Tracking Sources Of Fecal Contamination In Drinking Water
Waterborne pathogens can cycle between the environment and their human and animal hosts, causing illness in people and spreading disease between households. To determine how fecal matter contaminates groundwater, scientists use indicators that specify the source, identifying it as human or animal waste.
University of Wisconsin-Madison microbiologist Sharon Long discussed how she and staff at the Wisconsin State Laboratory of Hygiene use unique characteristics of microorganisms within or on a person's body — her or his microbiome — to track the source of fecal contamination of drinking water in a Nov. 5, 2014, talk at the Wednesday Nite @ The Lab science series on the UW-Madison campus.
Humans, cows, agricultural poultry and wild geese can shed pathogens through their urine and feces, Long said. Humans shed norovirus for up to six months after their 24- to 48-hour symptoms of vomiting and diarrhea subside. Pathogens from fecal waste can migrate into drinking water.
To determine whether drinking water is safe, it is monitored for what Long called "indicator organisms" that are highly associated with the pathogen sources. These indicators include total coliform bacteria, thermotolerant (fecal) coliforms, E. coli, fecal streptococci/enterococci and bacteriophages.
To track a pathogen's source — farm or human — Long and her staff use "fecal source tracking," a suite of about 25 tests that can identify sources of contamination and, at the very highest level, whether they were introduced by humans or animals. If trying to determine whether a pathogen was shed by cows or humans, Long would consider differences in their microbiomes. They might test a sample to determine whether it contains the artificial sweetener sorbitol, caffeine or milk. If it does not, the pathogen's source is likely nonhuman — the bovine menu does not include these items.
Long illustrated fecal source tracking by recounting her work in a residential area near where a farmer spread cow manure. After 3.5-inches of rain over 24 hours, several residents of a nearby housing development called the Wisconsin Department of Natural Resources to report their wells smelled like manure and asked the agency to test them.
The homes had private wells and septic systems.
"We wanted to make sure that we could distinguish between whether it was their own septic system that had gotten flooded so they needed to remediate that themselves, or if it was actually the farm field that had contaminated their well," Long said. "We have a human marker, a grazing animal marker, a human versus animal marker, a human versus veterinary marker, and then some animal markers."
Long found a lot of general contamination from grazing and other animals. Sorbitol was below detection limits, the caffeine marker was low, and other tests indicated dung from herbivores. The presence of atrazine suggested farm runoff. Ultimately, the fecal matter was traced to grazing animals and the homeowners qualified for state aid to remediate their wells.
Long's presentation was recorded for Wisconsin Public Television's University Place.
- Wisconsin has had outbreaks of hemorrhagic or toxigenic E. coli, legionella, norovirus, cryptosporidium and giardia.
- Unless one uses massive doses of antibiotics, E. coli are present in everyone's gut, but it is typically the benign form of E. coli, which are part of the human normal microbiota. Pathogenic E. coli are a variant that include toxigenic genes, especially the more publicized 0157:H7 serotype. Scientists classify six kinds of this toxigenic bacteria as "STEC," which stands for "shiga toxin-producing E. coli."
- Out of 100,000 people, on average of 0.72 individuals shed toxic E. coli, while and 36,000 of that same group shed norovirus for six months after their symptoms end.
- Calves are much more susceptible to pathogens than adult cattle. During calving season, up to 21 percent of cows shed E. coli, while 62 percent may have cryptosporidium. When calving ends, E. coli levels drop to 0.2 percent and cryptosporidium to 1 to 2 percent, depending on the farm. Meanwhile, wild geese can pick up cryptosporidium by foraging spilled grain — and about 15 percent of these birds are shedding cryptosporidium at any given time.
- After passage of the Safe Drinking Water Act, the U.S. Environmental Protection Agency established the total coliform rule requiring all public water utilities to regularly test their water for total coliform bacteria. Population size determines testing frequency — Milwaukee takes more than 400 samples a month, whereas a hotel's well might be sampled once per month. If a sample contains coliform bacteria, it must be retested for E. coli, enterococci or bacteriophages.
- Under the federal Clean Water Act, beaches are monitored for thermal tolerate or fecal coliforms, as well as specifically for E. coli.
- On risk of waterborne disease: "(T)he good news is we do a really good job in the United States at testing and monitoring and mitigating risk."
- On how silent shedding of pathogens confounds controlling them in the environment: "So one of the problems with norovirus is you're symptomatic for 24 to 48 hours, but many individuals can shed norovirus for up to six months after their infection without having any recurring symptoms."
- On how pursuit of a specific organism leads to creation of a fecal source tracking test: "So we've got some that indicate humans, some that discriminate between animals and non-human animals, humans and non-human animals, certain kinds of animals such as cows, the cryptosporidium genotypes can also identify raccoons and deer and different organisms like that, and then we've got a total microbial content."
- On using a selective and differential test method to isolate a problematic pathogen: "(If) I have a target, I want to do this because I want to make sure if it's going to be in domestic sewage and in septic systems but I'm not going to find it in the geese, in the cows, in the wild animals. ... So I've got to test that my target is where I think it is so I've got real positives and real negatives and not false positives."
- On a Door County restaurant with a broken septic system pipe: "I'm happy to report, they now have the cleanest water in Door County, and I would not hesitate to take my 3-month-old nephew to drink water there."
- On the value of fecal source tracking: "When used alone, traditional indicators are not enough to pinpoint a source of contamination. Fecal source tracking tools have to be used together in a weight of evidence approach to allow us to have certainty in our results."