Evaluation of Factors Influencing Efficacy of Vaccine Strain CVI988 Against Marek’s Disease in Meat-Type Chickens

Abstract

Marek's disease (MD) strain CVI988 is the most-protective commercially available vaccine against very virulent plus (vv+) Marek's disease virus (MDV). However, its use in meat-type chickens has been controversial. While several countries have been using CVI988 for more than 40 yr, others do not authorize its use or it is restricted mainly to layers. The use of CVI988 in meat-type chickens will be necessary in the future in areas where other vaccine protocols fail. The objective of this study was to evaluate factors (vaccine dose, vaccine origin, chicken genetics, age and route of vaccination, and combination with other MD vaccines) influencing the efficacy of CVI988 against MD in meat-type chickens. Three animal experiments were conducted in which various vaccine protocols using CVI988 were tested for their protection against challenge with vv+ strain 648A by contact at day of age. Experiments 1 and 2 were to compare the efficacy of CVI988 vaccines from three different origins (CVI988-A, CVI988-B, and CVI988-C) and evaluate the effect of vaccine dose and chicken genetics. Experiment 3 was to evaluate the effect of adding CVI988 vaccine to various vaccine protocols using other MD vaccines of serotypes 2 (SB-1) and 3 (rHVT). Our results show that, regardless of the origin of the vaccine, protection against early challenge with 648A was good when vaccines were administered at a high dose (>3000 plaque-forming units [PFU]). Differences among vaccines, however, were detected even when using a high dose in experiment 2 (vaccine CVI988-B conferred higher protection than did CVI988-C) but not in Experiment 1 (CVI988-B was compared to CVI988-A). The use of a fixed low dose (2000 PFU) of vaccine resulted in reduction in protection, and such reduction was more remarkable when using CV1988-A. No statistically significant differences were found when we compared the efficacy of CVI988 in two different genetic lines of broiler chickens (G1 and G2). Vaccination protocols that included CVI988 had better protection than protocols that only included MD vaccines of serotypes 2 and 3. This was true regardless of the vaccine protocol used (CVI988/rHVT+SB-1; CVI988+rHVT+SB-1/None; rHVT+SB-1/CVI988; wherein the vaccine before the slash (/) was administered in ovo at embryonation day 18 and the vaccine after the slash was administered at day of age, subcutaneously). When only vaccines of serotypes 2 and 3 were used, protection against early challenge with vv+MDV was higher when vaccines were administered in ovo (rHVT+SB-1/None) than if vaccines were administered at hatch (None/rHVT+SB-1). Monitoring vaccine DNA load in feather pulp (FP) samples at 1 wk was used to monitor vaccination, and results showed that differences in vaccine replication exist among vaccines but such differences were not necessarily related to protection (r = 0.41, P > 0.05). Monitoring load of challenge MDV DNA in FP at 21 days was conducted, and results correlated (r = 0.85, P < 0.05) with the percentage of chickens with MD lesions at the termination of the study,...

Comparison of Cvi and HVT Vaccine Strains Replication in Feather Tips in Different Marek´s Disease Vaccination Programs

“…In this study, MDV quantification in feather tips by real-time PCR enabled to show significant differences in the replication rate of both CVI and HVT components among the treatments analyzed (Table 2). Similar results have already been described in other scientific studies that seek to understand the intensity and dynamics of replication of different Marek's vaccine strains, especially the effect of combining conventional CVI component with rHVT vector vaccines (Gimeno et al, 2015a, Gimeno et al, 2019. Several factors interfere in rHVT replication, such as the age at vaccination, and in-ovo vaccination leads to increased replication when compared to subcutaneous vaccination (Gimeno et al, 2016).…”

“…The combination of serotype 3 and 1 has been an important and effective control strategy by means of vaccination of long-lived birds since it provides better protection against field challenges (Davison & Nair, 2005). The CVI component is considered the most effective strain for protection against the highly virulent Marek's viruses (Gimeno et al, 2015a).…”

Evaluation of Factors That Influence Dose Variability of Marek's Disease Vaccines

“…The requirement for licensing MD vaccines in the U.S. is a virus titer per dose of least 1,000 PFU [254]. However, most biologic manufacturers exceed this requirement as a precaution against lack of protective immunity, partly to counteract the reduction in the dose that occurs for the normally handling of the vaccine but also because every vaccine requires different levels of PFU to achieve the maximum protection [178,231]. Our results emphasize that MD vaccine doses should be given as a range of dose more than average PFUs, as dose variability will affect the dose of vaccines that chickens receive.…”

“…route and age of vaccination, dose of vaccine administered and the combination of vaccines used[116,178,231]. Vaccine replication in vivo can be tested by obtaining samples of feather pulp at 1 week of age and testing using real time PCR[116].Factors influencing the strength of the immune responses elicited by the vaccine 3.3.1.…”

“…The serotype 1 vaccine viruses have been adapted to grow in CEF and are exclusively used as cell-associated vaccines. Studies have demonstrated that there are differences in the level of antigenicity of various CVI-988depending on their origin[178][179][180]. Equally, differences have been shown on the ability to replicate in FFE and spread by contact[179].Conventional MD vaccines are live vaccines made using either nononcogenic strains from serotypes 2 and 3 or attenuated from serotype 1.…”