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chapter 2
“The Unholy Trinity”: the Soil-Transmitted
Helminth Infections Ascariasis,
Trichuriasis, and Hookworm Infection
As it was when I first saw it, so it is now, one of the most evil of infections. Not with dramatic pathology as are filariasis, or schistosomiasis, but with damage silent and insidious. Now that malaria is being pushed back, hookworm remains the great infection of mankind. In my view it outranks all other worm infections of man combined . . . in its production, frequently unrealized, of human misery, debility, and inefficiency in the tropics.
The neglected tropical diseases (NTDs) are the most common infections of the world’s poorest people, and the soil-transmitted helminth (STH) infections are the most common NTDs. The word helminth comes from the Greek έλμίνς, meaning “worm,”1 and the phrase soil-transmitted refers to the human acquisition of these worms through contact with soil contaminated with either parasite eggs or immature larval stages. STHs are also sometimes called intestinal helminths or intestinal worms because the adult stages of the parasite live in the human gastrointestinal tract. The STHs are also nematodes, a type of parasitic worm distinguished by their elongate and cylindroidal shape.
The three most important STH infections of humans, based on their prevalence and global disease burden, are:
Ascaris infection (also known as roundworm infection or ascariasis)
Hookworm infection (hookworm)
Trichuris infection (whipworm infection or trichuriasis)
Together, these helminth infections afflict more than 1 billion people in developing countries.
Humans have been infected with STHs since ancient times. We know this from accurate descriptions found in Egyptian medical papyri and the writings of Hippocrates in the 5th century BCE, including reports of large Ascaris roundworms being expelled from infected people and of the characteristic pallor and sallow complexion of people with hookworm.1 In addition, STH eggs have been recovered from coprolites, mummified feces thousands of years old, found in both the Old World and New World.1 Today, an estimated 800 to 900 million, 600 to 700 million, and 500 to 600 million people are infected with ascariasis, hookworm, and trichuriasis, respectively (Table 2.1).2 More often than not, a single individual living in a developing country, especially a school-age child, is infected with two and sometimes all three types of STH parasites simultaneously. Practically speaking, this observation means that the intestines of hundreds of millions of children living in Africa, Asia, and the Americas harbor a menagerie of worms. Harold Brown, the late former parasitology professor at Columbia University College of Physicians and Surgeons, frequently referred to Ascaris, Trichuris, and hookworms as “the unholy trinity” to indicate that it was extremely common for a child to be infected with all three parasites simultaneously. Typically, Ascaris roundworms and hookworms inhabit the small intestine, while Trichuris whipworms inhabit the large intestine.
How can we fathom the notion of approximately 1 billion people infected with STHs? To understand this concept better, we need to travel to a developing country where STH infections are endemic, meaning that the infections are constantly present in a particular region. Figure 2.1 shows children living in a rural village in Minas Gerais State, Brazil. The families of these children are mostly subsistence farmers involved with cultivation of manioc and beans. Looking at these children, one might not think that they appear terribly ill, unless one examines them more closely. The STH-infected children living in this Brazilian village are stunted in both weight and height because they are not growing normally. Moreover, they also do poorly on tests of cognition, memory, and intelligence. There is now strong evidence that such physical and mental disabilities result from the presence of intestinal worms.3
Table 2.1 The “unholy trinity”
