34 BULLETIN 809, U. S. DEPARTMENT OF AGRICULTURE. 



The results recorded on the foregoing pages relative to the resis- 

 tance of American foulbrood spores to heat, drying, fermentation, 

 sunlight, chemical disinfectants, and drugs, it will be observed, 

 are only approximate from a strictly technical viewpoint. For 

 practical purposes, however, they are in most instances entirely 

 adequate. In using any of these results in devising means for the 

 destruction of the spores in practical apiculture the time element 

 determined by the experiments should be somewhat increased in 

 each instance. 



MODES OF TRANSMISSION 



Observations made during these studies point to certain paths 

 by which American foulbrood is most likely to be transmitted. The 

 evidences obtained tend to support certain views which have been 

 entertained by beekeepers and to negative others which have been 

 suspected by some as possible. Inasmuch as the disease may be 

 produced in larvae by feeding a colony Bacillus larvae in pure cultures, 

 or from decaying remains of brood dead of the disease, it would 

 seem that the portal of entry of the virus is somewhere along the 

 alimentary tract of the larva. The fact leads at once to the sus- 

 picion that the food of bees contaminated with disease material 

 is a very probable source of infection. Were the water supply 

 likewise contaminated naturally it also would be a probable source of 

 infection. Two prerequisites for the appearance of American foul- 

 brood in a colony are (a) larvae of the feeding age, and (b) a suffi- 

 cient amount of disease material in the food or water supply of 

 bees. A small amount of brood or a heavy flow of nectar, therefore, 

 would tend to reduce the likelihood of colony infection under con- 

 ditions otherwise favorable for infection. 



Bees manifest a tendency to remove brood dead of American 

 foulbrood as shown by the remains of partially removed larvae and 

 pupae dead of the disease. The removal is done piecemeal and is 

 accomplished more readUy when the brood is recently dead than 

 when the decaying remains are at aU viscid or dry. Were the fate 

 of the removed fragments definitely known much more could be 

 said concerning the spread of the disease in the colony. It has 

 been observed that a small amount of infection — a dead larva or 

 pupa here and there — may be present in a colony for months, a 

 year, and even longer, without causing a heavy infection in it. There 

 is considerable evidence to support the belief that occasionally in 

 cases of light infection the disease may disappear unaided by treat- 

 ment. Such a phenomenon frequently takes place in sacbrood and 

 indeed should be expected to occur now and then in American 

 foulbrood. It should be emphasized that such a course for the 

 disease, if it occurs at all, is unusual. Although American foulbrood 



