Byerly T: Studies of resistance to pullorum diseases in chickens. Poult Sci

RESISTANCE to disease is a variable- factor. Operating together with changes in organism virulence, it is re-sponsible for the severity of any given dis-ease attack. Experimental evidence bring-ing more light to bear upon the conditions under which pullorum resistance varies may prove a valuable aid to brooding and flock management. Rettger, Kirkpatrick, and Stoneburn showed that pullorum resistance varies with age. Roberts and Card demonstrated the variation of pullorum resistance caused by genetic factors. These authors determined, among other things, that "Selection was ef-fective in producing strains of domestic fowl more resistant than... unselected stocks in respect to infection by Salmonella pullora." The same investigators also noted an in-crease of resistance under natural condi-tions in the presence of infection. They re-ported that among flocks of the Chia Gi variety in the vicinity of Peiping, China, pullorum reactors were present while no evidence of pullorum infection could be found among Langshams of Central China. Chicks of the Chia Gi variety showed a sur-vival of 72.7 percent after inoculation with pullorum organisms, while the inoculated Langshams showed a survival rate of only 10.6 percent. The resistance of the Chia Gi was concluded to be inherent rather than acquired since survival of progenies of infected birds was no greater than survival of progenies from non-reactors. Breeding experiments reported by Rob-erts and Card (1926, 1935, 1936) have thoroughly established the fact that in-herently resistant and susceptible strains may be established and maintained through many successive generations by means o