You're supposed to pick up after your dog. You know this. Everybody does. But it's late and you're tired and Fido just left a particularly nasty looking pile. Nobody's looking. Just leave it there. What's the worst that can happen?

Infection with a zoonotic disease, that's what. At least, that's according to US Environmental Protection Agency researcher Hyatt C. Green. In a recent issue of the journal Environmental Science and Technology, Green and colleagues examine the public health risks associated with what might be called the poo problem.

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Here's the issue, according to Green. "Canine fecal waste," he writes, "may represent one of the largest unregulated sources of aquatic fecal contaminants with zoonotic potential in areas impacted by urban runoff." Let's unpack that. There are nearly 70 million domestic dogs in the United States. Nearly two in five human diseases can also infect domestic pets, which means that dogs can act as reservoirs for nasty bugs like E. coli, Campylobacter bacteria, and the parasite Giardia duodenalis. And it's mainly on the honor system that owners pick up after their dogs.

And those infections can travel in both directions – from human to dog, from dog to human. In 2004, CDC researchers were called in to treat a 31-year-old nurse who had MRSA. The puzzling thing was that after being treated, the nurse became infected again. It turned out that the woman transmitted the bacterium to her dog, and then the dog re-infected its owner!

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What Green and his colleagues were interested in was the extent to which dog-derived diseases made their way into our waterways. There are, of course, multiple sources of water contamination, and multiple types of fecal contaminants. Besides dogs, in cities there's sewage (human fecal contamination) and in agricultural areas there is livestock waste to consider (cows, horses, pigs, sheep, and all the rest), plus the contaminants that might remain in manure. In some watersheds, wildlife like birds and deer may, as Green puts it, "also contribute to aquatic fecal loads."

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The most common method for identifying dog-related contamination in water is to look for evidence of the dogs' gut bacteria, some of which wind up in their feces. But the gut microbiome of pets and their owners overlap quite a bit, making it hard to know when a particular contaminant came from a human source or a canine one. So a more specific test was required.

After working their way through lots of canine and human poo, the researchers found themselves with just eleven DNA markers that were present in most of the canine samples, but absent in the human samples. That there was such a low success rate – 11 of 92 possible markers – also confirms the idea that humans and their pets have overlapping microbiomes.

To prove that their new, more specific test would more effectively screen water for traces the beasties living in dog poo, the researchers conducted a "proof of concept" study. They collected stormwater runoff from an urban garden known to be a favorite for dog owners during and after a 27-hour-long rainstorm. And it worked! They successfully detected many of the 11 genetic markers they identified as being primarily associated with dog poo.

Giardiasis, or beaver fever, involves diarrhea, vomiting, and stomach pains and can lead to lactose intolerance and to Vitamin B12 deficiency. Infection with E. coli can cause gastroenteritis, urinary tract infections, and in some cases neonatal meningitis. Campylobacteriosis can lead to bloody diarrhea (also known as dysentery), fever, and abdominal pain.

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When it comes to protecting the quality of the water we use, it's important to be able to identify the source of the contamination for any particular lake, river, pond, or stream. Only after the primary sources of contamination have been identified, can steps can be taken to prevent that contamination from continuing to occur.

The researchers point out that "despite the zoonotic potential presented by dogs, the management of dog waste is largely left up to voluntary owner responsibility, likely leading to a large proportion of dog fecal matter left in situ that can enter waterways via stormwater runoff." Because when nobody's looking, owners can get lazy. The solution is really, dizzyingly, mind-bogglingly simple: clean up after your pets.

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[Environmental Science and Technology]

Header photo: marcos ojeda/Flickr