Identifying American foulbrood in honey bee colonies

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The apiculture industry is experiencing higher levels of colony loss each year. The causes of honey bee illness and colony collapse, including American foulbrood disease, were addressed in a session at the 2023 Western Veterinary Conference (WVC) in Las Vegas.¹

Presenter Britteny R. Kyle, DVM, MSc, a PhD student in epidemiology with a collaborative specialization in One Health at the Ontario Veterinary College, University of Guelph in Guelph, Ontario, Canada, informed WVC attendees that although the average lifespan of a honey bee should be 5 to 7 years, with an annual loss rate of 10% to 15%. However, the past 15 years has seen the average winter loss rate of these pollinators to be about 40%.¹

Factors leading to colony collapse include bacteria, viruses, and fungal disease as well as pests, parasites, and noninfectious conditions. In her talk, Kyle addressed some of the most common of these factors affecting the health of beehives.¹

American foulbrood (AFB) is considered the most devastating honey bee brood, according to Kyle. AFB is a highly virulent bacterial disease caused by Paenibacillus larvae.^1,2 This disease is transmitted through common honey bee behavior and beekeeper activities, including use of apiculture equipment. “Those [bacterial] spores are heavily persistent and resistant,” said Kyle.¹

Identifying AFB

Clinical signs of the disease include a distinctive foul odor, a spotty brood pattern, honeycomb cappings that are sunken, greasy, or perforated; discolored larvae, pupal tongue, and larval remnant is gooey or viscous as well as a hard scale that is adhered to the bottom of a honeycomb cell.¹

Diagnostic testing can determine if AFB disease is present. According to Kyle, there are a variety of field diagnostics that can be performed, including the use of commercial test kits. Among these methods, Kyle discussed using simple matchstick to test the consistency of dead brood.¹

“When you stick something in that’s wooden, something with a rough surface, and pull it out, it stretches,” said Kyle, who added that a ropey substance that is gooey or viscous could indicate AFB. “If it stretches for 2 ½ cm…that’s considered to be diagnostic for AFB. It’s so reliable that the apiary inspection program where I am, they will use just this positive test, and say ‘kill the colony,’ based just on that test,” she added.¹

Diagnostics can also be performed in-clinic such as examining samples for spores or modified hanging drops, under a microscope; and bacterial cultures that test for antimicrobial susceptibility. Additionally, laboratory diagnostics including cultures and polymerase chain reaction tests can be used to detect AFB.¹

Managing AFB

AFB is not curable, and typically kills infected brood at the pupal or pre-pupal stage, although adults can spread spores.² “Most of the states have regulations in place, exactly how to deal with AFB, so it’s not going to be up to the veterinarians. In some states, there are no regulations, so veterinarians can make the decision, beekeepers can make the decision,” said Kyle, adding that AFB is a notifiable disease.¹

The best practice for dealing with a clinical case of AFB is to destroy the colony, including equipment such as frames and box parts.^1,2 “It’s typically destroyed by a fire,” said Kyle, adding that there are options available for treating infected equipment for reuse, but regulations vary by state.¹

Kyle said dousing a colony with diesel fuel, soap and water, or alcohol can also destroy the infected bees. “They do die quickly,” she said, noting that more research is needed to determine whether methods used to destroy a colony are humane.¹

Although the disease is uncurable, preventive measures can be taken to avoid a honey bee colony becoming infected with AFB, such as the use of antibiotics.¹ “It’s very common for commercial beekeepers to administer oxytetracycline, tylosin, or lincomycin once or twice a year in order to suppress the disease,” said Kyle. “It can stop the vegetative state and it can stop the production of the scale, which is where all the new spores are being produced.”

However, Kyle noted that additional research on antimicrobials is needed with AFB. Antimicrobials allow honey bees to develop resistance to the disease, but the treatments do not affect the spores. “We still have an infective source that we need to be aware of if we’re using antibiotics,” she said.¹

Another preventive measure is for beekeepers to check brood combs at least twice a year for signs of AFB. Replacing brood combs every few years can also help prevent infection.²

Takeaway

Despite data that shows honey bee colonies collapsing at a higher rate, Kyle dispelled the myth that honey bees are disappearing. “Honey bees are not going to disappear because they are an agricultural species. We breed them,” she told WVC attendees.¹

According to the Bee Informed Partnership, the number of honey bee colonies has remained stable over the past 20 years. As loss rates have risen, beekeepers are offsetting losses by creating new colonies.³ “The health status of honey bees is not good, but the population is controlled by supply and demand,” said Kyle.¹

References

1. Kyle B. When honey bees get sick: the most common diseases, pests, and parasites inside a hive. Presented at: Western Veterinary Conference; Las Vegas, NV. February 18-22, 2023.
2. American foulbrood. BeeAware. Accessed February 24, 2023. https://beeaware.org.au/archive-pest/american-foulbrood/#ad-image-0
3. Aurell D, Bruckner S, Wilson M, Steinhauer N, Williams G. United States honey bee colony losses 2021-2022: preliminary results from the Bee Informed Partnership. July 28, 2022. Accessed February 24, 2023. https://beeinformed.org/wp-content/uploads/2022/07/BIP_2021_22_Losses_Abstract.pdf