Cause: Nosema apis and Nosema ceranae, small microsporidian parasites that live in the digestive tract of honey bees.
Effect: Nosema disease is widespread and causes serious damage to adult honey bees thus reducing the life span of individual bees and weakening or killing colonies. Infected nurse bees do not fully develop and infected queens die off prematurely. The disease may be associated with Colony Collapse Disorder (CCD).
Symptoms: No symptoms are specifically indicative of Nosema disease. Inability of bees to fly, excreta on combs or lighting boards, and dead or dying bees on the ground in front of the hive may be manifestations of Nosema infection, but they may also be caused by other abnormal conditions. N. apis may cause the ventriculus of heavily infected bees to become white, soft and swollen while N. ceranae infections do not. A microscopic examination is the only reliable test for the presence of this disease.
Transmission: The spores of Nosema enter the body of the adult bee through the mouth and germinate in the gut. After germination, the active phase of the organism enters the midgut epithelial cells where it multiplies rapidly and new spores are formed. The cells rupture and shed the new spores into the midgut lumen where they pass down to different tissues or are voided in the excreta of the bee. The cycle begins over again when the spores contaminate the food of other bees. Spores may remain viable for many months in dried spots of excreta on brood combs. Near the end of winter, combs are often soiled with excreta from infected workers. Other bees become infected when they pick up the spores in the excreta as they clean the soiled combs during the spring expansion of the brood nest. Thus, the disease within the colony increases rapidly for a time, and a colony may dwindle in the spring because of the premature death of the overwintered bees. Usually, the colony survives and the proportion of infected bees begins to decline. This decline occurs because the excreta are normally voided away from the hive when regular flights become possible in spring. Nosema apis has been recognized in the U.S. for many years. Recently, it was found that a second species, N. ceranae was also present and that over 90% of Nosema infections in the U.S. are this second species. High infections of N. ceranae are frequently found during the summer months in production colonies.