The body's response to infection with one parasite may provide the perfect hiding place for another.

Many parasites modulate the host immune response to avoid being eliminated from the body or to limit the tissue damage they induce. Co-infection with different parasites is common, yet little is known about how infection with one affects the host's response to another.

Now a group from the London School of Hygiene and Tropical Medicine, led by Dr Quentin Bickle, has shown that mice infected with the parasitic worm Schistosoma mansoni (pictured) are unable to control infection with a protozoan called Leishmania donovani.

The immune response to S. mansoni results in the formation of granulomas – balls of inflammatory and immunomodulatory cells around eggs trapped in sites such as the liver. Interestingly, while co-infected mice did launch protective immune responses against Leishmania parasites present within the liver parenchyma, such protective responses failed to develop within the egg granulomas, which thus provided a protective environment for the Leishmania parasite to thrive and reproduce.

The results suggest that the presence of one parasite can have a significant impact on the success of a second species infecting the same host but that such effects may be very localised to the sites of infection.

Image: Parasitic worm, courtesy of the EM Unit/Royal Free Medical School.

External links

• Hassan MF et al. The Schistosoma mansoni hepatic egg granuloma provides a favorable microenvironment for sustained growth of Leishmania donovani. Am J Pathol 2006;169(3):943–53.