In the summer of 1988 — the year Aquatics International first went to press — 44 people who had visited a Los Angeles swimming pool were afflicted with a gastrointestinal illness characterized by watery diarrhea, severe cramps and fever.

The Los Angeles County health department investigated, and two years later the Centers for Disease Control (as it was known prior to 1992), which had been tracking waterborne disease outbreaks since 1978, confirmed the culprit — the parasite cryptosporidium. Though outbreaks of crypto from drinking water had been reported since 1984, these were the first known cases associated with chlorinated recreational water.

Over the next few years, crypto began to show up more frequently in CDC’s waterborne disease outbreak reports, mainly in cases related to drinking water contamination. The parasite’s resistance to chlorine was already recognized, and discussions about prevention focused on improved filtration. But a national alarm was sounded in 1993 when more than 400,000 residents of Milwaukee became ill with crypto after drinking city water — the largest outbreak ever recorded in the United States to this day.

The EPA turned its attention to drinking water standards, added crypto to the list of federally regulated contaminants, and strengthened standards for disinfection and filtration — efforts that prevented further massive outbreaks from public drinking water.

However, crypto began to emerge as the single biggest cause of recreational water illness, or RWI, in treated-water venues. After 2004, outbreaks associated with pool water rose dramatically, reaching an all-time high in 2007. In the CDC’s latest WBDO report (published in September 2011 for the years 2007 and 2008), crypto is listed as the cause of 12,154 cases of recreational-water-related gastroenteritis — 87 percent of all RWIs of any type. Of these cases, 12,137 — 99.9 percent — involved treated water venues.

“The emergence of cryptosporidium has changed everything,” says Michael Beach, PhD., head of CDC’s Waterborne Disease Prevention Branch and a member of the steering committee for the developing Model Aquatic Health Code. “Since the 1920s, we’ve relied on a method where you disinfect swimming pools with halogens and you filter. That doesn’t work anymore. These outbreaks continue, so we’ve got to adopt a new model for operation. But the good news is, if we keep crypto at bay, we also take care of the other pathogens — giardia, shigella, norovirus, E.coli, Legionella. Now, when we see the outbreak of those other bugs, we know it’s about poor pool operation, and we know how to fix it.”

As any trained pool operator knows, keeping crypto at bay is a formidable task, especially in the peak of summer, when the water is crowded. The tiny parasite has some big numbers on its side. For example, where giardia has a CT inactivation level of 45 (it takes 45 minutes to kill the parasite at a free chlorine level of 1 mg/L), crypto’s CT value is 15,300, which means its oocysts, or spores, can survive for up to 10 days — even in a properly chlorinated pool. Plus, according to research, it takes fewer than 10 oocysts to infect a swimmer, yet the number of oocysts deposited in a diarrhea release might number in the millions — from 10 million to 100 million, according to one study.  

With odds like these — and the likelihood that diarrhea released in a crowded pool might go undetected — it’s virtually impossible to totally eliminate the risk of a crypto infection, even in a perfectly maintained operation.

“It comes down to instilling in every employee the importance of their diligence to health and safety,” says Terri Stroupe, aquatic facilities and program director at the Raleigh (N.C.) Parks & Recreation Department. Stroupe manages nine public pools, four of which are year-round indoor facilities. All together, the pools average 314,000 swimmer visits a year. So far, there have been no RWI outbreaks, but Stroupe is far from complacent.

“We respond quickly when there’s a fecal accident. The staff immediately evacuates the pool, and they notify me or the other manager on call. We determine the course of action, depending on whether it’s a hard stool — indicating the swimmer is healthy — or diarrhea. If it’s diarrhea, we treat the water as if there were crypto present.” This means the pool will be shut down while the water is superchlorinated and then returned to swimmable levels. There’s a fecal accident — mostly solid — about every two weeks during the summer at one of the pools, and once every six weeks in the winter, according to Stroupe.

Crypto outbreaks can spread quickly through a community as infected swimmers move from one pool to another. From pools, the outbreak can then move into other kinds of facilities, such as day-care centers, where it continues to disperse through person-to-person contact.

This was illustrated by an outbreak in Utah in summer 2007, in which some 5,700 cases were documented around the state. But what happened after the illness ran its course is perhaps more instructive for public health officials and pool operators concerned with prevention. “Before the 2008 swim season started, Utah health officials put together a big education campaign,” says CDC epidemiologist Michelle Hlavsa, a coauthor of the Utah report and head of the CDC’s Healthy Swimming program. “And based on follow-up surveys, it looks like it was successful — that residents of Utah understand better than residents of other states what their role is in preventing crypto, how germs are spread in the pool, and how to protect themselves and others.” In addition to swimmer education, Utah took another step, installing secondary disinfection at key venues.

“They added either UV or ozone technology in 75 of the largest facilities in the Salt Lake area,” Hlavsa says. “And until this past summer, they have had no further RWI outbreaks in Utah.”

While the two-pronged approach seems to have paid off, no amount of existing technology can guarantee swimmer safety unless the swimmers themselves change their behavior.

“What research tells us is that there’s this 20 percent of swimmers who think it’s OK to swim with diarrhea,” Hlavsa says. “We need to find ways to reach them. We’ve tried to be proactive here at CDC. But if public health officials can’t get to them, if pool operators can’t get to them, maybe it’s about encouraging that mother sitting there watching her child to tell the mother in the next lounge chair, ‘You know, you really shouldn’t be bringing your kids into the pool with diarrhea.’ It’s about getting a critical mass of people who understand and help get the message out.”