Nosema (Nosematosis)
Lexikon
Nosematosis is a disease of the midgut of adult bees.
Nosematosis is a disease of the midgut of adult bees. It is caused by microsporidia of the genus Nosema, specifically by the native species Nosema apis and the introduced species Nosema ceranae. Older beekeeping literature refers to the pathogens as spore animalcules or protozoa. That is outdated: microsporidia are today classified as fungi. Infection occurs through spores that the bees pick up with food, water, or from contaminated surfaces. They germinate in the midgut, multiply in the gut cells, and are excreted again in the feces. Drifting, robbing and the beekeeper's own work carry the spores from colony to colony.
The familiar picture is brown to yellowish fecal spots on the landing board, hive walls and combs, along with flightless, crawling bees and a swollen abdomen. This, however, is where the most common mistake lies: fecal spots alone do not prove nosematosis. They also occur with the non-contagious dysentery, and Nosema ceranae often causes no fecal spots at all, showing itself instead through an inconspicuous shrinking of the colony. Only laboratory examination gives certainty; species identification requires molecular biological testing.
There is no approved medicine against nosematosis. All the work therefore consists of prevention and hygiene. Proven measures are: choosing a dry, sunny and wind-sheltered location, keeping colonies tightly clustered on their combs, ensuring good pollen supply, regularly introducing young queens, avoiding prolonged queenlessness, keeping water sources clean and not placing them in the flight path, and consistently renewing combs.
Soiled combs and old combs from affected colonies should be melted down, not reused. Diseased colonies should not be united with healthy ones. In cases of light infestation, an artificial swarm on clean new comb can rehabilitate the colony. One should not be guided by instructions for homemade disinfection solutions: comb treatment is subject to approval regulations that change over time, and the current status should be checked with one's own local bee institute.