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Egg Drop Syndrome ‘76

By Zac Williams

Egg Drop Syndrome ’76 or simply, EDS ’76 (EDS), is a virus that causes egg laying chickens to produce eggs with pale, soft or shell-less eggs. In most cases, hens appear perfectly healthy and show no signs of infection until eggs are laid, causing an overall drop in egg production for both commercial and backyard farms. Chickens that lay brown eggs are more commonly affected by this infection in contrast to white egg laying hens. The first recorded case of EDS ’76 was in Ireland in 1976, and was recently found in the United States

While the virus is naturally occurring in waterfowl such as ducks and geese, EDS occasionally finds its way into domestic chickens. Some have speculated that the virus was introduced to chickens via a contaminated vaccine that was grown in duck embryos. Regardless, the virus has recently found its way into commercial chickens in Pennsylvania, Indiana and Michigan and backyard chickens in Indiana. EDS has caused substantial problems with production loss and eggshell quality.

EDS does not cause illness in humans and is not a threat to human health.


There are three routes of infection in domestic chickens: classic, endemic and sporadic. In the classic cases of EDS, the virus is spread vertically from breeding stock to offspring. In these instances, the virus often remains undetected until the hens reach sexual maturity. The virus can then spread rapidly through eggs and waste excretions. Endemic transmission, also called horizontal transmission, occurs commonly in commercial egg layers. In this instance, contaminated equipment carries the virus from one flock to another. In commercial egg layers, horizontal transmission may occur through bird-to-bird transmission but more commonly, it is spread through equipment such as egg collection trays and shared egg packing stations. Sporadic transmission is caused by direct contact with waterfowl, either domestic or wild. In those cases, EDS is spread through use of a water supply that has been contaminated with droppings from wild birds.

Signs and symptoms

Typically, infected hens will show no signs of the disease, sometimes they can carry the virus from hatch to onset of egg production and show no outward signs. When infected hens come into production they will first lay eggs with pale shells, which is followed by thin or soft-shelled and even shell-less eggs. In most cases the internal quality of the egg is not affected though the thin albumen may be more liquid than normal. The virus is found in the shell gland where it replicates and subsequently causes damage to the surface epithelium. Eggs with ridges or other deformities and poor internal quality are not indicators of EDS. Sometimes flocks will also show signs of depression in the days leading up to changes in shell quality.

Eggs from infected hens that have poor shell quality are very fragile. Such eggs are prone to be broken, stepped on by other hens and either eaten or pushed into the litter. In these instances, infection can go overlooked at the very beginning. During infection the amount of saleable or usable eggs can decrease by 10% to 40%. In flocks that have experienced some spread of the virus, EDS more often presents as failure of that flock to reach production goals. These flocks tend to have small groups within the larger flock that are experiencing eggshell quality issues. This can be seen in caged layer facilities where transmission of the virus is typically slower and egg production may drop only a few percentage points.

EDS can be distinguished from other diseases that cause loss of egg production by the lack of outward symptoms and otherwise good health of the hens.

Treatment and prevention

Currently there is no treatment for EDS, only preventative measures. Vaccination, currently available on a provisional basis, with inactivated virus has proven successful in areas where the virus is considered endemic. These vaccines can be administered during the growing phase of the hen’s life cycle, typically between 14- and 18-weeks of age. As with any disease, biosecurity is key in keeping the virus out, especially with equipment that is shared or that handles eggs such as egg trays, egg packing stations, etc.

EDS is a virus that can cause significant economic damage through loss of useable eggs. Producers should watch for shell quality issues, such as pale, thin, or missing shells. A decrease in lay or failure to reach peak production is a reason to examine both your flock and eggshell quality closer. Diagnostic tests are available on a limited basis. While there is currently no treatment, vaccination during the growing phase can be successful in preventing this virus from causing egg production losses.

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