USAID's NTD Program
Hookworm
- Overview
- Epidemiology
- Distribution of Hookworm
- Life cycle
- Symptoms
- Diagnosis
- Treatment, Prevention, and Control
Overview
Human hookworm infection is a soil-transmitted helminthiasis caused by the nematode parasites Necator americanus (N. americanus) and Ancylostoma duodenale (A. duodenale). Mild infections with hookworm cause mild diarrhea and abdominal pain. More severe infections with hookworm can create serious health problems for newborns, children, pregnant women, and persons who are malnourished. Hookworm infection is the leading cause of anemia and protein malnutrition in developing nations, afflicting an estimated 740 million people.
Epidemiology
The largest numbers of cases of hookworm infection occur in impoverished rural areas of sub-Saharan Africa, Latin America, Southeast Asia, and China. In general, tropical coastal communities have the highest intensity of hookworm infection. N. americanus is the most common hookworm worldwide, while A. duodenale is more geographically restricted. There is no known animal reservoir for N. americanus or A. duodenale.
Unlike other soil-transmitted helminthiasis infections, such as ascariasis (roundworm) and trichuriasis (whipworm), in which the highest-intensity infections occur primarily in school-aged children, high-intensity hookworm infections frequently occur in adult populations. Up to 44 million pregnant women are estimated to be infected with hookworm. In pregnant women, anemia caused by hookworm disease results in several adverse outcomes for both the mother and her infant, including low birth weight, impaired milk production, and increased risk of death for both the mother and the child. In children, chronic hookworm infection has been shown to impair physical and intellectual development, reduce school performance and attendance, and adversely affect future productivity and wage-earning potential.
Distribution of Hookworm
| Estimated Number of Hookworm Infections (in millions) by Age Group, 2003 | |||||
|---|---|---|---|---|---|
| WHO Region | 0-4 Years |
5-9 Years |
10-14 Years |
15 Years |
Total |
| Africa | 9 | 18 | 29 | 142 | 198 |
| Americas | 1 | 3 | 5 | 41 | 50 |
| E. Mediterranean | 0 | 1 | 1 | 8 | 10 |
| South-East Asia | 4 | 10 | 16 | 100 | 130 |
| Western Pacific | 7 | 18 | 34 | 293 | 352 |
| Total | 21 | 50 | 85 | 584 | 740 |
Source: World Health Organization
Life cycle
 | |
| Source: CDC |
Step 1: Eggs are passed in the stool of an infected person. Hookworm eggs are not infective.
Step 2: Under favorable conditions (moisture, warmth, and shade), larvae hatch in 1 to 2 days.
Step 3: The released first stage (rhabditiform) larvae grow in the feces and/or the soil, and after 5 to 10 days they become third stage (filariform) larvae. These infective larvae can survive 3 to 4 weeks in favorable environmental conditions.
Step 4: Upon contact with the human host (mostly through the skin but larvae can also be ingested) the larvae penetrate the skin and are carried through the veins to the heart and then to the lungs. They penetrate the pulmonary alveoli, ascend the bronchial tree to the pharynx, and are swallowed.
Step 5: The larvae reach the small intestine, where they reside and mature into adults. Adult worms live in the lumen of the small intestine, where they attach to the intestinal wall, with resultant blood loss by the host. The presence of between 40 and 160 adult hookworms in the human intestine results in blood loss sufficient to cause anemia and malnutrition. Most adult worms are eliminated in 1 to 2 years, but they can live for several years.
Symptoms
Hookworm infection may be associated with a skin reaction (dermatitis), an increase of a certain type of white blood cell (eosinophils), a pulmonary reaction (pneumonitis), and skin rash (urticarial). Gastrointestinal symptoms include mild abdominal pain, nausea, vomiting, and anorexia. Iron-deficiency anemia due to blood loss is often associated with hookworm infection.
Hookworm-associated blood loss results from the destruction of capillaries in the intestinal mucosa; however, the level of iron deficiency and anemia depends on three major factors: worm burden, type of hookworm (A. duodenale causes more blood loss than N. americanus), as well as overall patient nutritional status, including the person’s iron reserves and diet. Iron deficiency signs will be found in a host with a high load of hookworms and include fatigue, poor concentration, and shortness of breath. Increased maternal and neonatal mortality has been found to be associated with hookworm iron-deficiency anemia.
Diagnosis
In general, hookworm is clinically diagnosed. Hookworm infection definitive diagnostic is established by identifying hookworm eggs in feces under light microscopy. Quantitative methods of egg count (e.g., Kato-Katz) can be used to provide information on the intensity of infection.
In the case of humans infected by some animal hookworms, negative fecal examination is often found. In those cases, definitive diagnostic is based on the identification of the parasite by endoscopy.
Treatment, Prevention, and Control
Regiments with mebendazole and albendazole are currently the treatment of choice for adult hookworms. Hookworm infection is treated with a single dose of 500 mg of mebendazole or 100 mg of mebendazole twice a day for 3 consecutive days. Albendazole is given at a single dose of 400 mg. Associated iron-deficiency anemia should be detected and treated adequately.
Hookworm infection control is achieved through the sanitary disposal of feces and educational campaigns about the proper use of latrines. The most cost-effective way to control hookworm infection has been through population-wide treatment with either albendazole or mebendazole. However, both children and adults usually become reinfected within a few months after deparasitation, which implies repeated and frequent use of the drugs. There is concern that heavy and exclusive reliance on albendazole and mebendazole might lead to drug resistance. hookworm/cycle.html
References
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- Brooker S., Bethony J., Hotez P.J. Human hookworm infection in the 21st century. Adv Parasitol. 58:197-288, 2004.
- Bundy D.A.P., Chan M.S., Savioli L. Hookworm infection in pregnancy. Trans R Soc Trop Med Hyg. 89:521-522, 1995.
- De Silva N., Brooker S., Hotez P., et al. Soil-transmitted helminth infections: Updating the global picture. Trends Parasitol. 19:547-551, 2003.
- Drugs for parasitic infections. Medical letter. April 2004.
- Hotez P.J., Zhan B., Bethony J.M., et al. Progress in the development of a recombinant vaccine initiative for human hookworm disease: The human hookworm vaccine initiative. Int J Parasitol. 33:1,245-1,258, 2003.
- Jelinek T., Maiwald H., Nothdurft H.D., et al. Cutaneous larva migrants in travelers: Synopsis of histories, symptoms, and treatment of 98 patients. Clin Infect Dis. 19:1062-1066, 1994.
- Mabaso M.L.H., Appleton C.C., Hughes J.C., et al. The effect of soil type and climate on hookworm (Necator americanus) distribution in KwaZulu Natal, South Africa. Trop Med Int Health. 8:722-727, 2003.
- Stephenson L.S., Latham M.C., Kinoti S.N., et al. Improvements in physical fitness of Kenyan school boys infected with hookworm, Trichuris trichiura, ascaris lumbricoides following a single dose of albendazole. Trans R Soc Trop Med Hyg. 84:277-282, 1990.
- World Health Organization. First WHO report on neglected tropical diseases: Working to overcome the global impact of neglected tropical diseases. World Health Organization, 2010.
- World Health Organization. Prevention and control of schistosomiasis and soil-transmitted helminthiasis. Report of a WHO expert committee, WHO Technical Report Series 912. World Health Organization, 2002.
- World Health Organization. Preventive chemotherapy in human helminthiasis: Coordinated use of anthelminthic drugs in control interventions – A manual for health professionals and programme managers [PDF, 1.6MB]. World Health Organization, 2006.