Transmission Dead prepupae appear as a sac filled with millions of viral particles. Young adult bees become infected when removing this sac and cleaning the cells. SBV also replicates in these infected adult bees which show no symptoms but act as virus reservoirs. These carrier bees then infect young larvae while feeding them. Within an apiary, the virus can spread from one colony to another through drifting and robbing. SBV can also be transmitted to pupae by Varroa destructor.
Conditions for occurrence All conditions that weaken larvae favour the occurrence of sacbrood: An imbalance between brood and adult bees due to the season (spring often combines cool temperatures with well-developed brood and too few nurse bees) or poisoning events that abruptly reduce the adult bee population. Nutritional deficiencies related to season or environment, which may lead to poor production of brood food. The presence of other brood pathogens. SBV is often a complication of European foulbrood. Varroa weakens immature stages (poor-quality brood food, haemolymph extraction) and can transmit the sacbrood virus.

Signs

At the cell level, the following can be observed:

• Sunken, perforated cappings (mortality occurs mainly at the prepupal stage).
• Pale yellow to brown prepupae with the head (usually grey to brown) oriented toward the cell opening and a sac-like body filled with liquid, easily removed with tweezers. The liquid may be clear, opaque or purulent.
• Dark brown, non-adherent scales shaped like flattened boats with raised ends (terminal stage).

At colony level, one may observe:

• a patchy brood pattern,
• in advanced stages, a weakened and less dynamic colony.

Diagnosis

A brood sample of 10 × 10 cm containing at least 15 affected prepupae can be sent to an accredited laboratory for PCR analysis to detect viral particles. The cost of the analysis is borne by the beekeeper. It is important to contact the reference laboratory (CRA) beforehand regarding approval, shipping conditions and cost.

Differential diagnosis

• American foulbrood: also affects capped brood with sunken or perforated cappings, but dead larvae adhere to the cell and a positive ropiness test helps identify this disease.

• European foulbrood: mainly affects open brood, causing early larval mortality. Larvae are flaccid, pale yellow to brown, show abnormal positions and sink to the bottom of the cell. They are non-adherent and turn into a semi-liquid mass; the head is not visible.

• Chalkbrood (ascosphaerosis): also affects capped brood and transforms larvae into a compact mass covered with white to dark grey mycelium. Note that these diseases may coexist with sacbrood in the same colony, particularly European foulbrood.

Treatment

• Depending on severity, various measures can be taken. If only a few cells are affected, remove the impacted frames. If many frames are involved, transfer the colony onto new foundation and destroy the old combs.
• Replace the queen with one from a less susceptible strain.
• If more than 20% of the brood is affected, the colony is weak, or wintering is imminent, destroy the colony.
• In all cases, carefully inspect all colonies in the apiary, as sacbrood may indicate other diseases (European foulbrood, varroosis).

Read our factsheet: Sanitary transfer of a colony Prevention of sacbrood relies on good beekeeping practices: Treat colonies against Varroa destructor with approved medicines at the appropriate time. Avoid nutritional deficiencies. Prevent the occurrence of other brood diseases. Promote the multiplication of less susceptible strains. Disinfect equipment (blowtorch, bleach). Limit drifting and robbing.

For more information:

Article “Sacbrood disease”, La Santé de l’Abeille, No. 254, pages 149–160.

Source: FNOSAD – National Federation of Departmental Apicultural Health Organisations

Photo credits: Jean-Paul Faucon, Anses Sophia Antipolis

See also

• Factsheet: 2.2 European foulbrood
• Factsheet: 2.1 American foulbrood
• Factsheet 1.1: Varroa concept
• Factsheet: 2.11 Sacbrood (SBV)