American FoulbroodFoulbrood is caused by a bacteria in the genus Bacillus. This bacteria reproduces using spores, and the spores are eaten by bees which then become infected. The bacteria multiply in the bees midgut, and will migrate around in the bees body eventually killing it.
The disease is thought to be spread by drifting bees such as drones who may visit a number of hives. Another method of spread may occur when beekeepers transfer infected comb from one hive to another. Infection occurs when spores are fed to brood less than three days old. Infected larvae are dull brown and eventually turn black and die. Cappings over diseased brood are sunken and may have holes in them.
This image is from Honey Bee Brood Diseases and this book is an absolute must for those checking to see if bees are healthy. The disease will progress to the "scale stage" in which only a small scale will be sticking out from the side of the infected cell.
Foulbrood is considered one of the most widespread and destructive brood diseases to which bees are susceptible. A few dead larvae may be the first signs of infection and a colony may not decline to dangerously low levels until the following year.
Since spores can remain in wax, honey or in the wood of the hive box, some states require that infected hives be burned to destroy any remaining spores. While used hive equipment is economical, the danger of acquiringfoulbrood must always be considered.
In the US, Oxytetracycline is the only accepted method of treatment. The use of grease patties containing this antibiotic is a common fall task for beekeepers wishing to avoid foulbrood infection.