Diseases of Honeybee

Bacterial Diseases

American Foulbrood Disease (AFD)

Beekeepers in temperate and sub-tropical regions around the world generally regard American foulbrood (AFB) as possibly the most destructive microbial disease affecting bee brood. The disease did not originate in, nor is it confined to, the Americas. It is widely distributed wherever colonies of Apis mellifera are kept. In tropical Asia, where sunlight is abundant and temperatures are relatively high throughout the year, the disease seldom causes severe damage to beekeeping operations. The disease is contagious and the pathogenic bacterium can remain dormant for as much as and more than 50 years. Therefore, beekeepers and extension specialists throughout Asia should be acquainted with the symptoms of this disease and know how to cope with it.

Cause: American foulbrood is a bee larvae affecting disease and is caused by Paenibacillus larvae. In U.S. it is the most destructive of all bee diseases. The pathogen is a rod-shaped, flagellate, motile bacillus highly resistant to heat, desiccation and disinfectants. Larvae of workers, drones and queens are all infected through ingestion of spores with their food. At the initial stage of colony infection, only a few dead older larvae or pupae will be observed. Subsequently, if remedial action is not taken, the disease will spread within the colony and can quickly spread to other colonies in the apiary as a result of robbing, drifting workers, or contamination caused by the beekeeper's hive manipulations. In the same way the pathogen agent can spread to other apiaries. Natural transfer mainly takes place within a radius of 1 km around the apiary. Often spores enter the bee colonies via foreign honey. Commercially available honey may be highly contaminated; therefore, special attention should be paid near honey processing enterprises and waste disposal sites.

Symptoms:

At the initial stage of AFB infection, isolated capped cells from which brood has not emerged can be seen on the comb. The caps of these dead brood cells are usually darker than the caps of healthy cells, sunken, and often punctured. On the other hand the caps of healthy brood cells are slightly protruding and fully closed. As the disease spreads within the colony, a scattered, irregular pattern of sealed and unsealed brood cells (see Fig.) can be easily distinguished from the normal, compact pattern of healthy brood cells observed in healthy colonies. The bee brood affected by AFB is usually at the stage of older sealed larvae or young pupae, upright in the cells. Often therefore, a protruding tongue can be found with the rest of the body already decayed. At first the dead brood is dull white in colour, but it gradually changes to light brown, coffee brown, and finally dark brown or almost black. The consistency of the decaying brood is soft. Once the dead brood has dried into scales, the test cannot be used. The dry brood lies flat on the lower side of the cell wall, adhering closely to it – in contrast to sac brood. This scale is usually black or dark brown and brittle. Often, a fine, threadlike proboscis or tongue of the dead pupa can be seen protruding from the scale, angling toward the upper cell wall.

The pathogen bacteria may be identified using a microscopic preparation or, more frequently, by cultivation on selective culture media.

A simple way of determining whether AFB caused the death of thebrood is the ‘stretch test’ (See Fig…). A small stick, match or toothpick is inserted into the body of the decayed larva and then gently and slowly, withdrawn. If the disease is present, the dead larva will adhere to the tip of the stick, stretching for up to 2.5 cm before breaking and snapping back in a somewhat elastic way. This symptom called ‘ropiness’, confirms American foulbrood disease, but it can be observed in decaying brood only.

Control : In several countries, where apiculture includes large commercial operations, frequent, efficient inspection services are particularly advanced and a ‘search and destroy’ strategy is adopted in an attempt to minimize damage to apiaries caused by this serious honey bee disease.

European Foulbrood Disease (EFB)

This disease was first reported in 1885 from U.K. in Apis mellifera and India in 1970 from Maharashtra. The range of distribution of European foulbrood disease is not confined to Europe alone and the disease is found in all continents where Apis mellifera colonies are kept. A. cerana colonies are also subject to EFB infection. The damage inflicted on honey bee colonies by the disease is variable.

Symptoms:

Honey bee larvae killed by EFB are younger than those killed by AFB. The diseased larvae die when they are four to five days old, or in the coiled stage. The colour of the larva changes as it decays from shiny white to pale yellow and then to brown. When dry, the scales of larvae killed by EFB, in contrast to AFB scales, do not adhere to the cell walls and can be removed with ease. The texture of the scales is rubbery rather than brittle, as with AFB. A sour odour can be detected from the decayed larvae. The clinical picture and the odour can vary depending on the kind of other bacteria involved (Bacillus alvei, Streptococcus faecalis, Achromobacter eurydice). Another symptom that is characteristic of EFB is that most of the affected larvae die before their cells are capped. The sick larvae appear somewhat displaced in the cells. When a scattered pattern of sealed and unsealed brood is observed in a diseased colony, this is normally an indication that the colony has reached a serious stage of infection and may be significantly weakened. However, this is the case with all brood diseases. EFB is transferred in the same way as AFB.

Control: The choice of an EFB control method depends on the strength of the infection, i.e. how many brood cells and combs are infested. If the infection is low, it is often sufficient to stimulate the hygiene behavior of the bees. Either they are placed at a good foraging site or they are fed with honey or syrup. In case of severe infection, removing the most infected brood combs is effective. Replace infected combs with empty fresh/ sterilized combs. Since the bees’ hygiene behavior is also genetically determined, replacement of the queen is also effective. Requeening can strengthen the colony by giving it a better egg-laying queen, thus increasing its resistance to the disease and interrupting the ongoing brood cycle giving the house bees enough time to remove infected larvae from the hive. Isolation of diseased colonies is also recommended for its control.

Chalk Brood Disease (Ascophaerosis)

In Asia including India, chalk brood is rarely considered to be a serious honeybee disease.

Cause: Chalk brood is a disease caused by the fungus Ascosphaera apis. As its name implies, it affects honey bee brood. This fungus only forms spores during sexual reproduction. Infection by spores of the fungus is usually observed in larvae that is three to four days old. The spores are absorbed either via food or the body surface. They cause mummification of the diseased larvae.

Symptoms: Initially, the dead larvae swell to the size of the cell and are covered with the whitish mycelia of the fungus. Subsequently, the dead larvae mummify, harden, shrink and appear chalklike. The colour of the dead larvae varies with the stage of growth of the mycelia: first white, then grey and finally, when the fruiting bodies are formed, black when infestation is heavy, much of the sealed brood dies and dries out within their cells. When such combs are shaken the mummified larvae make a rattling sound. In the laboratory the fungus can be identified by its morphology.

Control : As with other brood diseases, the bees remove the infected brood with their hygiene behavior, which is especially effective for white mummies. Though as soon as the fruit bodies of A. apis have developed, cleaning honey bees spread the spores within the colony by this behaviour. During the white mummy stage the fungus continues to develop at the hive bottom. If the mummies are not removed quickly, the spores may enter the brood cells carried there by circulating air. In most cases, the method of stimulating hygiene behaviour, already described is sufficient for chalkbrood control. The beekeeper should ensure that the colony has a strong worker population, and that the hive is well ventilated and free from accumulated moisture. At early stages of chalkbrood infection, adding young adult workers and hatching brood, combined with sugar-syrup feeding, often proves to be helpful.

Viral Diseases

Many virus types and strains have been recorded as disease pathogens of adult bees and bee brood in past, nearly all are RNA viruses. The damage caused to colonies by viral infection varies considerably according to a number of factors, which include the type and strain of virus involved, the strength of the colony, weather conditions, the season and food availability. Basically, bees are well - protected against infection with their chitin body shell and gut coating. Parasitic mites sucking the blood of the bees, however, can penetrate this protection. Therefore, increased infestation by parasites is often accompanied by increased virus infection.

Sac - brood disease

Sac brood is a virus disease attacking Apis mellifera brood. The diseased larvae appear sac like and hence the name. Sac-brood disease is perhaps the most common viral disease of honey bees. In Asia including India , at least two major types have been recorded. Sac-brood disease that affects the common honey bee Apis mellifera and Thai sac-brood disease of the Asian hive bee A. cerana. A new type of sac-brood virus has recently been reported in Asian colonies of A. cerana. It is highly probable that the virus is native to the continent and that it has been with the Asian hive bees over the long period of its evolution. Since its first discovery in Thailand in 1976, it has been found in association with A. cerana in India and known as Thai Sac brood virus disease (TSBV).

Symptoms:

Field inspection to determine whether the pathogenic virus has infected a colony can be easily carried out following symptoms.

• Diseased larvae fail to pupate after four days; they remain stretched out on their backs within their cells (distinct from the mostly twisted position of larvae affected by European foulbrood).
• The anterior section of the larva, consisting of its head and thorax, is the first part of its body to change colour, changing from white to pale yellow and finally to dark brown and black. On removing the larvae from their cell the inspector can easily observe that their skin is quite tough and that its contents are watery; the infected larva thus has the appearance of a small, watery sac.
• Dead larvae remaining within their cells eventually dry out to flat scales that adhere loosely to the cell floor.
• An another type Sac-brood disease caused by Thai sac brood virus (TSBV). This virus attacks specifically Apis cerana. The dead brood is found in prepal stage. The pupae turn into sac-like structures filled with lemon-coloured liquid at the posterior end. In advanced stage, the larvae change their appearance from yellowish to brownish to black colour. No discernible foul odour is noticed. Many Indian bee colonies were destroyed by TSBV in India during late nineties and caused severe loss to bee keeping industry.

Control: Prevention is better than cure. It is better to isolate the infected colonies. Combs from diseased colonies should not be used for any other purpose and dequeening of the colony for a few days followed by requeening with a healthy queen from a strong colony is effective. No agent is effective in preventing or controlling sac-brood disease. Colonies often recover from the infection without the beekeeper's intervention, particularly if the infection is not new to the geographic area. This mainly depends on the hygiene behavior of the bees, which may be stimulated as with other brood diseases as European foulbrood. Since the disease usually occurs when the colony is under stress, shortage of food, food storage space, unfavourable climatic conditions such as damp during the rainy season. or cold season, unhygienic hive interior, poor queen, infestation with other diseases, etc., the beekeeper should deal with severe cases by requeening the colony, removing infected brood combs and taking other management measures to restore colony strength, such as providing food and adding worker population.

Fungal Diseases

Nosema disease

This disease is caused by a Microsporadium, Nosema apis/ N. ceranae. It affects adult bees during cold/foggy and rainy weather. Nosema disease is generally regarded as one of the most destructive diseases of adult bees, affecting workers, queens and drones alike. Seriously affected worker bees are unable to fly and may crawl about at the hive entrance or stand trembling on top of the frames resulting into heavy mortality. The bees appear to age physiologically: their life-span is much shortened and their hypopharyngeal glands deteriorate, the result is a rapid dwindling of colony strength. Other important effects are abnormally high rates of winter losses and queen supersedures. In climates with pronounced long periods of flight restrictions, i.e. no flight opportunities even for a day, the infection easily reaches a severe stage that visibly affects the strength of the colony. Less obvious infection levels in other climates often go undetected. The damage caused by Nosema disease should not be judged by its effect on individual colonies alone as collectively it can cause great losses in apiary productivity.

Cause: The disease is caused by the Microsporadium, Nosema apis, whose 5 .0 mm sized spores infests the bees, is absorbed with the food and germinates in the midgut. After penetration into the gut wall the cells multiply forming new spores that infect new gut cells or can be defeacated. The nutrition of the bees is impaired, particularly protein metabolism.

Symptoms:

Unfortunately, there is no reliable field diagnostic symptom enabling a diseased worker bee to be identified without killing it, nor can the beekeeper recognize an infected queen. However, in severe cases of infection, it is sometimes possible to separate healthy from diseased bees, the abdomen of an infected worker often being swollen and shiny in appearance. On dissection, the individual circular constrictions in the alimentary canals of uninfected bees are clearly visible, while the constrictions cannot be seen clearly in diseased bees. Easy separation, after killing, of first abdominal segments with intestines attached, which shows white if strongly infected, versus a normal transparent, darker grey colour if there is no or only a low infection. In productive beekeeping, a healthy queen with a good egg-laying capability is always required, and Nosema disease in queens is therefore critical. The queen’s egg laying ability can be reduced possibly inducing her supersedures. She may also become the major cause of spreading the disease within the colony. On the other hand, beekeepers are naturally reluctant to destroy queens in the uncertain possibility that they are infected. The microscopic inspection of her feaces makes it possible to verify the presence or absence of the disease in the queen. Placed alone in a Petri dish, the queen will defecate in about an hour, the faces appearing as colourless drops of clear liquid. This liquid can be examined under the microscope for the presence of spores, without further preparation.

Control: Nosema disease can best be controlled by keeping colonies as strong as possible and removing possible causes of stress. Colonies and apiaries should receive adequate ventilation and protection from the cold and from humidity. The bees should have the possibility of for aging regularly in order to defecate. This prevents spreading of the spores within the colony. Beekeepers should also ensure that their colonies and queens come from disease-free stock. Hive equipment that is suspected of being contaminated by Nosema apis spores should be thoroughly decontaminated, preferably by heat treatment and fumigation. The best prevention is to change the combs once every two years. During normal wax processing the Nosema spores are killed.

Heat treatment and fumigation for Nosema Control

Heat treatment : Infected equipment is maintained at 49°C (120°F) for 24-hours, ensuring that hot air passes through all stacked combs during the entire period of treatment. The temperature must however be carefully regulated, because heat at levels higher than that specified will melt wax.

Fumigation : A pad of cotton or other absorbent material, soaked with 80 percent acetic acid, is placed over the top-bars of the comb in each hive. The hive bodies are stacked together, the entrance is closed, all cracks are sealed, and the stacks are placed in an open shed for about a week. After this period, the hives are opened and the pads of acetic acid are removed. The combs must then be allowed to air for 48 hours to rid them of acetic acid residue so that they can be used again. The spores in the food cannot be killed. Therefore, the food combs have to be centrifuged before decontamination. The food should not be used anymore for bees.

Source : National Bee Board