Foulbrood by Dr. Sandra Burnett

We have been corresponding with Dr. Burnett about Foulbrood and wanted to post what she said about it, especially with the current proposed honeybee bill in the Utah House of Representatives. Here are two emails. The first is one I sent to her with some questions and the second is her response. She gave permission for me to quote her here.

Dr. Burnett,
We’ve been talking a lot lately with the Dept of Ag about foulbrood epidemics. It is a hot topic this year with the proposed bill in the Utah House by Marc Roberts to make hive registration optional for hobbiest beekeepers (which we think might be nice because so many people are inadvertently breaking that law). Thanks for telling us about how there are foulbrood spores in every colony. It helps put things into perspective.
What stressors are causing foulbrood spores to infect the colony? It seems like some of our colonies just kind of get over it on their own. If they all have the spores already, how likely is a widespread outbreak?
Alicia Moulton

Alicia,

There are several factors involved in keeping spores down in hives, and in what causes outbreaks.  Obviously, if bees are healthy and strong as a colony, the risk of an outbreak is low, even though we can usually find spores somewhere in the hive if we look long enough (and sometimes it is quite easy to find spores, even in very healthy hives).
Healthy nurse bees can reduce spores being transmitted to larvae by the natural protection mechanism in their digestive tract.  Bees have the honey stomach, where they feed larvae from, and the honey stomach has a small opening to go to the bee’s regular stomach for it’s own digestion.  There are fibers in the honey stomach that are pointed downward that help push spores down the honey stomach and into the bee’s stomach.  If the spore gets to the adult bee’s stomach, it can not germinate and will pass through the digestive tract, and since bees so cleanly use the restroom outside, the spore is dropped outside the hive.  Success!
If the nurse bee isn’t well, her honey stomach will not be as effective at pushing spores down to the normal stomach, and she can potentially feed the spore to a larvae.  Even if a nurse bee is healthy, if she eats a very high load of spores, then it is too much in the honey stomach to move down to her digestive tract and she will feed spores to a larvae.  In the larvae, the spores germinate without a problem, and the bacteria gets overwhelming soon after the larvae is capped, which is why the larvae dies after capping (unlike European foulbrood where the larvae typically dies before capping).
So it seems that having healthy bees, and reducing exposure to heavy loads of spores is key to keeping a hive from getting AFB.
Stressors that can cause problems for the nurse bees include anything from weather problems (drought, too much rain causing reduced foraging time, heat, cold, etc), to animals bothering the hive, to mite loads, to nosema, to invading insects, to hive relocation stress, etc.  So any of these stressors can reduce the health of a nurse bee.
Exposure to heavy loads of spores occurs when foragers rob a sick hive and bring home tons of spores, which is why outbreaks occur within apiaries.  Once a hive is infected and larvae are dying in large amounts, spores are released in even larger amounts around each broken cap of dead larvae.  Once the bees are sufficiently sick and not yielding enough offspring to support protection of the hive, then raiders come and, along with stealing honey, they carry home hundreds and thousands of spores to their own hive, which puts their hive at risk of an outbreak due to the heavy spore load.
Sometimes, if there is just a short period of stress, or a short period of high spore exposure, it is possible that one or two cells will be infected, and reducing the stress and making sure that the nurse bees are eating well can help a hive recover.  On naturally recovering hives, we can find phages, and I suspect they are part of the natural recovery that occurs as well (which is why we also use samples from healthy hives to hunt for new phages).
Otherwise, if the infection spreads beyond just a few larvae, the infection can continue to spread because there are not only spores, but also active bacteria in the hive that cannot be stopped.  The hive will eventually fail – and that percentage of hive death is very high unless action is taken to treat for the bacteria, such as using Tylan or phages.
Hopefully these answers are helpful in understanding why all hives are positive for spores but not for AFB infection.  And why we can see a strong colony actually recover when there is a very low infection occurring.
Have a great day,
Dr. Burnett
Posted in Bee Biology, Beekeeping, the Honey Company and tagged , .

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