What Are Hookworms?

Hook worm is a parasitic nematode which is found in the small intestine of its host which may be a mammal like dog, cat or human. Two species of nematodes are known to infect humans and these are Ancylostoma duodenale and Necator americanus. The former is found commonly in the Middle East, India, northern Africa and southern Europe while the latter is found in the Americas, Sub-Saharan Africa, Southeast Asia, China and Indonesia. Hookworms are known to infect more than 600 million people all over the world. Hookworms are smaller in size as compared to the larger roundworms and the infestation of roundworm is less than the infection of the hookworm. The most significant risk of the hookworm infection is anemia followed by loss of iron from the gut. The worms are known to suck the blood continuously and damage the intestinal mucosa. However, the loss of blood in the faeces is not visibly apparent. The disease caused by Ancylostoma duodenale is known as ancylostomiasis which is characterized by severe anemia as iron is lost and blood is lost by continuous blood sucking habit of the parasite. Hookworm is responsible for the leading cause of maternal child morbidity in the tropical and subtropical countries. In susceptible children they cause intellectual, cognitive and growth retardation, intrauterine growth retardation, prematurity and low birth weight. Death because of the infection is rare but anemia is very intense in the infected individual.

No specific signs and symptoms are known for the hookworm infection. Invasion of the skin by the larvae may cause intense itching particularly in the areas of foot and legs and sometimes lesions similar to that of insect bite may be formed which lasts for a week or more. Coughing, sneezing and fever may result in those individuals in whom large number of larvae enters the body. Signs of intense infection are anemia, protein deficiency, emaciation, cardiac failure and abdominal distension with ascites. A. duodenale worms are grayish-white or pink with the head slightly bent in comparison to rest of the body. This bent forms a hook shaped structure and this is the reason why the worms are named as hookworms. Mouth is well developed with two pairs of teeth. Males are smaller in comparison to that of the females and measure about 1 cm in length. Males can be distinguished from the females by the presence of a characteristic copulatory bursa present at posterior end of the body. N. americanus is somewhat smaller than A. duodenale with males measuring 5-9 mm long and the females about 1 cm long. They have only a single pair of cutting plates in the buccal capsule. Hook is much more prominent in these animals.

Eggs of both the species are found in warm and moist soil where they hatch into the first stage larvae or L1. L1 is a feeding and non-infective stage known as rhabditiform larvae which feeds on soil microbes and ultimately transforms into the second larval stage or L2. The L2 is also known as rhabditiform larvae and they feed for about 7 days and then get transformed into the third larval stage or L3 or the filariform larvae. This stage is the non-feeding and infective stage of the parasite. They are extremely motile and wait in search of human host to penetrate their skin. They can survive for about 2 weeks without the host. The larvae of N. americanus are known to infect humans only by penetrating through the skin while the larvae of A. duodenale can infect humans by skin penetration as well as by oral route. After the successful entry of the L3 in the human host it passes through the subcutaneous venules and well as the lymphatic vessels. They finally enter the lungs through the pulmonary capillaries and break out into alveoli. Then they enter the trachea and are coughed and swallowed by the host. Finally they enter the small intestine of the host where they moult and become adults. The whole process form the penetration of the skin till the development of the adult takes about 5-9 weeks. After mating the female starts releasing the eggs which are passed out with the faeces of the host. After entering the soil the eggs will hatch into the larvae and the whole cycle will be repeated. The incubation period varies from weeks to months depending upon the number of hookworms entered in the human host.