American foulbrood (AFB, histolysis infectiosa perniciosa larvae apium, pestis americana larvae apium), caused by the spore-forming Paenibacillus larvae(formerly classified as B. larvae, then P. larvae ssp. larvae/pulvifaciens), is the most widespread and destructive of the bee brood diseases. P. larvae is a rod-shaped bacterium.
Larvae up to three days old become infected by ingesting spores present in their food. Young larvae less than 24 hours old are most susceptible to infection.
Spores germinate in the gut of the larva and the vegetative bacteria begin to grow, taking nourishment from the larva. Spores will not germinate in larvae over three days old.
Infected larvae normally die after their cell is sealed. The vegetative form of the bacterium will die, but not before it produces many millions of spores.
This disease only affects the bee larvae, but is highly infectious and deadly to bee brood. Infected larvae darken and die.
As with European foulbrood, research has been conducted using the “Shook Swarm”method to control American foulbrood, “the advantage being that chemicals are not used”.
When cleaning infected cells, bees distribute spores throughout the colony. Disease spreads rapidly throughout the hive as the bees, trying to remove the spore-laden dead larvae, contaminate brood food.
Nectar stored in contaminated cells will contain spores and soon the brood chamber becomes filled with contaminated honey.
As this honey is moved up into the supers, the entire hive becomes contaminated with spores. When the colony becomes weak from AFB infection, robber bees may enter and take contaminated honey back to their hives, thereby spreading the disease to other colonies and apiaries.
Beekeepers also may spread disease by moving equipment (frames or supers) from contaminated hives to healthy ones.
European Union law requires all infected hives and equipment to be destroyed. In the US, many State Apiary Inspectors require an AFB diseased hive to be burned completely. The spores can survive up to 40 years and are difficult to destroy.
A less radical method of containing the spread of disease is burning only the frames and comb and thoroughly flame scorching the interior of the hive body, bottom board and covers.
Dipping the hive parts in hot paraffin wax or a 3% sodium hypochlorite solution (bleach) also renders the AFB spores innocuous.
It is also possible to sterilize an infected hive without damaging either the structure of the hive or the stores of honey and pollen it contains by sufficiently lengthy exposure to an atmosphere of ethylene oxide gas, as in a closed chamber, as hospitals do to sterilize equipment that cannot withstand steam sterilization.
Antibiotics, in non-resistant strains of the pathogen, can prevent the vegetative state of the bacterium forming. Drug treatment to prevent the American foulbrood spores from successfully germinating and proliferating is possible using oxytetracycline hydrochloride (Terramycin).
Another drug treatment, tylosin tartrate, was approved by the US Food and Drug Administration (FDA) in 2005.
Chemical treatment is sometimes used prophylactically, but this is a source of considerable controversy because certain strains of the bacterium seem to be rapidly developing resistance.
In addition, hives that are contaminated with millions of American foulbrood spores have to be prophylactically treated indefinitely. Once the treatment is suspended, the American foulbrood spores germinate successfully again leading to a disease outbreak.
Thomas Brady at Brigham Young University and Heather Hendrickson at Massey University are currently studying phage therapy to treat American foulbrood.
Lactic acid-producing bacteria inhibit the growth of the spore and vegetative cells of Paenibacillus larvae under laboratory conditions. However, the bacteria made no difference to infected honey bee colonies.
Melissococcus plutonius is a bacterium that infects the midgut of bee larvae. European foulbrood is considered less serious than American foulbrood.
M. plutonius is not a spore-forming bacterium, but bacterial cells can survive for several months on wax foundation. Symptoms include dead and dying larvae which can appear curled upwards, brown or yellow, melted or deflated with tracheal tubes more apparent, or dried out and rubbery.
European foulbrood is often considered a “stress” disease—dangerous only if the colony is already under stress for other reasons. An otherwise healthy colony can usually survive European foulbrood.
Chemical treatment with oxytetracycline hydrochloride may control an outbreak of the disease, but honey from treated colonies could have chemical residues from the treatment.
The “Shook Swarm” technique of bee husbandry can also effectively control the disease, with the advantage of avoiding the use of chemicals. Prophylactic treatments are not recommended as they lead to resistant bacteria.
Scientific research showed, that the spread of the disease is density dependent. The higher the density of apiaries, the higher the probability of disease transmission.
The EFB disease cycle is as follows:
- EFB infection starts when larvae ingest the bacteria which can be present in the brood food, or can be transmitted from infected nurse bees.
- The bacteria then multiply in the mid-gut of infected larvae.
- The multiplying bacteria compete with the larvae for food, often causing the larvae to die before capping.
- The larvae then becomes a semi-fluid (sometimes described as molten) mass and changes colour from a healthy pearly white to a yellow then brown colour.
- The dead larvae slowly dry out becoming a ‘rubbery’ scale that adheres loosely to the cell.
- Nurse bees attempt to remove the dead or dying larvae. While removing infected larvae the mouth parts of nurse bees become contaminated with the bacteria.
- EFB is subsequently spread by the nurse bees to larvae while feeding.
- Occasionally some infected larvae will survive infection as larvae and become adults, which spread the bacteria in their faeces, further infecting the colony.