Infection of honey bees with Paenibacillus larvae (American foulbrood)

The brood comb looks mottled or patchy, with healthy capped cells mixed with uncapped cells and empty cells. Cappings over diseased larvae turn moist and dark, then sink and become perforated. The larva or pupa turns beige then dark brown, with a foul odour; its remains become glue-like and string out as a thread when a matchstick is inserted and withdrawn, though some remains are watery. About a month or more later the remains dry to hard, dark, brittle scales stuck firmly to the lower cell wall. A protruding pupal tongue is highly characteristic but rarely seen. Signs vary with genotype and colony strength.

American foulbrood is caused by Paenibacillus larvae, a Gram-positive, spore-forming rod. Only the spores are infectious. Young larvae swallow spores in contaminated food; the spores germinate in the midgut and multiply rapidly, and the bacterium secretes metabolites and chitin-degrading enzymes that break through the midgut epithelium, killing the larva and reducing it to a ropy mass. Each dead larva can yield over a billion spores. Five genotypes (ERIC I-V) differ in virulence: ERIC II-V kill most larvae within about 3 to 7 days, often before the cell is capped, whereas ERIC I takes around 12 days, so larvae usually die after capping and produce the classic signs.

The disease affects the larval stage of Apis mellifera and other Apis species and occurs worldwide wherever honey bees are kept. Worker, drone and queen larvae are susceptible, but the youngest larvae (about 12 to 36 hours after hatching) are most sensitive, and larvae cannot be infected more than 53 hours after hatching. Adult bees carry but are not harmed by spores. ERIC I and II are the genotypes commonly found in diseased colonies. Spores resist heat and chemicals and persist 3 to 10 years in honey, wax and equipment and about 35 years in dry scales. Spread between colonies is mainly by exchanging combs, feeding or robbing contaminated honey, package bees, queens and untreated wax. Human risk is generally low. It is a WOAH-listed disease.

European foulbrood, caused by Melissococcus plutonius, is the main disease to exclude. AFB is suggested by sunken perforated cappings, a positive matchstick (ropiness) test, tightly adherent scales and a pupal tongue, but these are not always present. Laboratory confirmation distinguishes the agents: culture on selective media, PCR on larval remains, catalase-negative or weak biochemical profile, microscopy of smears for spores, and the Holst milk test, which is used to differentiate P. larvae from M. plutonius.

Larval midgut and gut epithelium, whole larval and pupal body (tissue liquefaction), pupal tongue

Early detection; do not move combs, honey, wax or queens from infected colonies; burn infected colonies and hive material or use the shook swarm method (bees moved to a clean hive, brood combs destroyed); treat wax used for foundation.

Almost all larvae infected with virulent genotypes die. Spores survive decades, making control difficult. No vaccine exists; antibiotics do not give lasting recovery and have selected resistant strains, so infected colonies are often destroyed.