How is American foulbrood disease (AFB) primarily transmitted?

The primary mode of transmission for American foulbrood disease (AFB) involves the spores of the causative bacterium, Paenibacillus larvae. When the spores are ingested by honey bee larvae, they can germinate in the larval gut. This germination is critical because it allows the bacteria to multiply, leading to disease symptoms that can ultimately result in the death of the affected larvae.

Although spore transmission can also occur through adult bees or contaminated tools, the germination of spores specifically in the larval gut is the key process in the life cycle of AFB. This mechanism illustrates why AFB is particularly concerning for beekeepers, as it can rapidly spread within a colony once larvae become infected.

In contrast, while contaminated hive tools and spores carried by adult bees can also contribute to the spread of AFB under certain conditions, they do not represent the direct and immediate means of introducing the disease into the larval population. Pollens and nectar sources can harbor spores but are not the primary transmission route for the disease itself, as the infection necessitates ingestion by susceptible larvae. Thus, the correct answer underscores the critical role of larval ingestion in the transmission of AFB.