Effect of Different Levels of Maternally Derived Genotype VII Newcastle Disease Virus-Specific Hemagglutination Inhibition Antibodies on Protection against Virulent Challenge in Chicks

Abstract

Newcastle disease (ND), caused by the virulent Newcastle disease virus (NDV), is an acute, highly contagious, and economically significant avian disease worldwide. Vaccination is the most effective measure for controlling ND. In recent years, vaccines matched with the prevalent strains of genotype VII have been developed and are now commercially available. These vaccines can provide full protection for chickens against clinical disease and mortality after challenges with genotype VII viruses and significantly decrease virus shedding compared to conventional vaccines belonging to genotypes I and II. Vaccinated hens can transfer antibodies to their offspring through the egg yolk. Maternally derived antibodies can provide passive protection against diseases but can also interfere with vaccination efficacy early in life. This study was conducted on chicks hatched from hens vaccinated with a commercial genotype VII NDV-matched vaccine to investigate the correlation between hemagglutination inhibition (HI) antibody levels in chicks and hens and the decaying pattern of maternally derived HI antibodies, and to evaluate the protective efficacy of different levels of maternally derived HI antibodies against challenge with a virulent NDV strain of genotype VII based on survivability and virus shedding. The HI antibody titers in chicks at hatching were about 1.3 log₂ lower than those in hens, indicating an antibody transfer rate of approximately 41.52%. The estimated half-life of these antibodies was about 3.2 days. The protective efficacy of maternally derived HI antibodies was positively correlated with the titer. These antibodies could effectively protect chicks against mortality when the titer was 7 log₂ or higher, but they were unable to prevent virus shedding or infection even at a high titer of 11 log₂. The obtained results will greatly assist producers in determining the immune status of chicks and formulating appropriate vaccination schedules against ND.

Keywords: Newcastle disease; chick; efficacy; genotype VII-matched vaccine; hemagglutination inhibition antibody; maternally derived antibody; survivability; virus shedding.

Figure 1

Decaying pattern of maternally derived HI antibodies against NDV in chicks. The chicks were hatched from the hens at 22 weeks of age. The hens were vaccinated at 1 and 4 weeks using the combined live attenuated vaccine against ND and infectious bronchitis (VG/GA strain + H120 strain) (intraocularly and intranasally) and at 5 and 15 weeks using the inactivated recombinant NDV vaccine (A-VII strain, genotype VII) (intramuscularly). The chicks were not vaccinated, and no medication was administered during the experimental period. The HI test was performed using 4 HA units of the genotype VII NDV antigen. For each time point, the HI antibody titers are expressed as means ± SD based on the 20 chicks randomly selected from 50 chicks.

Figure 2

Survival curve of chicks with different levels of maternally derived HI antibodies following challenge with virulent genotype VII NDV strain JSC0804. The chicks were hatched from the hens at 25 weeks of age. The hens were vaccinated at 1 and 4 weeks using the combined live attenuated vaccine against ND and infectious bronchitis (VG/GA strain + H120 strain) (intraocularly and intranasally) and at 5 and 15 weeks using the inactivated recombinant NDV vaccine (A-VII strain, genotype VII) (intramuscularly). Each chick was challenged with 10⁶ ELD₅₀ JSC0804 strain intraocularly and intranasally and the survival status was monitored twice daily for 21 days.

Figure 3

Virus shedding in chicks with different levels of maternally derived HI antibodies at day 5 after the challenge with virulent genotype VII NDV strain JSC0804. Viruses in swabs were detected via virus isolation using 9- to 10-day-old SPF embryos. Oropharyngeal shedding rate = the number of birds positive in virus isolation from oropharyngeal swabs/the number of surviving birds × 100%; cloacal shedding rate = the number of birds positive in virus isolation from cloacal swabs/the number of surviving birds × 100%; total shedding rate = the number of birds positive in virus isolation from oropharyngeal or cloacal or both swabs/the number of surviving birds × 100%.

Figure 4

Pattern and duration of virus shedding in chicks with maternally derived HI antibody titers of 4 log₂ and 5 log₂ after challenge with virulent genotype VII NDV strain JSC0804. Viruses in swabs were detected by virus isolation using 9- to 10-day-old SPF embryos. Oropharyngeal shedding rate = the number of birds positive in virus isolation from oropharyngeal swabs/the number of surviving birds × 100%; cloacal shedding rate = the number of birds positive in virus isolation from cloacal swabs/the number of surviving birds × 100%. The shedding status was monitored for 31 days.

Figure 5

Serological response of chicks with different levels of maternally derived HI antibodies 15 days after the challenge with genotype VII NDV strain JSC0804. All birds in the group with the HI titer ≤ 3 log₂ died before 6 days pc. The HI test was performed using 4 HA units of the genotype VII NDV antigen. The HI titers after the challenge are shown as means ± SD.