TRANSMISSIBLE FOWL LEUKOSIS 19 



After the period from inoculation to death had become shortened to 14 days 

 or less the effect of the season could no longer be noted. 



Crank and Furth (26) have brought about death of experimental birds in two 

 or three days by transfusing them with large amounts of leukemic blood from 

 birds diseased with Furth's Strain 1 agent. The brief and fatal course of the 

 disease was apparently due to the autonomous growth of the inoculated leukemic 

 cells in the new host. 



Once a form of disease becomes manifest, that form usually persists until the 

 death of the affected fowl. There are, however, instances in which a transition 

 from one form of leukemic process to another occurs in the course of the disease 

 (Jarmai 99, Olson 155). 



Spontaneous recovery after development of leukosis in experimental birds has 

 been found infrequently by various investigators. There were only four instances 

 of spontaneous recovery observed in 98 cases of experimentally produced leukosis 

 (Olson 155). In one of these recovered birds the infectivity of whole blood was 

 demonstrated during the time that changes were still evident in the blood. Ober- 

 ling, Guerin, and Guerin (152) stated that the incidence of spontaneous recovery 

 following inoculation with a strain developed by them was less than 10 percent, 

 whereas they observed no recoveries following inoculation with a strain which 

 they had received from Engelbreth-Holm. 



Storti and Zaietta (187) have observed that an induced anemia in chickens had 

 the effect of increasing the number of successful inoculations and also of shorten- 

 ing the incubation period and the subsequent course of the disease. Their ex- 

 perimental birds were inoculated with the agent of leukosis after the count of 

 red blood cells and the hemoglobin value had fallen to about half the original 

 normal levels. The state of anemia was induced either by repeated bleeding or 

 by the repeated intravenous injection of 1 percent saponin solution. In the case of 

 acute anemia, from 40 to 50 cc. of blood was removed at one time from birds 

 weighing 1000 to 1400 grams; and in the case of chronic anemia, 10 to 15 cc. of 

 blood was removed daily for eight to ten days. They also noted that, in four 

 instances in which the experimental disease was pursuing a chronic course, 

 removal of blood led to the development of the blood picture of acute leukosis 

 in three or four days and to death within a week. Control animals, that is birds 

 affected with the chronic form of the disease but not bled, continued to live for 

 two or three months. 



Wallbach's (212) claim that he succeeded in changing erythroblastic leukosis 

 (produced with a strain received from Thomsen in Denmark) to granuloblastic 

 leukosis by the administration of arsylene or benzol is not convincing. He ob- 

 served a transitory granulocytosis in normal chickens as well as in leukotic birds 

 following administration of these substances. 



Pathogenesis 



The site of irritation of the exciting agent of fowl leukosis and the mode of 

 development of the disease are problems difficult to solve. In related fields con- 

 siderable work has been done in attempts to clarify the histogenesis of the Rous 

 sarcoma and similar tumors of the chicken. It is rather generally accepted that the 

 agents of these tumors are initially localized in the tissue of an inoculated bird 

 by cells of the macrophage type, but the subsequent role of these cells is not yet 

 settled. Some believe that the macrophage is then converted into the type cell 

 of the specific tumor whereas others believe that the macrophage liberates the 

 agent which is then capable of stimulating the type cell of the tumor into a state 



