Echinococcosis.

Echinococcosis is a human disease caused by the larval form of Taenia echinococcus, which lives in the gut of the dog, wild canides and other carnivorous animals which represent the definitive hosts and involves as intermediate hosts both domestic and wild animals. Humans become accidental intermediate hosts by ingesting Taenia eggs. The main species pathogenic for man are E granulosus causing cystic echinococcosis with worldwide distribution and endemic in sheep and cattle breeding countries, and E multilocularis causing alveolar echinococcosis, with preferential distribution in the northern hemisphere. After ingestion of contaminated food, hexacanth embryos migrate by the portal system to liver and later lung, brain and other tissues. Symptoms are related to both cyst location and size. E granulosus infection of the central nervous system (CNS) may be primary or secondary and has been estimated to be low (2%). Sharply demarcated, spherical and intraparenchymal, cysts may reach a large size causing neurological symptoms. Spilling of cyst fluid due to trauma or surgery may trigger anaphylaxis as well as disseminated infection. Host reaction is minimal in the brain but a foreign giant cell reaction may develop. E multilocularis develops within the liver as a rapid invasive pseudomalignant growth and may metastasize to the CNS, where estimated incidence reaches 5%. Hydatid antigens induce an immune reaction in the host which is helpful for the diagnosis. DNA probes and PCR may be applied to differentiate between Echinococcus spp. Although the host develops an immunological protection from reinfection, the parasite evades host immune attack. A wide range of evasion mechanisms have been advanced, including a barrier for host cells due to hydatid cyst laminated cuticle, polyclonal activation of lymphocytes by parasite soluble antigens, and depression of host cell immune responses. Chronic stimulation of the host by cyst fluid antigens leads to increased specific IgG4 production, which might act as blocking antibodies against anaphlaxis suggestive of host response immunomodulation.