Friday, 22 March 2013

Disseminated Strongyloidiasis - Interesting Things to Note

A recent case of disseminated strongyloidiasis, reported by the New England Journal of Medicine highlighted a couple of intriguing features of this catastrophic disease: in the absence of an effective immune response, worms can mature in the lungs as well as the intestine, and, there's something special about corticosteroid drugs that makes an unfortunate success of the worm.

Adult Strongyloides stercoralis in the Lungs

The case report of disseminated strongyloidiasis by Schroeder and Banaei describes adult worms, both rhabditiform and filarifom larvae, and ova containing active larvae in a tracheal aspirate. A similar case is reported by Bava et al. Typically, you'd find only filariform larvae in the lungs, and adults are almost never seen, even in the stool, because the adult females spend their time migrating through the tissues of the intestinal lining (and there are no parasitic males).

A larva of Strongyloides stercoralis. At a later stage, this larva would
be capable of penetrating and migrating through tissues like skin, or the
lining of the intestine. Image: CDC

I assumed that the adults found in this case had actually matured in the lungs rather than migrating there from the intestine. A 2004 paper in Clinical Microbiology Reviews agrees. Keiser and Nutman write “...findings suggest that filariform larvae develop into adults in the lungs... This hypothesis is supported by... autopsy studies showing adult worms in lung tissue.” In this scenario, the parasite could be multiplying very rapidly with new worms originating not only in the intestine, but in the lungs as well. One can only imagine the numbers of parasites that could be present within a short period of time.

Corticosteroids and Strongyloides stercoralis

We know that corticosteroids can initiate disseminated strongyloidiasis. But do they just give the worms a green light by suppressing immune response, or do they actually favor the parasite? Corticosteroids prevent production of eosinophils and cause the rapid destruction of eosinophils that already exist; these cells are part of the body's immune response to parasites. But it's thought that corticosteroids actually contribute to the success of S. stercoralis in another way. Gary Simon writes in Medical Parasitology that “they may stimulate female worms to increase larval output and promote molting of rhabditiform larvae into the invasive filariform larvae.”

Eosinophils in the blood are part of the
immune response to parasites. Image by
Iceclanl. (cropped)  CC BY-SA 3.0

So it looks like giving corticosteroids to a patient with S. stercoralis expands the “territory” in which the parasite can reproduce, hobbles the immune system's attempts to control it, and boosts the worms' fecundity and maturation. Given all of this, it's easy to see why it might be difficult to save a patient suffering from disseminated strongyloidiasis, unless the problem is discovered quickly. It also sheds some light on why other types of immunosuppression are relatively less catastrophic.



Bava  BAJ, Cecilia D et al. “Adult Female of Strongyloides stercoralis in Respiratory Secretions.”, Asian Pacific Journal of Tropical Biomedicine 3:4, April 2013, Pages 311–313

Keiser PB, and Nutman TB. Strongyloides stercoralis in the Immunocompromised Population.” Clinical Microbiology Reviews. 17:1, January 2004, 208–217.doi:10.1128/CMR.17.1.208-217.2004

Castelletto ML, Massey HC Jr et al. "Morphogenesis of Strongyloides stercoralis Infective Larvae Requires the DAF-16 Ortholog FKTF-1." , PLoS Pathogens 5(4): e1000370. doi:10.1371/journal.ppat.1000370

Schroeder L, and Banaei N. Strongyloides stercoralis Embryonated Ova in the Lung.” New England Journal of Medicine: March 21, 2013; 368:e15

Simon, G. “Strongyloidiasis.” In: Medical Parasitology. Satoskar AR et al eds. Austin: Landes Bioscience; 2009, pg 31

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