Paenibacillus larvae

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symptoms of Paenibacillus larvae infected honeybee brood (click on image to enlarge it)
Source: Wikimedia Commons
control of Paenibacillus larvae infected colony
Source: Wikimedia Commons

Paenibacillus larvae (White 1906) Ash et al. 1994 - (American foulbrood)

American foulbrood is an important disease of honeybee larvae and is caused by a Gram-positive, rod-shape and spore-forming bacterium. It has a worldwide distribution and its origin is unknown. The terms American foulbrood and European foulbrood are not related to their origins. Paenibacillus larvae is highly contagious and capable of destroying colonies. Infected colonies show irregular brood capping, with capped and uncapped cells scattered irregularly across the frames. The caps are dark, sunken and often punctured, emitting a ‘foul’ odor. The brown to dark brown remains of the dead larvae form a characteristic glue-like thread when removed with a wooden stick.

The disease quickly spreads through the whole colony. It can be transmitted to other hives by beekeepers, by robber bees entering the sick colony and removing contaminated honey, or by swarming bees. The spores remain viable for up to 40 years. Burning infected hives is the most common control strategy. Sterilization of beekeeping equipment is also considered effective. Treatment with antibiotics (oxytetracycline) is allowed in some countries, but banned in others. It only masks the disease and does not eliminate the infective spores. Moreover, it selects for resistant disease strains and the antibiotics will enter the honey and pose a risk to humans. Breeding for honeybee strains resistant to the disease is in progress. It is based on the hygienic behavior of the bees to remove infected broods. In some countries, control of the disease is mandatory by law and is monitored by government-funded inspectors. In England and Wales, this approach has reduced the disease incidence from over 2000 cases per year in the 1950s to 100-200 cases after 2000.

Vernacular names
• Deutsch: amerikanische Faulbrut
• English: American foulbrood
powdery scale
foulbrood of honeybee
• Español: loque americana
• Français: loque américaine

The spores are only infectious to young larvae (up to 3 days old) that become infected by consuming spore-contaminated food and die within a few days. As few as 10 spores are enough to infect and kill a larva. The spores germinate in the gut of the larvae, where they proliferate, apparently absorbing the larval food. At a later stage, the bacteria enter the hemocoel and finally form spores. A larva typically dies after its cell is sealed. A subspecies, Paenibacillus larvae pulvifaciens, causes the less virulent disease "powdery scale".

Bacillus larvae
Bacillus pulvifaciens
Paenibacillus pulvifaciens

For a review see Ashiralieva & Genersch (2006).