Early infection with Marek’s disease virus can jeopardize protection conferred by laryngotracheitis vaccines: a method to study MDV-induced immunosuppression

Abstract

Marek's disease virus (MDV) is a herpesvirus that induces lymphomas and immunosuppression in chickens. MDV-induced immunosuppression (MDV-IS) is divided into two phases: early-MDV-IS occurring mainly in chickens lacking maternal antibodies (MAb) against MDV and associated with lymphoid organ atrophy; and late-MDV-IS occurring once MDV enters latency and during tumour development. Our objectives were to document the impact of late-MDV-IS on commercial poultry (meat-type chickens bearing MAb against MDV and that were vaccinated or unvaccinated against MD) and to optimize a model to study late-MDV-IS under laboratory conditions. The impact of late-MDV-IS was evaluated by assessing the effect of early infection (day of age) with a very virulent plus MDV (vv+MDV) on the efficacy of chicken-embryo-origin (CEO) infectious laryngotracheitis (ILT) virus vaccine against ILT challenge. The CEO ILT vaccine was administered in water at 14 days of age and ILT virus (ILTV) challenge was done intratracheally at 30 days of age. Development of ILT was monitored by daily evaluation of clinical signs, development of gross and histological lesions in trachea, and quantification of ILTV transcripts in trachea. Infection with vv+MDV strain 648A resulted in total abrogation of protection conferred by the CEO vaccine against ILTV challenge even in chickens vaccinated at 1 day of age with either HVT, HVT+SB-1, or CVI988. Chickens exposed to vv+MDV prior to vaccination with CEO ILTV vaccine had similar (P < 0.05) clinical scores, gross lesions, histopathologic lesion scores, and load of ILTV transcripts in trachea after ILTV challenge, as chickens that were not vaccinated with CEO ILTV vaccine.

Evaluation of factors influencing the development of late Marek’s disease virus-induced immunosuppression: virus pathotype and host sex

“…Mortality was also recorded. Development of ILT-CS was assessed by ILT-CS score as previously described (Faiz et al, 2016a). Briefly, ILT-CS score was calculated as the total number of days a particular chicken showed CS (range from 0 to 6).…”

“…It is possible that the ability of MDV to induce immunosuppression (MDV-IS) has increased as MDV evolves in virulence. MDV-IS is a complex phenomenon which is divided into early-MDV-IS that occurs during early cytolytic infection in lymphoid organs (four to six days post-infection) and late-MDV-IS that occurs later in the pathogenesis (Calnek et al, 1998;Calnek, 2001;Schat, 2004;Faiz et al, 2016a). Early-MDV-IS is caused by the destruction of T and B cells in the lymphoid organs and it only occurs in chickens lacking MAb and that are not vaccinated against MDV; therefore, it does not have much relevance in commercial poultry (Churchill & Baxendale, 1973;Payne & Rennie, 1973).…”

“…The development of early-MDV-IS is influenced by MDV pathotype as vv and vv+MDV tend to induce more lymphoid organ atrophy than vMDV (Calnek et al, 1998;Witter et al, 2005). Late-MDV-IS, however, can occur in commercial chickens bearing MAb against MDV and even in chickens vaccinated against MD and properly protected against tumours (Faiz et al, 2016a(Faiz et al, , 2016b. Unlike early-MDV-IS, late-MDV-IS does not occur due to the destruction of B and T cells (Faiz et al, 2016a), but the mechanisms involved and the factors influencing it are poorly understood.…”

“…Late-MDV-IS, however, can occur in commercial chickens bearing MAb against MDV and even in chickens vaccinated against MD and properly protected against tumours (Faiz et al, 2016a(Faiz et al, , 2016b. Unlike early-MDV-IS, late-MDV-IS does not occur due to the destruction of B and T cells (Faiz et al, 2016a), but the mechanisms involved and the factors influencing it are poorly understood. Recently, we have developed a model to study late-MDV-IS under laboratory conditions.…”

“…Recently, we have developed a model to study late-MDV-IS under laboratory conditions. This model indirectly evaluates late-MDV-IS by assessing the effect of MDV infection on the efficacy of infectious laryngotracheitis (ILT) vaccine on commercial meat-type chickens (Faiz et al, 2016a). The objective of this study was to use the late-MDV-IS model to evaluate the effect of MDV pathotype on the ability to induce late-MDV-IS.…”