Nosemosis, or Nosema disease, is caused by the spore-forming fungus, Nosema apis and Nosema ceranae, Both species can infect worker bees, queen bees, and drones. Nosema spp produce spores which are ingested by adult honey bees through contaminated water or food, through food exchange with other honey bees, or from cleaning contaminated combs. Once absorbed, the spores germinate in the mid-gut of the honey bee and damages their ability to absorb nutrients from ingested foods. As a result, the honey bee's lifespan is shortened and an overall reduction in the health and performance of the colony. If the queen is infected, it will impair her egg laying ability which causes her possible supersedure.
Since the disease interferes with food digestion, the bees that develop a larger appetites are those that are most seriously affected. Adult bees have difficulty with controlling their fecal discharge. In heavy infestation, hive bodies are often smeared with fecal deposits on hive walls.
Nosema incidence in honeybee colonies peaks in early fall and early spring. The disease is often not detected because affected bees are either inside the colony (in winter) or in the field, where they die. Nosema is often confused with dysentery which produces similar symptoms. It occurs more frequently when colonies are under stress and poorly managed. The condition is often exacerbated when there is moisture build up and poor air circulation in the hive. Higher Nosema incidences have been reported with tracheal mite infestations.