Transmission in the colony: During brood care. The virus can also be present in honey and pollen and cause infections via this route. The sac brood virus can be transmitted from an infected queen to the offspring via the egg.
Transmission from colony to colony: through beekeeping activities (rehanging of combs) or through flight and predation of bees.
Situation in Austria
In Austria's honey bee colonies, sacbrood occurs widely. The occurrence of sac brood is not notifiable. Therefore, the frequency of occurrence is only surveyed on a random basis in the course of research projects. In 2015 to 2016, about 200 apiaries were checked for clinical symptoms of sac brood in the course of the project "Zukunft Biene". The frequency varied depending on the season. In summer 2015, bag brood symptoms were seen on 7.3% of the stands (95% confidence interval: 4.1-11.5%). In fall 2015 and spring 2016, bagbrood symptoms were seen on 1.6% of stands each (95% confidence interval: 0.4-4.1%).
Bag brood is a brood disease of bees caused by the bag brood virus (SBV). It is also detectable in adult bees and can be passed on through them. SBV is one of the most widespread bee viruses and belongs to the Iflaviridae family. Infection with SBV leads to the death of the bee larva, which subsequently acquires its typical sac-like shape and later dries into a scab. Infection of the adult bee is asymptomatic, affecting cells in the nervous tissue and hypopharyngeal glands (foraging sap glands). Infected nurse bees infect the young larvae by feeding them virus-contaminated foraging sap.
The sac brood virus survives in the broodless period in the foraging sap glands of the adult bees. These represent a permanent virus reservoir in the colony. Thus, the sac brood virus is latently present in many bee colonies for years without the disease breaking out with clinical symptoms in the brood.
In the colony: During brood care or removal of diseased brood, adult bees take up the viruses and excrete them again via the foraging sap glands without themselves becoming visibly ill. Young larvae (about two days old) are particularly susceptible to infection. Infected larvae die in the expanded maggot stage before the first pupal molt. Fluid is deposited under the last larval skin, which contains a large number of infectious virus particles. Subsequently, a sac-like shape develops. The infectivity decreases rapidly, brown dead larvae and black-brown boat-shaped scabs are hardly infectious.
Young worker larvae are most susceptible to infection, with virus transmission occurring through feeding on sap from infected nurse bees. Bees infected with sac brood virus show no outward change. However, the disease can shorten the life span of solitary bees. They no longer take pollen as food. As a result, they quickly become forager bees, but they will never bring in pollen.
The virus can also be found in honey and pollen and cause infections through this route. Bag brood virus can be transmitted from an infected queen to offspring via the egg.
From colony to colony: transmission from colony to colony occurs through beekeeping activities (rehanging of combs) or through flight and predation of bees.
In case of a severe sac brood disease of the colony, the brood nest is patchy, there are standing brood cells with cracked or holey and possibly sunken cell covers.
The remaining capped brood cells contain dead brood with typical symptoms: when the pupa dies, a bag-like shape is formed due to fluid accumulation between the old larval and pupal skin. In this condition, the larva can be pulled out of the cell with tweezers like a sack. After dying, the larva decomposes into a watery granular mass. It turns increasingly dark from front to back, the head bends upward, drying into a blackish-brown, boat-shaped scab with a hooked head end ("boat disease"). The scab lies loosely in the lower cell groove, and often the original segmentation of the larva is still visible.
A constant nectar supply is to be aimed for: The feed flow must never break off in the colony so that the larvae are always adequately supplied. In good weather conditions, spontaneous self-healing of the colony is usually possible. If there is a lack of feed during gaps in the tracht, feeding should be carried out. Stimulus feeding can also promote the cleaning instinct of the bees. Good hygienic behavior of the bees is important so that diseased brood is quickly cleared out. Reducing varroa infestation also helps preventively, as sac brood can be a side effect of varroosis.
There is no medication available for control. However, health-promoting (hygiene) measures can be taken. The following measures are recommended to reduce the amount of virus in the colony:
- Remove heavily infested brood combs from the colony and melt down the wax (the sac brood virus is sensitive to heat).
- Artificial swarming
- Cleaning the hive
- If sac brood is a concomitant effect of varroosis, effective varroa control must be carried out. Susceptibility to sac brood can also be genetically determined due to low hygiene behavior of the bees. In this case, re-pollination can help
- Maintenance measures such as confinement of the colony and stimulation of the cleaning instinct can also help. The stimulation can be done by stimulation feeding or by spraying the combs with a thin sugar solution.
- Killing heavily infested colonies helps to prevent the spread of the disease to other colonies.
The symptoms are typical and can usually be identified from the diseased brood. In addition, the sac brood virus can be detected by molecular biological methods (PCR) on adult bees as well as on brood stages. Virus detection by PCR will only be performed upon explicit request and if costs are covered by the sender.
Sample submission: Brood comb piece with diseased maggots, bees from diseased colonies.
Last updated: 17.02.2022