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Introduction:
Respiratory diseases are a health and economic problem for poultry production worldwide. Respiratory diseases caused by a single pathogen are the exception under field conditions, as co-infections involving multiple viral and bacterial pathogens are more commonly observed. IBV and AIV-H9 Co-infection generally increases the pathogenicity (intensity and/or duration) of the clinical signs caused by these pathogens when they occur individually.
Avian infectious bronchitis (IB) is a highly contagious disease of many avian species and is particularly responsible for devastating disease outbreaks in commercial poultry flocks in Algeria that incur huge economic losses to the national poultry industry annually. Respiratory, reproductive and renal lesions that may occur as early as 14 days of age characterize the disease in broilers. The damage include mortality of 10 to 25%, as well as losses due to secondary infections, treatment cost, poor performance and condemnations at slaughter.
H9N2 Avian influenza virus (AIV-H9) is considered the most prevalent AIV in poultry globally. Despite being classified as a low pathogenic virus, H9N2 AIV is responsible for substantial economic loss for the poultry industry. Infection is typically associated with moderate to severe respiratory symptoms, delayed growth and increased mortality, especially when co-infection with other pathogens is involved.
Pathogenesis of H9 and IBV co-infection:
H9 avian influenza virus was detected in clinically healthy chicken flocks with a history of no mortality during routine surveillance programs for respiratory viral infections in many countries around the world. It may reflect the low pathogenic nature of the virus, which permits the silent spread of such viruses in apparently healthy commercial chickens.
The increased pathogenicity of H9 infection could be explained by various hypotheses. Theses hypotheses are: secretion of trypsin-like proteases by IB viruses, stimulation of host cells to produce or secrete more protease, destruction of endogenous cell protease inhibitors, and suppression of immune system due to stress by the viral infection.
Consequently, the mortality rates varied according to the associated disease condition. They ranged from 0.5% to 70% in the affected broiler flocks. Based on the discrepancies between the high mortality in the field and the low pathogenicity of the H9N2 strains in the laboratory conditions, it is suggested that concurrent IBV viral infection caused severe mortality to 70% in flocks, which has mixed H9 & IBV infection.
Clinical and PM findings of H9 and IBV co-infection:
The clinical signs of the infected broiler flocks include respiratory signs such as Coughing, tracheal rales, sneezing, and difficult breathing, swelling of periorbital tissues with conjunctivitis, nasal and ocular discharges with mortality rates ranged from 0.5% to 70% and decrease in feed consumption.
The postmortem findings of the infected broiler flocks reveal tracheitis, congestion of lungs, exudation and cast formation at the tracheal bifurcation and lower bronchi. Pericarditis, perihepatitis, peritonitis and air-sacculitis are also observed.
Control of IBV infection :
It is well established that a single IBV vaccine serotype does not provide sufficient protection against heterologous infection. Therefore, producers administer more than one vaccine serotype in order to broaden the immune response of their flocks.
The principle of this approach is based on the fact that combined vaccination with multiple serotypes induces the production of neutralizing antibodies against each vaccine type individually, thereby conferring sterilizing immunity. This immunity also protects the tracheal cilia (by preventing ciliostasis) and reduces the risk of secondary infections caused by other pathogens.
However, for this strategy to remain effective, it is essential to know the pathogenic IBV serotypes circulating in the field in order to administer the appropriate vaccines. In the case of the emergence of a new variant, this strategy may fail to provide protection against that virus.
Continuous monitoring of IBV virus evolution and the constant need to adapt the vaccine panel therefore represent the key element for the success of the vaccination strategy.
Control of AIV H9 infection:
Commercial broiler chickens possessing maternally derived antibodies (MDA) against the H9N2 avian influenza virus have difficulty developing an effective immune response when vaccinated with an inactivated H9N2 vaccine on the first day of life, due to immune interference.
To overcome vaccination failures, it is recommended to: