Visual Indications of AFB
Wax cappings become sunken and perforated when adult bees nibble holes in them to try to remove the infected larvae underneath. These perforations tend to be jagged and irregular in shape.
Some cappings may become moist or greasy looking and slightly darker in colour than other cells.
At first only very few cells may show signs of disease and the colony will appear normal in other respects. Eventually much of the sealed brood will become affected by the disease, causing a patchy or ‘pepper pot’ brood pattern. There may then be an unpleasant smell associated with decomposition. At the sunken capping stage the dead larval remains are light to dark brown in colour and have a slimy consistency.
If a matchstick is inserted and slowly withdrawn, the remains can be drawn out in a brown, mucus-like thread or ‘rope’ 10-30mm long. This is called the ‘ropiness’ test and is a reliable test for the presence of AFB.
The ropy condition is followed by a tacky stage as the larval remains in the cell gradually dry up and the colour changes to dark brown.
Further drying leads to the final stage, which is a very dark brown, rather rough scale lying on the lower side of the cell and extending from just behind the mouth of the cell right back to the base.
The scales can be detected if the comb is held facing the light: they reflect the light from their rough surfaces and can easily be seen, even when their colour is almost the same as the comb itself.
American Foul Brood (AFB)
Bacterium: Paenibacillus larvae
American foulbrood is caused by a spore forming bacterium Paenibacillus larvae. Young honey bee larvae become infected when they consume P. larvae spores in their food. The spores germinate in the gut; bacteria then move into the gut tissues, where they multiply enormously in number. Infected larvae normally die after their cells are sealed. Millions of infective spores are formed in their remains, which dry to form ‘scales’ that adhere closely to the cell wall and cannot easily be removed by bees. Consequently brood combs from infected colonies are inevitably severely contaminated with bacterial spores. If the scales go unnoticed and infected combs are subsequently used or moved from colony to colony during routine beekeeping management, then infection has the potential to spread quickly.
The spores are very resistant to disinfectant and to extremes of heat and cold. They retain their powers of germination for many years in honey, in old combs kept in store or in derelict hives, skeps or boxes.
Once a colony is infected the disease will usually progress until most of the brood is affected. The colony then becomes unable to replace the ageing adult bee population, causing it to become weakened and, finally, to die out. The disease may develop for months before the colony succumbs.
Death may occur at any time of the year. Affected colonies are killed and combs are destroyed by fire.