In recent years, bee colonies worldwide have declined due to a combination of problems, including conditions such as varroosis, nosemosis and foulbrood diseases, and a number of viruses. As well as affecting both honey and wax production, this has also had a significant impact on crop pollination and the environment. Honey bees (Apis mellifera) are classified as food-producing animals but, to date, veterinary surgeons have played only a very modest role within the apiculture industry in the UK. However, the future of honey bee colonies will depend on practitioners working together with beekeepers to limit further losses. This article describes the most important pathogens affecting honey bees and outlines the options for treatment. In particular, it highlights how veterinary surgeons can help to promote and maintain bee health and food safety.
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Giovanni Formato is a veterinary surgeon based in Italy. He is a food safety specialist, with an interest in bee health and pathology. He is coordinator of the laboratory of apiculture at the Istituto Zooprofilattico Sperimentale delle Regioni Lazio e Toscana (IZSLT).
Antonella Comini is a veterinary practitioner specialising in animal welfare, educational farming and the control of diseases affecting honey bees.
Alessandra Giacomelli is a consultant beekeeping technician at the IZSLT, with experience in the control of diseases affecting honey bees, beekeeping management and training.
Arianna Ermenegildi holds a degree in natural science. She is a beekeeping technician at the laboratory of apiculture at the IZSLT, and is currently working on the diagnosis of diseases affecting honey bees using microbiological and biomolecular techniques.
Romano Zilli is a veterinary surgeon, and a food safety and public health specialist. He is head of the research, development and international cooperation unit at the IZSLT. He is president of the European Association of State Veterinary Officers.
Ivor Davis is a master beekeeper and holds the national diploma in beekeeping. He is a past president of the British Beekeepers' Association. He is an amateur beekeeper, but by profession is a chartered physicist, chartered IT professional and runs his own consultancy business.
Bees in the UK
The population of honey bees in the UK has fallen by almost 50 per cent since 1965, as has the number of beekeepers. There are currently an estimated 250,000 bee colonies in England and Wales, in addition to a number of feral colonies, but these are all continuing to experience a dramatic decline since the arrival of the parasitic mite Varroa destructor in 1992.
Honey bees have a poorly developed immune system and can be attacked by parasitic mites (eg, V destructor), fungi (eg, Nosema species), bacteria (eg, Paenibacillus larvae), numerous viruses and scavengers (eg, beetles) during any stage of their life cycle. While strong colonies can fight off the effects of some of these pathogens, prolonged attack can cause stress and ultimately result in the collapse of a colony.
V destructor is an ectoparasitic mite that attacks honey bees (Figs 1, 2). It can be seen with the naked eye on bees or on the bottom of hives (Fig 3). It is known to have affected bees for over 60 years, which is insufficient time for the host–parasite relationship to reach an equilibrium, thus resulting in the very large losses of bee colonies seen recently.
V destructor reproduces in sealed brood cells and feeds on bees by consuming their haemolymph. The mite itself does not harm bees, but the parasite has become an effective vector for a number of viruses affecting bees, and it is the resulting low levels of viral infection that eventually prove fatal for honey bee colonies. If left untreated, most colonies will die from Varroa infestation.
In the UK, V destructor was first successfully controlled using plastic strips containing the synthetic pyrethroids flumethrin (Bayvarol; Bayer) and tau-fluvalinate (Apistan; Vita Europe) (Fig 4). In recent years, however, populations of mites resistant to these chemicals have emerged and spread throughout the country. The only other licensed treatment, a thymol gel (Apiguard; Vita Europe), has variable efficacy and is not reliable in all parts of the UK. Protocols based on integrated pest management have therefore been developed to help overcome the lack of effective approved pharmaceutical agents, and the use of several unapproved substances has now grown to fill the therapeutic gap.
Oxalic acid is not licensed for use in bees in the UK and does not have a maximum residue limit (MRL) but has been shown to be an effective treatment for Varroa infestation. It is best applied topically during the winter when there is no (or at least very little) brood present (Fig 5). The efficacy and safety of oxalic acid is supported by a large body of evidence in the literature, and this product is now becoming more widely used. A number of approved products based on oxalic acid are available in Germany, Italy and Spain, and the Veterinary Medicines Directorate (VMD) is aware of this. Similar approval of such products, which would make them available on the general sales list (GSL) is required in the UK.
Two species of Nosema, a genus of microsporidian parasites, affect honey bees:
■ Nosema apis has long been known to infect western honey bees. It invades the midgut of adult bees, shortening the lives of infected individuals and reducing the ability of nurse bees to feed larvae;
■ Nosema ceranae is a newly recognised species and operates in a similar manner. It is able to cause a gradual depopulation of hives, resulting in decreased honey production and heavy autumn/winter colony deaths. Other than depopulation and death after a long incubation period, there are no specific outward signs of disease in infected bees. Diagnosis therefore requires light microscopy (Fig 6) or more sophisticated PCR-based molecular techniques.
Both species of Nosema have been found in the UK, and the National Bee Unit is currently attempting to establish their distribution throughout the honey bee population in the UK. Some studies have suggested that N ceranae played a part in recent multifactorial colony losses experienced in the UK, USA and Europe (Higes and others 2009).
Treatment of nosemosis is possible with Fumidil B (Ceva), but its availability is at risk as it does not have an MRL (which is required for medicines used in food-producing animals), despite the fact that it is licensed for use in bees in the UK. It is therefore important that treatment is optimised and other effective medication is identified to deal with this condition. Although Fumidil B has been used by UK beekeepers for nearly half a century to control N apis, there is no recent work that confirms its efficacy under current UK conditions.
Other treatments have been trialled against Nosema species in various countries, including plant extracts (Protofil; Institutul De Cercetare – Dezvoltare Pentru Apicultura SA), a formic acid and iodine formulation (Nosestat; Apinatura), a preparation of vegetable oil and vitamins (ApiHerb; Chemicals Laif), an extract containing beet extracts and molasses (Vita Feed Gold; Vita Europe), salicylic acid, garlic and thymol. While these products have been shown to be effective to varying degrees against N apis, they appear to have little effect against N ceranae, and this requires further research.
Box 1: Role of the veterinary practitioner in the beekeeping sector
Most medication for honey bees is currently sold on the general sales list (GSL) (Table 2). So, traditionally, veterinary surgeons have had very little to do with bees, while beekeepers have been responsible for:
■ Controlling the health status of their bee colonies;
■ Identifying proper treatment options for various pathogens (possibly involving laboratory analysis);
■ Ensuring that bee health and honey safety assurance systems are adhered to;
■ Keeping records as stipulated by European legislation.
Veterinary surgeons usually only get involved in honey bee health when the available medication appears ineffective or POM-V (prescription-only medicines – veterinarian) products such as antibiotics are required, or if non-licensed medication has to be given via the prescribing cascade to deal with emergencies.
However, legislation relating to medication for honey bees is likely to change in the future and this will require suitably qualified persons (SQPs) to store and supply medication. This move, driven by European rules, will in all probability change GSL medicines to POM-VPS (prescription-only medicines – veterinarian, pharmacist or SQP) products.
Veterinary practitioners will therefore have to become more involved with:
■ Diagnosis and treatment of notifiable diseases in bees;
■ Prescription of veterinary drugs, and any associated administration;
■ Sampling and testing of hive products to check for the presence of residues;
■ Planning and implementation of good farming practices and hygiene measures based on hazard analysis and critical control point (HACCP) principles.
In order to fulfil this role, clinicians must have a good understanding of bee health and disease prevention.
Veterinary surgeons working in an official capacity (eg, for government departments) need to have a solid knowledge of beekeeping practices and any associated animal and public health issues to ensure responsible inspection and auditing. In its Terrestrial Animal Health Code 2009, the World Organisation for Animal Health (OIE) indicated that ‘the permanent official sanitary surveillance of apiaries should be under the authority of the Veterinary Authority’ and should be performed by either representatives of this Authority or by those of approved organisations, possibly assisted by specially trained ‘health inspectors or advisers’ such as experienced beekeepers.
American foulbrood (AFB) is an infectious notifiable disease affecting honey bee broods. It is caused by Paenibacillus larvae, a spore-forming bacterium. AFB is widely distributed and destroys broods but the bacterium does not affect adult bees.
Young bee larvae become infected with Paenibacillus larvae spores via contaminated brood food. The spores germinate into the vegetative stage as soon as they enter the larval gut and continue to multiply until larvae die as a result of septicaemia (usually after the cell has been capped). Brood combs in an infected colony have a scattered and irregular pattern of capped and uncapped cells and show punctured and discoloured cappings (Fig 7).
AFB can usually be diagnosed in the field by carrying out a simple ‘ropiness test’. This involves stirring the contents of a cell with a match or similar probe and gradually pulling them out. If Paenibacillus larvae are present, their remains will form a fine elastic thread or ‘rope’ between 10 and 30 mm long (Fig 8).
After infected larvae die, they form spores (known as the sporulation process) that can remain viable for up to 40 years in hives. AFB spores can be easily transported and transferred by bees. As the infection weakens the affected colony, it can no longer defend itself against invasion from stronger colonies in the area. Spore-contaminated products (eg, honey, nectar and pollen) can be spread quickly from hive to hive. Contaminated beekeeping equipment can also transfer the disease between hives.
Current methods for the control of AFB involve destroying heavily infected colonies by burning. Tetracyclines, tylosin or sulphathiazole may be efficacious at the vegetative stage but are not suitable for killing off spores. No antibiotics are registered for use in bees within the EU, as there is concern about antibiotic residues in honey and the emergence of antimicrobial-resistant strains of the bacterium. For this reason, antibiotic treatments must not be used. Infection of the colonies can be prevented by good beekeeping practices such as not feeding colonies with honey and renewing brood combs every other year.
European foulbrood (EFB) is also a serious notifiable bacterial disease of honey bee broods, and has been seen internationally. Several bacterial organisms are associated with EFB infection, but the main cause is Melissococcus plutonius. The disease is thought to be linked to stress, but queen genetics, weather and geography may also be involved. In the UK, in severe cases, treatment involves destroying EFB-infected colonies. Low-level infections may be treated by an appointed bee inspector using oxytetracycline or by completely replacing a comb (called shook swarm).
Since 1963, when the first honey bee virus was isolated, 18 viruses affecting bees have been identified and characterised. These are mainly picornavirus-like in nature and appear as icosahedral particles, about 30 nm in diameter, and are morphologically similar when viewed under an electron microscope (Fig 9).
The most common viruses affecting honey bees are: deformed wing virus (DWV) (Fig 10a,b,c), black queen cell virus (BQCV), sacbrood virus (SBV), Kashmir bee virus (KBV), acute bee paralysis virus (ABPV) and chronic bee paralysis virus (CBPV). They can be differentiated using PCR-based molecular techniques. Another virus – Israel acute paralysis virus (IAPV) – has been suggested as a marker for colony collapse disorder (Cox-Foster and others 2007).
Viruses are transmitted in honey bees via both horizontal and vertical transmission pathways. V destructor is thought to be a carrier and reservoir for viruses affecting bees (Shen and others 2005) and may be responsible for higher infectivity rates. Nosema species may also be potential vectors for viruses (eg, N apis is known to be associated with BQCV, filamentous virus and bee virus Y) as may EFB.
Although virus particles are frequently present in a latent or asymptomatic form in bees, they are able to reduce the life span of honey bees and dramatically affect honey bee health under stress conditions (eg, in cases of high levels of Varroa infestation or in the face of other diseases, or due to poor weather conditions or management practices).
Viral infections cannot be treated. Instead, their impact may be minimised by:
■ Reducing stress to bees by implementing good beekeeping practices;
■ Providing stores, especially pollen, to the bees;
■ Keeping mite levels low by frequent monitoring and applying mite control treatments periodically;
■ Replacing the queen from another source;
■ Replacing combs.
Definitions and common beekeeping terms
■ Queen bee. Fertile female bee in a hive
■ Drone bee. Male honey bee, which develops from eggs that have not been fertilised. Its only function is to mate with the queen bee
■ Worker bee. Sterile bee specialised to collect food and maintain the hive
■ National hive. The most common type of hive used in the UK. It is square and capable of holding a colony of about 60,000 bees
■ Commercial hive. Larger hive that will accommodate up to 75,000 bees. It is simpler to construct but needs care when manipulating the bees. It is used by commercial beekeepers in the UK and often by beekeepers with many hives
■ Brood box. Main chamber in a hive where the queen will lay eggs and a brood develops into new workers. It is filled with frames of wax that are drawn out by the bees. Workers will nurture the brood in this area and also store pollen and nectar to feed the larvae
■ Supers. Boxes that hold frames of honeycomb where the bees are able to store honey for their future use or for the beekeeper to remove and extract at a later date. They are the same cross section as the brood box of a hive but less deep so that the weight, when full of honey, is not too great
■ Crown board. Separates supers from the roof. It is used to close a beehive and prevent bees from sticking the roof down
■ Queen excluder. Device with slots in it that will allow a worker to pass through but will prevent a queen or drones gaining access. It is placed above the lower box in a hive (the brood box) and thus ensure that the only bees that gain access to the rest of the hive are workers. This means that only nectar and honey are stored in the supers and that extracted honey is not contaminated with larvae and other hive debris
■ Eke. Shallow extension that can fit between the brood box and the supers to allow the beekeeper to create space for placing medication (eg, Apiguard) into the colony
■ Shook swarm/Bailey frame change. Management techniques that allow the beekeeper to replace all the brood frames in a colony. It is well known that pathogens build up in the wax in brood frames and that removal of all the frames in a short time helps to reduce stress and the possibility of recurrent disease in a colony
■ Open mesh floor. Device that sits at the bottom of the hive and allows small debris to fall out of the hive area onto the floor below. The mesh is small enough to prevent pests entering the hive but sufficiently large to ensure that varroa mites can drop out of the hive. An open mesh floor also provides excellent ventilation and can stop the build up of moisture during winter months
■ Feeder. Device placed above the brood box that allows the beekeeper to feed the colony with a sugar solution. The sugar can either be used as a stimulant in the spring or a means of providing the colony with sufficient stores to survive the winter. Feeding bees is also an excellent way of administering medications that are water soluble
■ Pepperpot pattern. The brood pattern in a healthy colony is uniform. If the pattern is patchy (ie, larvae at different stages are adjacent to each other), this can be a sign of disease and indicates that the colony may require further investigation.
With the continued worldwide decline in honey bee populations, veterinary surgeons are likely to have to play a greater role in the health and welfare of these animals (Fig 11) and to ensure the safety of the honey produced (Fig 12). In particular, practitioners will need to work with experienced beekeepers to ensure that bees are appropriately medicated and that the impact of diseases such as varoosis is limited.
■ British Beekeepers Association. Represents amateur beekeepers in the UK but mainly in England. www.britishbee.org.uk
■ Welsh Beekeepers Association. Represents amateur beekeepers in Wales. www.wbka.com
■ Scottish Beekeepers Association. Represents amateur beekeepers in Scotland. www.scottishbeekeepers.org.uk
■ Bee Farmers Association. Represents commercial beekeepers in the UK. www.beefarmers.co.uk
■ National Bee Unit. Part of the agency supporting the government in beekeeping matters. Provides a disease inspection service and has statutory rights to control notifiable honey bee diseases. www.nationalbeeunit.com
■ International Bee Research Association. Organisation that supports research into beekeeping and publishes research articles about honey bees. www.ibra.org.uk
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