HIV/AIDS, tuberculosis, and malaria, or “the big three,” are the three most recognized and targeted diseases afflicting developing countries. These diseases have high mortality rates, killing millions of people each year, and most international efforts to control infectious disease focus on these diseases.
I would like to address some tropical diseases which, although extremely common in developing countries, have lower mortality rates and receive far less international attention. These diseases, which are the 5 most prevalent Neglected Tropical Diseases (NTDs), are not as deadly as HIV/AIDS, tuberculosis, or malaria. However, these parasitical diseases have a great impact on child growth and development, have serious socioeconomic consequences, and can be chronically disabling and painful.
They are, in order of prevalence*:
- Ascariasis (roundworm), 807 million
- Trichuriasis (whipworm), 604 million
- Hookworm, 576 million
- Schistosomiasis, 207 million
- LF (lymphatic filariasis), 120 million
*Jotez, Peter. Forgotten People, Forgotten Diseases. 2008
More than one billion people are affected by one or more of these parasitical diseases. But only half a million people die from these diseases each year (as compared to 1 million each from HIV/AIDS and malaria and 1.8 million from tuberculosis). I will talk about each of these diseases briefly, focusing not so much on the medical details, which Wikipedia can supply, but on their socioeconomic impact. While the NTDs are products of poverty, they are also promoters of poverty.
1-3. Roundworm, Whipworm, and Hookworm
These three diseases are caused by soil-transmitted helminths (STHs), helminth being another word for parasitic worm. These worms infect the intestines of humans, and are spread when human feces containing worms are deposited onto soil, where the worms can infect new victims through ingestion of contaminated vegetables or water, or by burrowing directly through the skin. These worms thrive in warmer regions, where they can survive in the soil. The worms grow and mature in the intestines, where they rob the person of important nutrients and impair absorption of protein, fat, iron, vitamins, etc. STH infections can lead to other intestinal problems, such as colitis and rectal prolapse.
By causing malnutrition, STHs stunt the physical growth and cognitive development of children. Clinical studies have shown that STHs negatively affect children’s memory, cognition, and intelligence. They also reduce school attendance. Thus, STHs affect education, which ultimately has a negative impact on economic growth. For instance, studies have shown that “infection with hookworm during childhood is associated with a 43% reduction in future wage-earning capacity” (Jotez, 2008).
While the STHs promote poverty, poverty also promotes STH infections. Places where STH infections are endemic, afflicting much of the population, are also places where people lack sanitation systems and latrines. Dirt flooring in houses and a lack of shoes also allows for easier transmission. In endemic areas in Sub-Saharan Africa, India, and Latin America, anthelmintic drugs are only a temporary fix, because within months the person is usually infected again. Although dosing with anthelmintic drugs has been shown to have the greatest impact on reducing STH infections, economic development also has a great impact. Hookworm infections were once common in the rural Southern United States, but urbanization and economic development, combined with medication, eradicated hookworm.
4. Schistosomiasis
Schistosomiasis is caused by another type of helminth, called flukes. The flukes spend part of their life cycle in snails and are then released into water. When humans come into contact with the flukes, by bathing, swimming, fishing, working in irrigated fields, or drinking un-boiled water, the flukes penetrate their skin and enter the bloodstream, where they cause severe flu-like symptoms, known as “snail fever.” But over time, the flukes penetrate the bladder, kidneys, intestine, or liver, causing organ disease and anemia, and leading to chronic abdominal pain, malnutrition, and weakness. Schistosomiasis reduces a person’s work capacity, and in children, causes many of the same problems as STHs (stunted growth, impaired cognitive development). In many cases, schistosomiasis causes vaginal lesions, which increases the likelihood of HIV/AIDS transmission.
By decreasing children’s ability to learn and succeed in school and the ability of adults to perform both physical and cognitive labor, schistosomiasis, like the STHs, lowers the economic potential of a country. Due to use of praziquantel, an anti-fluke medication, schistosomiasis has been mostly eradicated in Egypt, China, and some Latin American countries. However, in sub-Saharan Africa, schistosomiasis is still prevalent in many countries, largely due to the fact that these countries cannot afford praziquantel (Jotez, 2008).
5. Lymphatic filariasis (LF)
LF is caused by the filarial parasitic worm Wuchereria bancrofti. The worm is spread by mosquitoes, in much the same way that malaria is spread. Unlike schistosomiasis and the STHs, the W. bancrofti parasites do not take up home in the intestines; rather, they infect the lymphatic system, where they release eggs called microfilariae into the bloodstream, so feeding mosquitoes can continue to spread the disease. LF can result in debilitating fever, hdyrocele (swelling of the scrotum), and lymphedema, the most dramatic and apparent symptom. Lymphedema occurs when the worms die, and causes severe swelling of the genitals or legs. Because of the elephant-like appearance of lymphedematous legs, this part of LF is sometimes referred to as “elephantiasis.”
Because the worms die around the time of adulthood, LF has great socioeconomic impact. Young men and women often lose their jobs because they are incapacitated. LF is also stigmatizing, and people with LF are frequently abandoned by their families. In his book, Dr. Peter Jotez describes a young woman with LF who lost her job and was abandoned by her husband.
Like the other parasitic diseases I have described, LF affects some of the world’s poorest people. LF was eliminated by widespread treatment with DEC (a drug that kills microfilariae) in China, Brazil, Japan, Tanzania, Taiwan, and Egypt. Meanwhile, in sub-Saharan Africa, there are many countries who lack the financial means to mass-administer DEC.
The good news is that, with widespread drug treatment, LF could be eliminated worldwide. Humans are the only reservoir for W. bancrofti. A single dose of DEC or ivermectin (another anti-parasitical drug) reduces the amount of microfilariae in the blood for a year. The fact that LF has already been eliminated from several other countries provides hope that one day, with enough drug administration, LF will be eliminated worldwide.
Source: Forgotten People, Forgotten Diseases, by Peter J. Hotez. ASM Press, 2008. Dr. Hotez is President of the Sabin Vaccine Institute in Washington DC, where his team is working on developing vaccines for hookworm and schistosomiasis.
This post was written by elizabeth.cozart
