Site of infection: Adult tape-worms lay in the lumen of the small intestines of their definitive hosts, attached to the mucosa only by their scoleces. Larval stages (metacestodes) may develop in a range of tissues and organs in their intermediate hosts, particularly in muscles, visceral organs and sometimes the brain.

Pathogenesis: Infections in humans by the large adult tape-worms generally only involve 1-2 worms, and often do not involve any distinct symptoms, although there may be vague abdominal pains, with mild intermittent diarrhoea or constipation, and generalized allergic manifestations, including urticaria, anal pruritis, and eosinophilia. Infections by the encysted larval stages (cysticerci) do not appear to cause any severe clinical disease in their normal hosts (cattle and pigs) even when present in relatively high numbers. The cysts often occur in skeletal muscle, connective tissues of the skin and the liver, and while they may occupy space, they generally do not cause organ enlargement, tissue displacement or untoward pressure on surrounding areas. Degenerating cysticerci tend to calcify and are palpable in the tissues. Heavy infections by live and calcified cysts impart a measly appearance to the flesh and may lead to the condemnation of the carcase. Unfortunately, humans may also be infected with T. solium cysticerci through the process of self-infection when eggs are accidentally ingested (and possibly by retrofection when eggs carried upwards by reverse peristalsis hatch in the gut). Cysticerci may develop in virtually every organ and tissue of the human body, although they show an affinity for subcutaneous connective tissue, eye, brain, muscles, heart, liver, lungs and coelom. Humans are quite susceptible to pressure necrosis, particularly when cysticerci develop in the brain (neurocysticercosis with cerebral signs, headaches, seizures, and coma) or eyes (ocular signs, pain, and loss of vision). Degenerating cysticerci may elicit severe acute, and even fatal, inflammatory responses before their eventual calcification.

Mode of transmission: These tape-worms have indirect life-cycles: involving predator-prey transmission where carnivores acquire infections by ingesting larval stages in meat. Adult worms produce thousands of eggs which are excreted with host faeces. The eggs are very resistant to desiccation and sewage treatment and can live for weeks on pastures. They are ingested by intermediate hosts with contaminated feed, drinking water, or are physically transferred to the mouth. The eggs hatch releasing the oncospheres which use their hooks to penetrate the gut wall into the circulation where they carried mainly to the skeletal muscles and connective tissues. Over 3 months, they metamorphose into thin-walled cysticerci; each containing a single tiny protoscolex invaginated into the lumen. These encysted larval stages are transmitted to their definitive hosts by carnivorism, when infected meat or offal is consumed. After ingestion, the outer bladder is digested away releasing the protoscolex which evaginates, attachs to the small intestinal mucosa and grows into an adult in about 10 weeks. Adult worms may live for as long as 25 years and they will produce billions of eggs in that time.

Differential diagnosis: Intestinal infections in humans are diagnosed by the detection of gravid segments or eggs in faecal samples. The eggs of T. saginata and T. solium are identical, but the gravid segments of T. saginata are more active than those of T. solium, and they have more lateral branches of the uterus (15-32 compared to 7-13). Infections by cysticerci can only be seen and felt when in superficial locations. Modern medical imaging techniques (magnetic resonance imaging (MRI) and computerized axial tomography (CAT) scans) may detect cysticerci in soft tissues, while X-rays generally only detect calcified cysticerci. Immunoserological tests have been developed to detect host antibodies against purified antigens and appear to be sensitive and specific.

Treatment and control: Anthelmintic treatment is effective in killing adult tape-worms but does not kill eggs. Single doses of praziquantel or niclosamide can cure infections in definitive hosts, while daily doses of praziquantel given for 1-2 weeks are effective against larval cysticercosis in intermediate hosts. Mebendazole and albendazole also appear to be effective against adult and larval stages. The prevention of infections involves breaking the transmission cycle; through stringent meat inspection for ‘measly’ meat, condemnation of infected carcases for human consumption, proper cooking or freezing of meat (pickling meat often does not kill larvae), sanitary disposal of faeces, prohibiting the use of sewage for fertilizing pastures, washing salad vegetables and strict personal hygiene.

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