Diseases & Pests of Apis Mellifera
Pathogen: bacterium Melissococcus pluton (formerly Streptococcus pluton)
Melissococcus pluton is a non-spore forming lancet-shaped bacteria only 0.5-0.7 microns by 1.0 microns thick. It does not affect adults, eggs, or pupae. It infects worker, drone, and even queen larvae, but larvae only. The name is European FoulBROOD. The infective stage is the bacterial vegetative stage. The pathogen, Melissococcus pluton, does not produce a toxin, does not invade tissue, nor do any damage. They simply live in the larvae’s food supply; they are parasites.
Symptoms: Brood diseases are generally easier to recognize than adult bee diseases. Brood comb from diseased colonies usually have a scattered brood pattern (pepperbox) and cappings are often darker in color, sunken, or punctured. A healthy brood pattern is uniform in color and convex (slightly rounded out).
European Foulbrood (EFB) is considered the fastest killing brood disease, but rarely kills the colony. It weakens it, spoiling it as a productive unit, but the colony generally survives from season to season. Diseased colonies fail to progress normally and the result is often a lack of surplus honey for the beekeeper. EFB is considered a stress disease and is usually most prevalent in the spring when the brood nest is expanding rapidly and rearing conditions are marginal (chalk brood, sac brood are similar). It will sometimes continue throughout the summer and occasionally the entire foraging season.
Transmission: The disease cycle begins when the bacterium that causes EFB is swallowed with contaminated food (usually less than 48 hours old). Larvae suffering from EFB appear to be undernourished. EFB is readily transmitted by nurse bees that inadvertently infect the larvae while feeding them. The bacterium multiplies as the disease progresses and grows in the midgut of the infected larvae, destroying the peritrophic membrane (similar to stomach lining in humans), and eventually invades the intestinal epithelium. The pathogen then infects the mouthparts of the nurse bees and the food that the larvae is fed contains this pathogen. If the larva is infected with a large number of pathogens, then its growth and multiplication will result in a midgut full of the pathogen. Larvae must compete for food with rapidly multiplying bacteria creating abnormal demand for larval food. Such a larva will starve, and unless it is getting a surplus of food it will starve before it pupates. The nurse bee will reject those larvae that require more than the usual amount of food. If, however, there is an unusually high number of nurse bees as is common prior to the spring nectar flow or in queen-rearing colonies, they may be able to supply the extra food, thus extending the length of the colony disease instead of quickly rejecting those diseased larvae and allowing them to starve. Once nectar flow commences, many of those nurse bees switch to foraging and those infected larvae are rejected by the remaining nurse bees. The sick larvae are displaced in their cells, just as they are when they are deprived of sufficient numbers of adult bees to feed them adequately.
Death normally occurs in uncapped cells with larvae still in the curled stage, usually when they are 4-5 days old (or 2-4 days old depending on the reference used). Healthy larvae are glossy and pearly-white in color. Dead larvae are seen as collapsed, coiled, or twisted remains in the cell that undergo gradual uneven color changes from dull white, to yellow, and finally dark brown in color. Especially as the larva turns brown, the trachea of the infected larvae appear as fine silvery tubes immediately below the skin. Melissococcus pluton does not form spores and so dead larvae are not sources of infection. Infected larvae decompose quickly and then dry to a brown and then black rubbery scale that is longitudinal in shape (not brittle like AFB, no pupal tongue in EFB). Scale of EFB is then easily removed from the cell by house bees.
The surviving larvae are sub-normal in weight because bacteria in the midgut have assimilated much of their food. As the larvae grow, proportionately less food will be taken by the bacteria, making the larvae less susceptible to the disease. If the larva survives and pupates, the bacteria that multiplied internally are discharged as feces and then deposited on the walls of the brood cell. The Melissococcus pluton bacterium over winters on the sides of the cell wall or in feces and wax debris on the bottom of the hive.
Package bees can serve as a temporary reservoir for EFB when shaken from a diseased colony. Severe outbreaks of EFB often occur after bees have been to blueberry plantations for pollination
Beekeeper Action: When the infection is light, treatment is usually not required. A good nectar flow will help speed up the recovery from EFB by forcing the bees to catch up with housecleaning chores. (same action seen in sac brood, nosema) When the adult population has diminished, treatment becomes necessary.
Provide a frame of both young and mature brood from another healthy colony and feed 1:1 syrup. The new brood will compete with the infected brood for attention from the nurse bees so the sick larvae will starve to death sooner and will be removed. The addition of syrup stimulates production of new brood to out-compete the infected larvae with similar results. Changing colony location may improve nectar flow and assist in clearing up the disease. Requeening gives the colony a more prolific queen which may be less susceptible to disease and also permits a time lag between brood cycles that allows the house bees time to remove diseased larvae from the cells. (as also occurs in sac brood) Requeening may also introduce a line of progeny that are less susceptible to the disease or have greater hygienic behavior.
Treatment: Terramycin is the only drug approved for use as preventive treatment against EFB. It is normally mixed with powdered sugar and sprinkled over the tops of the brood combs. Dusting reduces the likelihood of Terramycin getting mixed into the honey. Honey contamination is greater with syrup-fed Terramycin. Terramycin is relatively unstable (breaks down quickly) in honey and syrup. Cease treatment 4 weeks (28 days) prior to honey flow (another reference states 45 days). Remove all honey above the brood nest prior to drug feeding. Medication may not always be necessary and in some cases may slow its progress by helping infected larvae survive instead of dying and being removed.
Ethylene oxide gas (ETO) is a low temperature sterilizing gas that is used in some states to fumigate EFB contaminated equipment. Special fumigation chambers are required to control temperature, humidity, and pressure. Proper mixtures of ethylene oxide and carbon dioxide are effective in killing the pathogens that cause EFB. (ETO is also effective against AFB, chalk brood, & nosema) (States known to have ETO equipment include WV, MD, and NC)
Resistance, Geography, & Race: Differences in susceptibility has been reported in different strains of bees, but no strain is immune to the disease, although it was originally thought that Italians were somewhat resistant. EFB is found worldwide. The name European Foulbrood is not associated with the geographical distribution of the disease, but rather indicates the area of initial scientific research. Apis cerana is susceptible to EFB (and also AFB).
Confirmation: Beware the Ropey Test: EFB infected larvae will rope out similarly to AFB. However, it extends only slightly, usually less than 1 inch and is granular in nature. Named in 1906 to differentiate the “sour-smelling” disease (EFB) from the “stinking” disease (AFB). No single infected larva has been found with more than one disease pathogen. EFB will almost never be found in the same hive as AFB because the Paenibaccillus larvae of AFB produces an antibiotic to eliminate competition with other bacteria.
The sour odor associated with EFB varies with the different secondary invading bacteria and is not a reliable diagnostic test for differentiating between EFB and AFB. These secondary bacteria also make it difficult for scientists to isolate Melissococcus pluton from diseased EFB larvae. It is easiest to isolate when few, if any, organisms are present. Using laboratory techniques, EFB can be demonstrated to be present in apparently healthy colonies. EFB diseased larvae often harbor a complex of different bacterial organisms, which often affects the odor or consistency of the dead brood, in addition to the causative agent (Melissococcus pluton). Associated organisms, Bacillus alvei, Bacillus laterosporus, and Enterocococcus faecalis, do not cause EFB, but they do influence the odor and consistency of the dead brood.
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Dewey, M. 1999. Honey Bee Biology and Beekeeping. Wicwas Press.
Information compiled by Beekeeper Lonnie E. Campbell of The Loudoun Beekeepers Association.