MRSA: The equine “superbug”

  • Most vets working in a specialist equine practice will encounter the occasional horse with a persistent infection. There may be an injury and/or a wound that has failed to heal and continues to discharge pus. Gradually, the condition progresses and does not appear to respond to any treatment. In cases like these the so-called “superbug” MRSA may be responsible.

    The facts about MRSA?

    MRSA stands for methicillin-resistant Staphylococcus aureus. The bacteria Staphylococcus aureus is extremely common. It is present on the skin and frequently in the nose of most healthy humans and animals.

    The bacteria become resistant to methicillin, an antibiotic developed and used for human treatment nearly 50 years ago. The strains of bacteria resistant to this particular antibiotic, could not be eradicated by other forms of antibiotic. As a result many bacteria we have to deal with in human and veterinary diseases are no longer easily controlled and eradicated by antibiotics that have been tried and tested for years.

    Identifying treatment

    MRSA is not a single group of bacteria. By using advanced laboratory techniques, we can group MRSA into a variety of sub-types. This typing is extremely useful to understand where the infection is coming from, plus, most importantly, how we can control it.

    We know from the MRSA found in dog and cat infections that the small animal disease is very similar to that seen in human hospitals, with the carriage and transmission of the bacteria essentially the same.

    While MRSA infections are resistant to many common antibiotics, they are usually sensitive to some medicines. Therefore, it is worth taking samples to identify the bacteria present and the antibiotic to which it is sensitive. We know from work done in the USA that in excess of 80% of horses with clinical MRSA make a full recovery after treatment.

    From humans to horse?

    The MRSA seen in human hospitals and dog and cat infections has, so far, rarely been seen in the horse. Our understanding of how MRSA is carried and potentially transmitted in the horse is growing. The type found in most UK horse infections is not a common form and is rarely the cause of human MRSA infections.

    The UK bacteria appears to be rarely carried by people, and may, in fact, be a horse-adapted strain. What is not known is where those horses who end up with clinical MRSA infections managed to pick up the bacteria.

    There are gaps in our knowledge, in part because the disease seems to act differently in different parts of the world. In North America, there appears to be a type of horse-specific MRSA, only rarely seen to cause human infections, but is carried (in the nose) by people who work with or handle horses.

    Is my horse at risk?

    Fortunately, clinical MRSA disease in the horse is rare and sporadic, and therefore not a major cause for concern. That said, it is vital that owners and vets are aware of the possibilities of such infections and treat potential cases cautiously.

    Any horse with a bacterial infection that does not respond as expected to antibiotics should have samples taken to identify the possibilities of antibiotic resistance. If MRSA
    is confirmed, strict hygiene precautions are essential.

    Most precautions are no more than sensible nursing care, such as frequent hand washing, wearing of disposable gloves while dealing with the infected area, isolation from other horses to prevent ongoing transmission and bandaging of the infected area to prevent infected pus contaminating the environment.

    While treatment can be difficult and expensive, there is no reason why the majority of such cases should not respond and return to normal function.

    The future

    Our knowledge of the disease is increasing all the time. What is certain is that problems with bacteria are going to become greater, and the diseases will change over the coming years. The bacteria involved have the ability to alter and adapt to different conditions and the infections are likely to change in a short time.

    If we are to understand MRSA, it is vital that ongoing investigations identify how the bacteria are adapting over time. With this information, we can be best placed to tackle what is likely to be an increasing problem to vets in the future.

    This veterinary feature was first published in Horse & Hound (8 February, ’07)

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