Vaccine 28 (2010) 1294–1299 Contents lists available at ScienceDirect Vaccine journal homepage: www.elsevier.com/locate/vaccine Comparative evaluation of vaccine efficacy of recombinant Marek’s disease virus vaccine lacking Meq oncogene in commercial chickens Lucy F. Lee a,∗ , K.S. Kreager b , J. Arango b , A. Paraguassu b , B. Beckman b , Huanmin Zhang a , Aly Fadly a , B. Lupiani c , S.M. Reddy c a Avian Disease and Oncology Laboratory, Agricultural Research Service, 3606 East Mount Hope Road, East Lansing, MI 48823, United States b Hy-Line International, Dallas Center, IA 50063, United States c Department of Veterinary Pathobiology, College of Veterinary Medicine and Biomedical Sciences, Texas A&M University, College Station, TX 77843, United States article info Article history: Received 4 September 2009 Received in revised form 3 November 2009 Accepted 6 November 2009 Available online 24 November 2009 Keywords: Marek’s disease virus Recombinant MDV Meq oncogene MD vaccine abstract Marek’s disease virus (MDV) oncogene meq has been identified as the gene involved in tumorigene- sis in chickens. We have recently developed a Meq-null virus, rMd5Meq, in which the oncogene meq was deleted. Vaccine efficacy experiments conducted in Avian Disease and Oncology Laboratory (ADOL) 15I 5 × 7 1 chickens vaccinated with rMd5Meq virus or an ADOL preparation of CVI988/Rispens indicated that rMd5Meq provided superior protection than CVI988/Rispens when challenged with the very viru- lent plus MDV 648A strain. In the present study we set to investigate the vaccine efficacy of rMd5Meq in the field compared to several commercial preparations of CVI988/Rispens. Three large-scale field exper- iments, in which seeder chickens were inoculated with a very virulent plus strain of 686, vv+ MDV, were conducted in a model developed by Hy-Line International. In addition, comparisons were made with bivalent vaccine (HVT + SB-1), HVT alone and several serotype 3 HVT-vectored vaccines individually or in combination with CVI988/Rispens. Experimental results showed that addition of HVT to either of the two commercial CVI988/Rispens preparations tested (A or B) did not enhance protection conferred by CVI988/Rispens alone and that rMd5Meq was a better or equal vaccine compared to any of the CVI988/Rispens vaccines tested under the conditions of the field trials presented herein. Our results also emphasized the complexity of factors affecting vaccine efficacy and the importance of challenge dose in protection. Published by Elsevier Ltd. 1. Introduction Marek’s disease (MD) is an acute lymphoproliferative disease of chickens, resulting in T cell lymphomas in visceral organs, periph- eral nerves and skin [1,2]. The causative agent of MD, identified in 1968 and named Marek’s disease virus (MDV), belongs to the family Herpesviridae [3]. On the basis of its biological properties, MDV was first classified as a Gammaherpesvirus. However, based on its genomic structure MDV is now classified as an Alphaher- pesvirus [4,5]. MDV has three serotypes: serotype 1 includes all the oncogenic strains and their derivatives; serotype 2 includes non- oncogenic viruses isolated from chickens; and serotype 3 includes non-oncogenic viruses isolated from turkeys [6,7]. Within serotype 1 viruses, there are several pathotypes: mild (m), virulent (v), very virulent (vv) and very virulent plus (vv+) based on their pathogen- esis in protection tests [8]. ∗ Corresponding author. Tel.: +1 517 337 6836. E-mail address: lucy.lee@ars.usda.gov (L.F. Lee). MD has been controlled by the use of vaccines for almost four decades. A turkey herpesvirus (HVT) (MDV serotype 3), identified in 1970 [9], was widely used throughout the world between 1970 and 1980. However, the increased virulence of MDV field isolates led to the use of bivalent vaccines consisting of HVT and SB-1 (MDV serotype 2) [10]. This bivalent vaccine offered better protection against very virulent pathotypes of MDV. In the early 1990s, field strains of the vv+ pathotypes emerged in the United States and CVI988/Rispens [11,12], a serotype 1 vaccine, was introduced. At the present, CVI988/Rispens vaccine virus is used worldwide for controlling MD in the field and no better vaccine has appeared on the horizon. The continued evolution of MDV towards greater vir- ulence has prompted concern that the currently available vaccines will ultimately loose efficacy in controlling MD. This situation has led several investigators to try to develop more efficacious vac- cines, but it has been a difficult challenge [13,14]. Recently, Witter and Kreager [15] compared 10 candidate vaccine viruses and none showed better protection against the disease than CVI988/Rispens. They concluded that conventional vaccine development may have approached the biological threshold of vaccine efficacy and there- fore, new strategies are needed for vaccine development. 0264-410X/$ – see front matter. Published by Elsevier Ltd. doi:10.1016/j.vaccine.2009.11.022