Diagnosis of American foulbrood disease of honey bee brood
Note Number: AG1426
Published: September 2011
American foulbrood (AFB) is an infectious, notifiable disease of honey bee larvae and pupae. Infected bee colonies slowly become weak and die. Early detection of the disease is important because routine apiary management and interchange of hive components can easily spread it to healthy bee colonies. This Information Note provides detailed information to assist beekeepers diagnose AFB in bee hives.
Cause and Lifecycle
AFB is caused by the bacterium, Paenibacillus larvae. The spores of this organism are too small to be seen by the naked eye and are only visible under a high powered microscope.
The disease begins in honey bee larvae after they swallow AFB spores with their food. Within 24-48 hours, the spores germinate in the gut of the larva and develop into vegetative 'rods'. The rods grow and invade the haemolymph and body tissues, killing the infected larva before pupation, usually immediately after the brood cell is capped.
The final stage of the bacterium's lifecycle is reached when the vegetative rods form into spores. Approximately 2,500 million spores may occur in the remains of a single infected honey bee larva.
AFB spores are tough and can remain dormant for at least 50 years and possibly much longer than this. They can remain dormant on beeswax combs, used hives and components, honey, wax and propolis. They are very resistant to heat, direct sunlight, desiccation, fermentation, chemical disinfectants and veterinary drugs.
Inspection of Brood
People inexperienced in handling bees and collecting samples should first read the Agriculture Note AG1240 'Safe beekeeping practices'. It is essential that adequate protective clothing, including a bee veil, is worn and techniques for safe handling of bees are understood before opening hives and collecting samples.
Thorough inspections of brood should be conducted in early spring, during the main honey producing season, and in autumn when hives are prepared for winter. Additional inspections can be made at the time honey is removed from hives and when hives are requeened.
If you wear glasses to read, wear them while looking for AFB.
In healthy colonies, the brood pattern usually appears regular or uniform because the queen has methodically laid eggs across the comb, or in an area formed by a semi-circle or in concentric circles. Sometimes, there may only be a few empty cells interspersed in the area of brood. The caps of cells containing healthy brood are uniform in appearance, generally bright and convex. Some of these caps may not yet be fully built and consequently appear perforated. It is always good to look inside these cells to make sure the developing bee is healthy and it is not the beginning of a disease problem.
Photo 1. Healthy larvae. These are white and neatly curled at the base of their cells.
Photo 2. Healthy capped brood (pupae). Note the regular brood pattern, even appearance and convex caps.
Symptoms in the Combs
The caps of cells containing diseased larvae or pupae may be sunken, concave, dark and at times greasy-looking. Some caps may be perforated (Photo 3) and other caps may be totally removed, exposing the diseased individual.
Photo 3. Perforated cell caps in an American foulbrood infected comb.
In colonies with advanced AFB, the brood pattern is irregular or scattered (Photo 4). There may be many cells which appear empty. However, on closer inspection these cells may contain healthy larvae, sick and decaying larvae, or dried scales which are the remains of larvae. The comb surface may appear a little greasy. The darker sunken and sometimes greasy caps are scattered among the lighter, convex caps of cells that contain healthy pupae.
In the early stages of AFB, the brood pattern may look entirely normal. This is because AFB is usually slow to establish and only a few larvae will be affected at first. The absence of an irregular brood pattern does not mean that the disease is absent.
Irregular or scattered brood patterns can also occur when other brood diseases are present, or as a result of a failing queen or pesticide damage. It is very important to determine the cause of the scattered brood pattern and to determine if AFB is present.
Photo 4. A comb showing an irregular (scattered) brood pattern.
Symptoms in the Brood
Infected larvae change colour from the normal glistening white to off-white, light brown, coffee brown, dark brown and almost black. The colour change occurs after larvae have been capped in their cells. Infected individuals die in capped cells.
AFB diseased larvae and pupae always lie stretched out on their backs on the lower wall of their cells, from the back of the cell (the mid-rib of the comb) to the cell opening. If the cell opening is likened to a clock-face, the diseased individuals (and later scales) will be positioned at the bottom of the face between the figures 5 and 7.
The moist, decaying remains of a dead larva may be "roped out" of the cell with a match stick of a distance of about 25 mm or more (Photo 5). The match is inserted into the tacky larval remains which are then gently picked up (or gently scooped) to be slowly withdrawn from the cell. This match stick test is a very effective method of diagnosing AFB. The remains of larvae killed by European foulbrood disease may also stretch out, but usually not as far as AFB diseased larvae. If there is any doubt as to which disease is present, a laboratory diagnosis should be obtained. Full details for submitting samples for diagnosis are obtained in the Information Note 'Samples for Laboratory Diagnosis of Bee Diseases'.
Photo 5. The matchstick or ropiness test – an excellent field test for diagnosing American foulbrood.
Over time, the remains slowly dry to form dark-brown or black scales (Photo 7). In dull light, the scales, being dark, are not easily seen. It is best to hold the frame with the top bar held towards your stomach and tilted to allow sunlight to fall directly onto the lower wall of the cells.
Dried scales do not rope out with the matchstick test. Unlike scales of other brood diseases, they remain firmly attached on the cell wall and are impossible to remove without damaging the cell.
In some outbreaks, tongues of dead pupae may be seen pointing upwards from the remains, almost up to or even touching the roof of the cell (Photos 6 and 7). The presence of tongues is a very reliable AFB field sign, but the absence of tongues is not an indication that AFB is not present in a hive. Sometimes fully-formed bees that have died just prior to emerging from their cells may be observed with their tongue exposed. This is not a sign of AFB.
Photo 6. Some pupae infected with American foulbrood die after the tongue has formed. The infected pupa begins to decompose and dry to a thin scale, but the tongue remains pointing upwards and almost touches the upper cell wall.
Photo 7. The tongue of the pupa infected with American foulbrood dries and becomes thinner. Note the scales (dried remains of larvae) in the cells left of the tongue.
Odour or smell is not a reliable diagnostic tool because some cases of AFB have no discernible smell at all.
Confirmation of AFB
It is important to obtain an accurate diagnosis of AFB. Experience has shown that diagnosis provided by a fellow beekeeper may not always be accurate. An inaccurate diagnosis can lead to spread of the disease throughout the apiary and large losses of hives and honey production.
Confirmation of AFB may be obtained by laboratory examination of smears of infected larvae. Full details are found in the Agriculture Note AG1249 'Samples for Laboratory Diagnosis of Bee Diseases'.
Notification of AFB
The Livestock Disease Control Act 1994 requires that anyone knows or suspects that AFB is present in his or her apiary must notify a DEPI apiary officer or the Director, Animal Biosecurity & Welfare, within 12 hours.
Failure to notify is to break the law.
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This Agnote was developed by Russell Goodman and Joe Riordan, Biosecurity Victoria, September, 2011.