(1995; 152 pages) [French] [Spanish]
Cestode (tapeworm) infection
Intestinal taeniasis and cysticercosis
Two tapeworms that commonly infect cattle and pigs, Taenia saginata and T. solium, are responsible for intestinal infections in humans. The latter is also responsible for a systemic infection, cysticercosis. T. solium occurs in Central and South America, south-east Asia and some parts of eastern Europe. T. saginata occurs worldwide but its prevalence is particularly high in sub-Saharan Africa.
Intestinal infection occurs when the larval form (cysticercus) is eaten live in raw or undercooked meat. The evaginated scolex anchors to the mucosa of the small intestine where it matures into a segmented adult worm capable of living for many years. Distal segments loaded with eggs become detached and are excreted in the faeces. The transmission cycle is completed when eggs are ingested by pigs or cattle reared for human consumption.
Most infected individuals harbour only one worm, but this may become so long as to disturb gastrointestinal motility and secretions and to cause abdominal pains and nausea. Obstructive appendicitis or cholangitis also occurs occasionally in T. saginata infections as a result of aberrant migration of some segments. Otherwise, infection becomes evident only when segments are passed in the faeces.
Cysticercosis results when eggs of T. solium hatch in the intestine, and the larvae become invasive and spread systemically. Infection occurs when food or hands become contaminated with viable eggs or, rarely, when eggs from an intestinal worm are carried against the normal peristaltic flow towards the duodenum. Oncospheres liberated from hatched eggs penetrate the intestinal wall and are carried in the bloodstream to subcutaneous tissue, skeletal muscle and the brain (neurocysticercosis) or, less frequently, to the eye and the myocardium. Where infection is highly prevalent the disease is a common cause of epilepsy, raised intracranial pressure and focal neurological signs.
Prevention of transmission of T. solium is particularly important because of the risk of cysticercosis. This requires treatment of infected persons, health education, adequate treatment and disposal of sewage, and an effective system of meat inspection. Short-term chemotherapy of selected groups at high risk has also been used with good effect where the prevalence of the disease is particularly high.
Praziquantel is well tolerated and extensively absorbed. It kills adult intestinal worms in a single dose, and also T. solium cysticerci when taken for 3-7 days in high doses. It thus offers the prospect of a cure of neurocysticercosis, which was previously treatable only by surgery, anti-inflammatory corticosteroids and use of anticonvulsants. However, because dying and disintegrating cysts may induce localized cerebral oedema, treatment with praziquantel must always be undertaken in a hospital setting. Albendazole, which is an alternative to praziquantel, kills cysticerci, but only when administered at a daily dosage of 15 mg/kg for 30 days. The longer-established compound niclosamide acts only against the adult intestinal worms.
Hymenolepis nana, or dwarf tapeworm, commonly infects children in arid regions of the tropics and subtropics, particularly in Asia. Eggs are transmitted in food and water or on contaminated hands, either from person to person or in an autoinfective cycle. The eggs hatch in the intestine to release larvae which mature in the villi to release further generations of eggs. Autoinfection and rapid reproduction of the worms in situ result in a brisk increase in the worm population particularly in malnourished or immunocompromised children, who become more weak and emaciated and suffer from diarrhoea and abdominal pain.
Praziquantel is more effective than niclosamide, although resistance to praziquantel has already been reported. Repeated treatment may be necessary to cure intense infections or to eliminate the parasite within a family group or institution.
Larvae of Diphyllobothrium latum and D. pacificum, the fish tapeworms, are transmitted to humans in raw, infected freshwater fish. D. latum is endemic in unpolluted lakes and deltas of the northern hemisphere and D. pacificum occurs in south America.
The ingested larvae mature into adult worms after anchoring to the intestinal mucosa. Most infections are asymptomatic but, because the worm competes for vitamin B12, a few carriers develop a macrocytic, megaloblastic anaemia.
Prevention depends upon adequate inspection and cooking of fish.
Niclosamide or praziquantel in a single dose is highly effective. Hydroxocobalamin injections and folic acid supplements may be required.
Most cases of echinococcosis (hydatid disease) in humans are caused by two species of the Echinococcus tapeworm:
• E. granulosus, the cause of cystic echinococcosis, which is maintained worldwide in a domestic transmission cycle involving dogs and livestock, mainly sheep.
• E. multilocularis the cause of alveolar echinococcosis, which is endemic in the northern hemisphere in foxes, dogs and other carnivores and field rodents.
When eggs excreted by infected animals are ingested by humans they hatch in the intestine to release invasive oncospheres. These, according to the species, develop further by either expansion (cystic echinococcosis) or infiltration (alveolar echinococcosis) in the liver, lungs or other organs. The clinical sequelae of cystic echinococcosis usually result from pressure, but rupture of a cyst can invoke allergic reactions. Alveolar echinococcosis, particularly of the liver, can become metastatic and is frequently fatal.
The intensity of transmission of cystic echinococcosis has been reduced in some endemic foci by the destruction of stray dogs and by mass treatment of dogs and foxes with praziquantel to eliminate the adult tapeworm population.
Although surgery is still the treatment of choice for operable cystic disease due to E. granulosus, chemotherapy with benzimidazoles, such as mebendazole and albendazole, may be of value prior to surgery and in inoperable cases. Alveolar disease due to E. multilocularis may require both surgery and long-term treatment with either mebendazole or albendazole to inhibit metastatic spread.