Egg Drop Syndrome

By L. Loughlin and G. Lorenzoni

Etiology

EDS is caused by a double-stranded DNA virus called the egg drop syndrome virus (EDSV) or duck adenovirus 1. Only one serotype has been recognized.

Susceptibility of the agent

The EDSV is resistant to pH range 3–10 and to heating for 3 hours at 56°C (approx.132.8°F). Infection rate is lost after treatment with 0.5% formaldehyde or 0.5% glutaraldehyde.

Occurrence

EDS ‘76 has been recently reported in the US and in many other countries such as Ireland, England, and Brazil. All ages and breeds of chickens are susceptible to infection.

Pathogenesis

There are three patterns of viral transmission recognized in chickens. The first pattern with EDS ‘76 occurs when primary breeding stock are infected, and the virus is transmitted vertically through the egg. The chicks that hatch are contaminated with the virus which remains latent until the birds reach sexual maturity. Then, the virus multiply and are excreted in the eggs and fecal matter. The second pattern is seen through endemic EDS, which is the result of horizontal infection of the flock during lay. It is usually seen in commercial egg layers. The third pattern is sporadic and due to spread of EDS virus due to contact with domestic or wild ducks or geese or the use of a water supply contaminated with wildfowl droppings.

Clinical manifestations

The first sign of the disease is the production of pale-shelled eggs, followed by the production of soft-shelled and shell-less eggs. Birds tend to eat the thin-shelled and shell-less eggs. In flocks in which there has been some spread of virus and antibodies are present, the condition is seen as a failure to achieve predicted production targets. Birds with antibodies slow the spread of the virus throughout the flock.

Diagnosis

Production of pale thin-shelled and shell-less eggs by a flock that appears otherwise healthy suggests that EDS '76 is present. Transient mild depression and/or mild watery droppings may be seen. Ridged eggs and poor internal quality are not features of EDS '76. Poor eggshell quality at peak production in healthy hens should also raise strong concerns for EDS '76. With endemic or sporadic EDS '76, the disease can develop in laying hens of any age.

Confirmatory laboratory testing is needed for definitive diagnosis. Searching for evidence of seroconversion is the easiest diagnostic approach for nonvaccinated flocks. In addition, a hemagglutination-inhibition test using fowl RBCs, and ELISA, are the primary serologic tests of choice.

PCR-based tests and antigen capture ELISA tests have been used to detect EDSV DNA and antigens. It is important to select recently infected birds for testing, but these can be difficult to identify. Once of the easiest alternative methods is to feed affected eggs to antibody-free hens.

Relevant differential diagnosis

Similar diagnosis diseases of the EDS virus include failure to achieve adequate egg production levels or if there is a decrease in egg production in seemingly healthy birds. Shell-less, soft-shelled, and thin-shelled eggs are all characteristics that point to EDS; ridged and misshapen eggs are not.

Prevention and Treatment

There is no treatment for EDS '76, but preventative measures can be set in place to limit the chance of spreading the virus. Some preventative measures include washing and disinfecting plastic egg trays before use and separating chickens from other birds. A vaccine is available in some regions.