicrobial Resistance Surveillance System” (EARS Net) which is coordinated by the European Centre for Disease Prevention and Control (ECDC). According to this system, MRSA was identified in approximately 20 % on average of the Staphylococcus aureus isolates from septicaemia in hospital patients in Germany between 2007 and 2010.
Compared to other European countries, Germany is in the middle range when it comes to the prevalence of MRSA in hospitals. In Scandinavia and the Netherlands, intensive efforts have been undertaken for decades to control and monitor MRSA, and hence it is less widespread there. In the United Kingdom and southern European countries, the proportion of methicillin-resistant S. aureus in all S. aureus is far higher. Whereas in seven countries a downward trend was observed, in four countries, including Germany, the proportion of MRSA among all invasive Staphylococcus aureus isolates increased.
Together with the federal states, the BfR studies the prevalence of MRSA in animal populations and food. In the National Reference Laboratory for coagulase positive staphylococci including S. aureus, the bacteria isolated in the regional laboratories are then analysed to establish in more detail their characteristics and affinities.
As part of national and international research projects, the BfR develops, together with cooperation partners, methods for detecting and typing laMRSA. The BfR studies how MRSA are spread between animal populations and along the food chain. On the basis of both the scientific literature and insights gained directly, the BfR regularly updates its risk assessment. It makes this assessment available to agencies of both the federal and the state governments and also to the public.
Due to reports on MRSA in animals, especially production animals and animal-based foods, veterinary and food control agencies are currently conducting research at the national and international level on how prevalent methicillin-resistant S. aureus are in animals and foods. For example, as part of the zoonosis monitoring programme, the agencies of the federal and state governments undertook extensive studies in 2009 on the prevalence of MRSA in animal populations and foods. These annual studies provide important data on the extent of the contamination of animal populations and foods and on the properties of the MRSA types involved. The Federal Institute for Risk Assessment regularly updates its assessment of the risk for consumer health protection on the basis of emerging data and makes available this assessment to the Federal Government and veterinary agencies.
Under the auspices of the Federal Ministry of Food, Agriculture and Consumer Protection (BMELV), a task force was formed consisting of representatives from the world of science and from health, veterinary and food safety authorities. This task force is currently conducting research on strategies to curb MRSA in animal populations and to minimise the risk it poses to humans to the greatest extent possible. The Federal Ministry of Food, Agriculture and Consumer Protection (BMELV) and the Federal Ministry of Education and Research sponsor research projects on the presence and spread of livestock-associated MRSA. Additional studies are undertaken as part of EU research programmes. In these studies, universities and health research institutions closely cooperate with veterinary and food safety organisations. The BfR is directly involved in many of these projects.
The Hospital Hygiene Commission of the Robert Koch Institute already published recommendations in 2008 indicating that persons with exposition to production animal through their work should be tested for MRSA when admitted to hospitals. The committee for biological job safety has authored an opinion on the MRSA contamination of those who are in regular contact with production animals in a working environment.
In July 2009, it became compulsory in accordance with infection protection laws to report detected MRSA from blood cultures. In the years 2010 and 2011, 3341 and 4125 cases respectively were reported to the Robert Koch Institute. However, those cases only account for a small proportion of the overall findings of MRSA.
The point of contact for more information on the subject of “MRSA in Hospitals” is the Robert Koch Institute.