Amoebas — blob-shaped microbes linked to several deadly diseases — contaminate drinking-water systems around the world, according to a new analysis. The study finds that amoebas are appearing often enough in water supplies and even in treated tap water to be considered a potential health risk.
A number of these microorganisms can directly trigger disease, from a blinding corneal infection to a rapidly lethal brain inflammation. But many amoebas possess an equally sinister if less well-recognized alter ego: As Trojan horses, they can carry around harmful bacteria, allowing many types to not only multiply inside amoeba cells but also evade disinfection agents at water-treatment facilities.
Even though recent data indicate that amoebas can harbor many serious waterborne human pathogens, U.S. water systems don’t have to screen for the parasites, according to study coauthor Nicholas Ashbolt of the U.S. Environmental Protection Agency’s National Exposure Research Laboratory in Cincinnati. He coauthored a study of amoebas’ “yet unquantified emerging health risk” in the Feb. 1 Environmental Science & Technology.
He and Jacqueline Thomas of the University of New South Wales in Sydney analyze data from 26 studies conducted in 18 countries. All had identified amoebas in drinking-water systems. Some reports had focused on measurements at treatment plants, others in exiting water; some even extracted the parasites from tap water. Indeed, among 16 studies that looked for tap water contamination, 45 percent reported finding amoebas.
In 2003, Francine Marciano-Cabral of Virginia Commonwealth University in Richmond and her colleagues identified one species of amoeba that is directly lethal — Naegleria fowleri — in water throughout the plumbing of an Arizona home where two young boys had recently died. The amoeba explained the boys’ fatal encephalitis, a brain disease.
“We suspect they got it from submerging in the bathtub,” Marciano-Cabral says. The family’s private water supplies had not been chlorinated, a disinfection process that can limit amoeba contamination.
Thomas and Ashbolt reviewed six studies that together included data from 16 different water-treatment plants and probed for sources of the amoebas that the studies had turned up. Five of those studies reported finding a high prevalence of the parasites — in anywhere from 75 to 100 percent of the surface waters, such as rivers, that were sampled. After water treatment, often using carbon filtration or chlorination, contamination levels dropped somewhat, to fewer than 50 percent of water samples.
In general, the new analysis points out, water treatment appears to reduce amoeba concentrations to a tenth or one-hundredth of starting concentrations, “but breakthrough events do occur and release potentially high numbers of free-living amoebae” — roughly 110 of the parasites per liter — into drinking water distribution systems.
For instance, Megan Shoff of the Ohio State University in Columbus and her colleagues analyzed water from storage tanks above home toilets throughout Broward, Palm Beach and Dade counties in Florida. These free amoebas — ones not shielded by a slimy biofilm — turned up in 55 of 283 samples, or almost one in five. Eight samples contained Acanthamoeba, a type that other studies have associated with corneal infections in contact lens wearers.
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Such findings indicate that these amoebas either survived the upstream water-treatment plant or entered the community distribution system, perhaps through cracks in feeder pipes, Thomas and Ashbolt say.
Acanthamoeba is but one of several genera of amoeba that can harbor Legionella pneumophila, the bacterium responsible for virtually all cases of Legionnaires’ disease. Indeed, Ashbolt says, studies have shown that residing in an amoeba “increases the virulence of Legionella,” the leading source of waterborne disease in America. So if these bacteria have spent time in an amoeba host, he says, “they are more likely to be infectious in us.”
Gunnar Sandström of the Karolinska Institute in Stockholm is finding much the same with Vibrio cholerae germs. Cholera epidemics, he’s found, occur most frequently when the waterborne germs occur together with amoeba, including Acanthamoeba. And in the lab he’s shown that residence inside an amoeba increases the expression of 438 V. cholerae genes and reduces the expression of 396 others.
“We don’t yet know exactly what these genes do,” he acknowledges, but the end result is bacteria that survive better within amoebas — replicating to populations that can easily reach the 100 million cells that he says are needed to trigger human infection.
Sandström says his preliminary data show that “If we feed V. cholerae to one amoeba, the bacteria will grow until they reach around 100 cells. Then stop.” In the lab, if he then feeds one of those bacteria to a new amoeba, the bacteria won’t stop multiplying until populations inside the amoeba reach 10,000 cells. By continuing this iterative process, he has observed germ growth within a single amoeba of up to one billion cells.
He concludes that amoebas “appear to be a training ground for the Vibrio and key to the infectivity of cholera.”
The Thomas and Ashbolt paper “is a beautiful synthesis of prior work that was really much needed for progress on both the pathogenic-amoeba issue as well as for understanding Legionella disease” and the natural ecology of other bacterial diseases associated with home plumbing, says Marc Edwards of Virginia Tech in Blacksburg.
Amoeba contamination of drinking water probably should be regulated, Edwards contends, but can’t be until more data quantify the occurrence and risks associated with these pathogens. This new paper “is a critical first step” in that process, he says. Its synthesis of more than 100 studies “shows there’s just overwhelming evidence that this microorganism is occurring at levels that are a health concern in a large percentage of [water-distribution] sites.”