Proptosis caused by hydatid disease.

MODE OF DEVELOPMENT.-This disease is also termed echinococcosis, or hydatid cyst formation; it results from the ingestion of food contaminated with the eggs of the tape-worm Echinococcus granulosus. The embryo is released in the intestines, attaches itself to the mucosa, and makes its way into the portal circulation, by which it is carried to a capillary barrier, where it settles down, and, if not overcome by the reaction of the surrounding tissue, develops into a " hydatid cyst ". This hydatid cyst is the larval form of the tape-worm, man being the intermediate host. The adult worm is usually harboured by the dog, which is one of the main definitive hosts. The commonest site for hydatid cyst in man is the liver. Statistics in endemic areas show that in man hydatid cysts of the liver constitute 75 per cent. of cases. This is because, on account of their size, most of the embryos cannot pass through the capillary connection between the portal and hepatic veins and therefore settle in the liver. The smaller embryos are carried by the blood to the hepatic vein, reach the pulmonary circulation, and meet with a second barrier in the pulmonary capillaries. Here they are filtered and some have to settle and develop into hydatid cysts there. In man these form about 5 to 10 per cent. of the cases. Embryos that escape the two barriers of the liver and lung capillaries are carried to the left side of the heart and distributed to various parts of the body. Statistics show that the kidneys and brain are the two main organs infected in this way. The orbit is one of the rarest sites to be involved; according to Khalil (1939), the incidence of hydatid disease of the face, orbit, and mouth together, ranges between 0.8 and 2.3 per cent. in endemic areas.