Submitted to: Avian Diseases
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 19, 2012
Publication Date: September 30, 2012
Citation: Dunn, J.R., Silva, R.F. 2012. Ability of MEQ-deleted MDV vaccine candidates to adversely affect lymphoid organs and chicken weight gain. Avian Diseases. 56:494-500. Interpretive Summary: A recently developed vaccine for Marek's disease, a virus-induced cancer like disease of chickens, protects as good or better than a commercially available vaccine named Rispens, the current gold standard. Unfortunately, there have been reports that this vaccine causes shrinkage of lymphoid (immune system-related) organs. This study confirmed those reports and showed there was a strong correlation between the weight of these organs and the level of virus replication. This information will allow us to better understand how to create vaccines that grow well but don't negatively affect the immune system, and therefore have better contol of the disease.
Technical Abstract: CVI988 (Rispens) is currently the most effective vaccine used to protect against Marek’s disease, a lymphoproliferative disease of chickens. A MEQ-deleted Marek’s disease virus strain has shown promise as a vaccine candidate; however, unpublished results from vaccine safety trials suggest this candidate vaccine induces unwanted lymphoid atrophy. The current study evaluated lymphoid atrophy at multiple time points between 2-8 weeks post-inoculation and attempted to correlate results with virus replication in the thymus. Results confirmed reports that MEQ-deleted virus strains are able to cause thymus and bursa atrophy, most severe at 2 weeks post-inoculation. Both strains induced lower body weights and relative thymus and bursa weights compared to uninoculated and Rispens-vaccinated chickens at multiple time points between 2-8 weeks post inoculation. Both produced high levels of in vivo virus replication in the thymus at rates significantly greater than in Rispens-vaccinated chickens and were comparable to levels of RM1 virus, a Marek's disease virus, previously shown to induce severe thymus and bursa atrophy. Virus replication was highly correlated with relative thymus weights at each time point. Understanding this delicate balance of inducing maximum disease protection while preventing immunodepressive effects, is critical for the development of future Marek’s disease vaccines.