Egg drop syndrome, a viral disease of chickens and quail, is characterized by a decrease in egg production accompanied by a reduction in egg quality. This disease is primarily of economic importance, as the birds do not become ill.

Egg drop syndrome was first described in chickens in the 1970s. The causative virus, duck adenovirus A, has its reservoir in ducks and geese. The initial outbreak in chickens was probably caused by a contaminated Marek’s disease vaccine grown in duck embryo fibroblasts. This virus infected breeding flocks and spread to other flocks through infected eggs. Although it was eradicated from most commercial breeders, duck adenovirus A became endemic in chickens in many parts of the world. Rare outbreaks of egg drop syndrome are also caused by virus transmission from ducks and geese, either directly or through contaminated water.

Until recently, duck adenovirus A was thought to be avirulent in ducks and geese. However, in 2001, this virus was isolated from an outbreak of respiratory disease in young goslings, and the disease was reproduced by experimental infection of 1-day old birds.