Tuesday, June 14, 2011

MRSA from human to cows?

Do we have another case of animals being blamed for human disease, or in fact is the the opposite true?

Scientists, physicians, veterinarians and the general public are concerned about zoonotic diseases, or those diseases that are passed from animals to humans. A classic example is rabies, and much is known about how to treat and prevent zoonotic diseases.

Reverse zoonoses are diseases that are passed from humans to other animal species, and examples of those include mumps in humans that can become parotiditis in your family pet dog or tuberculosis in humans that is called the same disease if passed on to deer, dogs or elephants.

Another known reverse zoonoses is furunculosis (recurring boils) in humans that is caused by Staphylococcus aureus. When transferred to cows, it can cause Staph mastitis. Scarlet fever in humans is caused by Streptococcus pyogenes, and this Strep species can also cause mastitis in cattle.

There was a recent scientific journal article published in the prestigious journal, The Lancet Infectious Diseases in their Early Online Edition (doi:10.1016/S1473-3099(11)70126-8 http://bit.ly/kJzSAU) that identified a new kind of MRSA, and importantly it was found in both cattle and humans. MRSA is an acronym for Methicillin-resistant Staphyloccus aureus (http://bit.ly/WVHnK). There are many strains of Staph that fit in the MRSA category as they've become resistant to the methicillin antibiotics that have been used to treat them. Methicillin has been widely used in human medicine, but its use in veterinary medicine has been very limited.

The research article did a detailed DNA analysis on the Staph bacteria and it's clear that it’s not only a newly recognized MRSA, but that this unique strain was found in both cattle and humans. So far, it has only been found in the UK and Denmark. Much of the article was focused on the important implications for physicians who are trying to confirm a case of MRSA in a patient. It was estimated in the article, however, that approximately 2.8% of dairy farms may have this strain of Staph.

This important research article has triggered a number if 'news' items in publications like The Guardian, The Telegraph or The Independent, or online news aggregators like Natural Society or TopNews. Various blogs have jumped on the 'cow to human' claim and the 'Twitterverse' is rampant with links to this claim.

Only Reuters correctly indicated that the scientists did not know if humans were giving the MRSA bug to cows or if cows were a reservoir for humans.

The British group, Soil Association, is often referred to as the leading organization representing organic farming in the UK. They have published a policy paper on this topic that lists the limited evidence that the movement of the bacteria was from cow to humans.

Of course, prestigious journals like to tout their results, but back in 2002 and reported in 2007 the first case of identical MRSA in cows and humans was found in the Hungary.

The report in The Lancet was important for physicians trying to confirm diagnosis, but it seems to me that it's currently a bit early to say which direction this bug took. Whether it was from humans to cows or from cows to humans.

Hope that helps.