CHALKBROOD

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CHALKBROOD

Nature of the disease
Chalkbrood is an infectious disease of honeybee larvae caused by a fungus Ascosphaera apis. It causes the death and mummification of sealed brood with consequent weakness of bee colonies. It rarely kills a colony, but will weaken it, leading to a reduction in honey surplus.
Classification
SPC List D disease
Susceptible species
Honey bees Apis mellifera are susceptible and other Apis species can be affected. Only larvae are affected. A. apis does not multiply in adult bees.
Distribution
Chalkbrood is present on all continents and has spread extensively.
Clinical signs
Larvae die of chalkbrood after their cells have been capped.

They initially appear fluffy and swollen but later shrink and become hard. Some of the dead larvae remain chalky-white but others become dark blue-grey or almost black.

Young infected larvae do not usually die or show signs of disease. Infected larvae usually die within two days of having been sealed in their cells, otherwise they die as propupae.

Post-mortem findings
The disease is characterised by infected brood, called "mummies," which when removed from the comb, appear to be solid clumps, reminiscent of chalk pieces. The mummies can vary in colour from white to dark grey or black (when fruiting bodies are present).
Differential diagnosis
The nature of infection is very similar to that of Bacillus larvae (American foul brood). Chalk brood also needs to be differentiated from other brood diseases such as:
  • European foul brood
  • Stone brood
Specimens required for diagnosis
A piece of comb about 10cm by 7.5 cm containing dead and discoloured brood should be collected and forwarded for laboratory examination.
Transmission
Larvae ingest spores of A. apis with their food. Spread within a colony is usually limited. A. apis grows best when the brood is chilled, and temperature of the brood-nest is considered an important factor that checks naturally infection. Spread in the hive is due to the excessive accumulation of mummies overlaping the bees' capacity to remove their deads.

The disease has a low ability to spread between colonies and most spread occurs through the activities of beekeepers — exchanging equipment and bees, feeding contaminated honey or pollen, using contaminated tools and gloves.

The disease may recur in infected colonies, because spores remain infective for more than 3 years.

Risk of introduction
Chalkbrood could be introduced with contaminated honey or beekeeping equipment.
Control / vaccines
There is no recommended chemical control for chalkbrood; often requeening a colony will be effective treatment. Good hygienic behaviour by a colony, that is quick removal of the mummies by workers, appears to aid in clearing up the symptoms.
As chalkbrood is aggravated by chilling, practices that lead to loss of heat in infected hives (e.g. removing adult bees, or giving them extra brood to rear) should be avoided.
References
  • ROOT AI (1990), European Foulbrood In ABC and XYZ of Bee Culture, ed Root Company, p 127-129