Scientists nab water-polluting parasite.

Scientists nab water-polluting parasite

When students started pouring into her infirmary at a rate of 200 per day with diarrhea that lasted for weeks, physician Mary R. Miles, who directs the health center at West Georgia College in Carrollton, knew something was wrong. Most cases of "stomach flu" are caused by a virus and last just 24 hours, but this illness would flare and subside repeatedly for weeks.

Miles alerted state health authorities in mid-January 1987, triggering a state and federal investigation of what ultimately emerged as a widespread community illness caused by a parasite polluting the public water supply. Perhaps even more disturbing was the fact that Carrollton's water met safe-water standards set by the Environmental Protection Agency and the state.

"Even a filtered water system, operating under current EPA and state regulatory standards, can potentially be contaminated by this parasite," says Edward B. Hayes, the lead epidemiologist sent by the Atlanta-based Centers for Disease Control to investigate the case. Hayes and a team of state and federal scientists report in the May 25 NEW ENGLAND JOURNAL OF MEDICINE that the outbreak stemmed from a microscopic protozoan called Cryptosporidium. Lasting from Jan. 12 to Feb. 7, 1987, it struck an estimated 13,000 people in Carroll County, a community of about 65,000.

Hayes and his colleagues surveyed Carroll County residents by telephone from Jan. 31 to Feb. 2. They found that 299 (61 percent) of 489 household members exposed to the public water supply had experienced a bout of gastrointestinal illness in January. In contrast, only 64 (20 percent) of 322 people who lived or worked in areas not serviced by Carrollton's public water reported diarrhea during the same period:

Laboratory studies revealed Cryptosporidium in stool specimens taken from 58 of 147 patients with gastrointestinal illness during the outbreak. No other pathogen was implicated in the illness, the researchers say.

Hayes and his colleagues also identified Cryptosporidium in water samples obtained from the municipal water system on Jan. 28, Feb. 4 and Feb. 5. Three samples of treated water taken from a nearby town on a separate water system were negative for Cryptosporidium.

As for the source of the contamination, the probe never came up with a definitive answer. Hayes suspects the blame lay with infected cattle bathing in a river that supplies Carrollton's water. But an open sewage spill discovered later that winter near the treatment plant also contained Cryptosporidium, he notes.

The town's treatment plant met EPA safe-water standards during the entire episode, but the researchers believe inadequate filtering methods allowed the parasite to slip into the treated water, Hayes says. EPA requires plant personnel to measure water turbidity, a crude gauge of the particles floating in water. Carrollton's daily reading met EPA's standard, but the researchers suspect the plant's filters let occasional clumps of debris pass through. The clumps may have been diluted by the time they reached the turbidity monitor, allowing the plant to remain within EPA standards. Carrollton has since tightened its filtering methods and now gets high-quality water that far surpasses EPA standards.

The case fuels longstanding concerns about whether EPA's turbidity guidelines are tough enough to ensure that treatment plants snare passing microorganisms (SN: 7/6/85, p.4). "The Carrollton outbreak would seem to point out that if you're just meeting [EPA] standards, it's probably not adequate," says Dennis D. Juranek, an epidemiologist at the Centers for Disease Control and a coauthor of the paper. Cryptosporidium is an extraordinarily hardy microorganism with a tough, egg-like shell that makes it impervious to chlorine treatment, he says. Treatment plants must fine-tune their filtering methods to keep the parasite from contaminating public water supplies, he contends. Hayes adds that further research is needed to determine the extent of Cryptosporidium contamination in rivers and streams feeding public water systems elsewhere in the nation.