824 PATHOGENICITY 



shown that parasitic Protozoa differ not only in the response they evoke 

 from the host according to their standing as species, but also according 

 to strain properties. There is likewise abundant evidence that individual 

 hosts differ in their response to the same strain of parasitic protozoon. 

 The latter is in reality a statistical concept. It has been a general experi- 

 ence that when an attribute of an unselected group of individuals was 

 measured, the plotted measurements fell into the well-known frequency 

 distribution curve, either normal or skewed. The writer knows of no 

 data which have been plotted to demonstrate that quantitative data on 

 either individual resistance or susceptibility to adverse effects of parasit- 

 ism could be presented in a similar sort of graph, but there are many 

 facts, to support such a supposition. 



COCCIDIOSIS IN POULTRY 



For several years the writer (see Becker and Waters, 1938, 1939b) 

 has been testing the effect of the ration on the course of caecal coccidiosis 

 in chicks. While, in general, fatality was used as the criterion for com- 

 paring the effects of two rations, it has been possible to make a number 

 of additional hitherto-unpublished observations bearing on the variability 

 of host response to the disease. In one lot of thirty-three White Leg- 

 horn chicks experimentally infected with the same dosage, there were 

 three deaths by eight o'clock in the morning of the fifth day, and five 

 more during the remainder of that day. The next day nine succumbed, 

 making a total of seventeen. Nine others were noted to be in an ex- 

 tremely precarious condition, missed succumbing only by a narrow mar- 

 gin, but recovered to a considerable degree. Five others were observed 

 to be severely affected, but continued to move about and eat some feed 

 during the entire ordeal. One was quite active throughout, though its 

 comb paled significantly. One, a cockerel, continued to eat and move 

 about with undiminished vigor, and its comb did not pale perceptibly, 

 though the droppings were streaked slightly with blood. Similar observa- 

 tions have been common, and justify the assertion that fowls differ sig- 

 nificantly in the morbidity they exhibit in response to uniform dosage 

 with the same strain of Coccid'tum. 



The literature is replete with evidence that similar variability of host 

 response exists in the case of other protozoan infections. Walker and 

 Sellards (1913) early distinguished between "contact" carriers of En- 



