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Amoebas In Drinking Water: A Double Threat – Science News
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By Janet Raloff
Web edition : Friday, January 28th, 2011
Amoebas in drinking water: a double threat
Analysis reveals widespread, hidden contamination by the
sometimes lethal parasites
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 February 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
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.
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.”
SUGGESTED READING :
J. Raloff. Big water losses. Science News blog, October 22, 2008.
J. Raloff. The case for very hot water. Science News blog. October 23,
J.O. Falkinham, et al. 2008. Mycobacterium avium in a shower linked to
pumonary disease. Journal of Water and Health 6(2):209. Available
G.J. Kirmeyer and M.W. LeChevallier. 2001. Pathogen Intrusion Into
Distribution Systems [Project #436]. American Water Works
Association Research Foundation.
H.Y. Lau and N.J. Ashbolt. The role of biofilms and protozoa
in Legionella pathogenesis: implications for drinking water. Journal of
Applied Microbiology, Vol. 107, August 2009, p.
368. doi:10.1111/j.1365-2672.2009.04208.x. Abstract
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Amoebas In Drinking Water: A Double Threat – Science News
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CITATIONS & REFERENCES :
F. Marciano-Cabral, et al. Identification of Naegleria fowleri in Domestic
Water Sources by Nested PCR. APPLIED
ANDENVIRONMENTAL MICROBIOLOGY, Vol. 69, October 2003, p. 5864.
M.E. Shoff. Prevalence of Acanthamoeba and other naked amoebae in
South Florida domestic water. Journal of Water and Health, Vol. 6, p.
99. doi:10.2166/wh.2007.014. Abstract
G. Sandström, A. Saeed and H. Abd. Acanthamoeba polyphaga is a
possible host for Vibrio cholerae in aquatic environments. Experimental
Parasitology, Vol. 126, September 2010, p. 65.
J.M. Thomas and N.J. Ashbolt. Do Free-Living Amoebae in Treated
Drinking Water Systems Present an Emerging Health Risk?
Environmental Science & Technology, Vol. 45, February 1, 2011, p.
860. DOI: 10.1021/es102876y. Abstract at: