Schistosomiasis is the next most common NTD. It is caused by parasitic worms known as schistosomes that live in the veins draining the intestines or bladder. More than 90 percent of the 200 million cases occur in sub-Saharan Africa, with another few million cases in Brazil and several other countries. Female schistosomes release eggs equipped with tiny spears that invade and damage organs, including the intestine and liver or the bladder and kidneys, depending on the species. Roughly 100 million school-aged children and young adults pass blood in their urine or feces every day as a result. The inflammation produces pain, malnutrition, growth stunting and anemia. In women, schistosomes deposit eggs in the cervix and vagina, causing disabling pain during sexual intercourse and tripling the risk of acquiring HIV/AIDS [see “Fighting Killer Worms,” by Patrick Skelly; Scientific American, May 2008].
Two other important helminth infections are lymphatic filariasis (LF) and onchocerciasis. The worms that cause LF live in the limbs, breasts and genitals of 120 million people in Asia, Africa and Haiti. They lead to elephantiasis, a grossly disfiguring condition that prevents adults from working and leaves women, in particular, unable to marry or abandoned by their husbands. Onchocerciasis, or river blindness, causes a horribly itchy and disfiguring skin disease as well as blindness in middle-aged adulthood. Almost all of its 30 million to 40 million cases occur in Africa, except for a few locations in the Americas and Yemen.
The seventh important NTD, trachoma, is not caused by a parasitic worm but is a chronic bacterial infection caused by the Chlamydia microorganism. Occurring in 60 million to 80 million people, it is the leading infectious cause of blindness [see “Can Chlamydia Be Stopped?” by David M. Ojcius, Toni Darville and Patrik M. Bavoil; Scientific American, May 2005].
In a series of policy papers, my colleagues and I studied the repercussions of these seven NTDs. Together their global health damage, as measured by the number of healthy life years lost because of disability, is roughly equivalent to that of HIV/AIDS or malaria. Because of their devastating toll on child education and development, pregnancy, and agricultural worker productivity, these NTDs are a major cause of poverty. One case study by Hoyt Bleakley, a development economist, found that chronic hookworm infection in childhood reduced a person’s lifetime earning power by more than 40 percent; K. D. Ramaiah and others in India estimated more than $800 million lost annually from reduced worker productivity as a result of LF. Other studies have found similar effects for onchocerciasis and trachoma.
Curing the Sick with Salt
The good news is that these NTDs can be treated, or even prevented, simply and cheaply. In many cases, a single pill is enough. The available drugs have an excellent safety record, and each is either provided free of charge by multinational companies or available as cheap generics costing less than 10 cents per tablet.
In the early 20th century John D. Rockefeller sponsored mass drug administration to control helminth infection in the American South, and similar efforts began in the Caribbean. During the 1950s and 1960s several tropical medicine specialists started programs for other infections and locations. Among them was Frank Hawking, father of physicist Stephen Hawking, who in 1967 published the results of a study in Brazil in which he treated LF by adding diethylcarbamazine to cooking salt. In 1988 Merck & Co. began one of the first public-private partnerships for the mass treatment of river blindness. Various such partnerships have since been established, and today they reach tens of millions of people annually.