Staphylococcus aureus (S. aureus), often referred to as "staph," is a bacterium commonly carried on the skin or in the nose of healthy people. S. aureus typically causes a skin infection, but can cause infections in the bloodstream and major organs. Methicillin-resistant S. aureus (MRSA) occurs when the bacteria become resistant to the antibiotic, methicillin and other more common antibiotics such as oxacillin, penicillin and amoxicillin, making it more difficult to treat. The rate of invasive MRSA (infections in typically sterile sites like the bloodstream) is estimated to be 32 per 100,000 persons in the United States; the mortality (death) rate is thought to be about 6 per 100,000 persons. The risk of invasive MRSA infections is highest among older individuals, Blacks/African Americans, and men.
MRSA infections were initially limited to hospitals and nursing homes, especially among patients with weakened immune systems. Since the 1980s, community-acquired cases and outbreaks also have been reported. Community acquired cases are those not related to past year hospitalization or medical procedures like dialysis, surgery, or catheterization. These infections typically occur among otherwise healthy individuals and are more likely to be limited to skin infections. An increase in the virulence of MRSA bacteria in the past decade, however, has been responsible for more severe and sometimes fatal community acquired infections. More recently, MRSA has been identified in food animals and a few outbreaks have been ‘food-initiated’ or foodborne. In one such outbreak, those affected developed typical foodborne illness symptoms, such as vomiting and stomach cramps.
Sources and Transmission
The major site where people carry S. aureus is in the nasal passages. Approximately 25% to 30% of the population is colonized (when bacteria are present, but not causing an infection) with S. aureus, but only about 1% of the population is colonized with MRSA. The main mode of transmission of staph and MRSA is through hands, which may become contaminated by contact with colonized or infected individuals or through contact with colonized or infected body sites of other persons. Contact with devices, items, or environmental surfaces contaminated with body fluids containing staph or MRSA may also cause infection. Other factors contributing to transmission include close skin-to-skin contact, crowded conditions, and poor hygiene.
Studies in recent years have demonstrated that food-producing animals also carry MRSA. Studies conducted in the U.S. as well as several other countries, including Austria, Canada, China, Belgium, Denmark, France, Italy, South Korea, Taiwan, and The Netherlands, have isolated MRSA mainly from pigs. Other animals testing positive for MRSA have included chickens, cattle, and dairy cows. In addition, the testing of raw meat samples from slaughter houses and retail markets has revealed MRSA in several countries. This is likely due to the high use of antibiotics in food animal production. Estimates of the amounts of growth-promoting antibiotics used in U.S. animal production range from 3.1 million to approximately 25 million pounds annually. In some instances, animal strains are similar or identical to human strains. In fact, the presence of MRSA in food-producing animals has also led to the transmission of MRSA to farmers, their families, and veterinarians, resulting in human colonization.
Of even greater concern is the identification of MRSA in retail meats and food products, including pork, beef, and diary products. This has occurred in the U. S. as well as in Austria, China, The Netherlands, Portugal, and South Korea. However, few foodborne or food-initiated outbreaks have been reported. One ‘food-initiated’ outbreak in The Netherlands involved the transmission of MRSA from a colonized but healthy hospital dietary worker to a patient through food. The contaminated food (which tested positive for MRSA) was ingested by the patient who was severely immunocompromised, and the patient contracted a fatal infection. Transmission from that patient to other hospital workers and subsequently to other patients resulted in a major outbreak. In another food-related case, a community-acquired foodborne illness outbreak occurred in Tennessee. In that outbreak, a family developed typical foodborne illness symptoms after eating food prepared by a commercial foodhandler who was colonized with MRSA (see details below).
Community-acquired MRSA infections most often present as a skin or soft tissue infection such as a boil or abscess. Individuals frequently recall a “spider bite”. The involved site is red, swollen, and painful and may have pus or other drainage. MRSA infections also can cause more serious infections, such as necrotizing fasciitis, a rapidly progressive, life-threatening infection that involves skin, soft tissue, and deep fascia; bloodstream infections; or pneumonia, leading to symptoms of shortness of breath, fever, and chills. As seen in at least one outbreak, individuals with foodborne illness caused by MRSA may experience nausea, vomiting and stomach cramps.
According to the Centers for Disease Control and Prevention (CDC), individuals with MRSA infections that meet all of the following criteria likely have community-acquired MRSA infections:
1. Diagnosis was made in the outpatient setting or by a culture positive for MRSA within 48 hours after admission to the hospital.
2. No medical history of MRSA infection or colonization.
3. No medical history in the past year of:
b. Admission to a nursing home, skilled nursing facility, or hospice
4. No permanent indwelling catheters or medical devices that pass through the skin into the body.
Detection and Treatment
In general, a culture is obtained from the infection site (skin, blood, or urine) and sent to the microbiology laboratory. In the case of a skin Infection, a culture is taken from a small biopsy of skin or drainage from the infected site. A sputum culture should be taken in the event of pneumonia. If S. aureus is isolated, the organism should be tested to determine which antibiotics will be effective for treating the infection.
Staph skin infections, such as boils or abscesses, may be treated by incision and drainage, depending on severity. Antibiotic treatment, if indicated, should be guided by laboratory testing of the bacteria to determine antibiotic susceptibility.
Frequent handwashing is the most important way to prevent spread of staph in the community. If an individual has a skin infection, it should be kept covered. Keep wounds that are draining or have pus covered with clean, dry bandages until healed. Bandages and tape can be discarded with the regular trash.
Family members and others in close contact should wash their hands frequently with soap and water or use an alcohol-based hand sanitizer, especially after changing the bandage or touching the infected wound. Do not share personal items, such as towels, washcloths, razors, clothing or uniforms, that may have had contact with the infected wound or bandage. Wash sheets, towels, and clothes that become soiled with water and laundry detergent. Use a dryer to dry clothes completely.
Since little is known about the transmission of MRSA through food, other than through foodhandlers, there are no specific recommendations about the prevention of foodborne illness with regard to MRSA. In general, individuals should wash their hands with soap and water before preparing food, especially commercial foodhandlers; wash produce; wash cutting boards often; wash hands, utensils, and cutting boards after they have been in contact with raw meat or poultry and before they touch another food; and refrigerate food within 2 hours of serving. Use a food thermometer to cook meat and poultry to proper temperatures and keep hot foods hot; cold foods should be kept at 40°F or below.
Isolation and Characterization of Methicillin-Resistant Staphylococcus aureus Strains from Louisiana Retail Meats
Researchers in Baton Rouge, Louisiana investigated the prevalence MRSA in 120 retail meat samples from 30 grocery stores in Baton Rouge. The samples were randomly collected over a six-week period in early 2008. On each sampling day, five stores were chosen based on geographical proximity to each other. Three prepackaged pork chops and one beef steak in the refrigerated fresh meat section were collected from each store.
S. aureus strains were recovered from 45.6% of pork samples and 20% of beef samples; MRSA strains were isolated from 5% of samples (five pork samples and one beef sample). The majority (73%) of grocery stores surveyed had S. aureus-contaminated meats, and 10% sold MRSA-positive meats. All four meat samples (three pork samples and one beef sample) from one store (belonging to supermarket chain A) were positive for MRSA only. The two remaining MRSA-positive pork samples were obtained from two other stores (chain B). Five out of six MRSA-positive meat samples (all were pork) were chain-branded meats. The MRSA-positive samples were determined to be of two unique human epidemic strains; therefore, humans (foodhandlers) rather than animals were the likely source of contamination.
An Outbreak of Community-Acquired Foodborne Illness Caused by Methicillin-Resistant Staphylococcus aureus
A local health department in Tennessee investigated a report of foodborne illness in a family that purchased shredded pork barbeque and coleslaw from a convenience-market delicatessen. The pork was reheated in a home microwave, and three adults ate the food 30 minutes after it was purchased. Approximately 3 to 4 hours after eating the meal, the three adults―who had not eaten another common meal together in the preceding week—had nausea, vomiting, and stomach cramps. Two children at the dinner who did not eat barbeque or coleslaw did not become ill. Two of the three ill adults were taken to a hospital for evaluation, where they were treated and released.
Twelve cultures of S. aureus recovered from stool samples of the ill family members, food specimens, and nasal swabs of the food preparers were sent to the CDC for further testing. S. aureus was recovered from the stool cultures of the three ill persons, three samples taken from the barbequed pork, one sample from the coleslaw, and five nasal swabs from three food handlers at the convenience market. Five isolates were indistinguishable using molecular typing by pulsed-field gel electrophoresis (PFGE), including those from the stool cultures of three family members, the coleslaw, and the nasal swab of one food preparer. This strain produced staphylococcal enterotoxin C and was identified as being MRSA. Two different strains of S. aureus recovered from the nasal swab of the second food preparer also produced staphylococcal enterotoxin C and differed only slightly from the MRSA strain by PFGE. Those isolates were methicillin-sensitive S. aureus but were categorized as closely related subtypes of the outbreak strain. The S. aureus isolate recovered from the third food preparer was also methicillin-sensitive, produced staphylococcal enterotoxin A, and was determined by PFGE to be unrelated to the outbreak strain. Although the S. aureus isolates from the samples of pork barbeque each produced staphylococcal enterotoxin C, they were methicillin-sensitive and unrelated to the outbreak strain.
The barbequed pork and coleslaw were prepared at the store where they were purchased. An environmental inspection of the facility performed after the outbreak revealed no apparent lapses in technique or procedure that would have contributed to the outbreak. No additional cases of illness related to this outbreak were reported to the local health department. The foodhandler who was carrying the outbreak strain of MRSA performed various tasks at the store, including preparing foods and handling barbecued pork and coleslaw. She reported no recent gastrointestinal illness, chronic health problems, history of admission to a hospital, or use of antibiotics in the previous six months. She did intermittently visit an elderly relative who resided in a nursing home before the outbreak. She reported that this person had a staphylococcal infection and had subsequently died. Further medical information or isolates from that person were not available.
It appears that MRSA-contaminated food was the vehicle in this outbreak affecting low-risk persons within the community, and that this food was likely contaminated by a healthy carrier whose only apparent exposure were visits to a possibly infected relative in a nursing home. This outbreak could be a health-care-associated infection that spread to the community. The outbreak strain of MRSA, however, was resistant only to penicillin and oxacillin and was sensitive to all other antibiotics tested. A strain originating in a health-care facility more likely would have been a multidrug resistant organism.
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