Animal and Plant Health Inspection Service | Veterinary Services | 1

1. Disease Information

1.1 General Disease and Pathogen Information: American foulbrood (AFB) is a 

bacterial disease affecting brood (larval and pupal stages) of the western honey bee, 

Apis mellifera, and other Apis species, that is endemic in the U.S. The causative 

organism is Paenibacillus larvae (P. larvae), a gram-positive, spore-forming bacterium 

found throughout the world. P. larvae is passed to brood by unaffected adult bees, 

most commonly through spores suspended in food. When spores make their way to 

the bee brood gut, P. larvae germinates into the active, vegetative form and colonizes 

the gut, producing toxins and enzymes that ultimately kill the brood. AFB is considered 

infectious and highly contagious because the spore form of P. larvae is incredibly 

stable and hardy in the environment. Spores are resistant to heat and chemical agents 

and can survive for many years on scales, hive products, and other equipment. 

Furthermore, it takes only a few spores to cause disease in an otherwise healthy larva. 

Each infected larva can then produce over one billion spores. Combs of infected 

apiaries may show distinctive clinical signs that allow diagnosis of the disease in the 

field.

1.2 Clinical Signs: Brood comb of infected colonies has a mottled appearance from the 

mixture of healthy capped brood, uncapped cells with the remains of diseased larvae, 

and empty cells. Cell cappings appear moist and darkened, as well as sunken and 

punctured as infection develops. Dead brood are commonly older sealed larvae or 

young pupae whose color changes to creamy brown then dark brown to almost black 

with a soft consistency that becomes sticky or ropy. In some cases, the larval remains 

appear watery. Dead brood produce a slight to pronounced odor. The diseased brood 

eventually dries out to form characteristic brittle scales that adhere tightly to the lower 

sides of the cell. The development of a fine threadlike pupal tongue is a characteristic 

but rare sign of the disease. Subclinical infections are common and may require 

laboratory diagnosis.

2. Laboratory Criteria

2.1 Agent Isolation and Identification: Diagnosis of AFB is based on identification of the 

pathogenic agent and the presence of clinical signs in larvae. Identification techniques 

include microbiological characterization, and in particular the homogeneous presence 

of rod-shaped bacteria, polymerase chain reaction (PCR), biochemical profiling, 

antibody-based techniques, and microscopy. For culturing samples, five solid media 

are recommended: PLA (P. larvae agar), MYPGP agar, BHIT agar, J-agar, and 

Columbia sheep blood agar. Two PCR methods based on the 16S rRNA gene can be 

applied and used for confirmation of clinical disease and identification of bacterial 

Case Definition

American Foulbrood of Honey Bees (Paenibacillus larvae)

(Monitored) December 2023
Animal and Plant Health Inspection Service | Veterinary Services | 2

colonies after cultivation. Biochemical profiling of P. larvae can also be performed and 

is based on the catalase test, the production of acid from carbohydrates, and the 

hydrolysis of casein depending on the genotype. Antibody-based techniques that can 

be employed for identification are immunodiffusion tests and fluorescent antibody 

technique. Microscopic techniques used include gram staining of bacteria and carbol 

fuchsin staining of larval smears.

2.2 Agent Characterization: the bacterium is generally alone, without other infection.

2.3 Serology: NA.

3. Case Classification

3.1 Suspect Case: clinical signs, history, or epidemiology consistent with AFB in a 

susceptible honey bee hive.

3.2 Presumptive Positive Case: a suspect case that is positive on at least one AFB field 

test (lateral flow diagnostic kit, ropiness test).

3.3 Confirmed Positive Case: clinical signs and positive identification of P. larvae from 

culture and/or PCR.

4. Reporting Criteria: American foulbrood is a U.S. monitored disease that is reportable 

monthly under the APHIS National List of Reportable Animal Diseases (NLRAD).

4.1 NLRAD reporting in accordance with the NLRAD Standards for monitored diseases; 

and by APHIS to the World Organisation for Animal Health (WOAH).