Results from a recent study conducted by University of Georgia researchers highlighted the importance of careful diagnostic testing and monitoring for egg drop syndrome 1976 (EDS 76).
EDS 76 is caused by a virus called duck Atadenovirus A. It was once considered exotic to the US, but since 2018, it has been found in commercial layer and broiler breeder flocks, causing noticeable production losses. Birds affected by EDS 76 appear healthy but lay fewer eggs, and the eggs that are produced are often soft-shelled or shell-less.
Diagnostic challenges
“Diagnosing EDS 76 in the US is challenging and involves multiple diagnostic methods,” said Jenny Nicholds, DVM, MAM, DACPV, clinical associate professor, University of Georgia, and the study’s principal investigator.
Serology via hemagglutination inhibition and enzyme-linked immunosorbent assay is primarily used for confirmation in unvaccinated birds, while polymerase chain reaction (PCR) and real‑time PCR (qPCR) are used to detect viral DNA in suspect cases. Assessing the presence of viable virus by virus isolation relies on duck embryo fibroblast (DEF) cells and the limited availability of specific-pathogen-free duck eggs.
Unfortunately, EDS 76 qPCR cannot determine viral viability or assess the effectiveness of cleaning and disinfection. Furthermore, the role of mechanical vectors in EDS 76 transmission remains poorly defined.
Study details
This study had three main goals:
- Test which cell lines can grow the virus from field and environmental samples, including checking whether cleaning and disinfection practices work;
- Examine the antibody responses and viral shedding in vaccinated and unvaccinated flocks on farms with and without EDS 76; and
- Investigate possible ways the virus might be spread, such as through insects or other sources.
Results showed that two cell lines, DEF 141 and LMH 2177, could support replication of a reference EDS 76 virus, with visible changes in DEF 141 cells. However, attempts to grow the virus from field samples were unsuccessful, likely due to low viral loads or the timing of sample collection.
Serology tests on 33 flocks across seven states showed clear differences between vaccinated birds and those naturally exposed to the virus. Unvaccinated flocks had very low or no antibodies. Vaccinated flocks developed moderate antibody levels, and naturally exposed flocks had very high antibody levels. All vaccinated flocks tested negative for viral DNA.
Viral DNA was detected on eggshells, cardboard egg cartons, flies and beetles. Additionally, 45% of samples from live-bird market ducks tested positive. One-third of fly samples also tested positive, suggesting insects and market birds may help spread the virus.
Conclusion
Overall, this work provides important insights into EDS 76 in US poultry.
“Study results highlight the need for careful use of diagnostic tests, the limits of virus isolation in field conditions and the importance of monitoring both birds and the environment,” Nicholds said.
Findings also emphasize the need for strong biosecurity measures, especially around live-bird markets, and for insect control to reduce the risk of EDS 76 transmission. These results will help poultry producers better monitor, prevent and manage this emerging disease in commercial operations.
The research was funded by USPOULTRY and the USPOULTRY Foundation. Click here to view the industry summary.
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