PROTOZOA CAUSING DISEASE 417 



(Coccidium) avium, the cause of coccidiosis in domestic 

 fowls and in birds. The disease is widely distributed over 

 the United States and Europe, but has not been extensively 

 studied. The organism is taken into the body with food in 

 the form of a cyst which ruptures and allows the escape 

 of spindle-shaped protozoa. These burrow into the epi- 

 thelial cells of the intestinal walls or other membranes as in 

 the caeca. Upon entering a cell the spore rounds up into a 

 sphere and grows rapidly in size. The nucleus of this cell 

 breaks into a number of pieces and the protoplasm itself 

 divides into a considerable number of spindle-shaped cells. 

 These break away from the mother cell and in turn invade 

 new cells. This procedure may be repeated several times. 

 Eventually some of them develop as sexual reproductive 

 cells. Some of these are larger than the remainder, the 

 so-called macrogametes or eggs and the microgametocytes 

 which resemble macrogametes at first but divide into a 

 large number of very slender threadlike cells termed micro- 

 gametes. These are freed by the rupture of the cell and 

 the macrogametes are fertilized by the microgametes to 

 form an oocyte. This continues to enlarge and secretes a 

 firm wall (becomes encysted). When completely mature, 

 the contents of the cyst divide to form several spherical cells 

 or bodies. These elongate and become spindle-shaped, and 

 in each, two small, slender spores develop. When the cyst 

 is mature it is passed out with the feces of the bird. If 

 these cysts come in contact with food, they may be ingested 

 by a suitable host, whereupon the cycle begins again. 



Organisms somewhat resembling the form described are 

 known to produce coccidiosis in the rabbit, in cattle, in 

 sheep, and in the dog and cat. 



