18 MASS. EXPERIMENT STATION BULLETIN 370 



Furth's (72) Strain 1 of k-ukcsis showed an average incubation period of 47 

 days (13 being the minimum and 124 the maximum) and an average duration 

 period of 19 days. The length of life after inoculation was found by him to vary 

 from 21 to 204 days. One chicken with experimental leukosis has been observed 

 to live for 552 days after inoculation during which time there were remissions 

 from, the disease (Olson 155). In Furth's (74) studies on the concentration of 

 the agent in the blood plasma he observed a prolongation of the incubation period 

 when inoculations were made with small amounts of leukotic plasma. 



The disease-producing ability of the leukosis agent studied by Hamilton and 

 Sawyer (92) was particularly pronounced, as in the 12 serial passages of the agent 

 the longest internal between inoculation and death was only 27 days, the shortest 

 5 days, and the average 14.7 days. In addition, relatively few birds were found 

 to be resistant to the agent. 



Jarmai (102) has recently published some interesting figures which indicate the 

 increase in virulence of his strain of leukosis agent associated with animal passage. 

 These are: 



Average number of days 

 Year from inoculation to death 



1930 20 



1931 15 



1932 15 



1933 14 



1934 12 



1935 12 



1936 12 



1937 11 



Engelbreth-Holm and Rothe Me\'er (54) noted early in their experiments 

 with one of their strains that the incubation period was longer and the anemic 

 form of the disease more common in the fall and winter months than at other 

 seasons. The more severe form of disease developed in the summer months. 

 More recently, they (59) found that the seasonal variation had been manifest in 

 adult birds for three successive years, but that no such variation of susceptibility 

 could be detected in birds less than three months of age. The maximum of 82 

 percent successful inoculation of adult birds took place in April and May and the 

 minimum of 40 percent occurred in October and November. They have in- 

 vestigated several possible reasons for this seasonal variation of susceptibility, 

 including the matter of green feed in the diet, and the effect of more sunlight 

 during the summer. Neither of these factors seemed to be concerned. The Danish 

 workers also considered the possibility of hormonal variation with the season. 

 Engelbreth-Holm, Rothe Meyer, and Uhl (61) have since reported that growth 

 hormone did not influence the rate of development or character of disease in 

 chickens inoculated with their Strains E-S or T. In one experiment they demon- 

 strated that the hormone preparation was active as the birds receiving the hormone 

 showed a greater weight increase than the control birds. Gonadotropic hormone 

 did not inhibit the development of leukosis, but it did partially inhibit the growth 

 of sarcomas produced by the Strain E-S agent when administered either alone 

 or in conjunction with extract of the pineal body. Jarmai (102) noted that the 

 length of time between inoculation and death of the experimental animal was 

 usually longer in the first half of the year than in the last half of the year. This 

 was especially noticeable during the years 1930 and 1931 when the average of 

 this period was still 15 days and the variation was commonly one or two days. 



