I received the following comment in response to a recent E. coli O157 and petting zoos post, and thought that it merited a post of its own.
"Why doesn’t the petting zoo owners have a fecal swab sample taken from each animal in the petting zoo and submitted for STx PCR screen testing. If a positive is found isolate the animal and continue to monitor it. I would assume The University of Guelph’s extension service would have some information about this."
That’s a great question. When we start talking about infectious disease risks, people often ask about testing. However, testing is not always useful and I think that’s the case here. Here’s a few reasons why:
- Animals don’t shed E. coli O157 all the time. It’s been shown in cattle that if you sample animals regularly, you will find the bacterium in the manure some days but not others. Therefore, a single negative result does not mean that the animal is definitely negative.
- No test is 100% accurate. While current tests are quite good, it cannot be stated with absolute certainty that an animal that is negative on a test for E. coli O157 is truly negative.
- Even if the animals are all truly negative for E. coli O157, they may still be shedding other potentially harmful microorganisms (e.g. Salmonella, Campylobacter, Cryptosporidium ) for which people need to take the same kind of precautions as for E. coli.
- If petting zoo operators had to pay hundreds or thousands of dollars a year per animal for diagnostic testing (a reasonable estimate if they had to do multiple tests on each animal, possibly for multiple organisms), there wouldn’t be many petting zoos around.
For any test, whether it’s being used for screening or to make a diagnosis on a sick animal, it’s critical that it be thought of in terms of "what will I do with the results." In this case, negative results would not change recommendations for running or visiting a petting zoo. I’d assume that animals could still be shedding E. coli O157 intermittently, or that they could be shedding various other pathogens, and I’d still recommend use of good infection control practices like hand washing. Efforts are best spent working on petting zoo design and hand hygiene, rather than testing the animals, because these are more likely to have a positive impact by reducing the risk of disease transmission.
Photo source: http://www.microvet.arizona.edu/Faculty/songer/diag.htm