# Evaluation of the Serological Baseline Values of Broiler Chickens Jointly Vaccinated with Infectious Bronchitis H + 120 and GI-13 Preparations Under Field Conditions

## TL;DR

This study evaluates the effectiveness of a vaccination program using two vaccines in broiler chickens to protect against infectious bronchitis under real-world conditions.

## Contribution

The study establishes serological baseline values for a protectotype vaccination program using H-120 and GI-13 vaccines in broiler chickens.

## Key findings

- The vaccination program effectively protected flocks from clinical outbreaks after heterologous IBV infection.
- Serological baseline values were established for IDEXX and BioChek ELISAs in vaccinated chickens.
- The presence of heterologous VAR2 genotype did not negatively impact flock health or performance.

## Abstract

Infectious bronchitis (IB) is a highly infectious, viral disease in chickens contributing to very high production losses in the poultry industry worldwide. Specific prophylaxis is widely applied in chickens in order to control the damage caused by this disease. Despite the fact that numerous companies produce specific IBV vaccines, there are scarce field efficacy evaluation data available for them. The data by which to interpret serological results after the use of a given vaccination protocol against IB are also poor. Acknowledging the sparsity of data, this study sought to establish standards for serological evaluation (calculation of serological baseline values) of a vaccination program of broiler chickens against IB using Avishield IB H-120 and GI-13 vaccines. An additional goal of this study was to verify the efficacy of this program under field conditions. The presented results show that the vaccination protocol applied was very efficient in protecting flocks from clinical outbreaks after heterologous IBV infection. They also provide serological baseline values for the two most commonly used serological tests.

Infectious bronchitis virus (IBV) is a highly infectious disease of chickens posing a major threat to poultry production worldwide. Due to genetic and antigenic variability, it is difficult to establish one universal protocol for vaccination against IB. The protectotype vaccination strategy, combining antigenically distinct vaccine strains, offers broader cross-protection than homologous IBV vaccination approaches. This field study aimed to establish serological baseline values in broiler chickens in a protectotype vaccination program using Avishield IB H-120 and Avishield IB GI-13, and to evaluate the program’s efficacy under field conditions on commercial farms where heterologous IBV genotypes were detected. The study was conducted on 25 broiler farms in 2023–2024. Day-old chicks were vaccinated via coarse spray. Blood samples were taken from them at six weeks of age and analyzed using IDEXX and BioChek ELISAs, and cecal tonsils were tested by real-time RT-PCR. Production performance was assessed using the European production efficiency factor (EPEF), with a cut-off value of 360. No IB-related clinical signs were reported, and the mean EPEF of the included farms was 408.7. The detected IBV genotypes were Mass + 793B, 793B, and 793B + VAR2. The presence of heterologous VAR2 did not negatively impact flock health or performance. Preliminary serological baseline values for vaccinated, non-VAR2 farms were established at 891–1332 (in chicks tested with the IDEXX ELISA) and 4193–5204 (in those tested with the BioChek assay). The H-120 + GI-13 program demonstrated high field efficacy, reliable serological profiles, and cross-protection in the presence of heterologous IBV. Continuous monitoring of IBV genotypes remains essential.