Infectious Coryza is a severe, highly contagious upper respiratory disease primarily affecting chickens, caused by the bacterium Avibacterium paragallinarum. This infection can lead to significant economic losses in poultry operations due to reduced egg production and increased culling. The primary strategy for protecting flocks at risk of this bacterial disease is through a careful and consistent vaccination program.
Understanding Infectious Coryza
Infectious Coryza is an acute respiratory condition in chickens, sometimes referred to as roup or contagious catarrh. While the mortality rate is low when the disease is uncomplicated, the morbidity, or sickness rate, is high and spreads rapidly through a flock.
The incubation period is short, lasting only one to three days. Affected birds exhibit listlessness, sneezing, and a thin, watery nasal discharge that progresses to a thick, foul-smelling exudate. A hallmark sign is the swelling of the infraorbital sinuses and surrounding facial tissues, which can prevent the eyes from fully opening.
Transmission occurs through direct bird-to-bird contact and indirect means, such as airborne droplets or contaminated feed and drinking water. Birds that recover often become asymptomatic carriers for life, harboring the bacteria and serving as a continuous source of infection for susceptible flock mates. This carrier state emphasizes the importance of biosecurity and vaccination for prevention.
Vaccine Formulations and Serotype Selection
The vaccine used to combat Infectious Coryza is typically an inactivated, or killed, bacterin. This formulation stimulates the chicken’s immune system to produce antibodies without causing the actual disease. Since the vaccine is composed of non-living material, it prevents future outbreaks but has no therapeutic value for birds already suffering from an active infection.
Avibacterium paragallinarum is classified into three main serotypes: A, B, and C. These serotypes are immunologically distinct, meaning protection against one does not confer immunity against the others. Therefore, a vaccine must contain the specific serotype or serotypes circulating in the local poultry population.
Commercial vaccines are available as monovalent, bivalent, or trivalent preparations, containing one, two, or all three serotypes. Selecting the appropriate vaccine relies on local epidemiological data to match the prevalent field strains. A trivalent vaccine offers the broadest protection and is often chosen where the specific circulating strain is unknown or where multiple serotypes are present.
Practical Vaccination Protocol
Proper storage is essential to maintain vaccine efficacy. The vaccine must be stored in the dark at refrigeration temperatures (\(2^{\circ}\text{C}\) to \(8^{\circ}\text{C}\)) and must never freeze. Before use, the vaccine should be warmed to room temperature and thoroughly shaken to ensure the bacterial components are evenly suspended.
The standard administration method is by injection, either subcutaneously (under the skin) in the lower back of the neck or intramuscularly (into the muscle) in the breast or thigh. The dosage is \(0.25\) to \(0.5\) milliliters per bird, and all equipment should be sterile to prevent contamination. Care is required during injection to avoid damaging surrounding tissues or accidentally injecting the operator.
A two-dose regimen is required to establish robust, long-lasting immunity, particularly in layer and breeder flocks. The first dose is administered to young chickens around 8 to 12 weeks of age. A second, booster dose is given at least four weeks later, but no less than four weeks prior to the onset of egg production. Following vaccination, a mild, transient swelling may be observed at the injection site.