What is AFB and how is it caused?
American foulbrood disease is caused by the bacterium Paenibacillus larvae larvae. Until recently, the bacterium was known as Bacillus larvae, but scientists have now determined that the organism should be in its own unique genus (Paenibacillus).
Life cycle of AFB
The bacterium causing AFB exists in two forms (the spore form and the vegetative form), both of which are microscopic in size.
Bacterial spores can be thought of as seeds that assist the bacteria in spreading from one suitable host to another and resisting adverse conditions. Spores of Paenibacillus larvae larvae can survive outside a honey bee colony for more than 35 years, and are able to withstand very high temperatures, including boiling water.
The spores are also resistant to a range of disinfectants. AFB spores can survive more than 35 years, and withstand boiling water and many disinfectants.
A honey bee larva is usually infected by being fed AFB spores which contaminate the brood food placed in its cell by nurse bees. The larva eats the spores, which then germinate in the larval gut, and turn into the vegetative form of the bacterium.
The vegetative form is in the shape of rods. These rods penetrate the gut wall of the larva, where they multiply, consuming the larval tissues. Death of the developing bee usually occurs either in the pre-pupal stage or just after pupation. When the vegetative rods have consumed all of the larval tissues, they turn into spores again. A single diseased larva may contain more than 2.5 billion spores. House bees in the colony try to remove diseased larvae and pupae and in so doing become contaminated with spores. New larvae are infected when they are fed contaminated food.
Spores are the only form of the disease that can infect healthy larvae. As well, the spores can only increase in number by infecting a larva. They do not multiply in any other environment (e.g. honey or beekeeping equipment). AFB spores will only multiply inside a larva.