Hookworm (Ancylostoma) Infection
Hookworm (Ancylostoma) Infection
Ancylostoma (an-cy-LO-sto-ma) infection, also called hookworm infection, is an infection of one of two different roundworms: Ancylostoma duodenale or Necator americanus. Depending on maturity, the roundworms range from 0.3 to 0.5 inches (0.7 to 1.3 cm) in length.
Hookworms are parasitic roundworms, specifically infesting the intestines of their host. They have hooklike appendages, from which they take their name. Hookworms belong to the class Nematoda. Moderate infestation of hookworms in humans is considered by the World Health Organization (WHO) to be between 2,000 and 3,999 eggs per gram of feces. Heavy infestation is counted at 4,000 or more eggs per gram of feces.
Hookworm infection is most common with areas of rural poverty and low socioeconomic status, especially in southern China, the Indian subcontinent, and in parts of the Americas. The worldwide number of cases was first estimated in 1990 to be 740 million people. By 2005, the number of cases worldwide was about double that initial estimate.
Hookworms deposit eggs on ground containing warm, moist, shaded soil. Such conditions allow eggs to develop into larvae. The larvae are barely visible; however, they are easily able to penetrate human skin. They frequently enter the body through the soles of the feet or when humans handle feces. Children often are infected because they frequently play in dirt and go barefoot. Humans cannot infect other humans.
Once inside the body, larvae travel through the bloodstream to the lungs and respiratory tract, and on to the trachea. They are swallowed into the digestive tract and stomach where they end up in the small intestines. From skin to intestines, the trip takes, on average, about one week. At this point, larvae develop into adult worms about 0.5 inches (1.3 cm) in length. They attach themselves to the walls of the small intestine where they suck blood. The hookworm causes symptoms to its host when the worms drain blood and nourishment from the intestinal wall. One adult worm can produce thousands of eggs and live up to ten years. Eggs are expelled in feces. Under the proper conditions, the eggs hatch, molt, and develop into infective larvae after five to ten days.
WORDS TO KNOW
HELMINTH: A representative of various phyla of worm-like animals.
MORBIDITY: The term “morbidity” comes from the Latin word “morbus,” which means sick. In medicine it refers not just to the state of being ill, but also to the severity of the illness. A serious disease is said to have a high morbidity.
PARASITE: An organism that lives in or on a host organism and that gets its nourishment from that host. The parasite usually gains all the benefits of this relationship, while the host may suffer from various diseases and discomforts, or show no signs of the infection. The life cycle of a typical parasite usually includes several developmental stages and morphological changes as the parasite lives and moves through the environment and one or more hosts. Parasites that remain on a host's body surface to feed are called ectoparasites, while those that live inside a host's body are called endoparasites. Parasitism is a highly successful biological adaptation. There are more known parasitic species than nonparasitic ones, and parasites affect just about every form of life, including most all animals, plants, and even bacteria.
IN CONTEXT: DISEASE IN DEVELOPING NATIONS
The World Health Organization (WHO) states that hookworm infection “is a leading cause of anaemia and protein malnutrition. The largest numbers of cases occur in impoverished rural areas of sub-Saharan Africa, Latin America, South-East Asia and China.”
SOURCE: World Health Organization (WHO)
Most of the time, there are no symptoms. However, symptoms can occur at any point within the worm's life cycle. Initial symptoms include itching and a rash at the larvae's entrance site to the host. Asthma- or pneumonia-like symptoms may occur when worms are in the lungs. Symptoms from intestinal infection include anemia, loss of appetite and weight loss, excessive intestinal gases, cramps and abdominal pain, and diarrhea. In chronic infections, symptoms may include malnutrition, breathing difficulties, dizziness, pale complexion, tiredness and weakness, swelling and bloating, impotence, enlargement of the heart, and irregular heartbeat.
In children, physical development and growth can be slowed or not fully attained because of loss of sufficient amounts of iron and protein. Infection can be especially problematic for newborn and infant children, pregnant women, and people who are malnourished. Death is uncommon but can occur, especially in newborn and infant children.
Hookworm infection occurs mostly in tropical and subtropical regions of the world. Ancylostoma duodenale is found in China, India, Japan, and Mediterranean countries. Ancylostoma americanus is located in the tropical areas of Africa, Asia, and the Americas. According to the Division of Parasitic Diseases of the Centers for Disease Control and Prevention (CDC), approximately 1.3 to 1.6 billion people are infected worldwide, about one-fourth to one-fifth of the world's population.
Diagnosis is often accomplished by identifying hookworm eggs in a stool sample with the use of a microscope. A blood sample is also used because positive results show iron or protein deficiency.
Treatment consists of anthelmintic drugs (that is, drugs proven to effectively remove worms), along with iron supplements and a high protein diet. In particular, the drug mebendazole (MBZ) is used because it causes immobilization and eventual death of the worms by restricting the ingestion of nutrients. It is often branded under the names Antiox®, Ovex®, Pripsen®, and Vermox®. It cures the infection about 99% of the time when given twice a day for three days. Other drugs also given are albendazole (Albenza®) and pyrantel (Antiminth®), which are given once each day for three days. They should not be given to pregnant woman.
Hookworm infection is prevented by promoting safe sanitary practices. Feces should be disposed of properly and contaminated areas cleansed thoroughly. Wearing shoes, avoiding swimming in contaminated pools, and treating or boiling contaminated water before drinking also help prevent hookworm infection.
Hookworm infection is the leading cause of iron deficiency anemia in developing countries. In developing countries where food is scarce, people with heavy hookworm infections are sometimes unable to eat enough calories to compensate for those lost due to intestinal iron and protein depletion brought on by hookworms. In the past, hookworm infection has been neglected due to its concentration among the world's poorest peoples. Generally, in the past, international coordination involving the infection has not been accomplished. Over the decades of the 1990s and 2000s, however, there has been increasing concern over the global incidence of hookworm infection. International efforts are increasing to control the occurrence of hookworm, flatworm, and related helminth (parasitic worm) infections. The World Health Organization estimates that over two billion people worldwide suffer from illnesses associated with helminths.
Children are especially susceptible to hookworm infection because of the amount of time they spend outdoors. WHO estimates that about 400 million school-aged children are annually infected. Once infected, the children often suffer morbidity that includes physical and mental problems such as anemia, attention deficits, learning disabilities, and school absenteeism. Children who are not properly treated are permanently affected.
WHO adopted in 2001 a resolution to target all countries where helminth infections occur most frequently. The project called Partners for Parasite Control (PPC) aims to regularly treat at least 75% of all school children at risk by the year 2010. PPC also supports local health facilities so that they have adequate supplies of anti-helminth drugs and perform regular treatment to high-risk groups.
IN CONTEXT: REAL-WORLD RISKS
The Centers for Disease Control and Prevention (CDC), Division of Parasitic Diseases states that hookworm infection cause any serious health problems and that “The most serious results of hookworm infection are the development of anemia and protein deficiency caused by blood loss. When children are continuously infected by many worms, the loss of iron and protein can retard growth and mental development, sometimes irreversibly. Hookworm infection can also cause tiredness, difficulty breathing, enlargement of the heart, and irregular heartbeat. Sometimes hookworm infection is fatal, especially among infants.”
SOURCE: The Centers for Disease Control and Prevention (CDC), Division of Parasitic Diseases
Holland, Celia V., and Malcolm W. Kennedy, eds. The Geohelminths: Ascaris, Trichuris, and Hookworm. Boston, MA: Kluwer Academic Publishers, 2002.
Hotez, Peter J., et al. “Hookworm Infection.” New England Journal of Medicine. 351, 8 (August 19, 2004): 799–807.
Division of Parasitic Diseases of the Centers for Disease Control and Prevention (CDC). “Hookworm Infection.” <http://www.cdc.gov/ncidod/dpd/parasites/hookworm/factsht_hookworm.htm> (accessed March 14, 2007).
World Health Organization. “Partners for Parasite Control (PPC).” <http://www.who.int/wormcontrol/en/> (accessed March 14, 2007).