Scientists have shown how flesh-eating parasites¹ responsible for the disfiguring tropical disease leishmaniasis dupe the immune system.

科学家展示了可以导致热带地区疾病黑热病的食肉寄生虫如何骗过免疫系统。

The parasites are carried by the Sand fly

The parasites produce a gel（凝胶） which the latest study shows can fool specialised immune cells into feeding rather than killing² them.

It is hoped the findings could aid development of a vaccine³（疫苗） for a disease which affects 12m people a year.

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

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

Symptoms include disfiguring and painful skin ulcers⁴（皮肤溃疡）, and in severe cases the infection can also spread to the internal organs.

Patients with the infection often suffer from social exclusion⁵（社会排外） because of their disfigurement（外貌损伤）.

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（利什曼虫） parasites are carried in the guts⁶ of sandflies（白蛉）.

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（喷回，流回） this gel plug, which enters the skin alongside the parasites.

The latest study - carried out on mice - shows that the plug acts to entice⁷（诱骗，引诱） immune cells called macrophages（巨噬细胞） to the bite site.

Macrophages usually kill invading pathogens（病原体） by eating and digesting them.

But the gel persuades macrophages to engulf⁸（吞没） 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（利什曼病） infection because they injected parasites directly into tissue without including the gel plug.

He said: "Our research shows that Leishmania parasites are very cunning（狡猾的，巧妙的） - they make their own gel to control the human immune system so they can establish a skin infection."

Synthetic⁹ version

Dr Rogers said work suggested a synthetic（合成的，人造的） version of the gel might offer from protection against infection.

The researchers found that the gel attracted 108 times more macrophages to the bite site than a saline（含盐的，闲的） 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 Pharmacy¹⁰, said there had been several clinical trials of potential vaccines¹¹, 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 saliva¹²（唾液） 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 of new findings, the benefits from this work are likely to be long term."