Tapeworms Down Under and Elsewhere

While I was a graduate student in Canberra being introduced to the world of parasites, Australia was then battling more important parasitic diseases than the sparganum which I had studied, which affected only small communities in SE Asia. The sheep industry was very important to Australia’s economy and it faced several serious tapeworm diseases at that time. One of those was hydatid disease (or cystic echinococcosis) which is caused by the tapeworm Echinococcus granulosus (Fig. 2.1). This was a serious agricultural, medical and public health problem in Australia when I was a graduate student in the 1960s and it still remains the sort of public health issue that causes local health and veterinary authorities in Australia to post periodic health advisories in rural and farming communities. As tapeworms go, the adult tapeworm of E. granulosus is relatively small, only 6–7 mm (around a quarter of an inch) long, and lives in the small intestines of domestic dogs and in wild canids like wolves, jackals, hyenas and coyotes in different parts of the world. The scolex of the tapeworm is embedded firmly among the villi in the small intestines of the host by four suckers and a crown (or rostellum) of 28–50 hooks. There the adult tapeworm will mature and the segment (or proglottid) at the posterior end will produce and release eggs throughout its life in the intestines. The eggs are passed out with the dog feces and if they are swallowed by sheep, or the unfortunate human incidental host, a tiny oncosphere or hexacanth larva (so called because it has three pairs of tiny hooks) will hatch out and penetrate the gut of the sheep using the hooklets like the pitons of rock climbers to migrate in the tissues. They will eventually enter a blood or lymph vessel and be carried in the blood stream or lymphatic system, usually to the liver where the young embryo will proceed to grow within a cyst known as a hydatid cyst (Fig. 2.2). For the cycle to complete, the cysts will have to eventually be ingested by the canine host, where the hydatid cyst will evaginate (unfold), the scolex attach to the mucosal surface of the intestine of the dog and grow into a new tapeworm. Notice here that the Cyclophyllidean cestodes described here bypass the need for an aquatic stage and had evolved a life cycle that allows them to pass directly from intermediate to final hosts.