170 



THE RISE OF ANIMAL LIFE 



Fig. 9-n. A blood fluke normally infecting water birds occasionally enters human sicin, causing "swimmer's itch' 



(schistosome dermititis). 



cidia which penetrate the tissues of snails, 

 and follow stages of development similar 

 to those of Opistliorchis with minor varia- 

 tions. Instead of encysting on a fish, the 

 cercaria burrows through the skin of a per- 

 son who is unfortunate enough to be in the 

 vicinity and makes its way into the vascular 

 system. It passes through the heart, lungs, 

 and liver, eventually maturing in the blood 

 vessels ( veins ) that drain the intestines and 

 bladder. Here it grows rapidly, feeding on 

 blood, and when sexually mature lays its 

 eggs, thus completing the cycle. 



The several species of blood flukes infest 

 the populations of tropical America, many 

 parts of Africa, and the Orient, particularly 

 China. In some irrigated regions the infec- 

 tion nms as high as 90 per cent among the 

 adult males who are constantly in contact 

 with the water, hence exposed to the cer- 

 cariae. Treatment consists of giving large 

 doses of antimony compounds and, if the 

 patient can stand the treatment, he can be 

 cured. Like all of the complex parasites, 



blood flukes can be controlled by removing 

 the intermediate host. 



Water birds such as ducks, terns, and 

 Sulls, have their own varieties of blood 

 flukes which apparently cause them no par- 

 ticular harm. However, if this type of cer- 

 caria cannot find its proper final host, it 

 does penetrate the skin of any person who 

 happens to be near, causing a severe itch- 

 ing which has been called schistosome der- 

 mititis, or just "swimmer's itch" (Fig. 9-11). 

 The cercariae apparently are not able to 

 penetrate the tough thick mammalian skin, 

 but in their attempts to do so enter it and 

 cause intense irritation. There are several 

 different species of cercariae that follow 

 this pattern. Some are found on the sandy 

 bathing beaches in the lake regions of the 

 North Central states, especially Michigan 

 and Minnesota, where they sometimes be- 

 come such a nuisance that bathing is actu- 

 ally prevented, much to the disgust and 

 economic loss of resort owners. Elaborate 

 methods of treating the beaches with cop- 



