Egg drop syndrome virus is a naturally occurring adenovirus of ducks and geese which have infected and caused disease in chicken. It grows to very high titre in embryonated duck eggs but poorly in chicken embryo.
Transmission
Three patterns of Egg drop syndrome have become evident over time.
1. Classical egg drop syndrome: It follows introduction of virus into primary breeders stock most likely through a vaccine grown in duck cell. The main method of spread is vertically through the embryonated eggs. Such hatched chicks do not show clinical signs, spread virus or develop antibodies.
2. Endemic egg drop syndrome: it results from lateral spread between flocks. It is primarily due to contaminated eggs or egg trays and it is usually seen in commercial layers. Any age of laying flock may be affected and there is often association between flock and egg packing station.
3. Sporadic egg drop syndrome: It results from introduction from from duck, geese or any infected wild bird either through direct contact or indirectly through contaminated drinking water.
Clinical signs
The first signs are usually a loss of shell strength and pigmentation. This is quickly followed by thin shelled, soft shelled and shell less eggs. Eggs may show mineral deposits. The birds are apparently healthy but sometimes they appear slightly depressed for 48 hours or so.
Classical egg drop syndrome is manifested either by a sudden fall in egg production or around peak production or a failure to achieve or hold predicted production.
Diagnosis
A combination of a sudden fall in egg production associated with thin shelled, soft shelled and shell less eggs in a flock with apparently healthy birds is almost diagnostic. The easiest method to confirm diagnosis is by feeding affected eggs to susceptible hens which then produce affected eggs.
Also the pouch shell gland can be harvested and samples inoculated into embryonated duck eggs. Virus growth is detected by testing for haemagglutination. Detection of antibodies to Egg drop syndrome virus by the H1 test using fowl erythrocyte is sensitive and easy.
ELISA is also used to detect antibodies to Egg drop syndrome virus. Care must be taken to collect blood only from birds which have sero converted and are laying infected eggs.
Control
Since classic Egg drop syndrome is spread primarily by vertical transmission via the egg, birds should be derived from uninfected breeders stock.
Endemic Egg drop syndrome can be controlled by vaccination with inactivated oil-adjuvanted vaccine given at 14-18 weeks of age. As the main method of horizontal spread is via the infected egg and egg trays, plastic trays should be washed and sterilized before returning to the farm.
Sporadic Egg drop syndrome can be controlled by segregation of chicken from ducks and geese and other wild birds. If surface water is given to birds, the water should be chlorinated and replaced with borehole derived water.