A closer look: AFB SPORES & VEGETATIVE RODS

摘要

American foulbrood (AFB) is a widespread disease of honey bee larvae caused by the spore-forming bacterium Paenibacillus larvae subsp. larvae (formerly known as Bacillus larvae). Spores are the infectious stage, and shortly after being ingested, theygerminate into vegetative rods in the midgut. The rods then migrate into the hemolymph (blood), where they cause septicemia and death of the larval host. Spores are resistant to desiccation, radiation, high temperatures and disinfectants; therefore onlythe vegetative stage of P. I. larvae can be successfully controlled with antibiotics. While older larvae have physiological characteristics that slow or even prevent septicemia from occurring (Alippi 1999), individual larvae that ingest as few as 10 spores before they are 24-hours old will not recuperate once infected. After a larva has succumbed, the bacteria sporulate in the decaying corpse, which becomes characteristically brown and mucilaginous (Shimanuki 1997). Finally, the larva desiccates, leaving a dark scale on the bottom side of the cell. The scale contains millions of P. l. larvae spores, which re-enter the cycle when they are picked up by house-cleaning bees. It is known that large numbers of spores are required to establish infections in honey bee colonies and it has also been shown that colonies can maintain relatively large numbers of spores over several seasons without manifesting any clinical symptoms of AFB. In order to understand the epidemiology of the disease, it is important to understand the distribution and virulence of spores within the honey bee population and hive, and the susceptibility of the vegetative rods to various treatments.