Egg Drop Syndrome in Chickens

Egg Drop Syndrome (EDS) is an infectious disease caused by a virus which mainly affects laying hens. The disease results in egg quality issues in infected hens. Egg Drop Syndrome Virus (EDSV) or Duck adenovirus-1, originated in waterfowl and was likely introduced to commercial chickens through a contaminated vaccine.

Ducks and geese are the natural hosts for the virus, and are often asymptomatic carriers. The virus is spread both vertically (hen to chick) and horizontally (between flock members). The primary site of replication of the virus is the pouch shell gland. In infected embryos or young birds, the virus is latent until sexual maturity and the start of egg laying.

The first sign of infection is a change in egg shell colour, followed by production of smaller, shell-less, thin-shelled, or soft-shelled eggs, and either a rapid or extended loss in egg production of up to 40%. Symptoms are mainly related to egg production.

Transmission 

• Contaminated eggshells or poop
• Infected birds or contaminated equipment
• Vertical transmission: Infected breeder hens can pass the virus to their offspring via the egg

Waterfowl such as ducks and geese can carry the virus without showing signs, acting as reservoirs and spreading the virus to chickens in nearby flocks.

Symptoms

The hallmark sign of EDS is a sudden, unexplained drop in egg production, often within a few days. Other clinical signs may include:

• Pale or discoloured eggs
• Eggs with thin, soft, or shell-less shells
• Decreased egg weight or size
• No apparent signs of illness in the bird itself

The hens typically appear healthy, with usually no change in appetite, behaviour, or mortality, making diagnosis challenging.

Diagnosis relies on a combination of:

• Flock history: sudden drop in egg production, especially in previously healthy birds
• Laboratory testing:
  • Serology to detect antibodies
  • Virus isolation in duck embryos
  • PCR testing to identify viral DNA

Diagnosis requires eliminating other conditions that cause egg production issues, such as Infectious Bronchitis Virus (IBV), Newcastle Disease, or nutritional imbalances, is essential.

Prevention 

While no treatment exists, it is preventable through good biosecurity and vaccination.

Vaccination

• Inactivated vaccines are available and effective at controlling the disease but don’t prevent viral shedding. 
• Administer to pullets around 14–18 weeks of age, before sexual maturity.
• A single dose usually provides long-lasting protection.

Biosecurity

• Prevent wild birds and waterfowl access to coops and runs.  
• Use clean water sources and disinfect equipment regularly.
• Quarantine new birds before introducing them to the flock.

Monitoring Breeder Stock

• Testing breeder flocks helps prevent vertical transmission to chicks.
• Only source chicks from EDS-free parent flocks.

Although most outbreaks are reported in commercial poultry farms, backyard flocks can be affected especially if waterfowl share the same environment or if biosecurity is lax. In small flocks, a few abnormal eggs or missed days of laying are normal, but persistent issues with shell quality or a sharp decline in laying can be a sign of more serious issues.

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