The multiplication and spread of sacbrood virus of bees

Sacbrood virus multiplied without causing symptoms in adult bees when it was injected into them or when it was fed to young individuals. More virus accumulated in heads of infected bees than elsewhere in their bodies, and much was in their hypopharyngeal glands. The extract of each infected head contained about 10²medial lethal doses (LD 50s) of sacbrood virus for larvae when given in their food. The infective dose of sacbrood virus by injection for adults was about 10^-4of the LD 50 in food for larvae. The infective dose by mouth for adults was about 10²LD 50s for larvae, but bees older than 4–8 days could not be infected this way. Infection did not spread between adults but is probably transmitted in nature from infected adults to larvae and back to young adults that ingest remains of larvae killed by sacbrood. The youngest adult bees infected with sacbrood ate little pollen and lived only about 3 weeks in the laboratory, as did bees receiving no pollen, whereas bees fed with ample pollen lived 9 weeks.