Taenia solium Pork Tapeworm Figure 236

Taenia solium is the most important human tapeworm, because it is the only intestinal parasite responsible for human cysticercosis as well as pig cysticercosis. T. solium continues to be an endemic parasitic disease in many countries of Central and South America, South Africa and Asia. Humans are the only natural definitive host. Tapeworms attach to the epithelial wall of the small bowel and grow in segments (proglottids) which contain male and female sex organs. Its importance resides in the capacity of the embryos to traverse the intestinal wall and lodge in muscle masses or in the brain, where they develop into the larval (metacestode) stage of the

Fig. 23.5 Life-cycle of Taenia saginata

parasite. In humans, neurocysticercosis (NC) is by far the most important disease caused by this parasite. Pigs are the intermediate host for the larval stage, which they acquire by ingesting feces containing adult tapeworm proglottids. The life-cycle thrives in rural areas with poor sanitation, without water or drainage and where pigs are left to roam and scavenge on human excrement and garbage. It has been recognized for many years that the larval stage can survive for long periods in the host before being destroyed or attacked by the immune response. The classic work of Dixon and Lipscomb (1961), showed that British soldiers returning in 1948 from India, a country with a high prevalence of Taenia solium, took an average of 2-5 years to develop symptoms of NC, suggesting that the parasite, lodged in the nervous tissue, either does not release antigens or has evasion mechanisms that allow survival for long periods.

The adult worm has an armed scolex (Figure 23.7) which consists of a rostellum bearing two rows of hooks (22-32). Recent experimental evidence, obtained by infection of golden

Metacestode (Cysticercus)in brain, connective tissue, eye, muecles, lungs

Circulation

Adult in small intestine

* Scolex attaches to intestine

Oncosphere hatches penetrates intestinal y WO II

Gravid proglottid in feces or environment

Auto-infection

Embryonated eggs or proglottids k occasionally ingested

Metacestode (Cysticercus) in muscle ( infective stage)

in feces

Circulation

Embryonated eggs or proglottids ingested

Oncosphere hatches, penetrates intestinal wall

Fig. 23.6 Life-cycle of Taenia solium

Fig. 23.7 Light micrograph of Taenia solium whole worm preparation, to show scolex with four suckers (s) and rostellum with double row of hooks (arrow)

hamsters, has shown that the scolex implants in the upper third of the duodenum (Merchant et al., 1998), by engulfing intestinal villi in the four suckers (Figure 23.8) and burrowing the rostellar pad into the crypts of Lieberkuhn of the sub-mucosa, similar to what has been described for the dog tapeworm, Echinococcus granulosus (Figure 23.9).

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