Worms track us down with a chemical trail

Parasitic worms infect millions of people in the developing world, by following chemical trails to their hosts. Isolating the substances that attract them is the key to finding cheap and practical ways of preventing infection.

Throughout the day, our skins are constantly sending out messages that we can neither see nor hear. The message is written in chemical form and it says, “Here I am. Come and get me”. We neither see nor hear these signals. But to those that can, they act as shining beacons, sending them crawling, swimming and slithering in our direction.

HookwormsThese creatures are nematodes, a group of worms that are some of the most common animals on the planet. The vast majority of nematode species are parasites, and hundreds of species count humans among their potential hosts.

In the developing countries of the tropics and sub-tropics, these parasites pose a major health problem, causing illness, stunted physical and mental development, anaemia and more. Just two species, the hookworms Ancylostoma duodenale and Nector americanus, infect over 600 million people around the world

Unlike bacteria or viruses, nematodes actively seek out their hosts, rather than waiting to be transferred by water, a sneeze or a bite. Unfortunately, we know very little about how these parasites track down their prey, giving us few options for preventing infections.

This may be because they are a relatively minor problem in the world’s affluent nations. But mostly, our lack of knowledge reflects how difficult it is to study these animals. Human experiments are out of the question, and animal tests are largely impossible since most human nematodes do not equally infect other species.

Strongyloides stercoralisThe threadworm Strongyloides stercoralis is an exception. It can be relatively easily raised in laboratory conditions and also affect primates and dogs. Daniel Safer and colleagues from the University of Pennsylvania have exploited these traits to work out how this species finds a potential victim.

Safer found extracts prepared from the skins of host animals, like dogs and gerbils, had a magnetic attraction for Strongyloides, drawing the worms from distances many times greater than their own body length. Extracts prepared from non-host species, like cats had no effect.

By separating the chemicals in these extracts into more and more specific fractions, Safer eventually identified the substance that was attracting the worms – urocanic acid.

Urocanic acid is found in animal skin where it acts as a natural sunscreen. But it is most common on the sole of the foot, where where levels can be five to ten times higher than other body parts. Each of our footsteps creates a trail of this acid that leads the soil-dwelling Strongyloides to the part of our body closest to the ground.

This acid is not the only substance that attracts Strongyloides. Temperatures within those of mammalian bodies summon them too, as do the high carbon dioxide levels in the air we exhale. But while we cannot control our body heat or the gases we breathe out, we can do something about urocanic acid.

Urocanic acid sticks to metal ions, and Safer reasoned that this could be a way of stopping the parasites from tracking it. Sure enough, adding manganese, calcium or magnesium successfully hid otherwise attractive extracts from the sniffing worms.

This is encouraging news. It means that applying creams containing these metals to vulnerable body parts like feet, could provide a cheap and practical way of preventing Strongyloides infection.

Foiling this species along would have great implications for global health, and doing so cheaply and easily would be ideal for the developing countries where the worms pose the greatest threat. Currently, over 300 million people are infected every year. Strongyloides can cause anaemia, but in people with weakened immune systems, the worms can breed to a point where they become fatal.

Drugs can kill the adult worms, but do nothing against the scores of larvae that travel through infected bodies. Preventing infection is best option. Safer’s work provides hope for a breakthrough, and for finding molecules that other parasites use to track their hosts.

Safer, Brenes, Dunipace & Schad. 2006. PNAS 104: 1627-1630.

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