Pest Management Strategy - Elimination of American Foulbrood Without the Use of Drugs
A Practical Manual for Beekeepers, by Mark Goodwin and Cliff Van Eaton

5 Natural Progression of AFB in a Honey Bee Colony

5.1 Differing Scenarios for AFB Progression

It is often assumed that the progression of AFB is both rapid, and in one direction, with the presence of spores in the colony inevitably leading to visual symptoms (ie, diseased brood), and eventually to colony death. Colony death occurs either because the adult bee population is no longer replaced, or because the adult population becomes so small that the diseased colony is robbed out by foraging bees from other colonies.

Although this rapid one directional progression does occur, other scenarios are also possible. A trial was carried out at Ruakura that looked at the development of AFB infections in honey bee colonies that initially had less than 5 brood cells exhibiting clinical AFB symptoms.¹² The colonies were checked weekly and the number of cells with disease symptoms were counted.

In about one third of colonies studied the infection developed rapidly. For instance, Figure 7 shows a colony observed in that study where the number of diseased larvae increased exponentially.

Figure 7. The number of cells exhibiting clinical AFB symptoms in a honey bee colony where the number of diseases cells increased exponentially over a 30 day period.

However, in about a further third of colonies, the AFB symptoms disappeared and did not reoccur even though the colonies were followed for the next 4 years. Figure 8 shows what happened in one of the colonies in this group.

Figure 8. The number of cells exhibiting clinical AFB symptoms in a honey bee colony where the AFB infection disappeared after 50 days.

In the last third of colonies studied, AFB symptoms disappeared for between one and three weeks. However, after this time the disease underwent exponential growth. Figure 10 shows what happened to one of the colonies in this group. A beekeeper conducting an AFB check during the time this colony was not exhibiting clinical symptoms would have assumed that the colony was not diseased. If frames of brood or honey were removed, the frames could act as a means of spread of the disease to other colonies.

Figure 9. The number of cells exhibiting clinical AFB symptoms in a honey bee colony where the AFB infection disappeared for three weeks and then reappeared and underwent an exponential increase.

5.2 Contaminated Colonies

An AFB infection does not occur simply because a single bacterial spore enters a colony. Infections only result when millions of spores are introduced. Many of the spores either end up stored in honey or find their way into the stomachs of adult bees where they are removed from the colony when the bees defecate outside. Some spores may be fed to larvae, but may not be fed in high enough numbers to create an infection. Many colonies can be found that contain small numbers of AFB spores but never have a diseased larva or pupa.

The presence of AFB spores in a honey bee colony, or even in the gut of a larva, does not necessarily mean that the colony or the larva is diseased. A larva is not diseased until the bacteria kills the larva, either before or after pupation. Likewise, a colony is not diseased until it contains a diseased larva or pupa. Colonies that contain spores but not diseased larvae can be thought of as being "contaminated", rather than diseased.

5.3 Infected Colonies

A colony is said to have AFB when one or more of its larvae are fed enough spores to kill a larva or pupa. In some colonies, the beekeeper may never be aware that a diseased larva or pupa has been produced. The house bees may remove diseased larvae as fast as they become infected, so all the beekeeper sees is a spotty brood pattern. The house bees may do this effectively enough that they eliminate the infection from a colony (see above).

Infections where no visual symptoms appear in the colony are called "inapparent" or "subclinical" infections. However, it is not possible to differentiate between a colony that is contaminated with spores and has no diseased larvae or pupae, and one that has a subclinical infection. We may never know how many colonies develop subclinical infections of AFB and eliminate the disease by themselves.

Many beekeepers have noticed colonies that appear to recover from AFB. Having found a colony with a single diseased larva or pupa, they check the colony a few days later before burning it and are then unable to detect any sign of the disease.

The danger for beekeepers is that although some colonies may eliminate the disease completely, for other colonies the recovery will only be temporary. Because there may still be large numbers of AFB spores in such colonies, they may develop visual symptoms of the disease at a later date, and the symptoms will be severe enough to destroy the colony.

Obviously, colonies with subclinical infections are a factor in the spread of AFB. Even if beekeepers carefully check every frame of brood for diseased larvae and pupae before removing frames to place in other colonies, they cannot guarantee that they are not going to spread the disease.

5.4 Time Period to Develop Visual Symptoms

The average time between a larva being fed spores and it starting to exhibit clinical AFB disease symptoms is about 12.5 days. This is therefore the earliest that an infection can be detected visually. However, most colonies need considerably longer periods of time before they develop observable disease symptoms. The following table shows the length of time it took for honey bee colonies in one study to develop AFB disease symptoms after they were fed spores.

Months after spores
were fed Percent of colonies
developing AFB symptoms

1 19

2 30

3 11

4 14

5 11

6 6

>6 9

Table 1. Length of time taken for honey bee colonies to develop AFB symptoms after being fed AFB spores.

Just under half of the colonies developed disease symptoms within 2 months of being fed spores. However, approximately one third of the colonies didn't show any symptoms until 3 months or longer.¹³

If the infections had come from spores in contaminated honey supers that were unknowingly removed from infected hives in the autumn and placed on clean hives the following spring, then the most a full brood check the next autumn would reveal would be about 85% of the infections that were created. The remainder would hopefully be identified the following spring, one and a half years after the honey supers were removed from the original AFB hives.

5.5 Hygienic Behaviour

The disappearance of disease symptoms in a colony is due to the hygienic behaviour of honey bees. Adult house bees can be very efficient in recognising and removing diseased brood. For instance, in one trial 50% of the diseased larvae were removed before they were 11 days old, which is before such larvae show any visible symptoms of the disease and before spores are produced.¹⁴

As well, a colony may produce diseased larvae and pupae containing spores, but the house bees can be so efficient at cleaning the cells that most of the new larvae reared in those cells will not become diseased.

Hygienic behaviour in honey bees is controlled at least partly by two recessive genes, one for uncapping of cells, and the other for the removal of brood remains. The degree of hygienic behaviour varies between colonies, both because of the genetic composition of the worker bees, and because of the strength and age distribution of the colony population. The more hygienic a honey bee colony is, the more resistant it will be to AFB infections. It is possible to select for resistance (see section 9.4).

Pest Management Strategy - Elimination of American Foulbrood Without the Use of Drugs
A Practical Manual for Beekeepers, by Mark Goodwin and Cliff Van Eaton

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Months after spores were fed	Percent of colonies developing AFB symptoms
1	19
2	30
3	11
4	14
5	11
6	6
>6	9