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Worm long, slender, soft-bodied, legless, bilateral invertebrates including the flatworms, roundworms, acanthocephalans, ribbon worms (nemerteans), and annelids. Many of the worms are parasitic, but there are also a large number of free-living forms occurring in the soil and in aquatic habitats.

Filarial worm Long and thin nematode parasite belonging to the super-family Filarioidea. Among those causing harm, Wuchereria bancrofti infection occurs in varying degrees in parts of Bangladesh. But the northern districts appear to have a high prevalence. Brugia malayi was reported once only from the Chittagong Hill Tracts. Both W. bancrofti and B. malayi appear in humans, and are transmitted by the mosquito, produce the disease elephantiasis or filariasis. Some other filarial worms are found in monkeys in Bangladesh. W. bancrofti and B. malayi live in the lymphatic system. W. bancroti males are about 40 mm long and the females are about 82 mm. Females can produce thousands of larvae which are called microfilariae. The microfilariae leave the lymphatic ducts and appear in the blood stream between 10 PM and 2 AM. The appearance in the blood stream coincides with the appearance of the mosquito at night.

When mosquitoes feed on the blood, microfilariae are ingested also. They undergo development in the mosquito and enter a person during a blood meal. The microfilariae mature in the lymph vessels. Adult worms may block the flow of lymph and cause the lymphatic ducts to swell. Elephantiasis is the enlargement of the scrotum, breasts or legs. Hydrocoele is a condition where the lymph fluid is forced into the lining of the testes causing swelling. Varicose develops when lymph fluid return is obstructed. Milky urine develops due to lymph being forced out.

Several genera of mosquitoes transmit W. bancrofti. In Bangladesh Culex quinquefasciatus is the known vector and Cx. pipiens fatigans has also been incriminated with filariasis. Prevalence in them can vary from 0.2 to 45%.W. bancrofti infection has been reported from Dhaka city, Thakurgaon, Nilphamari, Dinapjur, Rangpur, and Rajshahi districts. Fifteen to 30 year old people are most vulnerable. The prevalence of infection can range from 7.2% to 32.8%. The northern districts appear to have a higher prevalence rate than other areas. B. malayi and W. bancrofti look alike, but the male B. malayi is smaller (13 mm). The microfilaria are also different. The worms live in the lymphatics and elephantiasis can occur chiefly in the arms and legs. The vector is a mosquito belonging to the genera Mansonia, Aedes, and Culex. [Joseph D'Silva]

Hookworm Any of a number of blood sucking nematodes in the phylum Nematoda, order Strongiloidae that live as parasites in humans and other mammals, and attach themselves to the host's intestine by means of hooks. The body of these worms is slightly curved anteriorly; hence the name 'hookworm'. Hookworm infection in man is caused by infestation with Ancylostoma duodenale or with Necator americanus. A number of species infest cattle, pig, dog, cat and other animals. Heavy infections lead to bleeding, anaemia, and loss of iron. Males are 8 to 11 mm and females 9 to 13 mm long. A female hookworm can lay 5,000 to 10,000 eggs daily. In Bangladesh hookworm infections have been reported from Comilla, Noakhali, Dhaka, Barisal, Dinajpur, Rangpur, and Mymensingh. Eggs are found both in soil and in stool. Prevalence may be as high as 50%. Adults are more susceptible than children; males tend to be more infected than women, perhaps because of their preoccupation with farm work. Infection by N. americanus is more common than by A. duodenale. Ignorance, poor hygiene, and general living conditions allow hookworm infection to be present in Bangladesh.

Hookworm infection is more prevalent in rural areas where people do not wear shoes or stockings, and where the soil is contaminated by human excrement. The larva of the hookworm, living in moist soil or mud, easily penetrates the exposed skin; usually the sole of the foot, and is then carried by blood to the lungs. The early sign of hookworm infestation is a dermatitis at the site of entry, commonly known as 'ground itch'. The human hookworm causes anaemia, weakness, and abdominal pain.

A. caninum is the common hookworm of dog, reported in cat and jackal. Incidence in dogs may be as high as 28%. Bunostomum bovis infests cattle. The rate of infection is about 5% in Bangladesh. [AWM Shamsul Islam and Joseph D'Silva]

Roundworm Common name for members of the phylum Nematoda; in popular parlance however, the term refers to the worm appearing in man, namely, Ascaris lumbricoides. But roundworms in man should also refer to Trichuris trichura, the whipworm; Enterobius vermicularis, the pinworm; Ancylostoma duodenale, the hookworm; and Strongyloides stercoralis, the threadworm all of which occur in the intestine.

In Bangladesh, rural and urban populations, children and tertiary level students harbour one or more of these roundworms. Most of the data comes from studies carried out in Dhaka city and its periphery, especially on children. The occurrence of roundworms has been linked to nutritional deficiencies, low weight and low haemoglobin. Poor sanitary conditions, illiteracy, ignorance, environmental conditions, perception of mothers all contribute to the high rate of roundworm infection in Bangladesh. The rate of A. lumbricoides infection can be as high as 92% or as low as 25%; infection occurs when its eggs are swallowed. The eggs are voided in the faces and carried in the water.

Open latrines in the villages are the biggest source of infection in Bangladesh. Eggs can appear in soil and also on leafy vegetables. Uncooked vegetables, water and unwashed hands can be sources of infection. Each female A. lumbricoides can lay up to 250,000 eggs per day, and eggs can remain viable buried in the soil for several months. Whipworm infection is due to a roundworm T. trichura. It is called a whipworm because the posterior end is rather large and the anterior two-third of the body is threadlike. The worms are 30 to 50 mm long, the male being slightly shorter than the female. Each female can lay 3,000 to 50,000 eggs per day. The eggs enter the body when swallowed with food or water. [Joseph D'Silva]

See also ascariasis; nematode.