An international team of scientists announced they
had unravelled the genome of the parasitic worm that causes
bilharzia, a disease that each year claims hundreds of thousands of
The breakthrough could throw open the way to new drugs to fight
a curse that rivals malaria and tuberculosis in many tropical
countries, they said.
In fact, the blood fluke's genetic profile suggests it may be
vulnerable to treatments already used for other diseases, the
Bilharzia, also called schistosomiasis or snail fever, occurs
when people bathe or wade in river or lake water inhabited by tiny
snails that are the parasite's intermediate hosts.
How bilharzia spreads
The snails release the microscopic, fork-tailed parasites into
the water, and these burrow into the bather's skin. From there, they travel in the blood to the urinary and intestinal organs, including the liver, where they mature. The
worms can grow up to a centimetre long and live
five to 10 years on average in humans, although the record is 40
Once inside the body, the parasites reproduce -- the female worm
lives inside the thicker male -- and release thousands of eggs.
The eggs are shed in urine and faeces, which enables them to
find their way back into snail-inhabited water, and thus the cycle
starts again. People infected with the fluke can suffer internal bleeding,
organ damage, diarrhoea and anaemia.
Around 210 million people have been infected in 76 countries,
according to a 2006 estimates, and the death toll is 280 000 each
year in sub-Saharan Africa alone. Parts of the Middle East, Brazil,
Venezuela and some West Indian islands are also badly affected.
How the research was done
The new probe, published in the British journal Nature, gathered
scientists from the United States and Europe in a years-long
endeavour to crack Schistosoma mansoni's genetic code.
The sequence "catapults schistosomiasis research into a new
era," said Matthew Berriman of Britain's Wellcome Trust Sanger
Institute, who co-led the research.
"It provides a foundation for understanding aspects of the
parasite's complex biology as well as a vehicle to try to
immediately identify new targets for drug treatment."
The creature's DNA comprises 363 million nucleotides, or
"letters" in the genetic code, encompassing at least 11 809 genes.
Analysis suggests that for all of its simplicity, the worm has
become a smart survivor in murky waters.
Evolutionary pressure has given it sophisticated sensors,
enabling it to detect a potential host through changes in
chemicals, light levels and temperature.
Current drug doesn’t prevent re-infection
Publication of S. mansoni comes alongside that of S. japonicum,
another source of the disease, by Chinese scientists. It marks the first time that members of Lophotrochozea, a
poorly-explored group of intervertebrates, have had their genome
A cheap, effective drug to tackle bilharzia already exists -- a
treatment called praziquantel.
The worry, though, is that it does not prevent people from
getting re-infected by bathing in infested waters, and reinfection
offers plenty of opportunities for the parasite to become
Opening up the genome thus opens up new paths for drugs that can
block the parasite at various points in the cycle -- the moments of
human infection and replication, for instance.
But there could already be promising parasite-busters in
existing drugs. By matching parasite genes to known drug molecules, the team
found more than 90 candidates, although these first have to be
thoroughly tested to see if the hunch is right.
"Chronic infection with schistosoma parasites makes life
miserable for millions of people in tropical countries around the
globe, and can lead to death," said Anthony Fauci of the US
National Institute of Allergy and Infectious Diseases (NIAID),
which supported the research.
"New drugs and other interventions are badly needed." – (Sapa, July 2009)