What is sacbrood?
Sacbrood is a viral disease that affects honey bee brood. It is often confused with AFB, since brood killed by the virus always dies at the late larval (prepupal) stage, when the larva is fully extended along the bottom wall of the cell. This is the same position in which a larva infected with AFB dies. Unlike AFB, however, sacbrood infected brood never die in the pupal stage.
Cells containing sacbrood can be either capped or uncapped. If the cells are capped, perforations in the cappings appear in the same way as for AFB, since in both cases the perforations are created by house bees that have detected diseased larvae underneath.
Decaying larvae killed by sacbrood tend to remain more rounded in shape than those killed by AFB, at least in their early stages of decomposition. The skins of sacbrood infected larvae become tough and plastic-like (Fig. 43), and retain the segmentation visible in healthy brood. AFB infected larvae do not show this clear segmentation.
Sacbrood infected larvae go through a series of colour changes, from yellow, to brown, to grey, and finally to black. AFB infected larvae also go through a series of colour changes, but the change is from light coffee-brown colour to darker coffee-brown colour, and finally to black. Sacbrood infected larvae at the brown colour stage are the ones most often confused with AFB infected larvae.
Inspect the head
In sacbrood infected larvae, the head-end of the larva usually remains darker than the rest of the body, and the head-end is often raised (Fig. 44).
AFB infected larvae do not generally display two-tone colouration, although at the onset of the final drying down of AFB larva into scale, the head-end of the larvae can become drier than the rest of the body, giving it a darker appearance.
The head-end is not raised in AFB infected larvae.
Unless house bees remove them, sacbrood infected larvae eventually dry down to black-coloured scale, which are similar in appearance to AFB (although no pupal scale or associated tongues are ever found). Unlike AFB scale, sacbrood scale can be easily removed from the cell wall, often in one piece. Sacbrood scales are also generally found only in colonies suffering heavy infections. Such infections are rare.
At the early stage of decomposition, larvae infected with sacbrood can usually be removed with a stick or twig as whole larvae (Fig. 45), and will hang in a sack-like fashion (hence the name “sac” brood). If the larva is punctured, a watery fluid will come out. AFB infected larvae do not display such symptoms.
Ropiness test not to be relied on in this case
Beekeepers often use the ropiness test when they find sacbrood infected larvae in the brown colour stage (after the larvae have collapsed, but before drying down to scale). It can be difficult to make a visual diagnosis of such larvae, since the symptoms so closely resemble AFB. Generally, sacbrood infected larvae at this stage will not show the degree of roping associated with AFB and the rope will be lumpy and uneven in colour. The roping is probably caused by other bacteria feeding on the larval remains.
Because ropiness can also be variable in AFB infected larvae, it is important to look for other contrasting symptoms when making a differential diagnosis for sacbrood. The remainder of the brood should be checked carefully for more definitive AFB symptoms (and especially pupal scale). If no further symptoms can be found, a sample of the suspect (roping) larva should be taken for laboratory testing (see laboratory diagnosis of AFB).
Some hives are more susceptible to sacbrood than others. Requeening will normally cause the symptoms to disappear.
Unlike AFB, sacbrood does not affect pupae. Sacbrood larvae do not rope out like AFB, and have a watery consistency.
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