Location: Exotic & Emerging Avian Viral Diseases Research
Project Number: 6040-32000-081-007-I
Project Type: Interagency Reimbursable Agreement
Start Date: Aug 1, 2020
End Date: Jul 30, 2025
Objective:
Egg drop syndrome (EDS) of chickens is caused by an atadenovirus. It was originally identified in 1976 and is also commonly referred to as EDS76. Clinically the virus causes production of pale, thin -shelled, and shell-less eggs, and the primary economic effect is because of the decrease in usable eggs. The virus is thought to naturally occur in ducks where there are mixed reports on whether it causes clinical disease. It can also infect geese and experimentally quail, but the disease in chickens remains the main economic concern. The virus has been detected in many countries, and it was recently reported from layer chickens in the United States with clinical disease. Although this was the first detection of the virus associated with clinical disease, evidence of atadenovirus virus circulation in the U.S. dates back to the 1980s. As well as the economic concerns of drops in egg production, trade issues have occurred with some countries blocking U.S. poultry exports because of concern of poultry meat being a potential source of virus introduction into their country.
The EDS virus likely is transmitted from asymptomatic ducks to poultry, but once introduced the virus can persist. The virus is believed to be vertically transmitted through the egg from the hen to the chick at low numbers, but the virus can spread horizontally to infect most birds within a flock. The virus is also persistent, particularly in the reproductive tract, which contributes to the main pathology and spread to the next generation.
Most of the research on EDS was conducted in the 1970-80s, and no studies were performed specifically looking at whether meat or other poultry products can be a source of introduction of virus through trade. The concern is that raw poultry scraps that came from an infected chicken will be feed to backyard poultry, which could introduce the virus into a country with no known previous infection. In addition, immunologic studies will examine the importance of the fiber and hexon proteins to provide protective immunity in chickens. This may allow for the use of subunit vaccines and the differentiation of vaccinated and naturally infected birds.
Approach:
1) Evaluate the risk of EDS virus being found in broiler chickens at slaughter
EDS virus is thought to be transmitted by both vertical and horizontal transmission. Infected breeding hens can lay eggs infected with the virus at a low prevalence, but the infected chicks can after hatching spread the virus horizontally to uninfected hatch mates so that eventually most birds in a flock will become infected. The proposed challenge experiment will follow the pathogenesis of the virus in white rock broiler chickens inoculated at different ages. We also propose to infect two 1 day old chicks and move the infected birds to a naïve group of twenty same age chicks.
2) Pathogenesis study of EDS virus in layer chickens
EDS virus if introduced into a layer flock causes a persistent infection with the virus having an affinity for the reproductive system that can result in layer chickens having decreased egg production and/or malformed eggs. These egg abnormalities can cause important economic losses to the producer. A pathogenesis study is planned to examine a representative EDS virus from Pennsylvania, which appears to be the center of the current outbreak in the U.S.
3) Evaluate the immune response to different EDS proteins
Based on the existing literature of EDS and related adenoviruses, the surface proteins, fiber and hexon, will produce an antibody response that is neutralizing in cell culture. We plan on evaluating in more detail the ability of antibody to each of these proteins alone and together to protect chickens from infection.
4) Vaccine challenge experiment
A vaccine challenge experiment will be conducted to evaluate the subunit vaccines in comparison with traditional whole virus inactivated vaccines. The subunit vaccine may include baculovirus expressed protein or the viral vectored vaccine. The whole virus vaccine will be produced in house using a commercial adjuvant. Vaccine protection will be evaluated by reduction of clinical signs (egg quality and quantity) and virus titers in blood and cloacal shedding. For the subunit vaccines, it will be evaluated if a DIVA strategy can be developed.
5) Diagnostic test evaluation and development
Evaluation of real-time PCR (RT-PCR) to demonstrate sensitivity and specificity. In addition to the molecular testing, serologic testing will be evaluated. We will also look at expressing the hexon and fiber proteins in a baculovirus system to determine if these are good antigens for use as a diagnostic test, specifically an ELISA format.
6) Next Generation Sequencing
The use of Next Generation Sequencing (NGS) will be used with the goal of optimizing sequence from clinical samples. We plan on exploring different methods of host depletion including specifically targeting rRNA sequences from chickens to try and increase the sensitivity of NGS as well as using custom targeted bait hybridizations methods to specifically increase sequence from specific pathogens as a way to increase sensitivity.
