The Threat of Waterborne Illness: Battling the Bugs

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In September last year, four people died as a result of Legionnaires’ disease they caught at a state fair in Fletcher, N.C. In all, 141 people were reported sick and 94 were hospitalized.

In the final report from the North Carolina health department, released a week ago, a hot tub display was identified as the prime suspect, although forensic proof was impossible to obtain. This incident and others like it are stark reminders of what can happen when opportunistic waterborne pathogens are inadvertently set loose. People get sick, and sometimes they die. Legionella, the cause of Legionnaires’ disease and Pontiac fever, arguably tops the list of dangerous waterborne organisms, but it is far from alone.

RELATED: Final Report Says Hot Tubs “Most Likely” Source of NC Legionella Outbreak


Roy Vore is technology manager for Biolab and a Ph.D. microbiologist with decades of experience in the pool and spa industry. He is passionate about the world of microorganisms and is one of the industry’s leading authorities on the pathogens common to pools and spas and how to kill them.

Legionella is the one pathogen that scares him. “Of all the bugs we commonly deal with in this industry, Legionella is the only one that has a universally recognized high fatality rate,” Vore says. “I’m a microbiologist and I don’t scare easily, but when we’re talking about under-disinfected spas/hot tubs, yeah, Legionella scares the hell out of me.”

RELATED: Lower the Risk of Legionellosis

In addition to Legionella’s proven lethality — according to the CDC, about one in 10 people who get it die — the way the disease is spread and its incubation period make it problematic to track and diagnose. “The pattern is different than what we see in some of the other diseases,” Vore explains. “A lot of these outbreaks occur when someone travels, which is why they call it ‘Traveler’s Legionella.’ They go somewhere where they’re exposed and maybe they’re only there for a few hours or a couple of days, but the symptoms don’t hit until three, four or five days later.

“It shows up like viral pneumonia, but the treatments don’t work for Legionella — and some even make it worse. So in an outbreak, they may not even identify what’s happening until they’ve lost three or four people. Later on, once they’ve identified it as Legionella, then subsequent patients get proper treatment and get better. But in a major outbreak it’s usually the first few patients that don’t make it.”

Once an outbreak has been identified, it can be extremely difficult to pinpoint the exact source. The outbreak that gave the disease its name, which occurred at a 1976 American Legion Convention in Philadelphia, was caused by a system, one of the most common sources. Others include shower plumbing systems, public fountains, solar water heating systems, drinking water systems and spas/hot tubs. In 2016, there were 6,100 reported short of the actual number due to confusion in diagnosis.

Legionella exists in natural bodies of water and is widespread across all types of water systems, natural and man-made. It has a history of surprising outbreaks including, as an example, an ongoing problem with truckers in Japan in which Legionella was developing in water in potholes and when splashed into the air, truck drivers were getting sick when inhaling the contaminated water droplets.

“That’s how insidious it is,” Vore explains. “Legionella is actually a parasite that invades amoebas, so it hides as it’s being spread. It’s the combination of factors that makes it so dangerous.”


While spas and hot tubs are not the most common source, they are unfortunately well suited to Legionella when not properly sanitized, due in part to the warm temps, oxygenated water, organic compounds in the form of bather waste and aerosolizing action of the bubbles and jets.

Like many waterborne microorganisms, Legionella often develops and thrives in biofilm where it is virtually impervious to chemical attack.

“Even if you raise the free chlorine level in a spa to 100 ppm, it only oxidizes the upper layer of the biofilm and the microbes you don’t kill use the oxidized compounds to grow and get stronger,” Vore explains. (He also points out that biofilm does form in properly sanitized water, often within seconds on a microscopic level.)

While Legionella present in biofilm is safe from sanitizers, it only becomes hazardous when released, or becomes “planktonic,” which can happen for a variety of reasons such as a change in water pressure or flow or even a change in water chemistry. “If that happens when the water is unsanitized, and somebody breathes it in, that’s when you have a problem,” Vore says. “That’s when people get sick.”

RELATED: Lethal Legionella Outbreak in North Carolina Linked to Hot Tub Display

The good news is that when Legionella is floating free, unprotected by biofilm, it is easily killed by recommended chlorine levels of 1-4 ppm. In other words, all roads in the battle against Legionella lead back to proper sanitization, Vore says. “But we know that there are going to be lapses in treatment at places like vacation homes,” he says. “This is where we need to raise awareness. Maintaining a recommended sanitizer level takes care of it, but the risk of under-sanitized water is greater than most people realize and as an industry we need to take this very, very seriously.”

The CDC promotes preventive measures in its “Operating Public Hot Tubs” fact sheet, including maintaining free chlorine at 2-4 ppm and bromine at 4-6 ppm, frequent testing, maintaining water balance, scrubbing surfaces and plumbing to remove biofilm and enforcing bather limits, among many other common practices.


In terms of public awareness, there’s no question the star of infamy shines bright on cryptosporidium, i.e. crypto. The CDC estimates that approximately 750,000 people are infected with crypto each year, with only about 2% reported. Crypto can cause weeks of severe diarrhea and can be fatal in immune-compromised or elderly patients.

While not as manifestly deadly as Legionella, the nature of crypto — its omnipresence, its complex oocyst lifecycle (it develops in the intestines and spreads through fecal release incidents in pools) and its resistance to chlorine and bromine make it a formidable adversary. In addition, crypto outbreaks draw extra media attention with gruesome stories about “poop in pools,” the unsavory means by which crypto is spread.

As is the case with Legionella, the time between exposure and becoming symptomatic makes immediate detection almost impossible, meaning the presence of crypto in a body of water can go on for days or even weeks, exposing countless people. That nature of the disease makes it difficult to discuss.

“Let’s face it,” Vore says, “people have a hard time talking about body issues, especially when it involves an issue like diarrhea. But that has to be part of the solution, making people aware of the problem and getting them to avoid going in the water if they’ve had symptoms.”

RELATED: Ozone Mixing Key to Disinfection

Naturally, accidents will happen regardless of public awareness and the big challenge with crypto is that in any practical sense, it is completely resistant to chlorine. “About 14,000 times more so than Legionella, for example,” Vore says. “To get a two-minute kill rate for crypto, you’d have to fill a pool with 100% bleach. It just doesn’t work.

“So, we have to do something different,” he explains. “And, that means secondary disinfection as defined in the CDC’s Model Aquatic Health Code, which is ozone and UV. While those systems won’t eliminate crypto outbreaks completely because it takes time to treat the entire volume of water, they will limit the outbreak to that particular day.”

Of course, the big challenge in using ozone and UV is the cost of the systems, which can easily reach well into five or even six figures. “But what’s the cost associated with an outbreak in your facility, the legal exposure, the downtime and everything that goes along with making people sick?” Vore says. “People need to ask how much is health and safety worth?”

Another possible, rarely discussed solution, Vore says, is to filter out crypto. “It can be done with old-fashioned low-rate sand filtration and flocculation with alum. The problem, of course, is that low-rate sand filtration was replaced by high-rate sand filters in the ’60s. But high-rate doesn’t work, probably because the water simply moves too fast. But it is possible to do it with low-rate sand filtration.

“One way or another, dealing with crypto does mean we have to do something different,” he concludes. “These bugs have a billion years evolution on their side, and all sorts different characteristics we need to consider if we’re going to stop them from making people sick.”

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