Repeated Challenge with Virulent Newcastle Disease Virus Does Not Decrease the Efficacy of Vaccines.

Area of Science:

• Veterinary Immunology
• Avian Pathology
• Vaccinology

Background:

• Poultry producers observe Newcastle disease (ND) in well-vaccinated flocks, contradicting experimental findings.
• A hypothesis suggests repeated exposure to virulent Newcastle disease virus (vNDV) in field settings overwhelms vaccine-induced immunity.

Purpose of the Study:

• To evaluate if repeated high-dose challenges with vNDV can overcome vaccine immunity in chickens.
• To investigate the efficacy of existing Newcastle disease vaccination under simulated field challenge conditions.

Main Methods:

• Well-vaccinated chickens were challenged daily with high doses of vNDV for 10 days.
• Sham-vaccinated chickens served as controls.
• Clinical signs, antibody titers, and mortality were monitored.

Main Results:

• All sham-vaccinated control birds succumbed to the challenge by day 4.
• No clinical signs, morbidity, or mortality were observed in NDV-vaccinated birds up to 14 days postchallenge.
• Repeated high-dose vNDV exposure did not overcome the immunity provided by vaccination.

Conclusions:

• Existing Newcastle disease vaccination provides robust protection against virulent strains, even under conditions of repeated high-dose exposure.
• The hypothesis that repeated field challenges overwhelm vaccine immunity was not supported under these experimental conditions.