Lymph nodes found to host malarial parasites
Jan 23, 2006 - 4:03:37 PM

Malarial parasites develop not only in the liver - as believed until now - but also in lymph nodes, says a new study.

When a mosquito infected with plasmodium bites a mammal, the immature parasites travel to the animal's liver, which until now scientists thought was the only place where they could develop.

But researchers led by Robert Ménard of Pasteur Institute in Paris found that the parasites developing in an unexpected place: the lymph nodes, according to a report published on the website of the Howard Hughes Medical Institute (HHMI).

The researchers infected mosquitoes with fluorescent-tagged plasmodium parasites and then allowed the mosquitoes to bite a mouse. From each mosquito bite, they found an average of 20 fluorescent parasites embedded in the animal's skin.

They found that the parasites moved through the skin in a random, circuitous path at a speed that is amongst the fastest recorded for any migrating cell. After leaving the skin, the parasites frequently invaded blood vessels. However, many of the parasites also invaded lymphatic vessels, they found.

About 25 percent of the parasites injected by the mosquito bites were drained by lymphatic vessels and ended up in lymph nodes close to the site of the bite. Their journey seemed to stop there.

Within about four hours of the mosquito bite, many of the lymph node parasites appeared degraded. They were also seen interacting with key mammalian immune cells, suggesting that the immune cells were destroying them.

A small number of the parasites in the lymph nodes, however, escaped degradation and began to develop into forms usually found only in the liver.

Up to now, researchers believed that although both blood and lymphatic vessels take up plasmodium parasites, they all end up in the liver, Ménard said. "Nobody had proposed that they actually might stop" in the lymph nodes and develop there, he observed.

Understanding the intricacies of the mammalian immune response to plasmodium infection might help scientists create better vaccines, including vaccines that target parasites before they develop in the liver, Ménard said.

Parasite development in lymph nodes could even be one reason why there is so much tolerance to these parasites, he suggested.

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