Clinical Traits and Pathology of Avian Influenza Infections, Guidelines for Farm Visit and Differential Diagnosis

Abstract

Influenza A viruses have been grouped into two distinct pathotypes on the basis of the severity of the disease they cause in susceptible chickens. Highly pathogenic avian influenza (HPAI) viruses cause severe disease in chickens and other gallinaceous birds. Low pathogenicity avian influenza (LPAI) viruses cause a much milder clinical disease. HPAI has been associated only with some strains of H5 and H7 (occasionally some H10) avian influenza viruses. The infection causes a lethal disease in most birds, particularly in galliforms (chickens and turkeys) and is characterised by very high flock mortality. Some birds, such as ostriches as well as domestic and wild waterfowl, may be resistant clinically to disease, although they harbour the virus and are permissive to viral replication. LPAI is caused by all 16 haemagglutinin subtypes (H1–H16) of the virus. Low pathogenicity strains are usually responsible for mild or inapparent disease. Viral replication is restricted to the respiratory, digestive and urogenital tracts; therefore, the clinical appearance of infection consists of respiratory signs, gastroenteric disorders and a decline in egg production.