Americans love their dogs, and most people clean up after their pets when they are out on a walk, but some do not: people who claim they wouldn't pour toxic chemicals or medicines onto the ground because they recognize it gets into waterways delude themselves into believing dog excrement is "natural" and will be okay in waterways.

But it isn't. Bacteria and anti-bacterial strains from dogs can make people sick from dogs just like it does humans, and we recognize that humans should not go to the bathroom on the ground near a lake.

A new genetic test helps quantify how much dogs are contributing to this health concern. Orin C. Shanks, Hyatt C. Green and colleagues explain that our waterways are susceptible to many sources of fecal contamination, including sewage leaks and droppings from farm animals and wildlife. Contamination from dog feces is a concern because it can harbor antibiotic-resistant strains of E. coli and other bacteria and parasites that can infect humans — and there are nearly 70 million domesticated dogs just in the U.S. 

Scientists have had few tools to determine the extent to which waste from dogs is adding to the pathogens in rivers, lakes and beachfront surf. Current methods look for certain genes from gut bacteria that end up in dog feces. However, this is not foolproof — the microbiota of humans and the canine pets they live with often overlap, making the analysis complicated. So Shanks' team set out to create a more specific test.

The researchers developed a new genetic testing method to specifically detect canine fecal contamination in water. They identified 11 genetic markers that were common among most of the dog samples but missing from the human ones. To determine whether their method would work for real-world monitoring, they sampled storm water from a rain garden where people often walk their dogs. The technique successfully detected some of the same markers they had identified as evidence for canine waste.