They say fact is stranger than fiction, and with Halloween just around the corner, what better time to explore the creepier side of science? You don’t need special effects, makeup and scary masks to feel a shiver down your spine, because all you need to do is dive into tropical waters to encounter an invisible threat. Microscopic creatures swimming in fresh water, waiting to find a human host to invade with the secret weapon they bring with them: the ability to penetrate the skin without leaving a trace, without causing pain, without the victim being able to notice anything. It sounds like the plot of a horror film, but instead it is daily reality for millions of people who live with one of nature’s most insidious parasites.
Silent invasion
Like a ghost walking through a wall, the parasitic worm Schistosoma mansoni it can pass through human skin without triggering alarm signals in the body. A study published last August in the Journal of Immunology reveals how the worm can achieve this by suppressing the neurons that transmit information, and avoid both pain and itching as it penetrates the skin.
It usually infests humans who dive into infected waters – even just dipping your toes in a lake can cause an infection – then the adult worms and larvae migrate through the host’s bloodstream to weaken the host’s immune system.
A global threat
Schistosoma mansoni belongs to the trematode family and represents one of the main causes of schistosomiasis, a parasitic disease that affects approximately 240 million people worldwide and more than 700 million living in endemic areas, concentrated mainly in sub-Saharan Africa, the Middle East and South America. The World Health Organization classifies it as the second most widespread parasitic disease after malaria, responsible for over 10 thousand deaths annually.
The life cycle of the parasite is complex. The eggs are expelled with human feces and, if they reach fresh water, they hatch, releasing the larvae that infect specific species of water snails. Inside these molluscs, the parasites multiply and transform into cercariae, larval forms equipped with a forked tail that allows them to swim.
The infiltration technique
When a person comes into contact with contaminated water, cercariae attach to the skin within minutes, then release enzymes that dissolve the outer layer of the epidermis and actively penetrate the dermis. The process takes about fifteen minutes and occurs in total biological silence.
As mentioned, research has shown how the parasite is able to secrete molecules capable of blocking pain and itch receptors, an evolutionary strategy that allows the worm to complete the invasion before the immune system can organize an effective response. Once inside, the parasite loses its tail and migrates through the blood vessels until it reaches the lungs, then the liver and finally the intestinal veins, where it matures and reproduces.
Symptoms of the disease
In the first few hours after infection, some people develop a dermatitis called “swimmer’s itch,” with small red spots on the skin. After four to six weeks, when the worms begin to lay eggs, more serious symptoms appear: fever, muscle pain, cough and bloody diarrhea. Eggs that become trapped in tissues cause chronic inflammation.
Over the years, schistosomiasis can cause permanent damage to the liver, intestines, bladder and other organs, while in children the chronic infection impairs growth and cognitive development. The disease can be treated with praziquantel, a cheap and effective antiparasitic drug, although cases of reinfection are very frequent in endemic areas.
A hope against pain
There could also be a silver lining, as scientists believe the molecules the worms use to block neuronal signals could inspire new pain-relieving techniques and represent alternatives to opioid drugs.
The opioid crisis has pushed research into safer painkillers, and the mechanisms that Schistosoma mansoni has perfected over millions of years of evolution could offer a natural model for muting pain without the devastating side effects of addiction.
