Protection provided by Rispens CVI988 vaccine against Marek's disease virus isolates of different pathotypes and early prediction of vaccine take and MD outcome

Abstract

We tested the level of protection provided by the Rispens CVI988 (Rispens) vaccine against challenge with a virulent Marek's disease virus (MDV) pathotype (vMDV) and a very virulent pathotype (vvMDV) and the accuracy of a range of predictive measures of Marek's disease (MD) incidence and vaccine take. Commercial layer chicks (n = 236) were vaccinated (or not) with 4000 plaque-forming units (pfu) of Rispens vaccine at hatch and challenged (or not) with 500 pfu of each challenge virus five days post vaccination. The vvMDV pathotype FT158 induced higher MD incidence (65%) and mortality (33%) when compared with the vMDV pathotype MPF57 (39% and 8%, respectively). The protective index provided by the Rispens vaccine against FT158 (61%) did not differ significantly from that against MPF57 (66%). This provides additional evidence that protection provided by the Rispens vaccine is not influenced by pathotype determined in studies using vaccines of other Mardivirus species. The challenge viruses did not differ in MDV or Rispens viral load in spleen at 14 dpc (days post challenge) determined by specific quantitative polymerase chain reaction test. MDV load in peripheral blood leucocytes at 7 and 14 dpc, splenocytes at 14 dpc, feather cells at 14 and 21 dpc and isolator dust at 21 dpc were significant early indicators of subsequent MD incidence to 56 dpc. These are potentially useful as the sampling can be carried out well before the onset of MD and some measures are non-invasive. The Rispens viral load in both invasive and non-invasive samples was more useful as a measure of vaccine take.

Newly detected mutations in the Meq oncogene and molecular pathotyping of very virulent Marek’s disease herpesvirus in Tunisia

“…Sporadic epidemics of MD have been reported worldwide, even in vaccinated flocks [ 16 ], indicating that the virulence of MDV has increased in recent decades, and some of the more recent isolates are more pathogenic to chickens than older isolates [ 17 , 18 ]. Thus, the increased use of vaccines has led the virus to evolve toward higher virulence, which can overcome the protection conferred by currently available vaccines [ 19 ]. It has been suggested that the widespread use of the CVI988 vaccine strain may have led to the emergence of new, more virulent pathotypes [ 20 ], and there are serious concerns that such pathotypes are circulating in Tunisia, especially due to the excessive use of vaccines.…”

Managing Marek’s disease in the egg industry

“…All model parameters were extracted from available data (Atkins et al, 2013a, 2011a; Cui et al, 2016; Kennedy et al, 2018; Ralapanawe et al, 2016b, 2016a; Zhang et al, 2015). Parameters associated to barn characteristics where obtained from communication with Burnbrae Farms in Ontario Canada, or the literature.…”

“…We assume dust shed by MDV infected laying-hens contain viral particles proportional to the virulence level of the MDV strain (Ralapanawe et al, 2016a). To obtain the viral shedding rate, we took Atkins et al (2011) model of daily dander shedding for a typical broiler bird and arameterized it according to a recent study on dust shed from MDV infected layer chickens vaccinated with Rispen CVI988 (Atkins et al, 2011a; Ralapanawe et al, 2016b, 2016a). Thereby, we obtain the viral shedding rates κ for each MDV strain from data on the dust shed from a typical laying-hen (Table S4), and the viral copy number (VCN) of MDV per milligram of dust for vMDV (pathotype MPF57) and vvMDV (pathotype FT158) (Table S3), and over each cohort duration (older chickens will shed more viral particles) (Table S5) (Bell, 2003; Witter et al, 1968).…”

Out of Sight, but Not Out of Mind: Aspects of the Avian Oncogenic Herpesvirus, Marek’s Disease Virus

“…MDV was the only poultry live vaccine whose vaccination uptake efficacy was studied by demonstrating its presence in feather tips. The presence of live vaccines was demonstrated in the feathers of experimentally infected and commercially vaccinated layers in Australia [23][24][25]. By developing a novel nested real-time PCR assay we were able to characterize the vaccine application efficacy in commercially vaccinated chicks in Israel [13] and to trace back MDV-affected chicken flocks to problematic MDV vaccine application [14].…”