Introduction

American foulbrood (AFB) is the most serious, contagious notifiable bacterial disease of honeybee brood of international proportion. The causative organism is a virulent spore-forming bacterium, Paenibacillus larvae var. larvae. AFB can appear and spread quickly through a colony and if left untreated may result in the death of the hive in a short space of time.

 
Infection

As with many disorders, apparition of disease is exacerbated with supplementary stress conditions such as lack of food, water, space or additional disease or pest attack. P.larvae var larvae forms spores which are resistant to desiccation and to antibiotic treatment. Spores can remain dormant for many years on hive and beekeeping equipment and in honey or wax. AFB spores can readily be transported and transferred by bees or through the unwitting manipulations of the beekeeper to new colonies.

Young bee larvae become infected by Paenibacillus larvae var. larvae spores which may be already present in the cell, from housecleaning bees or through contaminated brood food. Once inside the larval gut the spores germinate and the bacteria multiply rapidly, moving from the gut into the surrounding tissues of the bee.

Bacterial proliferation is so great and so fast that infected larvae die within a few days, usually after the cells have been capped. The cadaver dries to form an infective “scale” in the bottom or on the side of the cell.

Untreated, an American foulbrood infection will spread rapidly through a colony killing much of the bee brood. At the end of a season this could result in a small, weak population to constitute the wintering colony.

It is also at this time that AFB can be spread by robbing of the weak, infested colonies by bees from stronger, more healthy hives. Swarms from infected colonies may also transport the disease to new locations. Each scale resulting from American foulbrood infection will yield millions of infective spores.

 

 
Diagnosis
   
Signs of American foulbrood infection
Signs of American foulbrood infection can appear at any time of the year. It is vitally important to detect AFB as early as possible. Vita has developed with Pocket Diagnostics a quick and effective Vita Diagnostic Test Kit. This enables every beekeeper to test their hives at the first suspicion of the presence of AFB. In severe infections the colony exudes a foul, rotten odour (hence foulbrood). Cell cappings may be dark and greasy-looking, sometimes sunken and perforated where the housecleaning bees have attempted to open them.
As in European foulbrood (EFB) infections, the brood pattern in AFB-affected colonies can appear very patchy and irregular, sometimes termed “pepper-pot” in distribution
As in European foulbrood (EFB) infections, the brood pattern in AFB-affected colonies can appear very patchy and irregular, sometimes termed “pepper-pot” in distribution. The queen will not lay eggs in cells previously contaminated with American foulbrood so the subsequent brood may be scattered sparsely over the combs in the remaining “clean” cells.
Bee larvae infected with American foulbrood die in the late larval stage with the body stretched out.

Bee larvae infected with American foulbrood die in the late larval stage with the body stretched out. As the cadaver decays (later drying to a dark sticky “scale” which is difficult for the housecleaning bees to remove) it becomes gelatinous.

If probed with a matchstick or similar implement the body can be ”roped-out” which is a key diagnostic symptom of AFB disease.

 
Treatment

Good husbandry in beekeeping practice will help reduce other stresses on the honeybee colony and possibly limit the extent of any American foulbrood infection. In some countries, such as UK and New Zealand for example, the policy is to destroy colonies infected by American foulbrood by burning. This absolute measure in the UK has reduced the incidence of AFB dramatically.

In some countries, such as UK and New Zealand for example, the policy is to destroy colonies infected by American foulbrood by burning.

In many other countries, administration of the antibiotic oxytetracycline (also known as terramycin) is permitted as a preventative as well as a curative treatment.

However, P. larvae spores can survive antibiotic treatment so application of antibiotic can only serve to suppress the vegetative stage of the bacterium. Spores can survive on combs, on bees, in honey and on any associated beekeeping equipment to reinfect the following or subsequent season. In North and in South America, bee colonies are often treated routinely with oxytetracycline, whether the colonies are overtly infected or not as American foulbrood is endemic practically world-wide.

Difficulties associated with this type of regime include cost to the beekeeper, residues of antibiotics in hive products, effect on the bees and more recently the emergence of oxytetracycline-resistant bacteria.

 
Ongoing Research

Vita (Europe) Limited in conjunction with the National Bee Unit and Cardiff University have won UK government funding for a research project evaluating a new biological control agent for foulbrood. A harmless bacterium found as a commensal in beehives has been shown to control Paenibacillus larvae var. larvae (as well as Melissococcus plutonius) infections under laboratory conditions. Studies on the toxicity and palatability of the bacterium show no effect on the bees.

Field trials against both European foulbrood and American foulbrood are underway and current progress is positive.

A new, natural product may be available from Vita (Europe) Limited for the treatment of foulbrood within the next few years.