Bailey wrote in 1977

Anyone who relies on the old accounts of bee diseases, and even on the most up-to-date books on beekeeping, will conclude that when bees are not obviously sick they must be free of pathogens; conversely, when a visibly sick colony is found to be infected with a pathogen this will be blamed for causing the disease; and should a normal colony become infected with a pathogen then an obvious sickness will be anticipated. It all seems very straightforward. 

When I began work on bee diseases, this was the general idea, and I fully expected to cause sickness easily for my experiments. However, my bees usually continued to look well. It was not that the agents of these alleged diseases were not in my bees; I could find them quite easily, but usually they didn’t produce any striking symptoms. The fact soon became clear that most, probably all of the wide variety of bee pathogens can occur in colonies that nevertheless can continue to appear healthy. 

Indeed, there is nothing unusual in this, especially with free-living populations of insects and of other wild animals. They survive, in spite of being infected to some degree by a great variety of pathogens; the multiplication and spread of these are usually checked or balanced in wild populations by a variety of natural processes. 

However, once infected, individual insects are much less able than vertebrate animals to resist the multiplication of pathogens. Therefore, processes that decrease the spread of pathogens and hinder their invasiveness are the most important means of defense insect populations have against diseases. Such processes in bee colonies are the ones most easily upset by beekeeping practices.

European foulbrood (EFB) illustrates once more, the usual fundamental characteristics of bee diseases. ... The bacteria multiply only within the food mass in the intestine of young larvae and although these are thereby deprived of some of their food, they usually survive and void the bacteria, with their excrement, into their brood-comb cells when they pupate. Adult bees clean most of the bacteria away but some get to more larvae. Occasionally, a severely infected larva dies, but it’s usually quickly ejected from the colony by house-cleaning bees. This dynamic balance can go on indefinitely and the beekeeper may see nothing untoward. 

However, the balance depends on a normal seasonal growth and development of colonies. When colony development is checked, as it is most frequently in nature by changeable weather in spring, the ratio of nurse bees to larvae increases temporarily, and severely infected larvae, that would die and be ejected under normal circumstances, are kept alive by surplus brood food secreted by the nurse bees. These larvae spread many more bacteria than usual when they pupate. 

In this way, contamination increases and often spreads quicker than house-cleaning bees can clear it away. Eventually, an outbreak of dead and dying larvae occurs, especially when the main nectar-flows begin. Infected larvae are then most likely to die because the amount of brood relative to nurse bees suddenly becomes large, as the colony tries to grow, and the demand on brood food makes this sufficient only for healthy larvae.

The effect of checks in colony development, especially in spring, coupled with the fact that infection is carried on combs, and can pass unseen even when it is actively spreading, makes EFB a disease that beekeeping can easily aggravate and spread. It should be regarded seriously and beekeepers should try to earn how to recognise it.

BAILEY, L.  Good Beekeeping and Bee Health. A lecture given to the Central Association of Bee-keepers on 1st October 1977

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