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
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 http://www.nejm.org/doi/full/10.1056/NEJMicm1204579
Simon, G. “Strongyloidiasis.” In: Medical Parasitology. Satoskar AR et al eds. Austin: Landes Bioscience; 2009, pg 31