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Man may become accidentally infected with E. granulosus by contact with infected dogs or by ingesting food, water, or soil containing eggs deposited in the feces of the dog, wolf, or other canines. Children, shepherds, and agricultural workers whose play and leisure habits or duties promote intimate human-canine relationships are particularly vulnerable. Contaminated uncooked foods (e.g. watercress or other salad vegetables) may play an occasional part in transmission. There is indirect evidence that direct inhalation can occasionally cause lung hydatids in man, and it has been shown that eggs administered to sheep via a tracheostomy develop into lung cysts

Once within man or other intermediate host, proteolytic enzymes in the digestive juices dissolve the external coats of the ingested eggs, which hatch in the duodenum to release oncospheres (true larvae) that attach to the jejeunal mucosa by hooklets and suckers. Oncospheres burrow into the submucosa by means of rhythmic muscular movements involving the body and hooklets, aided by secretions of the so-called penetration gland. Oncospheres then enter veins or lymphatics and those that travel in the portal system lodge in the capillary bed of the liver, which acts as an effective filter for most of the larvae; if, however, that barrier is overcome, the larvae pass through the inferior vena cava into the right side of the heart and then to the lungs. This accounts for the predominant involvement in those organs (liver approximately 50-70%, lungs 15-30%). If the worm is not trapped in either the liver or lungs, or if it travels in lymphatics and bypasses the liver and lungs, it may lodge virtually anywhere in the body, most notably in the peritoneum, spleen, kidneys, heart, brain, spine, bony skeleton, and muscles. Not every larva develops; over 90% are overcome by the host reaction.

The encysted oncosphere grows 1-5 cm in diameter per year, depending on the density of the host tissue. The cyst produces protoscolices which are released when a carnivore ingests the viscera of the intermediate host. The swallowed protoscolices evaginate to become scolices which attach to the proximal small bowel of the primary host, completing the life cycle.

If the larva is not destroyed, it develops into a tiny cyst containing a small amount of clear fluid (Fig. 3.5). This mainly hollow cyst grows steadily over a period of years, at times reaching enormous proportions before it becomes clinically evident (Figs. 3.6, 3.7, 3.8). The average rate of growth varies from 1 to 5 cm a year, being partly determined by the pressure resistance of its anatomical location. Thus, its rate of growth is more rapid in the lungs than in the liver, spleen, or other solid organs, and slowest in bones. If the cyst has room to develop easily, as happens in the liver, lungs or peritoneum, it almost always grows as the classic unilocular cyst. It can remain latent for a long period, up to 53 years having been reported.

Fig. 3.5 Larvae of E. granulosus and E. multilocularis. In the E. granulosus larva (left), note the outer ectocyst, the acellular hyaline cuticle, the germinal membrane lining the endocyst and, within the endocyst itself, the brood capsules with scolices and daughter cysts: note also the scolices with their heads evaginated or invaginated (lower left). By comparison, the E. multilocularis larva (right) buds outwards exogenously into the host tissue without the formation of an enveloping capsule. Note the numerous brood capsules containing invaginated scolices.

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Copyright: Palmer and Reeder