Page last updated at 23:00 GMT, Tuesday, 25 August 2009 00:00 UK

How flesh bug fools immune system

Sand fly
The parasites are carried by the Sand fly

Scientists have shown how flesh-eating parasites responsible for the disfiguring tropical disease leishmaniasis dupe the immune system.

The parasites produce a gel which the latest study shows can fool specialised immune cells into feeding rather than killing them.

It is hoped the findings could aid development of a vaccine for a disease which affects 12m people a year.

The study, led by Imperial College London, appears in PLoS Pathogens.

This study may well prove to be of significant benefit
Dr Tim Paget
Medway School of Pharmacy

Leishmaniasis is a serious problem in tropical and sub-tropical countries.

Symptoms include disfiguring and painful skin ulcers, and in severe cases the infection can also spread to the internal organs.

Patients with the infection often suffer from social exclusion because of their disfigurement.

There is currently no vaccine against the disease and, although treatments are available, they are not always effective and access is limited in many areas.

Leishmania parasites are carried in the guts of sandflies.

The parasites produce a gel which turns into a plug which effectively blocks up the fly's digestive system.

When an infected fly bites a human it regurgitates this gel plug, which enters the skin alongside the parasites.

The latest study - carried out on mice - shows that the plug acts to entice immune cells called macrophages to the bite site.

Macrophages usually kill invading pathogens by eating and digesting them.

But the gel persuades macrophages to engulf the parasites, and feed them rather than digest them.

This happens within the first few days following infection, enabling the parasites to establish themselves and infect the skin.

Lead researcher Dr Matthew Rogers said previous studies might have failed to explain leishmaniasis infection because they injected parasites directly into tissue without including the gel plug.

He said: "Our research shows that leishmania parasites are very cunning - they make their own gel to control the human immune system so they can establish a skin infection."

Synthetic version

Dr Rogers said work suggested a synthetic version of the gel might offer protection against infection.

The researchers found that the gel attracted 108 times more macrophages to the bite site than a saline solution.

They also showed that the number of parasites that survived the first 48 hours following infection, and the number of host cells that were infected, were both eight times higher when the gel was present.

Dr Tim Paget, an expert in microbiology at Medway School of Pharmacy, said there had been several clinical trials of potential vaccines, but they had generated mixed results.

He said: "This study may well prove to be of significant benefit.

"It is known that vaccines raised against proteins from the saliva from the sandfly can give protection to infection.

"Thus it is very likely that this gel could be used a target for the development of a novel vaccine.

"However, like all new findings, the benefits from this work are likely to be long term."

Professor Simon Croft, an expert in parasitology at the London School of Hygiene and Tropical Medicine, said: "This is very exciting research and helps us understand a lot more about how Leishmania parasites establish an infection in mammalian hosts."

However, he added: "This is a long long way from a vaccine, but this type of knowledge could be useful in thinking about the design of vaccines for leishmaniasis."

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