Ancylostoma duodenale

Human hookworms are found in tropical and subtropical regions between 30° north and south of the equator. Ancylostoma duodenale is found in the Mediterranean region, Southeast Asia, and scattered in the Southern Americas. (Beigal, Greenburg, and Ostfeld, 2000; Changhua et al., 1999; Roberts and Janovy Jr., 2000)

Along with its range of definitive hosts, Ancylostoma duodenale also has a range of paratenic hosts of canids and felids, where it may remain for intervals of time until it reaches the definitive host. In the paratenic host it may survive in the muscles where it is then transferred to humans via undercooked meat, including rabbit, lamb, beef, and pork. The eggs of Ancylostoma duodenale are still within the muscle and are ingested with the meat, allowing for the adults to develop within the intestinal tract.

Juveniles of the species reside in the warmer regions of the world where the soil is preferably humus and loose with reasonable water drainage and good aeration. Oxygen is necessary for the development of the eggs, whose metabolism is aerobic.

Hookworm eggs derive their nutrition from the host feces via absorption. Therefore they must live in areas with soils of neutral pHs and in shady areas, such as coffee, banana, and sugar plantations where the feces will remain intact long enough for them to develop into juveniles. They are extremely sensitive to sunlight, which can ultimately kill the juveniles. Juveniles are also sensitive to high salt concentrations and acidic pHs of soils.

After penetrating the skin, juveniles attach to blood vessels and begin to feed until reaching the adult stage. Adult females remain attached and the males detach to find their mates. Continual reinfection is promoted by repeated defecation by infected individuals in the same locals where they were originally infected. This may even lead to epidemics of Ancylostoma duodenale infections in arid regions of the world. (Chilton and Gasser, 1999; Roberts and Janovy Jr., 2000)

Ancylostoma duodenale is an S-shaped worm because of its flexure at the frontal end. The worm is pinkish-white. Adult male hookworms range in size from 8-11 mm long, whereas adult females range in size from 10-13 mm long. This species is dimorphic, with the males having bursa characteristics and needle-like spicules with small tips, which are distally fused. Females have a vulva located approximately one-third of the body length from the posterior end. Both male and female hookworms have two powerful ventral teeth in the adult forms of the parasite, one along each side of the buccal capsule; smaller pairs of teeth are located deeper in the capsule.

Hookworm eggs have a thin shell and the larvae possess amphids, large paired sensilla on each side of the mouth, which allow them to locate their host. The larvae are rod-shaped and are about 0.004 cm long. (Ashton, Schad, and Li, 1999; Carson-DeWitt, 1999; D.W., 1980; Roberts and Janovy Jr., 2000; Williams, 1969)

The hookworm life cycle is composed of seven steps, which are as follows. First, the Ancylostoma duodenale eggs are passed into the feces of the host. Second, the embryo passes via and develops within the feces. The first stage rhabditiform juvenile then hatches once the egg is outside of the host. Next, the filariform or infective juvenile develops after two molts. This stage is characterized by an arrest in development until a new host is reached. Humans may be infected via the oral cavity by ingestion of undercooked meat. Filiform juveniles infect by directly penetrating the skin of the host, usually a human. Fifth, the juveniles then migrate through the circulatory system until they reach the lungs. Sixth, once they have reached the lungs, the juveniles leave the circulatory system by finding their way into the alveoli and then migrating to the small intestine via the trachea. It takes about 5-6 weeks for the hookworm to reach the small intestine from the lungs. Finally, the adult worms develop in the small intestine where they mate, and produce eggs that are sent off in the feces of the host to begin the process once more. Adults form about 6 weeks after the initial infection.

A possible alternate root of infection may occur if juveniles are swallowed and develop normally without moving into the lungs. However, this is a very rare occurrence. (Beigal, Greenburg, and Ostfeld, 2000; D.W., 1980; Roberts and Janovy Jr., 2000)

The juvenile stages of the parasite move around in the outside environment prior to locating the host. The adult worms can move from one place to another along the intestine once inside of the host, thus increasing blood loss through the wounds that are left behind in the intestinal linings.

The larvae of the infective stage are usually stationary, until they sense vibrations in the soil as heat or carbon dioxide. They use environmental signals to flag their host and prepare for ingestion during their third larval stage. They do so by using neurons with dendritic processes that resemble cilia, which are mechanosensory, thermosensory and chemosensory. Adult human hookworms move by flowing within the bloodstream from one local to another and then attach to the intestinal walls where they feed. (Ashton, Schad, and Li, 1999; Roberts and Janovy Jr., 2000; Williams, 1969)

The larvae of the infective stage are usually stationary, until they sense vibrations in the soil as heat or carbon dioxide. They use environmental signals to flag their host and prepare for ingestion during their third larval stage. They do so by using neurons with dendritic processes that resemble cilia, which are mechanosensory, thermosensory and chemosensory. (Ashton, Schad, and Li, 1999; Roberts and Janovy Jr., 2000; Williams, 1969)

The definitive host is where the parasite reaches sexual maturity. Humans are the definitive hosts of Ancylostoma duodenale. Recent research shows that other definitive hosts may exist because of the ability to cross-infect different hosts. For example, Ancylostoma duodenale posses the ability to cross-infect from humans to canines, whereas its close relative, Ancylostoma caninum, cannot cross-infect to humans.

Hookworm eggs gain nutrition via the host feces. After penetrating the skin, juveniles attach to blood vessels and begin to feed.

The larval stage is free-living where there is independent existence in the soil. They then penetrate the host's skin by the secretion of digestive enzymes that dissolve the skin.

Young and adult worms feed on blood from the walls of the host's intestine by attaching to the intestinal lining via their sharp buccal cavity teeth, which they also use to break open small blood vessels so that they can suck the blood from them. Ancylostoma duodenale possess anticoagulant substances that are secreted to prevent blood clotting to the blood flowing from the wound. (Brinksworth et al., 2000; Chilton and Gasser, 1999; D.W., 1980; Roberts and Janovy Jr., 2000)

These parasites are probably not preyed on directly, but are ingested from host to host. Larval mortality is high as most of the parasites do not reach appropriate hosts.

Ancylostoma duodenale mainly infects humans but paratenic hosts include canids and felids, where it may remain for intervals of time until it reaches the definitive host.

Infected individuals are susceptible to malnutrition, protein and iron drain from the diet. Other effects include stunted growth and below-average intelligence in developing children, lowered antibody response to infectious agents, and anemia due to heavy blood loss and iron-deficiency among other side-effects. In some cases, heavy infestations may lead to fatalities because of infection of other worms or malaria as well as excess blood loss and other types of complications. Infants were recently recognized in the field of public health as being vulnerable. Hookworm disease is more prevalent in females than males.

Tourists visiting areas where local sanitation is a problem should be careful of infestation, especially in regions with humid climates.

Treatment is fairly simple with Mebendazole, Albendazole, and Levamisole. The use of dietary supplementation is important to compensate for the loss in nutrients. (Beigal, Greenburg, and Ostfeld, 2000; Bennett and Guyatt, 2000; Changhua et al., 1999; Roberts and Janovy Jr., 2000; Sen-Hai, Ze-Xiao, and Long-Qi, 1995)