Mark Crislip, MD


June 29, 2011

Discussion: Leishmania

What is cool about Leishmania is that some strains are infected with a double-stranded RNA virus, LRV1. Strains that have LRV1 are more likely to cause metastatic disease. The pathophysiology has been described by Scott.[8] Leishmania parasites are transmitted to the mammalian host by a sandfly. Once inoculated into the skin, the parasites in the promastigote stage infect macrophages and transform into the amastigote form. The parasites live and divide within an endosomal compartment of the macrophage.

A recent study showed that some Leishmania parasites contain an RNA virus, termed LRV1, that promotes the production of proinflammatory molecules such as tumor necrosis factor α, interleukin-6, CXCL10, and CCL5. The response to the virus is mediated by the TLR 3 gene product (TLR3), an RNA receptor that is expressed within the same endosomal compartment as the parasites. It is hypothesized that viral RNA is released by dead parasites soon after infection; the binding of viral RNA to TLR3 results in the production of cytokines and chemokines that enhance inflammatory responses and thus exacerbate disease.[8]

This viral parasitism has been demonstrated only for Leishmania guyanensis in South America but is an interesting example of how pathogens can have pathogens that affect their virulence.

"Great fleas have little fleas upon their backs to bite 'em,
And little fleas have lesser fleas, and so ad infinitum."
-- Augustus de Morgan[9]


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