I have heard of three cases of American Foulbrood (AFB) today.
Two cases in Minneapolis and one in Lake Elmo.
The incident in Lake Elmo was a hive with all equipment purchased new. The beekeeper was an experienced beekeeper and the bees more than likely robbed out an infected colony.
The Mpls AFB, I believe was on used equipment.
Purchasing used equipment while sometimes you can save money, there is the pitfall of equipment with AFB scale. Early on in my beekeeping career I purchased used equipment. The beekeeper said hive equipment was good and he never had any problems with disease. Well, his equipment was full of AFB scale and I quickly had a major AFB problem in my beekeeping outfit. I will say that experience is the best teacher. My experience in this AFB fiasco did educate me about buying used equipment and I quickly learned of what AFB scale looks like in the frame. Since that incident, I have discovered many AFB infected frames that beekeepers have brought to me to check out.
AFB infected larvae will start to discolor, turning a yellowish color, then will darken to a milk chocolate color. The larvae will turn gelatinous and sink to the bottom of the cell it is in. You will see the larvae laying flat on the bottom of the cell. The tongue of the bee may be sticking up in the cell. As the larvae desiccates (dries out) the larvae forms a hard scale on the bottom of the cell. This scale is highly infectious and contains over 2 billion of infectious AFB spores. That is just one cell. Imagine a frame of a couple hundred cells of AFB scale. The cappings of the larvae cells will have a shrunken appearance and the cappings may be perforated will small holes. Beekeepers always think they will smell the foulbrood. The odor only gets pronounced when there is a large infection in the colony.
As more larvae die, the colony will get weaker. While the colony still will try to carry out its normal function of foraging for honey and pollen. As the colony weakens it is more susceptible to being overcome and robbed of its honey stores. Normally, colonies will get robbed out in the spring or late summer, when there is not a nectar flow. Some honey in the AFB hive may have been stored in cells with AFB scale. Now the infected honey is brought back to a healthy colony. The healthy colony now has a good chance of getting AFB.
The trick with AFB, is knowing that you have it before it gets out of hand. Looking at your larvae and making sure it is pearly white. If the larvae is any color other than pearly white, you have a brood issue. There could be many reasons for discolored brood, chilled brood in the early Spring, AFB, European Foulbrood (EFB), Parasitic Mite Syndrome (PMS), The Crud, these are the most common. The only thing that makes AFB different, is that when the larvae is in the milk chocolate color stage, the larvae will rope out with a toothpick or small stick. To do this, a small stick, toothpick , or matchstick is punctured into the milk chocolate colored diseased larvae. The wooden instrument is slowly drawn away from the larvae. The goo attached to the wood will start to string out. If it is AFB, the string of goo will be an inch or more before it detaches from the stick. If it does not rope an inch or more, it is not AFB.
The treatment for AFB is shaking all the bees on new foundation. Feeding the bees syrup for them to draw out the comb will flush out the AFB spores out of their bodies. When the bees start feeding the new brood in the hive on the new foundation, the AFB spores should be gone. A treatment also with Terramycin will get the bees off to a good start. Terramycin can only be purchased with a prescription from a Veterinarian. Getting rid of the infected frames is imperative. That is where the problem lies. Infected comb.
AFB spores are long lasting. They have an active infectious life of well over 50 years. The Univ of MN took some AFB spores from some frames they had that I think were over seventy years old. They purposely infected some colonies trying to see if the AFB was still infectious. All of the colonies came down with AFB. The infected old frames have to be burned. The boxes can be reused but the inside of the boxes should be blackened with a torch. The only thing that kills AFB spores is a direct flame over 1000 degrees or irradiating the scale with radiation at an irradiation facility. I think there is an irradiation facility in the Chicago, IL. area.
Here is a link that accurately talks about all aspects of AFB:
http://beeaware.org.au/archive-pest/american-foulbrood/#ad-image-0