Sacbrood is a viral condition that affects bee broods. The virus can also be detected in adult bees and is passed on by them. The sacbrood virus (SBV) is round, with a diameter of 28 nanometre (1 nm = one millionth of a meter) and was discovered by Bailey in 1964. It infects fat and muscle cells and the trachea cells of larvae, as well as cells in the neural tissue and the hypopharynx gland of adult bees. Larvae become infected after feeding from contaminated food produced by worker bees with undetected infections.
The virus is ingested by bees during brood rearing or while removing infected parts of the brood. It is then secreted via the hypopharyngeal glands without infecting the bee itself. Young (ca. two day-old) larvae are especially susceptible to infections. Infected brood die before they can pupate. During the sac stage, the brood is highly infectious (Image 1). Infectiousness decreases rapidly, though. Brown larvae and blackish-brown, gondola-shaped scales are rarely infectious anymore.
SBV survives during the broodless period in the salivary glands of adult bees. Such bees are a permanent virus reservoir within the colony. As a result, the sacbrood virus is present latently in many bee colonies over many years, without actually affecting the brood and displaying no clinical symptoms.
Bees infected with the sacbrood virus show no visible changes, but the disease can shorten the lifespan of the individual bee. Infected bees do not feed on pollen. As a result, they become forager bees that will never bring pollen back to the hive.
SBV is endemic throughout the world. Sacbrood occurs more often in spring when there are fewer younger bees and the number of infected, nursing bees is especially high. Additionally, varroa mite infestation or a lack of food (starvation periods) also result in higher sacbrood incidences.
Most colonies recover spontaneously, when there are honey plant pastures in the vicinity. SBV is very common in bee colonies. However, large numbers of infected larvae are rare, as they are identified early and removed. Even the queen can be infected with SBV (SBV particles detected in the ovaries, eggs, head), but only a small number of its offspring will be infected directly by her. SBV is also found in varroa mites. However, Varroa mites do not seem to be a cause of sacbrood outbreaks, but accelerate its spread in previously infected colonies.