The following information comes from the Ag Dept. Victoria and the full article can be found at: Nosema disease of honey bees | Honey bee pests and diseases | Animal diseases | Biosecurity | Agriculture Victoria
Infection does not normally pass directly from infected bees to the next generation of adults. Instead, young bees become infected when they ingest spores as they clean contaminated combs.
During the summer months, most honey bee colonies carry a few infected bees with little or no apparent effect on the colony. Spores may also persist on the combs. As the weather in autumn changes, these spores may initiate an outbreak of nosema. Losses of bees at this time of the year may be very heavy.
Winter losses can also be heavy. Infected bees confined in their hives due to bad weather may defecate inside the hive soiling the combs and hive interior with excreta and spores. This, together with spores produced in the preceding autumn causes infection in spring.
Spring outbreaks usually begin in late August or September, when temperatures begin to rise. They may last until late spring or early summer.
When the warm weather comes, the disease begins to decline due to improved flight conditions. The source of infection is largely removed because the bees are able to defecate outside the hive thereby reducing the contamination of combs.
Fortunately, serious nosema outbreaks do not occur every year. Research has indicated that the following conditions appear to be associated with serious autumn outbreaks and epidemics of nosema:
- heavy summer rainfall
- an early autumn break in the fine weather about mid-March to early April
- bees working grey box (Eucalyptus microcarpa), red ironbark (E. sideroxylon) and white box (E. albens).
The exact reasons for these apparent relationships are not known. In these epidemics, strong colonies may be seriously weakened before winter. They may be reduced to the size of a nucleus colony in a matter of days. Infected colonies that survive the winter may require a long build-up period for the population of adult bees to reach normal numbers.
Losses caused by nosema disease are not confined to areas of Victoria having the field conditions mentioned above.
Spores of Nosema apis may occur in honey or pollen. Research reports indicate that honey bee workers can transmit nosema to queens in queen mailing cages, queen banks and queen mating nuclei.