Avian influenza (AI) is a highly contagious viral infection of wild and domestic birds worldwide. Avian influenza viruses are classified as low pathogenic avian influenza (LPAI) or highly pathogenic (HPAI), based on their pathogenicity in domestic chickens. LPAI viruses cause milder disease, while HPAI result in more severe, multi-systemic infections. Once infected, the AIV replicates in the respiratory, intestinal, renal, and/or reproductive organs of the host.
Clinical signs of AI are highly variable and the severity of the infection depends on age, sex, host immunity, pathogenicity of the infecting virus, presence of secondary pathogens, and where the virus replicates in the body.
- LPAI: In chickens, most infections with LPAI viruses affect the respiratory tract, especially the sinuses. Gross lesions are characterized as catarrhal, fibrinous, serofibrinous, mucopurulent, or fibrinopurulent inflammation, often accompanied by secondary bacterial infections. The bird's infraorbital sinuses may appear swollen and filled with mucous-to-mucopurulent nasal discharge. Hens may start laying abnormal eggs, often misshapen, abnormally fragile, and show loss of pigmentation. They may develop egg yolk peritonitis as a result of secondary complications due to the virus.
- HPAI: HPAI viruses frequently replicate and cause damage to multiple visceral organs, and cardiovascular and nervous system. However, the presenting clinical signs will vary from bird to bird, depending on the amount of damage induced to the organs. Respiratory signs are less likely to occur then with LPAI virus infections. Nervous system signs most often include head and neck tremors, wry neck, inability to stand, and abnormal tilted head positions. There will be overall less activity among the birds who will appear very depressed and show reduced appetite.
AIV is excreted through the nasal and respiratory secretions (nares, mouth, conjunctiva), cloaca, and feces of infected birds into the surrounding environment. The virus is transmitted by direct or indirect contact with infected birds through aerosol droplets or exposure to virus-contaminated fomites.
Wild waterfowl are the primary hosts of influenza A viruses (IAV), especially mallard ducks. They act as asymptomatic carriers of LPAI viruses, and will shed high amounts of the virus in their feces, into the body of water they're swimming in. Since wild waterfowl are often carriers of the low pathogenic influenza virus, they are often a source of entry in flocks.
AIV is diagnosed in chickens through the use of several types of common laboratory diagnostic tests. Swabs taken from the chicken's oropharynx or cloaca work best. Virus isolation is considered to be the "gold standard" in confirming that birds have been infected by the AI virus.
The incubation period for avian influenza virus is 3-5 days and is dependent on the dose of the virus, the route of exposure, the species exposed, and the ability to detect clinical signs.