Egg drop syndrome (EDS) is caused by a viral infection in laying hens. It is characterized by the production of soft-shelled and shell-less eggs in apparently healthy birds and leads to a sudden drop (10-40%) in recorded egg production or
It can be difficult to identify the early stages of the disease as hens will eat the shell-less eggs, and the only evidence that may remain in the membranes is a sign that is easy to miss. In flocks where some birds have acquired immunity due to the spread of the virus, a failure to reach expected production targets is observed. Clinical signs include diarrhea and brief loss of shell color and egg yolk pigment before the production of soft-shelled eggs and mortality is usually negligible. Ducks and geese are the natural hosts for the EDS virus and are asymptomatic carriers.
EDS can be distinguished from Newcastle disease and influenza virus infections by the absence of illness, and infectious bronchitis by the eggshell changes that occur at or just before the drop in egg production.
What causes egg drop syndrome?
EDS is caused by infection with the EDS virus which is an adenovirus. The incubation period is three to five days and the course of the disease is four to 10 weeks. The virus is transmitted through any of the conventional means of viral disease spread and is also transmitted on and in the egg (horizontal and vertical transmission).
The main method of horizontal spread is through contaminated egg trays, however, droppings are also infective. Contact with wild ducks or geese, or water or ranges frequented by these birds, maybe a source of infection. Humans and contaminated fomites (such as crates or trucks) can spread viruses, which can also be transmitted by needles when vaccinating and drawing blood. Insect transmission has not been proven but is considered possible.
Chicks hatched from infected eggs may develop antibodies and excrete the virus however it is more common that the virus will remain latent (alive but inactive) and subsequently the bird will not produce antibodies. The virus reactivates and grows in the oviduct when the hens go into lay, at which point the viral cycle begins again. Birds that are immune to the virus (already have antibodies) reduce the rate of spread of the virus. EDS does not affect the fertility or hatchability of eggs that are suitable for the setting.
Treatment and prevention of EDS
There is no successful treatment for EDS. The classical form has been eradicated from primary breeders and the maintenance of EDS-free breeding stock is the main control measure. In layers, induced molting will restore egg production after an episode of EDS infection.
The prevention of horizontal spread relies on good biosecurity and washing and disinfecting plastic egg trays before use can control the endemic form. The sporadic form can be prevented by ensuring that chicken flocks do not come into contact with other birds, especially waterfowl. As such, potentially contaminated water should be chlorinated before use and general sanitary precautions should always be followed. There are vaccines available to prevent infection, and if appropriately produced and administered, these inactivated vaccines can prevent clinical disease but will not prevent virus shedding.
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