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A deadly parasitic worm that quietly inhabits the guts of humans the world over has, according to some researchers, been the most neglected of the neglected tropical diseases—and health experts are finally hoping to raise its profile to stamp it out.
Infecting an estimated 370 million people, the tiny worm surpasses the worldwide reach of malaria each year. And in the right conditions, it can abruptly turn deadly, Australian health experts note in a commentary this week on The Conversation. Though it’s largely linked to the developing world, it can make its way into anyone, anywhere. In some Indigenous Australian communities, the infection rate is as high as 60 percent, the experts report. And it pops up in disadvantaged areas of the US and Europe.
The worm is Strongyloides stercoralis, a “threadworm” nematode that causes strongyloidiasis. S. stercoralis has a peculiar life cycle that involves unfortunate humans coughing up, then swallowing larva after they invade the blood stream. From there, adult S. stercoralis discreetly inhabits mucus tunnels in the small intestines, sometimes for a person’s whole life. It often causes no symptoms. But when it does, symptoms are often mild and vague, such as cramps, diarrhea, weight loss, or a rash.
That all changes if the infected person suffers a blow to their health that compromises their immune system or if they take immunosuppressive drugs, such as some steroid drugs. In those cases, the worm can abruptly cause a life-threatening “hyperinfection” and spread throughout the body. In a hyperinfection, the worm’s reproduction shifts to ludicrous speed, resulting in an overwhelming number of worms. These can disseminate, spreading far beyond the intestines, causing massive full body infection. A disseminated hyperinfection can cause death in up to 87 percent of cases.
A parasitic drug called ivermectin can defeat the S. stercoralis, but it may be a tough battle. Some worms are showing up with drug resistance now, researchers report. And if a single worm remains, it can asexually reproduce and reignite an infection. Also, the immune system doesn’t develop an effective response to the worm, so a person can be re-infected.
It’s unclear how exactly S. stercoralis survives in environments to infect or reinfect a human host. But researchers think it can survive for a long time outside, including in contaminated soil or water. Researchers think it often enters a host through skin—it burrows in and makes its way to the bloodstream. Once there, it hitches a ride to the lungs, where it gets coughed out into the throat and swallowed. The worm then settles into the small intestines.
S. stercoralis is tricky to detect in humans. Blood tests for an immune response to the worm can be inaccurate. And searching through stool for worms can also result in false negatives because the worm is not consistently shed into feces.
Taken all together, there are too many questions, unknowns, and infections to ignore, the Australian health experts conclude. They call for more research on the worm’s spread and drug resistance.
“However, the biggest challenge in fighting this disease,” they write, “is that many people have not even heard of the strongyloides worm.”