Page last updated at 23:04 GMT, Saturday, 14 June 2008 00:04 UK

Study cracks amoeba attack tactic

An amoeba in the colon
An amoeba is capable of evading the immune system for a long time

The tiny creature behind tens of thousands of dysentery deaths each year has a crafty method of slipping past our immune system, claim researchers.

US scientists say amoebae can get rid of giveaway chemicals on their surface.

The study in the journal Genes and Development suggests a similar technique helps malaria parasites get into human cells.

A UK specialist said amoebic dysentery, once diagnosed, is curable but the findings could aid vaccine development.

In theory, this idea could help people who are trying to work on a vaccine
Dr Graham Clark
London School of Hygiene and Tropical Medicine

It is suspected that the number of people infected by amoebae amounts to millions worldwide.

Most of them will never suffer bloody diarrhoea, which is the first sign of amoebic dysentery, an infection which kills approximately 70,000 people each year.

In most symptomless cases, the body's immune system eventually gets rid of the infection, but it can persist for years on end.

Researchers from Johns Hopkins and Stanford universities in the US believe they have found out why the single-celled organism is capable of evading the immune system for so long.

Existing research on the plasmodium malaria parasite revealed that it used a type of cell chemical called a "rhomboid enzyme" to help it get into the host cell.

A scan of the DNA of other parasites revealed the same chemical in amoebae, and led to the discovery this chemical was capable of getting rid of a protein called lectin found on its surface.

Surface security

Normally the immune system works out the difference between friend and foe by looking for "foreign" surface proteins and, by cutting them loose, the amoeba is able to stay undisturbed.

Dr Sin Urban, who led the study, said: "This is the first enzyme to be identified which looks like it could mediate immune system evasion."

Now the hunt could be on for drugs which specifically target the rhomboid enzyme.

Dr Graham Clark from the London School of Hygiene and Tropical Medicine, said that while effective treatments for amoebic dysentery did exist, it was often hard to identify, and could be mixed up with bacterial infection or even Crohns disease.

"In theory, this idea could help people who are trying to work on a vaccine.

"But if you understood how these proteins are being 'sloughed off', that could help you get around this process."

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