Parasites adapt to host's immune system, study shows

The parasites that cause diseases like elephantiasis and river blindness develop and reproduce faster and in greater numbers when they encounter hosts with a strong immune system, new EU-funded research reveals. Writing in the journal PLoS Biology, the scientists from France, the UK and the US note that the findings could have implications for efforts to develop vaccines against these debilitating diseases.

EU support for the work came from a Marie Curie grant as well as from the VARBO (Vaccination against river blindness) and SCOOTT (Sustainable control of onchocerciasis today and tomorrow) projects, which were funded under the international cooperation budget lines of the Fifth and Sixth Framework Programmes (FP5 and FP6) respectively.

Some species are able to adapt their development and reproduction strategies to their environment. For example, the water flea Daphnia adjusts the age and size at which it becomes mature in response to the presence of predatory fish. For some time, scientists have suspected that the tiny filarial parasites behind diseases that blight the lives of millions of people worldwide may have a similar strategy.

To find out, the researchers altered the immune systems of mice in different ways and then infected them with filarial parasites. Their experiments revealed that the parasites are indeed able to adjust their development and reproduction strategies to the presence of immune cells called eosinophils that attack the invading parasites.

When the host's immune response is strong, meaning that the parasites' mortality levels are likely to be high, the worms produce offspring earlier and in greater numbers. Furthermore, the parasite determines which strategy to apply within hours of infection.

The findings have implications for the development of vaccines against these diseases. Currently, most vaccines are designed to boost the immune system to help it fight off the parasites.

"Most vaccines mimic the natural immunity of people, but our research suggests this approach could be counterproductive for some parasitic diseases," explained Dr Simon Babayan of the University of Edinburgh in the UK, the lead author of the paper. "We hope this latest finding will help inform the design of future vaccines against these infections. Clinical trials analyse the impact of potential vaccines on host health; we suggest they should also focus on their impact on parasite life history."

According to the researchers, further work is needed to fully understand the mechanisms involved in the parasite's ability to adapt its reproductive strategy to the strength of the host's immune system. Meanwhile their findings form an important contribution to an international project to create a vaccine against these diseases.

Filiarial parasites infect around 200 million people worldwide, causing devastating diseases such as lymphatic filariasis and river blindness. The minute worms are transmitted by flies and mosquitoes. In lymphatic filariasis, the parasites lodge in the lymphatic system where they cause unpleasant swellings that make it hard for those affected to work and have a social life. According to the Global Alliance to Eliminate Lymphatic Filariasis, over 120 million people worldwide are infected with the parasite, and 40 million of them are incapacitated or disfigured by the disease.

River blindness (onchocerciasis) is caused by a parasitic worm called Onchocerca volvulus that spreads throughout the skin and eyes of affected individuals. When the worms die, they cause lesions in the eye as well as terrible itching. Eventually, these lesions can lead to blindness while the skin problems can leave patients disfigured.

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