gl4 ANIMALS AND THEIR ENVIRONMENT 



intracellular in the vertebrate host. These are responsible for a variety 

 ot tropical sores and ulcers where the skin and underlying tissue have 

 been destroyed. One species, L. donovani, invades the inner body tis- 

 sues, especially the spleen, producing a disease known as kala-azar. 

 Fever, jxan and anemia are lol lowed by progressive emaciation ot the 

 body while the spleen becomes enlarged. Untreated cases are 95 per 

 cent fatal. Within the last twenty years, however, drugs have been 

 found which reduce the mortality rate to 5 per cent or less. 



Sporozoans are common parasites ot the intestinal tract of arthro- 

 pods, infecting the individual cells of the lining. Other species infect 

 the intestinal cells of vertebrates, including all the domestic mammals 

 and birds. The most important of these belong to the order Coccidia 

 and produce a disease called coccidiosis. fn wild animals they are not a 

 serious problem because the spores are shed in the feces and must be 

 eaten to cause reinfection. Domestication often forces animals into a 

 closer association with tlieir excrement than is natural, and the con- 

 tamination of food by feces is common. Chickens particularly suffer 

 from the conditions imposed upon them. If too many of the intestinal 

 cells are destroyed at once the animal suffers weakness, diarrhea, bloody 

 feces, loss ot appetite, and often death. 



Another group of sporozoans, the order Haemosporidia, pass a 

 part of their lite cycle as intracellular parasites ot blood cells and an- 

 other part in an arthropod bloodsucker. The malarial parasites of man, 

 described earlier (Fig. 6.1), belong to this group, in regions where 

 malaria is common it is typically a chronic disease. Those infected 

 suffer periodic relapses of fever, weakness, and a general decrease in 

 resistance to other diseases. The fever produced wlien malarial parasites 

 burst from one set of blood cells and infect a new set is liigh enough 

 to be deleterious to other parasites, notably the bacterial spirochete 

 producing syphilis. In tact, several tropical tribes liave been found in 

 which all the individuals have both syphilis and malaria. The people 

 have some resistance to malaria so that it is not a serious illness, and 

 suffer very little from syphilis because the malarial fevers keep it under 

 control. When some of these individuals were cured of malaria their 

 syphilis immediately became worse. Before the discovery of penicillin 

 a mild form of malaria was used in American hospitals as one means of 

 controlling advanced cases of syphilis. 



Intracellular nematodes are common and sometimes serious para- 

 sites of plants. 



The insidious parasitic diseases of man which have a widespread 

 distribution are preponderantly blood diseases. Malaria, caused by an 

 intracellular parasite of red blood cells, has been the most serious 

 world-wide parasitic disease but modern medicine has somewhat re- 

 duced its importance. Schistosomiasis, caused by trematodes which live 

 in blood vessels and eat blood, remains a medical challenge. The extent 

 of its damage in regions where most people are infected is vuiknown. 

 Hookworm disease, caused by bloodsucking parasites in the intestine, 

 and amebiasis, caused by Entamoeba histolytica eroding the intestine 

 and eating red blood cells, are both extremely widespread diseases. 



