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.

 

Sources

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

Beauveria bassiana - A Fungus That Kills Bed Bugs

Beauveria bassiana is a fungus that is well known for killing insects. Spores of B. bassiana adhere to the cuticle (the outer protective covering) of the insect, begin to grow, and work their way through to the inner tissues. There the fungal growth continues, taking nutrients from the host's body until the insect dies. Because B. bassiana is lethal to many insects, including their larvae, it has been grown and distributed commercially for use in agricultural control of insect pests. It makes sense to wonder whether it could be used to control bed bugs as well.

Beauveria bassiana is grown for control of agricultural
pest insects. Image courtesy of Keith Weller.

Beauveria bassiana Kills Bed Bugs

A study by Alexis Barbarin et al tested B. bassiana against bed bugs, and the results indicate that the fungus is lethal to the pests: not only does it kill virtually all bugs that come in contact with it, infected bugs can carry it back to daytime hiding places and pass it on to other bugs that have not been otherwise exposed. Barbarin el al propose that B. bassiana might rid a bed bug infested building of its bugs.

Bed Bug Traps Using Beauveria bassiana

In their study, Barbarin et al exposed bed bugs to a mixture of oil and fungal spores on various surfaces, and found that jersey knit cotton transmitted the infection most effectively. Though further research is required, they propose that a fabric bed skirt impregnated with B. bassiana spores might be an efficient means of infecting a resident bedbug population. Presumably any trap designed so that all bedbugs climbing onto or leaving the bed would have to pass through it could be used to infect them with the fungus.

Is Beauveria bassiana Safe for Humans?

Beauveria bassiana is generally regarded as safe for humans and it's already being used for insect control applications all over the world without dire consequences for human health. This fungus is already naturally occurring in the environment. However, there's reason to be cautious with this approach. A study that tested fungi isolated from poultry barns found that B. bassiana has several virulence factors that potentially “increase [its] survival, growth, and propagation... in animal tissue.” Authors Taira el al comment, quite correctly, that otherwise harmless fungi can cause serious infections in people whose immune systems are already compromised. Such people include AIDS patients and organ donor recipients among others.

These grasshoppers were killed by Beauveria bassiana.
Fungal growth is visible on the insects' remains.
Image courtesy of Stefan Jaronski.

Cases of both deep tissue infection and skin infection caused by B. bassiana have been reported in the medical literature (Figueira et al). The possibility of skin infection, in particular, prompts second thoughts. Beauvaria bassiana does not wipe out bed bugs on contact: it takes time for the infection to kill. Therefore, bugs that have contacted the fungus will still visit the sleeping host to feed. And while feeding, they will create a break in the skin, often with severe irritation resulting, and possibly introduce fungal spores. This does not seem like a good plan.

Beyond the possibility of skin infection arising from contact with the bugs, a spore impregnated bed skirt would presumably contaminate a living space with fungal spores pretty thoroughly, and fungal spores are as hardy and as hard to eliminate as bed bugs are. Under the right conditions, they could remain viable for a very long time, possibly protecting against reinfestation by bed bugs, but also a potential hazard for the immunocompromised occupant of, or visitor to, the space.

Sources

Barbarin AM, Jenkins NE et al. “A Preliminary Evaluation of the Potential of Beauveria bassiana for Bed Bug Control.” Journal of Invertebrate Pathology 111 (2012) 82–85

Figueira L, Pinheiro D et al. “Beauveria bassiana Keratitis in Bullous Keratopathy: Antifungal Sensitivity Testing and Management.” European Journal of Ophthalmology 22:5 (2012) 814-818

Taira CL, Marcondes NR et al. “Virulence Potential of Filamentous Fungi Isolated From Poultry Barns in Cascavel, Paraná, Brazil.” Brazilian Journal of Pharmaceutical Sciences 47:1 Jan./Mar. 2011