PIG INTERNATIONAL: Dr Burch, what exactly is MRSA and why is more attention suddenly being paid to possible links between working with pigs and the chances of carrying the bacteria?
DAVID BURCH: The term MRSA refers to staphylococci with resistance to methicillin, which is an advanced antibiotic related to penicillin. All possible connections are being explored after an apparent increase in reports of the resistant bacteria in pigs in various countries. The major concern with MRSA (methicillin-resistant Staphylococcus aureus) previously has been in human hospitals. The Staphylococcus organism can be found on the skin of patients and it also colonises the nasal cavity. However, most of the time it has no clinical effect. But it can cause the death of the patient if it is able to penetrate into the body of someone whose immunity is compromised or affect a person who is undergoing surgery. The issue of drug resistance first came to light when S. aureus bacteria emerged that were resistant to penicillin. Then methicillin was developed, which was active initially against those penicillin-resistant bacteria. Now resistance has developed to methicillin — and this resistance also applies to all the penicillins and cephalosporins which are commonly used in hospitals to combat life-threatening infections.
PI: Why the connection to pigs?
BURCH: In some countries, notably Holland and Denmark, patients entering hospital are screened for MRSA before their admission or other healthcare procedures. These places that did routine screening noticed in 2004 that some pig farm workers were carriers of an unusual form of MRSA. Known as ST 398, it was not one of the usual human hospital epidemic strains. This alerted medical teams to try to discover where the organism was coming from. It led to Dutch surveys which identified that a high percentage of pigs and pig farmers were colonised by this strain of MRSA. Similar findings were also reported from a number of other countries in Europe, such as Germany. As a result, the European Com mission has initiated a survey this year covering the complete European Union area to see how far the situation has spread. Sweden appears to be clear and Ireland and Denmark have a very low incidence.
PI: Do pigs affected by MRSA become ill?
BURCH: Not necessarily, but when they do it is commonly associated with spread via the blood to joints, lung, heart and spleen, with joint infections accounting for 48 percent of cases.
PI: From the evidence, is working with pigs a health risk for producers, their employees and advisers?
BURCH: The health risk to farm staff has not been quantified. What the evidence has shown to date is that, in those countries where a high level of infection exists in pig herds, there is an increased incidence of carrying MRSA among pig producers and employees and veterinarians. Probably this is because the organism can be found in dust on pig units. In Holland, where they have a screening programme for people going into a hospital, any pig farmer being prepared for a surgical procedure is first put into isolation for testing and treatment. Individuals carrying MRSA can be treated to remove the infection prior to surgery.
PI: What can anyone do to reduce the risk at farm level?
BURCH: There do not appear to be many options at present to control the situation in heavily contaminated farms. The ongoing surveys have found countries which appear to have a low incidence or possibly no incidence. For them, perhaps a control on the imports of breeding pigs should be considered. This could involve either purchasing the animals from a known free source or insisting that they are tested before shipment. Bought-in breeding stock could also be tested in quarantine before introduction, but keeping the infection out is preferable if possible. The rapid dissemination of drug resistance in pig units has now been shown in Holland to be due to a spread down the breeding pyramids and this is why it has raised questions over breeding stock supplies. At the same time, the fact that the same strain of MRSA is appearing in many countries around the world, such as Canada and the USA, has prompted some speculation that this might be due to the international movement of breeding pigs, although this is by no means certain.
PI: Are concerns over MRSA related only to pig production?
BURCH: No. The bacteria have been found in other animal species — veal calves and poultry — as well as in pigs. Different strains, related to human strains, have been found in companion animals such as dogs and also horses.
PI: Is it an old problem linked to contact with animals or has it surfaced recently?
BURCH: We can say for sure that MRSA is an old problem. However, the new strain which has surfaced in animals is thought to have started in about 2004. This is when the Dutch screenings started to pick it up. There is a version of the same ST398 strain that is sensitive to methicillin and is thought to have been in pigs for much longer. But the development of methicillin resistance seems to be more recent.
PI: What could be the reason for an increase in these bacteria?
BURCH: That is a million-dollar question. The Dutch have suggested it is antimicrobial usage in general that has caused the problem. This is a little naïve as antimicrobials have been used for many years in animals without the problem emerging. In one study there appeared to be a higher incidence of MRSA in farms that had used broad-spectrum penicillins, but such a finding would not be surprising as the treatment presumably killed susceptible S. aureus. Some observers have postulated that using higher penicillins or cephalosporins, as in the human hospital situation, will select for MRSA. This has not been proven yet at a farm level, although early reports from Holland do suggest it may well be true.