Egg drop syndrome affects laying flocks that otherwise look completely healthy. There is no cough, no lethargy, no change in feed intake. The only clue is what turns up in the nest box: pale, thin-shelled, soft-shelled or shell-less eggs, followed by a fall in saleable egg numbers of 10 to 40 per cent (Smyth, 2024). Because the birds show no illness, farm staff often notice the eggs before they notice anything wrong with the flock.
What causes egg drop syndrome
The disease is caused by Duck adenovirus 1, also known as the egg drop syndrome virus (Smyth, 2024). Ducks and geese are the natural hosts and usually carry it without symptoms. The virus reached commercial chickens through a contaminated vaccine grown in duck cells, then established itself in breeder flocks (Smyth, 2024; Lera, 2021).
Three infection patterns matter for a layer operation. Classic egg drop syndrome passes vertically from infected breeders through the egg, and stays latent until the pullet reaches sexual maturity. Endemic infection spreads horizontally during lay, usually through shared or reused filler-flats, and is the pattern most commercial egg producers actually encounter. Sporadic infection follows direct or indirect contact with wild ducks or geese, often through contaminated water (Smyth, 2024). The disease can also pass through crates, trucks, personnel and even vaccination needles, and droppings remain infective throughout (Lera, 2021).
Symptoms of egg drop syndrome in chickens
The first visible sign is usually a loss of shell pigment, quickly followed by thin-shelled, soft-shelled or shell-less eggs (Lera, 2021). Because these fragile eggs are often eaten by the hens or lost in the litter, the true scale of the problem is easy to underestimate. Some birds show transient diarrhoea or mild depression for a few days, but these signs are inconsistent and mortality is not increased (Lera, 2021; Smyth, 2024). Internal egg quality, fertility and hatchability are unaffected, and flocks generally return to normal production within four to ten weeks (Lera, 2021).
A flock infected before or at the start of lay will simply fail to reach its expected production peak, rather than showing an obvious drop.
Diagnosis
Suspicion should follow any otherwise healthy flock producing soft-shelled eggs, but confirmation depends on laboratory testing. The haemagglutination-inhibition test and ELISA are the standard serological tools, with PCR available on cloacal swabs or shell gland tissue (Smyth, 2024).
Egg drop syndrome is distinguished from infectious bronchitis by the absence of respiratory signs and ridged or malformed eggs, and from Newcastle disease and avian influenza by the absence of any illness in the birds (Smyth, 2024).
Prevention and vaccination
There is no treatment once a flock is infected (Smyth, 2024; Lera, 2021). Control relies on keeping the virus out and, where it is already established, vaccinating before the onset of laying.
Egg trays are one of the main routes of horizontal spread, so dedicating trays to a single farm, or washing and disinfecting them before reuse, is a basic control step (Lera, 2021). Keeping layer flocks away from wild or domestic waterfowl, and chlorinating any water supply that could be contaminated by their droppings, addresses the sporadic route of infection.
Inactivated oil-adjuvant vaccines are given during rearing, usually between 14 and 18 weeks of age, often alongside a Newcastle disease vaccine (Smyth, 2024). Vaccination protects against clinical disease but reduces rather than eliminates virus shedding, so biosecurity around trays and equipment remains necessary even in vaccinated flocks.
Egg drop syndrome rarely threatens bird welfare directly, but the loss of saleable eggs and the difficulty of spotting early cases make it an expensive disease to overlook.
References
- Lera, R. (2021). Egg Drop Syndrome. Hendrix Genetics Laying Hens. https://layinghens.hendrix-genetics.com/en/articles/Egg_Drop_Syndrome-is-a-disease-caused-by-an-adenovirus-affecting-laying-hens-characterized-by-a-fall-in-egg-production/
- Smyth, J. A. (2024). Egg Drop Syndrome ’76. MSD Veterinary Manual. https://www.msdvetmanual.com/poultry/egg-drop-syndrome-76/egg-drop-syndrome-76