Allen Dick and I have spent a few nights talking intensely on the issue of how AFB might cycle through its life in a colony even though no visible signs of disease can be seen in the brood nest.  The conversation about AFB cycling and no disease symptoms usually comes up when we are talking about antibiotic resistance, because Allen questions how resistance could possibly have evolved as a consequence of selection from continued antibiotic use, when during antibiotic use you never see the disease, and presumably, no generations of AFB pass.

I have seen things over the last few months that make me think it is not so crazy that AFB does cycle below levels where beekeepers can spot it.  I am helping out on a big experiment looking into different ways to detect AFB spores from colonies, and it is from this I am seeing things about AFB I did not know about.

As many of you know, spores are not only found on ropey larvae or scale, but they are also in honey, on wooden ware and on adult bees.  For years people have noticed that even if you find spores in colonies, you might not actually see any diseased larvae.  Where do these spores come from?  They could be immigrants, having floated in on the occational drifting bee returning from an unclean drum of honey or having hitched a ride on a frame that was previously housed in an infected colony.  Another theory is that larvae are continually being infected with the disease and workers are ejecting them before we can see them, but not soon enough to prevent a cloud of spores in the colonies.  Camilla Brodsgaard, in Denmark, and her collegues state this theory the following way in the discussion section of one of their papers:

"There may be several reasons explaining the lack of visible signs of AFB when P. l. larvae spores are present in the honey of honey bee colonies.  The present study suggests that, the ability of the nurse/cleaning bees to remove infected brood can be an important factor".

In the trial I am involved right now we have set up 5 different yards and have inoculated 10 x 10 cm patches of 1st instar larvae with spores of increasing concentration at each yard (yard 1 has no spores and yard 5 has 2 billion spores applied to each patch).  Each week we have carefully examined the brood nest and have taken honey samples, from both the surplus honey and the brood nest, from adult brood nest bees or from swabs of the spaces between frames in the brood nest.  I have noticed the following remarkable things:

1) Larvae that have recently died from AFB are sometimes still white, hold their shape and do not rope but are packed with spores (I only new this because I grew some larvae out that were slightly funny looking... I would not have picked them out of the line-up otherwise)
2) Many larvae that are inoculated with spores are ejected before they show signs of symptoms
3) Scales can form within a week of first observing ropey larvae and can be completely removed within another week,
4) Following a flush of infected larvae from the inoculation there is often a one week period where no symptoms can be seen, followed by a second week were symptoms are ambiguous, followed by a 3rd week (which hasn't actually happened yet, but I am expecting it) with diagnostic symptoms.

Although we are waiting for a slow time to grow out the samples, I expect there will be many times we go to the colonies and do not see clear AFB symptoms but there are many spores floating around.

Allen, I hope I have not misrepresented you.  If I have, know that it is because I did not understand you properly, likely because you had given me to much mead that particular night.  I don't want to get into the evolution of antibiotic resistance, beacuse you long ago showed me how complicated those questions are, but I thought you all might appreciate the observations.

Regards,
Adony