Colony dwindling and underperforming in spring. Mimics several other conditions. Need positive identification of Nosema spores to confirm diagnosis.
Nosema, the most common disease of adult bees, is a single celled microsporidian, once believed to be a protozoan but currently classified as a fungus. Two species occur in honey bees: Nosema apis and Nosema ceranae, the latter replacing N. apis in dominance since 1996. The microsporidians live and reproduce in cells of the midgut (ventriculus) of adult worker, queen and drone bees. Worker bees that ingest spores when they are less than a week old do not digest food well and are not capable of producing brood food secretions. They become early (precocious) foragers and die young. Queens ingesting spores are superseded or the queen dies overwinter, leaving the spring colony queenless. Although as much as 50% of sampled bees may have Nosema, when the infection rate exceeds one million spores per bee, colony winter losses are elevated. In N. apis, elevated spring spore numbers drop drastically, but in N. ceranae, the spore levels remain higher year-round, with higher seasonal losses. Diagnose by sampling bee guts and looking for or counting spores using a haemocytometerhaemocytometer:
a counting-chamber device originally designed and usually used for counting blood cells; consists of a thick glass microscope slide with a rectangular indentation
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The spread of Nosema from hive to hive occurs by transferring contaminated combs between hives, feeding bees with honey contaminated with spores, and by splitting and uniting contaminated hives. Robbing of honey from infected hives, drifting, installing infected package bees, and an infected queen and her attendant workers are other causes of transmission. Bees with Nosema have higher levels of black queen cell virus.
Dysentery during winter and early spring is considered a sign of Nosema apis. Nosema ceranae is less likely to display this characteristic. (Most dysentery is thought to be related to poor winter stores.)
Standard treatment for Nosema has been the antibiotic Fumagillan, but this antibiotic is no longer indicated for treatment as Nosema levels seem to bounce back at higher levels following use.
parasitic mite syndrome; weak spring colony
Burnham AJ. 2019. Scientific Advances in Controlling Nosema ceranae (Microsporidia) Infections in Honey Bees (Apis mellifera). Frontiers in Veterinary Science 6:79. https://www.frontiersin.org/articles/10.3389/fvets.2019.00079/full
USDA. 2016. Nosema Disease. USDA Agricultural Research Service, Bee Research Laboratory. Accessed 2023. https://www.ars.usda.gov/northeast-area/beltsville-md-barc/beltsville-agricultural-research-center/bee-research-laboratory/docs/nosema-disease/
