30 BULLETIN 431, U. S. DEPARTMENT OF AGRICULTURE. 



the pores of the Pasteur-Chamberland filter also, it is therefore fil- 

 terable and is very properly referred to as a " filterable" 1 virus. 



In considering the virus of sacbrood it is suggested that the bee- 

 keeper think of it as a microorganism 2 which is so small or of such 

 a nature that it has not been seen, and which will pass through the 

 pores of fine clay filters. This conception of it will at least make it 

 more easily understood. 



WEAKENING EFFECT OF SACBROOD UPON A COLONY. 



The first inoculations in proving that sacbrood is an infectious 

 disease were made on June 25, 1912. Two colonies were used, 

 each being fed with material from a different source. The inocu- 

 lation feedings were made on successive days. Sacbrood having 

 been produced in the colonies, the inoculations were continued 

 at intervals throughout July and August. During this period, a 

 large amount of sacbrood was present in both colonies. By the end 

 of July these colonies had become noticeably weakened, and by the 

 end of August they had become very much weakened, as a result of 

 the sacbrood present in them. On September 5 one of the colonies 

 swarmed out. 



The brood (PL IV) of this colony, large in quantity, was practically 

 all dying of sacbrood. The other colony, when examined on Sep- 

 tember 16, was found to be very weak. At this time, however, most 

 of the dead brood had been removed and healthy brood was being 

 reared. This colony increased in strength and wintered successfully. 



The results obtained from the inoculation of these two colonies 

 demonstrated not only that sacbrood is an infectious disease, but 

 also that the disease in a colony tends to weaken it. The results 

 indicate also that a colony may be destroyed by the disease, or it 

 may recover from it, gam in strength, and winter successfully. 



Each year since 1912 two or more colonies have been fed sacbrood 

 material at intervals during the brood-rearing season for the purpose 

 of obtaining disease material for experimental purposes. The inocu- 

 lated colonies in all instances have shown a tendency to become 

 weakened as a result of the inoculations. 



The death of the worker larvae is the primary cause for the weak- 

 ness resulting from the disease in a colony. Another point to be 

 thought of is that dead sacbrood larvae remaining in the cells for 

 weeks, as they not infrequently do, reduce the capacity of the brood 

 nest for brood rearing, which has a tendency also to weaken the colony. 



1 In searching the tissues of larvae dead of sacbrood and the filtrates obtained from them nothing has been 

 discovered by the aid of the microscope, or culturally, which has j*et been demonstrated as being the infect- 

 ing agent. This being true, the virus could be spoken of tentatively as an "ultramicroscopic virus." It 

 is preferable, for the present, however, to refer to it simply as a filterable virus. 



2 There is some question whether, in the case of diseases having a virus which is filterable, the infecting 

 agent is in every instance a microorganism. The evidence is strong, however, that it is. 



