36 MASS. EXPERIMENT STATION BULLETIN 370 



CONCLUSIONS 



Transmissible fowl leukosis is a usually fatal, specific disease which occurs 

 naturally in the chicken. It is caused by a filter-passing agent whose ability 

 to produce disease is enhanced by successive animal passage. The different 

 strains of fowl leukosis agent vary in their action. Some cause stimulation of 

 erythroblastic cells only; some have the ability to stimulate the granuloblastic 

 cells; others under certain conditions are capable of producing sarcoma. The 

 existence of agents capable of inducing fowl paralysis and lymphoid tumors as 

 well as leukosis is an unsettled question. Fowl leukosis is not highly if at all 

 contagious and the possibility that the disease agent is of spontaneous origin in 

 the affected animal must be seriously considered. The disease is readily trans- 

 mitted when the pathogenicity of the particular strain of agent in question has 

 become established. The bile, urine, and feces of affected birds are either devoid 

 of the agent or contain it in an inactive form. The agent can be demonstrated in 

 most of the tissues of diseased chickens by inoculation of the material into other 

 birds. Strong doses of roentgen rays do not destroy the activity of the agent. 

 The agent is relatively resistant to drying, freezing, and to glycerin solutions. It 

 is thermolabile and susceptible to inactivation by o.xidation. Many chickens 

 have a natural relative resistance to the agent. Some few recover from the disease 

 and gain a variable degree of resistance thereby. The serum of such recovered 

 birds is capable of neutralizing the action of the agent in vitro, as is also the serum 

 of ducks which have been subjected to a series of injections of the agent. Some 

 strains of the agent have produced the disease experimentally in pheasants, 

 guinea fowl, and turkeys, although other species of fowl are resistant to their 

 action. No practical therapeutic agents have yet been found for this disease. 



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