Bad synergy

Simultaneous parasite infections act in concert to raise risk of anemia in children

Hookworm and other parasites that strike children can have a synergistic effect that increases the risk of anemia beyond the damage expected from having multiple parasitic infections, a study in the Philippines shows.

Researchers tested 507 children ages 7 to 18 for parasitic worms. Nearly all were infected with at least one, and lab tests delineated mild, moderate and severe infestation based on the number of worm eggs per gram of stool sample.

Children moderately or severely infected with hookworm (Necator americanus) and a flatworm called blood fluke (Schistosoma japonicum) were 13 times as likely to be anemic as children with no parasite infections or with only mild cases. Children who harbored moderate to severe levels of hookworm plus whipworm (Trichuris trichiura) were more than five times as likely to be anemic, the researchers report in the June PLoS Neglected Tropical Diseases.

These data suggest that moderate-to-severe parasite infestations with hookworms and blood flukes seem to triple the risk of anemia when compared with that would be calculated by simply adding the risks of these worms together, says study author Amara Ezeamama, an epidemiologist at the Health Effects Institute in Boston.

This synergistic effect may result from chronic inflammation caused by ongoing low-grade parasitic infections that disrupt the iron content in the blood, tilting the mix of iron away from the usable ferrous form and toward the less accessible ferritin. This iron deficiency limits red blood cells’ ability to transport oxygen to the body’s cells, the authors say. Worms also cause anemia when a patient loses blood through the stool without knowing it.

Hookworms enter the skin directly by contact. They were largely wiped out in the West with treatment and by discouraging children from going barefoot. Schistosome larvae are waterborne and infect directly through the skin. Whipworms are typically ingested with food.

The new study supplies evidence of an intensified risk of anemia that tropical disease experts have suspected for a long time, says Donald Bundy, an epidemiologist and parasite expert at the World Bank in Washington, D.C. Anemia makes children weak, drowsy and unable to concentrate. “These infections are particularly intense in school children. They have a profound effect on education,” he says. “The children are unable to learn.”

Bundy says the study shows that yearly doses of anti-parasite drugs, some of which are very inexpensive, could engender long-term improvement in areas endemic for these parasites.

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