When Parasites Go Pop!

Lathering up with topical creams before each swim may no longer be necessary to prevent schistosomiasis, a water-borne parasitic infection that kills an estimated 800,000 people a year. A mix of red cedarwood oil and surfactant--a compound that makes oil spread evenly on the water surface--can kill schistosome larvae by making them swell and explode, according to a new study.

Schistosomiasis, also known as bilharzia, infects 200 million people in Africa, the Middle East, and Asia. Repeated infections can cause severe and eventually deadly damage to the liver, intestines, lungs, bladder, and the brain.

After being released from infected aquatic snails, schistosome larvae swim to the water surface, where they wait for a mammalian host, pierce its skin, and slip into its bloodstream. There are topical creams that repel the larvae, but applying them every day is so cumbersome that most people at risk don't use them. Another strategy--killing the snails that carry the parasite--has produced mixed results so far. That's why Johns Hopkins microbiologist Clive Shiff and his post-doc Jean Naples decided to go after the larvae themselves, with a strategy that exploits their host-seeking behavior.

When larvae sense lipids in the water--a sign that hosts have arrived--they shed the outer coat that helps regulate their water uptake. If penetration doesn't ensue quickly, they swell and burst. Shiff and his colleagues observed that parasites also undressed when exposed to the unsaturated fatty acids of red cedarwood oil, a food and fragrance additive approved by the U.S. Food and Drug Administration.

Microscopic analysis revealed the fatal swelling, and while mice tails exposed to untreated schistosome-infested water were infected with an average of 12 parasites, only a single parasite was able to enter from infected water treated with cedarwood oil. A combination of the oil with a surfactant known as Tween 80 was even more effective, the team reports in the November issue of the American Journal of Tropical Medicine and Hygiene. The next step, Shiff says, is to begin field tests to show whether the product can be applied easily under real-world circumstances, probably in Africa.

"What we really need now is a simple method of control," says Peter Neal, an urologist studying schistosomiasis at Marshfield Clinic-Indianhead Center in Rice Lake, Wisconsin. "Red cedarwood oil and surfactant are readily available, nontoxic, stable, and easy to use. So, if this test works in the field, it could help a lot of people."

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