Two types of hookworm are responsible for hookworm disease in humans. Necator americanus and Ancylostoma duodenale have similar life cycles and similar methods of causing illness. The adult worm of both Necator americanus and Ancylostoma duodenale is about 10 mm long, pinkish-white in color, and curved into an S-shape or double hook.
Both types of hookworm have similar life cycles. The females produce about 10,000–20,000 eggs per day. These eggs are passed out of the host's body in feces. The eggs enter the soil, where they incubate. After about 48 hours, the immature larval form hatches out of the eggs. These larvae take about six weeks to develop into the mature larval form that is capable of causing human infection. If exposed to human skin at this point (usually bare feet walking in the dirt or bare hands digging in the dirt), the larvae will bore through the skin and ride through the lymph circulation to the right side of the heart. The larvae are then pumped into the lungs. There they bore into the tiny air sacs (alveoli) of the lungs. Their presence within the lungs usually causes enough irritation to produce coughing. The larvae are coughed up into the throat and mouth, and are then swallowed and passed into the small intestine. It is within the intestine that they develop into the adult worm, producing illness in their human host.
Ancylostoma duodenale is found primarily in the Mediterranean, the Middle East, and throughout Asia. Necator americanus is common in tropical areas including Asia, parts of the Americas, and throughout Africa. Research suggests that at least 25% of all people in the world have hookworm disease. In the United States, 700,000 people are believed to be infected with hookworms at any given time.
Causes and symptoms
Hookworms cause trouble for their human host when the worms attach their mouths to the lining of the small intestine and suck the person's blood.
An itchy, slightly raised rash called "ground itch" may appear around the area where the larvae first bored through the skin. The skin in this area may become red and swollen. This lasts for several days and commonly occurs between the toes.
When the larvae are in the lungs, the patient may have a fever, cough, and some wheezing. Some people, however, have none of these symptoms.
Once established within the intestine, the adult worms can cause abdominal pain, decreased appetite, diarrhea, and weight loss. Most importantly, the worms suck between 0.03–0.2 ml of blood per day. When a worm moves from one area of the intestine to another, it detaches its mouth from the intestinal lining, leaving an irritated area that may continue to bleed for some time. This results in even further blood loss. A single adult worm can live for up to 14 years in a patient's intestine. Over time, the patient's blood loss may be very significant.
Anemia is the most serious complication of hookworm disease, progressing over months or years. Children are particularly harmed by such anemia, and can suffer from heart problems, mental retardation, slowed growth, and delayed sexual development. In infants, hookworm disease can be deadly.
Diagnosis of hookworm disease involves collecting a stool sample for examination under a microscope. Hookworm eggs have a characteristic appearance. Counting the eggs in a specific amount of feces allows the healthcare provider to estimate the severity of the infection.
Minor infections are often left untreated, especially in areas where hookworm is very common. If treatment is required, the doctor will prescribe a three-day dose of medication. One to two weeks later, another stool sample will be taken to see if the infection is still present.
The prognosis for patients with hookworm disease is generally good. However, reinfection rates are extremely high in countries with poor sanitation.
Prevention of hookworm disease involves improving sanitation and avoiding contact with soil in areas with high rates of hookworm infection. Children should be required to wear shoes when playing outside in such areas, and people who are gardening should wear gloves.
Plorde, James J. "Hookworms." In Sherris Medical Microbiology: An Introduction to Infectious Diseases. 3rd ed. Ed. Kenneth J. Ryan. Norwalk, CT: Appleton & Lange, 1994.
Stoffman, Phyllis. The Family Guide to Preventing and Treating 100 Infectious Diseases. New York: John Wiley & Sons, 1995.
Farley, Dixie. "Treating Tropical Diseases." FDA Consumer (Jan./Feb. 1997): 26+.
Hotez, P. J., and D. I. Pritchard. "Hookworm Infection." Scientific American (June 1995): 68+.
Centers for Disease Control and Prevention. 1600 Clifton Rd., NE, Atlanta, GA 30333. (800) 311-3435, (404) 639-3311. <http://www.cdc.gov>.
Rosalyn Carson-DeWitt, MD
Alveoli—The small air sacs clustered at the ends of the bronchioles, in the lungs in which oxygen-carbon dioxide exchange takes place.
Anemia—Any condition where the oxygen-carrying capacity of the red blood cells is reduced; symptoms often include fatigue.
Host—The organism (like a human) in which another infecting organism (like a worm) is living.
Larva—An immature form of an organism, occurring early in that organism's development.