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Controlling American foulbrood without antibiotics
Cliff Van Eaton, Apiculture Consultant, Tauranga, New Zealand
In most parts of the world, antibiotics have been the recommended treatment for the control of American foulbrood (AFB) for at least the last half century. They have been accepted readily by many beekeepers, probably because antibiotics have been seen as a quick and easy solution to one of the most economically significant bee diseases.
The problem with administering antibiotics to bees is the same as giving them to other animals. If they are prescribed by a professional who first makes a competent diagnosis, and then applied according to label recommendation, symptoms of the disease are likely to be significantly reduced, if not altogether 'cured'. If, on the other hand, they are administered routinely, and especially at a dose that does not kill all the disease-causing bacteria, eventually a strain of the bacteria will develop that is resistant to the antibiotic. We have seen this many times with diseases in humans, and during the late 1990s a strain was detected of the bacteria that causes AFB, Paenibacillus larvae subsp larvae (PL), that was resistant to oxytetracycline.
At the same time, food safety scares in Europe, together with increased sophistication in analytical equipment, meant that government heath authorities in Europe and the USA began to take increased interest in chemical residues in bee products. Beekeepers heard about the problems China experienced with Chloramphenicol in honey (see BfDJ 63), and now, honey and other bee products destined for human consumption are tested routinely for antibiotic residues.
We now realise that using antibiotics to control AFB has become problematic. The big question remains for beekeepers - how do I control this disease that can be so dangerous to my honey bees without the use of drugs?
New Zealand (NZ), with a beekeeping industry that is overwhelmingly commercial, and which produces the world-famous manuka honey, has always had a legal ban on the feeding of antibiotics to honey bees. However - just because something is illegal does not mean it does not happen. Yet, over the years, extensive government inspections of NZ bee hives, as well as routine tests of NZ honey, confirm that beekeepers in NZ have not used the various antibiotics available to control AFB.
There are a number of reasons why this is so, including strict control of such drugs by dispensing veterinarians. But perhaps the most important reason is simply that NZ beekeepers developed management practices that kept the number of AFB hives at low levels, and they did not see any good reason to change something that worked.
Understand the life cycle
The NZ AFB control system is based on an understanding of what causes a honey bee colony to develop visual symptoms of the disease, and how the disease spreads from hive to hive. PL is a spore forming bacteria, and it is the spores that cause an infection. The spores find their way into their host (a young larva) in the food provided by nurse bees. Once the spores have been ingested, they hatch and produce the vegetative (or growing) stage of the bacteria. The vegetative stage lives in the stomach of the young honey bee larva, and feeds on the larva until the larva dies. At this stage, the bacteria produce spores, and the vegetative stage dies. The new spores are picked up by nurse bees when they attempt to clean up the remains of the dead larva. Thus the spores are spread around the hive during the constant exchange of food between all the adult bees in the hive. The cycle is finally complete when a nurse bee transfers the spores to another healthy young larva during feeding.
PL spores are like tiny seeds. They have a very strong outer coat, and can remain viable that is, can hatch out into growing bacteria again, for many years. What they cannot do, however, is hatch out again until they find their way back into the stomach of another young honey bee larva, because this is the only place in nature where PL can live. As it turns out, there needs to be a lot of these spores in food that is always being transferred around the colony, and this food needs to be fed to a larva by a nurse bee, before there is a good chance for one or more of the spores to hatch.
So here is an important point - spores of PL are not particularly infective, and they need to be in high concentrations (at least 50 million/L) before a larva will become infected. Also for the infection to occur, the spores have to be in something that has a likelihood of coming into direct contact with nurse bees, and through them, the larvae themselves. If we can accept these two important facts about PL, we can start to think a bit differently about our beekeeping management. And that is just what NZ beekeepers have done. Their AFB control method is to carry out management practices that lower the levels of PL spores in their hives.
What are the materials we use in beekeeping that have a high risk of carrying large concentrations of PL spores in a form that will likely end up in the stomachs of young larvae? It will be obvious that combs with brood, honey and pollen can contain lots of spores. They are therefore always considered to be a high risk. House-cleaning bees, because they clean up dead larvae, and (possibly AFB infected ones), can have high numbers of spores, and because bees continually transfer food between each other, all the adult bees in the hive will be high risk. Also bees constantly lick the insides of the hive, so supers, lids and floorboards could have lots of spores, and are therefore high risk.
If honey is removed from a hive that has AFB the honey itself may have lots of spores. If this honey is extracted and does not come into contact again with nurse bees, it is low risk. However, the 'wet' frames (frames after honey extraction), which still have residual honey, could contain spores in a form that could be fed to nurse bees, and then to larvae. NZ experience suggests that 'wet' frames should always be considered high risk.
Bees drifting between hives could carry spores in their honey stomachs, but there is not usually, naturally, a major transfer of bees between hives. Studies conducted in NZ have shown that drifting bees from an AFB hive have quite a low risk of spreading the disease to other hives.
Beekeeping books tell us that it is very important to sterilise our hive tools and gloves, but when you look at these materials from a hazard point of view, you see that they represent a low risk of transferring large numbers of spores in a form attractive to nurse bees. The same can be said for the earth in front of hives.
In NZ, feral colonies were traditionally blamed as a source of AFB. However, studies were conducted that tested bees from feral colonies in NZ for the presence of PL spores. The results showed that very few such colonies had spores: so few in fact, that feral colonies were at much greater risk of getting AFB from managed colonies than the other way around.
Finally, there is the problem of robbing bees. This can certainly be a risk when the hive being robbed is seriously depleted of adult bees because so many larvae are killed by AFB. In that case, there are large numbers of spores in the honey, which is taken back in large amounts to the other hive, where it is probably fed to nurse bees and young larvae. What this means, therefore, is that it is important to periodically inspect hives for AFB, and not to allow any to get so weak that they become a source of infection to other hives.
When you look at the list of hazards, and what are the most infective sources of PL spores, a truth emerges that is sometimes very difficult for beekeepers to accept: AFB may be a disease of honey bees, but it is almost always spread as a result of the actions (or inactions) of beekeepers. The good news is that since that is the case, if we change what we do with beekeeping materials that are at risk of containing high concentrations of spores, over time we should be able to reduce the spore concentrations in all the hives of our apiaries. And, as the spore levels go down, the number of colonies that will become infected with AFB will also go down.
The major skill that must be learned is the ability to identify the visual symptoms of AFB, and, as important, be able to tell those symptoms apart from the symptoms of various other brood diseases and abnormalities that may be present. There are many books that provide good descriptions and photographs of these brood diseases. Most important is to be able to tell the symptoms of AFB from those caused by sacbrood. Everything I am describing here by way of management techniques to control AFB are extremely low-tech. However, there is one 'medium-tech' item that can be added to assist in AFB diagnosis, and that is the use of a microscope to visually identify AFB spores in diseased larvae. The technique is quite straight forward, but is generally only suitable for beekeeping co-operatives or government veterinary labs.
Once you know how to visually identify the symptoms of AFB, the key is to be diligent in inspecting your hives. In NZ, an inspection of all brood frames in every hive is carried out at least twice a year, at the beginning and end of the beekeeping season. Whenever a beekeeper wants to move anything (combs, supers, etc) from one hive to another, each hive is given a brood inspection. This is also true at the time honey is removed from the hive, especially if the honey is being removed as full supers, to be extracted away from the apiary, and then the supers are possibly returned to another apiary to be filled again, either immediately, or the following year. It may be difficult to carry out an inspection on the day of removal. If so, supers should be marked to identify them to the hive and apiary of origin, and an inspection should be carried out as soon as possible.
Inspection is the means of identifying the disease.
Quarantine is the way to control how colonies are treated so they do not become a source of spores to others.
The first type is apiary quarantine, where nothing is moved between apiaries. In NZ this is used routinely every year, starting at the end of the honey season and continuing until full inspections have been completed at the beginning of the next season. This ensures that no hives containing high numbers of spores are moved to any other apiaries during the time when inspections are not usually carried out.
The second type is hive quarantine, where nothing is moved between hives. This is used whenever AFB is found. No bees or brood are taken away from any of the hives, and nothing is moved to any other apiary. If honey supers are removed, the honey is extracted, and the frames are kept together in the super. The super is marked to identify it to the hive, and it is not placed on any other hive. Hive quarantine continues for 18 months after the last AFB hive was found in the apiary.
When an AFB hive is found, it is immediately dealt with to ensure there is no chance of the large number of spores it has produced coming into contact with other hives. This is probably the most difficult thing that beekeepers who have used antibiotics find to accept, because to make sure spores are not transferred, the bees in the hive have to be killed (usually with a small amount of petrol), and the contents of the hive (bees and brood frames) have to be destroyed by fire.
Burning the hive contents ensures that nothing will be left that will be attractive to foraging bees from other hives, and also means that the beekeeper will not inadvertently put diseased materials into or on any other hives. Honey can be removed first, and extracted, provided that the honey does not come into contact with other frames. The extracted frames must then also be destroyed by burning.
Supers, bottom boards and lids are a major cost to beekeepers in NZ, just as they are everywhere. Therefore NZ beekeepers have devised a sterilisation technique that has been proven to kill the very resistant PL spores, and at the same time help preserve the woodenware from rotting. The technique involves immersing the woodenware in paraffin wax heated to 160oC for at least 10 minutes. At shorter times,
or lower temperatures, it has been found that not all PL spores will be destroyed.
How effective is the NZ system?
NZ has had an organised bee disease control programme and kept detailed records for many years. By law, beekeepers are required to report all cases of AFB. They also make an annual declaration of apiary locations and hive numbers. Statistics show that the annual incidence of AFB in NZ is less than 0.3% per annum. This is extremely low by world standards, especially since most countries do not have good information on AFB rates or even hive numbers. AFB control in NZ is supported by a national programme funded by beekeeper levies, and managed on behalf of the National Beekeepers Association by a state-owned agricultural services enterprise. Apiary inspections are carried out by employees of the enterprise. These inspections have always confirmed the low incidence of AFB reported by beekeepers.
Some understanding of AFB incidence, and the overall level of PL spores circulating within a country's beehives, can also be judged by analysing retail samples of honey. In a random survey of 45 samples of NZ retail honey in 2007, no spores of PL were found. This compares to studies elsewhere in the world where up to 56% of samples were found to contain spores.
The spread and control of American foulbrood BfD Journal 76
GOODWIN,M; VAN EATON,C. (1999) Elimination of American foulbrood
Information portal on the BfD website
The enterprise had two significant outbreaks of AFB during that time, the first beginning in 1973, and the second beginning in 1989. Both were associated with the purchase of honey supers from other beekeepers.
Nevertheless, using only hive inspections, hive and apiary quarantines, destruction of AFB infected colonies and hive woodenware sterilisation, the enterprise managed to operate within the internationally accepted veterinary standard for 'disease freedom' (0.2% of animals/hives per herd/enterprise per annum) in 27 of the last 35 years, and in each of the last 10 years. Significantly, this low level of AFB incidence was achieved in a large-scale, migratory beekeeping enterprise consisting of 7,000 hives. A considerable number of the enterprise's hives are placed each year in apple and kiwifruit orchards for pollination, in close proximity to other pollination hives (and therefore potential sources of AFB infection) belonging to a range of other beekeepers. The enterprise also migrates all of its hives to at least two honey flows. In the last four years shown on this graph, the enterprise did not record a single case of AFB.