MORTALITY STUDIES IN R. I. REDS 3 



Single Comb White Leghorns and Rhode Island Reds appeared to be the most 

 susceptible. Under natural infection the disease occurred in about one-fourth 

 of the pullets in families where it appeared. This fact pointed to resistance as 

 being due to a single dominant gene. Artificial inoculations were also made and 

 these tended to support the idea of dififerences in resistance between families. 



McClary and Upp (1939) concluded that the iritic type of paralysis was trans- 

 mitted through the egg. They also noted a higher incidence of the disease in 

 progeny of iritic parents. 



Hutt, Cole and Bruckner (1941), in four years of selective breeding for high 

 and low resistance to the avian leukosis complex, have shown an incidence of 

 26 percent in the low line compared with 12 percent in the high line between 

 the ages of 160 and 500 days. 



Jeffrey, Beaudette and Hudson (1942) report results of selective breeding of 

 White Leghorns for high and low resistance to the avian leukosis complex through 

 seven generations. A marked decline in the incidence of the complex was ob- 

 served in both lines, but the line selected for freedom from the complex had a 

 significantly lower incidence than the line selected for high incidence. Mortality 

 from all causes was still excessive in both lines at the close of the experiment. 



Taylor, Lerner, DeOme and Beach (1943) bred two lines of Leghorns: one for 

 high resistance and another for low resistance to the avian leukosis complex. 

 A significant difference was observed between the lines throughout the period. 

 There was no decline in the incidence of the disease in the resistant line through- 

 out the period. After one year of selective breeding for low resistance no sig- 

 nificant change in incidence of the disease appeared. There was also a parallelism 

 between the two lines in the incidence of the disease. 



DeOme (1943) concluded, after extensive injections of lymphomatous nerve 

 tissue into resistant and susceptible chickens, that the relative resistance or 

 susceptibility of the two strains was due to the action of numerous genetic factors. 



Lee and Wilcke (1941), by inoculations with blood from chicks from stock 

 affected with iritis into susceptible chicks, produced the disease in 67 percent of 

 these chicks up to twelve months of age. Control chicks not inoculated showed 

 no manifestation of the disease. 



Work at the Regional Poultry Research Laboratory (Winton, 1943) showed 

 that the avian leukosis complex may be transmitted through the egg and that 

 there is a striking difference in the incidence of the disease in different families. 

 There is also evidence that disease-free families may be developed through 

 selective breeding and quarantine and that males are less susceptible to natural 

 infection than females. 



Marble (1939), through five years of selective breeding with White Leghorns 

 and Barred Plymouth Rocks, improved the viability during the growing period 

 and reduced the mortality rate for females in the laying house by 50 percent. 



Bearse and McClary (1939) report the results of eight years' selection to pro- 

 duce high and low viability lines in White Leghorns. The two lines differed 

 significantly in mortality from all causes during the growing period and during 

 the first laying year. 



Sturkie (1943) selectively bred White Leghorns to establish a line showing 

 high viability during the first laying year. The initial mortality rate of pullets 

 in the laying house was 89 percent. In five years the mortality rate had been 

 reduced to 27 percent. At the beginning of the experiment about two-thirds of 

 the deaths were due to leukosis and in the last generation about 14 percent died 

 from this cause. It was concluded that superior inbred families were developed 

 that exhibited high livability and satisfactory egg production. 



