SACBEOOD. 27 



scarcity. It has been produced experimentally in colonies under 

 equally varying conditions in regard to the quantity of food. 



While it is possible that the quantity or quality of food may influ- 

 ence somewhat the course of the disease in the colony, the r61e played 

 by food in the causation of sacbrood must be slight, if indeed it con- 

 tributes at all appreciably to it. Practically, therefore, for the 

 present it may be considered that neither the quality nor quantity 

 of food predisposes to this disease. 



EXCITING CAUSE OF SACBHOOD. 



That sacbrood is an infectious disease was demonstrated by the 

 wi'iter (1913) through experiments performed during the summer of 

 1912. This was done by feeding to healthy colonies the crushed 

 tissues of larvse dead of sacbrood, suspended in sugar sirup. The 

 experiments were performed under various conditions, and it was 

 found that the disease could be produced at will, demonstrating 

 thereby that it was actually an infectious one. 



In the crushed larval mass no microorganisms were found either 

 microscopically or culturally to which the infection could be attrib- 

 uted, although the experiments had proved that the larva dead of 

 the disease did contain the infecting agent. This led to the next step 

 in the investigation, which was to determine whether the virus was 

 so small that it had not been observed, and whether its nature would 

 permit its passage through a filter. The first filter used for this 

 purpose was the Berkefeld. 



The process by which the filtration is done is briefly this: Larvae 

 which have been dead' of sacbrood only a few days are picked from 

 the brood comb and crushed. The crushed mass is added to water in 

 the proportion of 1 part larval mass to 10 parts water. A higher 

 dilution may be used. This aqueous suspension is allowed to stand for 

 some hours, preferably overnight. To remove the fragments of the 

 larval tissues stiU remaining, the suspension is filtered, using filter 

 paper. The filtrate thus obtained is then filtered by the use of the 

 Berkefeld filter ^ (fig. 26) properly prepared. The filtering in the 

 case of the coarser filters especially can be done through gravity 



alone. 



To determine whether any visible microorganisms are present 

 in this last filtrate, it is examined microscopically and culturally. 

 When f oimd to be apparently free from such microorganisms, a quan- 

 tity of it may be added to sirup and the mixture fed to healthy colo- 



1 The Berkefeld filter consists of a compact material (infusorial earth) in the form of a cylinder. A glass 

 mantel (A) in which is fixed the filter forms a cup for holding the fluid to be filtered. Havmg filtered 

 the aqueous suspension of crushed sacbrood larvae through paper, the filtrate is then filtered by aUowmg 

 it to pass through the waUs of the Berkefeld cylinder (B). The filtrate from this filtration is collected 

 into a sterile flask (F) through a glass tube (D) with its rubber connection (C). In flltermg m this instance 

 gravity is the only force used. 



