Parasites hinder immunity against cholera
of Article: http://www.sciencenews.org/view/generic/id/42368/title/Parasites_hinder_immunity_against_cholera_
hitchhikers may render people more vulnerable to the microbe and might
lessen vaccines’ effectiveness
By Nathan Seppa
edition : Tuesday, March 31st, 2009
Harboring intestinal parasites
seems to limit a person’s ability to fend off cholera, a study conducted in
Bangladesh shows. The finding might explain why vaccines against cholera
have shown only spotty effectiveness and also suggests that vaccination
campaigns should be preceded by programs to wipe out parasites,
particularly intestinal worms. The study appears in the March PLoS Neglected Tropical
The bacterium Vibrio cholerae causes
cholera and is spread in unsanitary food or drinking water. In the past,
scientists had been puzzled when experimental cholera vaccines that induced
a strong response in Western volunteers failed to generate consistent
immunity when given to people in the tropics. Researchers initially
suspected poor nutrition or genetic differences, says pediatrician Jason
Harris of Harvard Medical School and Massachusetts General Hospital in
Boston. But in recent years, some came to wonder whether other infections,
including those caused by parasites that are prevalent in poor tropical
countries, might be hindering the immune response and therefore a vaccine’s
Harris and his U.S. colleagues
teamed with researchers at the International Centre for Diarrhoeal Disease
Research in Dhaka to collect and analyze blood and feces samples from 361
people who were brought to hospitals with severe cholera from 2001 to 2006.
The scientists found that 53 of the patients also had parasitic infections.
Those with intestinal worms had
markedly poorer antibody production against the toxin made by the cholera
microbe than did those without worms, the researchers report. These
responses against the actual microbe would suggest a similar effect in
people getting vaccinated.
The Vibrio cholerae toxin causes severe
diarrhea when it comes into contact with the intestinal lining, and
hospitalized cholera patients receive fluids to prevent life-threatening
dehydration. People naturally develop antibodies against the toxin and
against the microbe itself to the point that adults in endemic countries
such as Bangladesh become largely immune to cholera after repeated
exposures. Children initially exposed are most vulnerable.
The precise mode of action by
which intestinal worms lessen antibody production against the toxin is
poorly understood, the researchers say.
“Intestinal parasites do seem to
have influences beyond their own little niche,” says Thomas Nutman, an
immunologist at the National Institute of Allergy and Infectious Diseases
in Bethesda, Md. “This is among a growing list of studies that have
addressed the bystander effect — a spillover effect, if you will – of these
[worm] parasites,” he says.
Medication can clear intestinal
worms from the body, including roundworms, the parasites most frequently
detected in this study. Campaigns using drugs to de-worm large numbers of
people are under way in many parts of the tropics as health officials seek to
improve school-age children’s growth and cognitive development, Nutman
The new findings suggest that
these de-worming programs could have “an unexpected benefit” in communities
with endemic cholera by improving the effectiveness of cholera vaccination,
Cholera infects roughly 5 million
people worldwide each year and causes about 100,000 deaths.