Tapeworms belong to the subclass Cestoda. These are flat worms measuring a few millimetres (Echinococcus granulosus) to several metres (Taenia saginata) in length. Structurally they consist of a head that is adorned with suckers and hooks (Taenia solium) or suckers alone (T. saginata). The head is attached via a short slender neck to several segments or proglottids that form a chain-like structure or strobila. The terminal proglottide is the most mature. The entire worm is covered with a continuous elastic cuticle. Tapeworms are devoid of a gastrointestinal tract or vascular system; nutrients are absorbed directly through the cuticle. They are hermaphrodites and crossfertilization between proglottids is frequent. Adults live in the intestinal tract of vertebrates, whereas the larvae (oncospheres) exist in the tissues of vertebrates and invertebrates. Infection is transmitted to humans by ingestion of meats infected with larval forms. Four tapeworms commonly infect humans: T. saginata, T. solium, Diphyllobothrium latum and Hymenolepsis nana.
Taenia saginata (beef tapeworm)infection
T. saginata measures up to 10 m in length, inhabits the upper jejunum, and is prevalent in humans in all beefeating countries. The majority of patients are asymptomatic. Symptoms are mild, with vague epigastric and abdominal pain, and occasional diarrhoea and vomiting. Weight loss is unusual. Rarely, appendicitis and pancreatitis due to obstruction of the appendix and pancreatic ducts, respectively, by the adult worms may occur. The commonest symptom is the presence of proglottids in the faeces, bed or underclothing.
The presence of proglottids, which are visible macroscopically, or the eggs, which are seen microscopically, in the faeces or perianal region is diagnostic. A higher positive yield is obtained by examining perianal clear adhesive tape swabs for ova, in which case the scolex (the head) or proglottids are required to establish the species.
Niclosamide 2 g as a single chewed dose is effective. Praziquantel 10 mg kg'” as a single dose is effective but is not available on the UK market.
Prevention is easily effected by careful inspection of beef for cysticerci (encysted larval forms). Refrigeration of beef at O°C for S days or cooking it at S7°C for a few minutes destroys the cysticerci.
Taenia solium infection and cysticercosis
T. solium (the pork tapeworm) measures up to 6 m in length. It has a worldwide distribution but is seen most frequently in Eastern Europe, South East Asia and Africa. In the adult form it lives in the human upper jejunum. The clinical features are similar to those caused by T. saginata. Treatment is similar to that for T. saginata. However, because release of ova can occur during treatment and, theoretically, could be carried back into the stomach, releasing the intermediate larval stage, treatment for T. solium should be followed by a saline purge.
Cysticercosis occurs after autoinfection or heteroinfection by eggs of T. solium and invasion of tissues by the intermediate larval form-cysticercus cellulosae. Cysticercosis is most commonly seen in parts of Asia, Africa and South America. Cysticerci may develop in any tissue in the body. Most commonly, however, three clinical forms are recognized:
1 Cerebral cysticercosis may present as various forms of epilepsy, as a space-occupying lesion or as focal neurological deficits including hemiplegia and behavioural changes.
2 Ocular cysticercosis may present as retinitis, uveitis, conjunctivitis or choroidal atrophy. Blindness may ensue.
3 Subcutaneous cysticercosis presents as small, pea-sized, hard nodules in the subcutaneous tissue. The diagnosis is established by biopsy of a subcutaneous ule and demonstrating the characteristic translucent embrane. Radiography may demonstrate calcified enerating cysticerci. CT brain scan should be pered when subcutaneous cysticercosis has been diag- Indirect haem agglutination tests are useful.
Treatment involves surgical excision of the cysticerci if possible. Praziquantel is the drug of choice for cyscosis; steroids are given during therapy to avoid reaction. Antiepileptic drugs are usually necessary for cerebral cercysticercosis.
Diphyllobothrium latum infection
DiphyIlobothriasis is particularly prevalent in Scandinavountries, the Baltic region, Japan and the lake region – itzerland. Infection in humans, the definitive host, from ingestion of fish that contain the infected ercoid form. The adult tapeworm measures several in length. The proglottids differ from those of tacnia in that they are more wide than long. The adult usually attaches itself to the jejunum.
clinical features are usually mild and consist of abdominal discomfort, anorexia, nausea and vomiting. Megaloblastic anaemia, due to competitive utilization of ingested vitamin B12 by the parasite, may occur small percentage of patients. Rarely, intestinal obstruction occurs.
Trearment is similar to that described for T. saginata.
Hydatid disease occurs when humans ingest the hexacanth embryos of the dog tapeworm Echinococcus granulosus or of E. multilocularis. Human infection with E. granulosus frequently occurs in early childhood by direct contact with infected dogs, or by eating uncooked, improperly washed vegetables contaminated with infected canine faeces. In the duodenum the hexacanth embryos hatch, penetrate the intestinal wall, enter the portal system and are then carried to the liver. Further dissemination of embryos to the lung and to almost every organ in the body may occur, where they form hydatid cysts. The disease is seen in all parts of the world particularly in those countries where sheep and cattle-raising constitutes an important means of livelihood. It is rare in the UK but is seen in the Middle East, North and East Africa, Australia and Argentina. These animals perpetuate the life-cycle of the parasite. Symptoms largely depend upon the site of the unilocular hydatid cyst. The liver is the commonest site for cyst formation (60%), followed by the lung (20%), kidneys (3%) and brain (l %). In the liver the majority of cysts are situated in the right lobe. The symptoms are those of a slowly growing benign tumour. Pressure on the bile ducts may cause jaundice. Rupture into the abdominal cavity, pleural cavity or biliary tree may occur. In the latter instance, intermittent jaundice, abdominal pain and fever associated with eosinophilia result. A cyst rupturing into a bronchus may result in its expectoration and spontaneous cure, but if secondary infection supervenes a chronic pulmonary abscess will form. Haemoptysis, dyspnoea and chest pain may lead to a mistaken diagnosis of malignancy. Focal seizures may occur if cysts are present in the brain. Renal involvement produces lumbar pain and haematuria. Calcification of the cyst occurs in about 40% of cases.
The alveolar hydatid cyst caused by E. multilocularis results from its larval stage. E. multilocularis is seen in parts of Canada, the former USSR and Alaska. Foxes and small rodents constitute the intermediate hosts; humans are accidental hosts. The majority of the lesions are in the liver and metastasis may occur. The diagnosis and treatment of hydatid disease.