Some adult nematodes live within the intestinal lumen. The disease is spread to humans:
PASSIVELY by ingesting infective eggs, as occurs with Ascaris lumbricoides (’roundworm’), Trichuris trichiura (whipworm) and Enterobius vermicularis (threadworm) ACTIVELY by percutaneous spread of filariform larvae that penetrate the skin (hookworm and Strongyloides) Ascaris deviates from the simplified life-cycle shown in that it invades the duodenum and enters the venous system, via which it reaches the lungs. The worm is eventually expectorated and swallowed, entering the intestine where it completes its maturation. Strongyloides is the only nematode that is able to complete its life-cycle in humans; its rhabditiform larvae, which hatch in the intestine, are able to reinfect the host by penetrating the intestinal wall and entering the venous system.
Strongyloides stercoralis is found worldwide but is particularly common in warm, wet regions such as parts of Central America and South-East Asia. Infection can persist for decades and is still being discovered in war veterans, particularly prisoners of war who worked on the Burma- Thailand railway and veterans from Vietnam. Adult worms inhabit the crypts of the small intestine, causing little damage, but in heavy infection worms are embedded in the mucosa, and cause an inflammatory response with mucosal injury. The worms are passed in the stools and autoinfection is common.
After penetration of the skin by the filariform larvae, a local reaction occurs characterized by itching, erythema, oedema and urticaria. This subsides within 2 days. A week later, migration of the adolescent worms causes irritation of the upper airways, producing cough and occasionallymore severe respiratory symptoms. After about 3 weeks, intestinal colonization occurs, often leading to abdominal discomfort, intermittent diarrhoea and constipation. These symptoms can be mild and may pass unnoticed. However, in some individuals, heavy infection may lead to persistent diarrhoea, nausea, anorexia and evidence of intestinal malabsorption, notably steatorrhoea. Hypoalbuminaemia and weight loss also occur. Disseminated strongyloidiasis is a very serious and often fatal condition and has been described in patients receiving cortico-steroid or other immunosuppressive therapy or in those who are irnmunocompromised for other reasons.
Motile rhabditiform larvae can be detected in fresh stool or in duodenal aspirate. Eosinophilia is common. In heavy infection, anaemia and biochemical evidence of malabsorption are found.
Treatment consists of thiabendazole 1.5 g twice daily for 2 days. Therapy should be given for at least 5 days (often longer) in the hyperinfected patient with disseminated disease. Albendazole 400 mg kg” for 3 days is also effective. Repeated therapy may be required. The mortality is high in the hyperinfected group owing to an accompanying Gram-negative septicaemia and treatment should include i.v. broad-spectrum antibiotics. Hookworm infection Hookworm is seen worldwide and affects approximately 25% of the world’s population. Ancylostoma duodenale is found in Europe, the Middle East and North Africa, whereas Necator american us is found in the Western Hemisphere, sub-Saharan Africa, South-East Asia and the Far East.
Adult worms inhabit the small intestine and attach firmly to the intestinal mucosa by the teeth or cutting plates in their large buccal capsule. Blood loss is approximately 0.2 ml daily in A. duodenale infection (five- to tenfold less with N. americanus); in heavy infection it has been estimated that up to 100 ml of blood is lost daily.
Local irritation at the site of larval entry in the skin is known as ‘ground itch’, but this rapidly disappears to be followed some 2 weeks later by mild and transitory pulmonary symptoms. Most patients are asymptomatic once the larvae have reached the small intestine. Some patients experience ulcer-like symptoms and those with heavy chronic infection eventually develop symptoms and signs of anaemia. Hookworm infection is the commonest cause of iron deficiency anaemia worldwide. A. braziliensis (dog hookworm) causes characteristic patterns of subcutaneous infection in children.
Hookworm ova appear in the stool, the number of eggs present giving a guide to the severity of the infection. Early in the infection, eosinophilia may be found in the peripheral blood. This is followed later by the appearance of iron deficiency anaemia.
Mebendazole 100 mg twice daily for 3 days is effective in ooth types of hookworm, although the infection may not be cleared with a single course of treatment.
Ascaris lumbricoides (’roundworm’)
A lumbricoides is a large worm that is found orldwide but is particularly common in poor rural communities where there is heavy faecal contamination of the immediate environment. Infection may be entirely asymptomatic, although heavy infections are associated with nausea, vomiting, abdominal discomfort and anorexia. Worms may obstruct the small intestine, the commonest site being at the ileocaecal valve. Worms occasionally invade the appendix, causing acute appendicitis, or the bile duct, resulting in biliary obstruction and suppurative cholangitis. Larvae in the lung may produce pulmonary eosinophila. The nutritional impact of Ascaris infection in children is controversial, although it is very likely that heavy infection in malnourished children compounds the situation, largely by competition for host nutrients.
Ascaris eggs may be identified in the stool and occasionally adult worms emerge from the mouth or the anus.
Mebendazole 100 mg twice daily for 3 days or a single dose of piperazine 100 mg kg-lor pyrantel pamoate 10 mg kg-l are effective. Surgical or endoscopic intervention may be required for intestinal or biliary obstruction.