I received a flyer from Zoologix, a company that offers various (typically unvalidated and unproven) PCR tests for animals. The flyer headline was "Pets can carry MRSA – but testing can help."
Testing in certain situations is useful, but this is almost always limited to diagnosis of animals with active infections (i.e. they’re sick). PCR is not a good way to make such a diagnosis, because the test doesn’t tell you anything about the bacterium’s susceptibility to other antibiotics. Screening of pets just to determine whether or not they carry MRSA is rarely needed, and currently there is no evidence that PCR is a reasonable test for this.
There are no validated PCR tests for MRSA in animals. We looked at using a human test in horses and it failed miserably. There are validated tests for use in people, and they are quite good: they accurately identify MRSA and differentiate it from other methicillin-resistant staphylococci and from methicillin-susceptible S. aureus. That’s critical, because you have to know what a positive test really means.
I called the company and asked what the test actually detects. They said it detects the mecA gene, the gene that confers methicillin-resistance to staphylococci such as S. aureus. However, this gene can be present in other staphylococci that can be found in many healthy dogs and cats (10-30% in some studies). It does not actually detect MRSA and a large percentage of samples that give positive results will be false positives. The tests that are used in humans are specifically designed to look at two things in combination: whether S. aureus is present and whether it has the mecA gene (methicillin-resistance). This is the right approach because it excludes all those other false positives. Detecting mecA alone is completely useless. It’s interesting that the flyer states "PCR testing is fast, effective and accurately differentiates MRSA from other bacteria – even other Staph strains." Based on what the company told me over the phone, with regard to the test they’re advertising, that’s a blatant lie.
This is an example of a combination of bad science and bad ethics. This company has no business marketing this test. It’s false advertising, because the test isn’t an MRSA test. Their justification for using it is similarly weak. Anyone thinking about using this test should run away quickly! The issues with this test (and others) should also be considered when deciding whether to use this company for any tests.
More (and accurate) information about MRSA can be found on the Worms & Germs Resources page.