Methicillin-resistant Staphylococcus Aureus (MRSA)
Occurrence: In many pig rearing countries.
Age affected: All ages can be colonised.
Causes: Methicillin - resistant Staphylococcus aureus (MRSA).
Effects: Colonisation alone does not cause disease in pigs, but it may be present in abscesses, osteomyelitis and skin disease. Pig colonisation is of great public health importance.
The parent species, Staphylococcus aureus, is an occasional cause of abscesses, arthritis, osteomyelitis, mastitis and skin conditions in pigs. MRSA can be isolated from the same conditions and sites. The strain present in the pig is mostly CC/ST 398, one of a group of Livestock-Associated MRSA (LA-MRSA) found in farm animals and is indistinguishable in culture from the antibiotic sensitive strains until its sensitivity to antibiotic is determined when it is found to be resistant to methicillin.
The strain carries both the nuc and mec genes unlike the sensitive members of the species. Its resistance to antimicrobials is the reason for its importance in the pig and key to its relevance in public health, where humans infected with LA-MRSA may develop wound and skin infections which are difficult to treat.
Mode of transmission
MRSA is present on the skin and in the nasal passages of clinically normal pigs on an infected farm and is shed in nasal secretions and skin scales. It survives drying and can be found in dust, as an aerosol in dusty premises, on clothing and on pen furniture. It may be transmitted to piglets from the sow vagina or skin at birth, by contact with colonised pigs and from contaminated pen furniture, vehicles and clothing and hands of workers. Movement of the organism between farms is mainly by the movement of colonised pigs and contaminated vehicles and clothing but temporarily – colonised humans could also be sources of infection. Aerosol transmission for 300 m has been demonstrated, but the importance of this route is not known.
Contact with colonised pigs can result in humans being colonised, and the organism can also be found on meat intended for human consumption.
Colonisation does not give rise to any disease in most cases. Those clinical signs that occur are the same as those seen in infections with antimicrobial-sensitive S. aureus, and only laboratory demonstration can confirm that MRSA is present, rather than the sensitive variant. The clinical conditions in which it has been found include swollen ears, skin lesions, umbilical abscesses, subcutaneous abscesses, foot lesions, arthritis, osteomyelitis, mastitis metritis and vegetative endocarditis. It has been found in enteritis, but it is not clear whether it can cause it.
Staphylococcal infections can be suspected when exudative epidermitis is seen (usually S. hyicus) and when abscesses filled with creamy pus are found. The presence of a staphylococcus can be seen microscopically in samples, but in most cases, its identification follows bacterial culture. Only after further cultural or PCR examination, can it be confirmed as S. aureus. PCR for the mec gene or antibiotic sensitivity testing reveal it as MRSA. It can then be typed as an LA-MRSA. Surveys can be carried out using selective media or PCR testing for MRSA on nasal, vaginal or skin swabs.
Lesions in which MRSA may be found include those of exudative epidermitis, other skin lesions, subcutaneous abscesses, foot lesions, arthritis (where the joint cavity is usually filled with creamy pus), osteomyelitis and vegetative endocarditis. The post-mortem findings are only suggestive of a staphylococcal infection and is confirmed as involving MRSA by PCR or bacterial culture. Only laboratory testing can identify LA-MRSA contamination of carcasses or retail pigmeat.
Treatment and prevention
LA-MRSA remains sensitive to some antimicrobials, and infected wounds in pet pigs caused by the organism could be treated. Treatment to eliminate MRSA colonisation from a commercial pig or a herd is unlikely to succeed and has not been described. Hygiene, isolation of pig populations, all in, all out management and thorough cleaning may reduce the levels of colonisation. The spread to clean farms or areas can be prevented by strict isolation, only allowing the inward movement of pigs known to be uninfected, coupled with rigorous hygiene for visitors, workers and supplies.
As the importance of this organism is primarily to public health, those working on pig farms or in frequent contact with pigs, pig carcasses or pig meat should make the fact known to any medical institution where they might pose a risk to the hygiene status of the institution and infect others. This is to their benefit, as their own surgery or treatment could be put at risk by the LA-MRSA they carry. Although the organism can be recovered from pig meat, colonisation of the general public by this route is rarely reported. The organism appears to be cleared from the nasal passages of visitors to infected farms within 24 hours, but the wearing of masks may reduce the risk of carriage still further.