ROBERT HEGNER 675 



and nowhere can it be considered common. Some natural resistance of the human 

 host apparently accounts for the small number of infections, but just what factors 

 are responsible for this resistance is not known. 



Under extraordinary conditions Balantidium coli may attack the tissues of the 

 intestinal wall, producing ulcers that resemble the lesions caused by Endamoeha his- 

 tolytica and resulting in balantidial diarrhea or dysentery. It is not certain just how 

 these ulcers are produced. Walker (1913) believes that the ciliates bore their way 

 mechanically into the intestinal wall and then dissolve away the surrounding cells by 

 means of ferments which they secrete. Certain other investigators believe that the 

 organisms penetrate the intestinal wall by breaking down the cells, as Endamoeba 

 histolytica is supposed to do. Carriers of Balantidiiwi coli exist among human beings, 

 but Aguilar (1926) has reported evidence indicating that all individuals infected 

 exhibit symptoms at some time during the infection. 



According to our present knowledge of Balantidium coli, this species exists not only 

 in human beings but in a large percentage of pigs in various -parts of the world, and 

 occurs in several species of monkeys. There is some evidence that the ciliate may be 

 pathogenic to pigs, but this apparently is not the usual condition, Brumpt (1909) and 

 Walker (1913) proved that specimens from monkeys were infective to pigs and that 

 specimens from pigs were infective to monkeys. Walker was also able to infect mon- 

 keys with specimens from man. According to Scott (1925), the balantidium that 

 occurs in guinea pigs is morphologically indistinguishable from Balantidium coli and 

 may actually be the same species. 



Epidemiological studies of balantidiosis in man indicate that in most cases human 

 beings are infected by ingesting organisms from pigs. Strong (1904) found that 25 

 per cent of the cases described in the literature up to that time had been associated 

 with pigs or had eaten or prepared fresh sausage. Since then many cases have been 

 reported in persons who had worked with pigs. Infection can thus be prevented by 

 the observance of greater cleanliness by those associated with pigs and by the 

 thorough cooking of pork products before their use as food. 



HOST-PARASITE RELATIONS OF COCCIDIA 



The human coccidium, Isospora hominis, is known only in the oocyst stage but 

 il probably resembles in its life-cycle the similar parasite, Isospora Jells, that occurs 

 in cats. The oocysts are extremely resistant and probably remain alive for a con- 

 siderable period outside the body. During this time they may contaminate food and 

 drink and be ingested by human beings. Human infections, however, are very rare. 



It is known that Isospora felis of the cat attacks the epithelium of the small 

 intestine and is thus always pathogenic. Isospora hominis no doubt similarly attacks 

 the intestinal wall of man. The best account of a human infection with this species is 

 that of Connal (1922), The incubation period in this case lasted for six days, at which 

 time diarrhea commenced. Thirty-one days later stools became formed. Oocysts ap- 

 peared in the feces twenty-two days after the beginning of the diarrheic condition and 

 were found daily for thirteen days. No more oocysts were observed and no more 

 symptoms exhibited by the patient after the end of this period. The appearance of 

 symptoms in cases of coccidiosis before oocysts are present in the feces is a peculiarity 



