Hookworm is a parasitic worm found in tropical and subtropical zones around the world, which today is estimated to infect about 600 million people worldwide.

Hookworm is an parasitic nematode worm, which lives in the intestinal tract of a number of animals. The hookworm spends much of its life feeding off of the host's blood. There are two species of hookworms that live in humans: Necator americanus and Ancylostoma duodenale. Epidemiologically these two are largely the same; notable differences are that A. duodenale draws ten times as much blood per worm per day as does N. americanus; only A. duodenale appears to be able to lie dormant in the host for an extended period of time; and N. americanus lives three to five times as long as does A. duodenale. Besides these differences, there are clear morphological differences in the mouthparts that allows for identification between the species in grown worms.

Hookworm was first described in 1838 by an Italian physician attempting to determine the cause of miner's anemia. The coal mines of Europe were warm and damp enough to allow the propagation of hookworm, and the close quarters and lack of sanitary facilities encouraged the spread from person to person. This European hookworm was A. duodenale; it has since been found in Africa, Asia, the Americas, and the Middle East. Hookworm in America was first reported in 1900 by Dr. Bailey Ashford, who found severe cases in Puerto Rico migrant workers. He sent samples of these to the zoologist Charles Stiles, who in 1902 announced that they were a unfamiliar species of hookworm, which he named Necator americanus, meaning 'American killer.' Within three years, N. americanus was identified in a comparatively isolated population native to central Africa; Stiles quickly admitted that N. americanus was not unique to the Americas, but the name stuck. N. americanus has since been found in Asia and Australia.

The effects of hookworm infection vary with the parasite load. A light hookworm infection, consisting of ten or twenty worms, may cause no noticeable effects on the host. In adults, a load of 25 to 100 worms will cause light symptoms; slight weakness, stomach aches, and the heart racing upon exertion. With a 100-500 worm load, iron deficiency anemia and protein deficiency set in. The victim turns apathetic and passive, may experience nausea and vomiting, constipation and diarrhea, dizziness and aches, yellowing of the skin, and aches in the joints, head, and chest. At this point an infected person may also start eating dirt and clay, or other non-food substances. Parasite loads over 500 worms cause swelling of the belly, swollen ankles, loss of nervous reflexes, irregular fever, blurred vision, pain in the muscles, and an enlarged heart overworked from trying to pump enough blood to supply the body with oxygen. At this point, any secondary infection may be fatal, although hookworm itself rarely kills. Chronically infected children may suffer severe, sometimes permanent, setbacks in their cognitive and mental development.

Hookworm eggs are released into the environment when the host defecates. At this stage they are only four- or eight-cell eggs, but under favorable conditions they will molt twice and develop into infective larvae within two days. These larvae will exhume themselves, climb a blade of grass or other elevated point, and stand on their tail, swaying, as they wait for a potential host to brush against them. Hookworm infects a human host through a patch of exposed skin, usually the sole of the foot. Burrowing through the skin causes an immune response in the host; this irritation is known as ground itch. The larvae migrate through the blood stream until they reach the lungs, where they puncture an alveolus to gain entry. At this point, they undergo another molt, which causes minor irritation of the lungs; this causes the host to cough them up, after which they are swallowed. The larvae finally settle in the small intestine, where it molts one final time, resulting in an adult hookworm. The hookworm bites into the intestinal wall, sucking out 0.03 (N. americanus) to 0.15 (A. duodenale) milliliters of blood each day. The hookworm only feeds on the blood plasma, releasing the rest into the intestine. This rejected blood, composed primarily of red blood cells, is lost to the body. At about two months of age, the female is able to start producing eggs; A. duodenale produces 10,000 to 30,000 eggs per day, N. americanus only 5,000 to 10,000. The eggs are released into the feces, starting the cycle again.

There are some interesting qualifications to this account of hookworm lifecycle: A. duodenale larvae can lie dormant in the body for months before reaching the intestine; A. duodenale can be passed through mother's milk; A. duodenale larvae can infect through the mouth, and therefore through being eaten; and hookworms cannot embryonate outside the host below certain temperatures. The arresting temperature is about 20 degrees Celsius (68 degrees Fahrenheit). If conditions are not favorable for embryonation, hookworm eggs can remain in a state of arrested development for months before dying. Normal adult life span is one year for A. duodenale, and three to five years for N. americanus, although N. americanus has been observed to live to the age of fifteen years.

There are a number of drugs that can cure hookworm infection by loosening the parasite and causing it to be eliminated from the body, and dietary supplements can help relieve the symptoms (most of the symptoms arise from iron and protein deficiency). But these are only stopgap measures; there are no vaccinations to prevent reinfection, and the body's immune system does not learn to recognize hookworm as a pathogen. Prevention is the only long term solution to the hookworm problem, and has proved successful in many places, including in the American south.

Hookworm only becomes a problem in human populations when the soil is polluted with human feces, and this soil is then brought into contact with the skin or food supply. While hookworm can be spread through food sources, possibly including domesticated animals, this is not usually a major point of infection. It is common in many cultures to defecate out-of-doors, often behind whatever cover is most convenient. If the same areas are used continuously by multiple people, this encourages hookworm spread. While humans are pretty good at avoiding stepping on feces, even decomposing ones, in warm humid climates the traces of feces may disappear before the hookworm larvae do. Another common cause of hookworm infection is the collection and use of night soil. The best solution is usually to build latrines and improved the composting methods of night soil.

This was originally part of a paper I wrote for my Heath and Disease in Human Populations class. I've cut out most of the boring bits ("edited it for readability"), and then cut off the second half of the paper; that was the part where I looked at the appearance of Hookworm in the Americas. (Basically, I concluded that it was a mystery; quite possibly a trans-Pacific crossing from Japan. It does seem to have appeared before Columbus did).

Along with this paper, I had an extensive bibliography. After looking it over, I see only one item that I feel any need to pass along to you:

Williams, Greer
1969. The Plague Killers. New York: Charles Scribner's Sons.

This gives a great history of the fight against hookworm. It is U.S.-centric, but then, the battle against hookworm pretty much started in the U.S.

http://www.dpd.cdc.gov/DPDx/ is also a useful site for all your parasitic-disease research needs.

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