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American foulbrood and European foulbrood are perhaps two of the most commonly misunderstood diseases among beekeepers. And understandably so. After all, both diseases share many of the same characteristics even down to their names. However, there are just as many differences between the two that make identification much simpler than we often think it is. Here’s a breakdown of those similarities and differences and how to obtain an accurate diagnosis when either foulbrood is suspected. 

What is Foulbrood? 

Both American foulbrood (AFB) and European foulbrood (EFB) are highly contagious diseases caused by two different bacteria. These bacteria may be brought into an unsuspecting, healthy colony by adult honey bees, robbers, contaminated equipment, and through the use of contaminated honey and pollen. 

Transmission throughout the colony of both diseases occurs via nurse bees feeding infected food to larvae roughly two days old or younger. Sick, dying, and dead larvae turn from a glistening white color to yellow to brown and even black. Shotgun brood patterns will also be noted as diseased larvae are removed by nurse bees. To make identification even more difficult, both diseases emit foul odors of decay as the brood dies. 

Left unchecked, both diseases can completely destroy not only a single colony but an entire apiary in a short period of time. 

American Foulbrood 

AFB is arguably the most alarming of the two diseases. Caused by a spore-producing bacterium called Paenibacillus larvae, adult honey bees carry the disease-causing spores on their bodies as they go about their daily activities and bring them back to the hive. Spores ingested by larvae quickly germinate and multiply within the honey bee’s midgut until the gut wall ruptures causing bacteria to flood the body cavity, thus killing its host. 

Visible signs of AFB include discolored (yellow to brown to black) brood in the late larval stage to the early pupal stage. These larvae typically die in an upright position shortly after their cell is capped. Capped cells appear as sunken or somewhat caved in, often with a greasy look. Cell cappings often present with holes and tiny perforations as well.  

When death occurs during the pupal stage, it is possible to witness what is known as “false tongue.” This false tongue gives the appearance of the honeybee’s tongue sticking from one cell wall to the opposite cell wall. The body of the pupa essentially melts down into the cell leaving behind this false tongue. This sign is unique to AFB but is not always present. 

After about a month, desiccated pupae turn black and become what is known as scale. This scale is stuck so efficiently to the lower cell wall that removal is impossible without damaging the wall. Transmission throughout the hive continues as house bees attempt to clean out these infected cells.  

The final, most confirming sign to look for without a diagnostic test is the rope test. Larvae infected with AFB become gooey before they fully dry out. This gooey matter readily sticks to a toothpick, twig, or even a matchstick that has been swirled into the goo. As the toothpick is pulled from the cell larval remains will stretch out and appear ropey if AFB is present. However, the absence of this ropiness is not an indicator that all is well with the colony, so don’t rely solely on this field test.  

Due to the extreme economic impact of untreated colonies infected with AFB, most states require beekeepers report AFB breakouts. Most often, a state bee inspector will visit the apiary in question and run a more thorough diagnostic test and will then advise on what the state requires to prevent further transmission of this horrible disease. There are times when guidelines may allow for the torching of larger equipment like hive bodies, lids, and bottom boards rather than burning them. Alternative treatments/remedies are highly variable making contact with your state agency a necessity to keep your bees and other beekeepers’ bees safe from AFB. 

European Foulbrood 

Caused by the non-spore producing bacterium, Melissococcus plutonium, European foulbrood is not nearly as devastating as AFB with proper intervention. However, transmission of EFB is the same as AFB — adult bees, equipment, and food sources — with heavy colony losses possible if left untreated. The good news is there are plenty of signs available to help differentiate EFB from AFB with the naked eye.  

For instance, EFB tends to be more prevalent during the early spring when colonies are stressed from cool, wet weather, during nectar dearths, in the presence of high varroa counts, as well as other common stressors. Colonies weakened by these stressors become more susceptible to this bacteria just as people become more susceptible to colds and the flu when our bodies become too stressed.   

Symptoms of European foulbrood include a lack of sunken, greasy looking cappings. Larvae fed brood food infected with EFB bacteria do not live as long as those infected with AFB because the bacteria growing inside the larva’s midgut multiplies and begins competing with the larva for nutrients rather than bursting the midgut. This battle for nutrients results in larvae requesting extra feedings, alerting the nurse bees of trouble. However, rather than continue feeding these overly hungry larvae, nurse bees promptly discard them. This removal of infected larvae prior to cell capping results in the typical shotgun appearance as also seen with AFB.  

There are occasions, however, when EFB infected larvae do die after cell capping or larvae are not removed by nurse bees as quickly as some colonies may. Infected larvae still exhibit the typical yellow to brown coloring as seen in AFB; however, larvae not removed prior to capping are often seen twisted inside their cells, dead larvae do not rope out, no false tongue is visible, and the odor is not nearly as foul as with AFB. The scale left behind in EFB, while similar to AFB, has one distinctive difference — removal is easy to accomplish without destroying the cell wall due to the lack of adhesion to the cell walls.  

European foulbrood, just like AFB, is a reportable honeybee disease in most states. However, the good news is that destruction of the colony is not necessary as treatment options for EFB do exist. One of the simplest ways to turn an EFB infected colony around is to shake the colony’s adult bees onto new foundation while discarding the old, infected comb. Feeding with sugar syrup and protein patties while adding additional adult bees and brood from a stronger, healthier hive is another option as increased feed and populations relieves the stressors that encourage many cases of EFB. Requeening with hygienic stock is another option. In more difficult or more advanced cases, treatment with the antibiotic Terramycin is useful. However, any antibiotic use in honeybee colonies must be under the guidance of a veterinarian.  

Differentiating between both American and European foulbrood can be daunting at best for many due to their many similarities. However, both diseases have enough differences to help take the guesswork out of diagnosing suspicious colonies. Once a potential diagnosis is made, state inspectors and purchased testing kits can help take the final guesswork out of the equation.

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Originally published in the Spring 2023 issue of Backyard Beekeeping and regularly vetted for accuracy.