Egg Drop Syndrome ’76: Keep on your rule out list
Emily Martin, Davor Ojkic
Animal Health Laboratory, University of Guelph, Guelph, ON.
AHL Newsletter 2023;27(3):18.
A disease that was originally described as “Egg Drop Syndrome ’76” is caused by duck adenovirus 1 (DAdV-1) infection. The agent, DAdV-1, is still often referred to as “egg drop syndrome virus” – EDSV. Previously DAdV-1 was called “group III avian adenovirus” but is now classified as a member of Duck atadenovirus A species of the genus Atadenovirus.
Since 2019 there have been multiple outbreaks of “Egg Drop Syndrome” associated with DAdV-1 infection in the USA affecting commercial layer and broiler breeder chickens. EDS has also been previously identified in commercial ducks in Ontario (2009) and Quebec (2019). The identification of this virus in the USA and Quebec since 2019 is concerning given that this virus is known to exist in wildlife, therefore, this virus should continue to be a rule out in cases of egg production drops. Infection primarily affects the oviduct of laying chickens replicating in the oviduct, nasal mucosa and spleen. This results in the production of pale, thin-shelled, soft-shelled or shell-less eggs. There is also a drop in egg production. Shell-less eggs may be missed if they rupture (leaving only membranes) or if they are eaten by the birds.
Clinical presentation: The affected birds typically have no other overt clinical signs other than poor egg quality and production problems. The fall in egg production can be variable (rapid or weeks, up to 40% drop), outbreaks last 4-10 weeks, and there is no effect on fertility or hatchability. The age of initial infection can affect the clinical signs (i.e., vertical, day of age, in lay,) resulting in 3 possible presentations (Table 1).
Table 1. Egg Drop Syndrome ‘76 patterns of disease.
Table 1. Egg Drop Syndrome ‘76 patterns of disease.
Transmission: Vertical transmission can result in a latent infection. When birds come into egg production the virus can be reactivated, excreted and disseminated through lateral spread. The virus can also be spread by fomites (equipment, egg trays), or reused needles during vaccination. Sporadic spread can occur from domestic or wild ducks or geese through contaminated droppings or drinking water. The incubation period is 7-9 days.
Gross lesions: Potential lesions include mucosal edema and exudate in the shell gland, mild splenomegaly, flaccid ovules, and multiple eggs in various stages of development in the celomic cavity. Alternatively, birds could have regressed ovaries and oviducts.
Diagnosis: It may be difficult to select birds for examination due to the lack of clinical signs, slow spread through flock, and short duration of virus replication/lesions. DAdV-1 (EDSV) PCR testing is available at the AHL; suitable samples include shell membrane of shell-less eggs or cloacal swabs. A positive PCR and detection of antibodies coincident with a drop in egg production or shell abnormalities is considered diagnostic. As serological testing is not available at the AHL, serum samples for a hemagglutination inhibition test will be forwarded to another commercial laboratory.
Since December of 2012, when the DAdV-1 (EDSV) PCR was introduced, 200 samples have been tested and only 2 commercial duck samples have been positive (Table 2).
Table 2. AHL DAdV-1 PCR results since 2012.
Out of these 200 samples, 126 were also tested for IBV, another agent commonly suspected in egg production issues, and 90 (71.4%) were IBV-reactive (e.g., inconclusive and positive).
Table 3. DAdV-1 samples from Table 2 also tested for IBV.
Control: There is no treatment available. Control is by eradication or vaccination of the breeder flocks. Laying hens provide maternal antibodies to the chicks. Ensure birds are not exposed to ducks, geese or open untreated water sources (ponds, rivers).
All species and ages of chickens are susceptible to developing disease. Other species that can be infected include quail and guinea fowl. The virus is resistant to inactivation and resistant to pH range 3–10. Treatment with 0.5% formaldehyde and 0.5% glutaraldehyde could neutralize infectivity. EDS ’76 is immediately notifiable under the Ontario provincial Animal Health Act. AHL
References
1. Gingerich E, Blough B. Outbreak Of Egg Drop Syndrome (EDS) In Northern Indiana Layer Flocks. Proceedings of 22nd WPDC, 2022 March: 11-93.
2. Brash ML, Swinton JN, Weisz A, Ojkić D. Isolation and identification of duck adenovirus 1 in ducklings with proliferative tracheitis in Ontario. Avian Dis 2009;53(2):317-20.
3. Smyth JA. Atadenovirus (Egg Drop Syndrome of Chickens and Related Infections). In: Swayne DE, ed. Diseases of Poultry, 14th ed., Vol I. Wiley Blackwell, 2020:332-339.
4. Smyth JA. Egg Drop Syndrome '76. Merck Veterinary Manual. https://www.merckvetmanual.com/poultry/egg-drop-syndrome-76/egg-drop-syndrome-%E2%80%9976
5. Chénier S, et al. First reported outbreak of Duck atadenovirus A tracheobronchitis in 3-week-old ducklings in Québec including whole genome sequence of the virus. CVJ 2019;60(12):1285-1288.