Although American Foulbrood (AFB) is not as common as EFB, it is the most serious and destructive brood disease of honey bees. It is caused by the spore-forming, gram + bacterium, Paenibacillus larvae, specific to honey bees. It is highly contagious and will weaken and ultimately kill most infected colonies. This bacterium can produce over one billion spores in each infected larva! What makes this disease so dangerous is that the spores are resistant to heat and chemical agents and can exist for decades in nature, in old equipment and in hive products. There is no cure; antibiotics are not a cure or a treatment for AFB, they have no impact on spores. I do not recommend antibiotics for the vegetative state because that just masks the symptoms and can lead to an even more serious infection.
Unfortunately, the clinical signs of AFB are diverse. Variable signs depend on the genotype (virulence), the season, the stage of the disease, the strength of the colony, and bees' tolerance. Clinical signs can be a malodorous hive, a sunken and mottled brood pattern, dead larvae, a positive "ropey"stick test (see picture) from capped larvae cells and/or a pupal tongue. You or your veterinarian should perform an antibody test as soon as possible and send a sample of brood to the Bee Lab. Another field test is the Holst Milk Test, devised in 1946, it is a simple and inexpensive test that can identify AFB but only if there are active spores.
Prevention is the only strategy. Most AFB is transmitted by us beekeepers so we must incorporate biosecurity into our management practices. Once a colony is infected with spores it has to be destroyed by burning or irradiating. AFB is the reason that state apiary programs were created. Check with your own state to know if this is reportable. In my state of Michigan it is not. However, I would recommend contacting your state inspector if you have a confirmed case. It's the right thing to do.