Sacbrood

When inspecting the hives the week before last I found some unusual looking larvae which I eventually identified as having a viral infection called Sacbrood. It's called that because basically it turns the larvae (or brood) into sacs of fluid

Healthy larva look like fat little white "c" shapes stuck to the vertical back wall of their cells. The most obvious sign larvae aren't healthy during a cursory inspection is that they're no longer stuck to the cell back wall but have fallen to the cell floor. You see this with Chalkbrood, European Foulbrood and no doubt a few other brood virii too.

The segmentation of their bodies should clearly visible in a way best described as looking like the Michelin man -in fact taking that a little further if you think of a healthy larvae as looking like the Michelin Man then think of the unhealthy larvae as looking like Ghostbuster's Mr Stay Puft.

Healthy Larvae

As well as losing definition larvae with sacbrood change shape with the head section staying small and raising up slightly, then with time the dead larvae darkens. In the UK these infected larvae are usually  described as being "Chinese slipper shaped" although to be fair I've not seen any Chinese slippers or shoes looking like diseased dead insect larvae and I very much doubt chinese beekeepers ever describe the condition as "my slipper shaped". I think the infected larvae look more like little crossiants really.

Larvae with Sacbrood

There is no cure for Sacbrood, but it's currently not thought to be a particularly major virus. Whilst it kills a few brood the general consensus is that a healthy colony should be able to weather it and it often disappears in spring anyway. The virus is actually carried by nurse bees who then transmit it to the larvae they're feeding. If it does become widespread enough to be a threat to the colony the only 'treament' advised is requeening the colony with a couple of weeks between removing the old Queen and adding the new one. There doesn't seem to be much real information available on why this works but it may be that in the time between losing the old queen and the new Queen starting to lay the virus is unable to infect new larvae and may die out.

As bees age their duties actually change and my theory is that in the interim between the old and new Queens the infected nurse bees are unable to pass the virus to new larvae and graduate to become forager bees by the time the new Queen's eggs are hatching. As they're then not feeding brood any more if they do still carry the virus they don't get to transmit it to larvae anymore. It's possibly a little simplistic but seems to fit with the requeening idea.

A couple more larvae with Sacbrood

As you can see from the images it's only affecting a small number of larvae at present, these are located in two small patches on frames 7 and 9 of Hive2. When I checked the hive again a week later I could see there was still some larvae with sacbrood present but they were still in the same patches of the same frames. This leads me to wonder if nurse bees work a specific area of brood rather than wandering all over the hive feeding all brood they encounter. If they do work in some sort of brownian motion which is pretty much what I thought they did then I'd expect to see the infected larvae spread evenly through the hive rather than concentrated in two specific patches.

The worker bees are supposed to remove and dispose of the infected brood -by dispose I mean they drag the ill larvae from their cells and throw them out. I may give them a little help with some tweezers next inspection and pull a few out myself. It's thought that Sacbrood is normally present in colonies but in levels so low it's often not noticed. Reasons it can increase are if the balance of the hive shifts for example if the bees are spread too thin withing the hive and I suspect that may indeed be the case in Hive2. They're working on 2 supers at present which does mean less bees available to work in the brood box, however the Queen is still laying like crazy so bee numbers are still increasing. I'm currently assuming as numbers increase the bees will be better able to manage the sacbrood and when I remove the supers the population in the brood box will increase substantially anyway.

Time will tell, and if need be I do have a spare Queen living in the nucleus.