What Causes American Foulbrood? How Is It Spread?

When Paenibacillus bacteria find their way into your hive it can have devastating effects on the colony. An understanding of what causes American foulbrood and how the spores are transferred will help beekeepers reduce the spread.

How does AFB spread inside a hive?

Once foulbrood spores are inside a hive, nurse bees unknowingly feed them to their developing larvae in tainted food sources. The bacteria then activate and move to the larval intestines. Under capped cell, they feed on the larva resulting in the developing bee’s death.

As nurse bees uncap the cells, they discover dead larval goo and scale that was once a healthy larva. The workers get busy cleaning out the cell which is now riddled with new highly contagious spores. This housekeeping contaminates brood food which further spreads the disease.

The crushing final blow is when honey from the brood is moved into the supers, contaminating the entire hive with infectious spores.

Tip: Foulbrood is devastating for bees, but it’s not the only challenge for beekeepers. Find out some of the biggest threats to honey bees here.

How is American foulbrood spread between hives?

As a beekeeper, you may be wondering how American foulbrood shows up at your hive in the first place. Here are some of the common ways AFB spreads between hives.

1. Transferring beehive parts between hives

American foulbrood can transfer to new hives in a range of ways, but beekeepers are the biggest cause of spread. Modular hives like Langstoths make it easy to move supers from one hive to another. Years later, if sufficient spores are present, the disease will usually overwhelm the new hive.

Moving contaminated brood and honey frames is also a major cause of transfer. Any sharing of resources such as splitting or combining colonies doesn’t bode well for the healthy bees.

Important: Find out how to treat American foulbrood if your hive is displaying symptoms.

2. Honey from another hive

AFB spores can live in honey for years without being detected. Feeding honey to bees from an unknown source is a big risk and should always be avoided. Pasteurized honey is no exception.

Check out our advice on preventing AFB here.

3. Robbing

A diseased hive soon becomes weak; rearing bees to maturity is extremely difficult. As colony numbers dwindle, the contaminated hive becomes susceptible to robbing from other bees.

Robber bees enter the hive and collect honey to take back to their own colony. This begins a new cycle of destruction in the new hive.

Drifting spreads AFB in a similar way to robbing.

4. Swarming

American foulbrood takes time to weaken a hive, especially thriving ones. If a healthy hive gets infected by the disease and then swarms soon after, it may take spores with them.

Building out a new hive is never easy. Weighed down by a crippling bacteria like foulbrood, their odds of success are low.

5. Queens and package bees

There is a thriving industry of buying and selling queens and bee packages. It’s an essential part of the beekeeping and thankfully, AFB isn’t a common disease. But if you’ve been in the beekeeping game long enough, you’ll probably come across someone that’s been sold bees with foulbrood.

6. Beekeeping equipment

Beekeepers can unwittingly spread spores on equipment they take from hive to hive. Hive tools and gloves are likely offenders.

If you’re visiting a nearby hive, then use their hive tool instead of taking your own. Alternatively, make sure it’s carefully cleaned with a bleach and water solution.

Bees wax foundations and even varroa strips for treatments can assist spore transfer.  

7. Mites

Mites are easily transferred from one hive to another. While their presence is enough to make a beekeeper lose sleep at night, they can also introduce unwanted spores.

Researchers have studied the role of mites in bacteria transferal. One study investigated whether Varroa destructor can transfer Paenibacillus larvae spores from infected to healthy bee colonies. Scientists concluded that they can carry sufficient spores to infect a healthy bee colony.

Read more about their research here.

How long can American foulbrood live?

It is estimated that American foulbrood spores can survive up to 80 years and still be capable of infecting honey bee larvae. The bacterium is extremely resistant to chemical and environmental stressors.

Does foulbrood kill adult bees?

AFB doesn’t kill adult bees, but it can decimate bee brood. By cutting off this stage of a honey bees lifecycle, the colony is unable to expand. Over time, the hive is likely to die out without assistance.

Quick facts about American foulbrood

• A single bacterial spore isn’t enough to infect a colony. Controlled research has found that millions are required to infect larvae.
• The bacteria are capable of multiplying rapidly.
• The spores are resistant to sunlight, fermentation, heat, dehydration, freezing, and most chemicals.
• Larvae less than one day old are most susceptible to AFB.

Summing up

American foulbrood is much less of a problem than it was in the 1920s. But it’s still out there and if your hive contracts the disease, the results are usually catastrophic.

It is essential for beekeepers to have a basic understanding of how the disease works and the main causes of spread. There are many ways the spores transfer to new hives, but the biggest cause is the beekeeper. Activities like swapping parts, moving bees, and using equipment across multiple hives all increases risk.

Beekeepers should understand the signs of AFB. Early detection could save many surrounding hives from being destroyed.