Division 2 Clinical signs in birds infected with HPAI virus

The clinical signs are very variable and are influenced by factors such as the virulence of the infecting virus, the species affected, age, sex, concurrent diseases and environment.

Early signs can include inappetence, reduction in water intake and relatively low mortality. However, alternatively the disease may appear suddenly in a flock and many birds may die either without premonitory signs or with minimal signs of depression, inappetence, ruffled feathers and fever. Generally the longer birds survive the more marked are the clinical signs. The timeline for the development of signs depends on the virus, host and initial infecting dose together with husbandry system. The virus spreads more slowly in caged layers or outdoor birds compared to in broiler houses.

Hens infected with the HPAI virus may at first lay soft-shelled eggs, but soon stop laying. Sick birds often sit or stand in a semi-comatose state with their heads touching the ground. Combs and wattles are cyanotic and oedematous, and may have petechial or ecchymotic haemorrhages at their tips. Profuse watery diarrhoea is frequently present and birds are excessively thirsty. Respiration may be laboured and excessive lacrimation may be seen. Haemorrhages may be seen on unfeathered areas of skin. The flock mortality rates vary from 50 to 100 %.

In broilers, the signs of HPAI are frequently less obvious than for other poultry and usually include severe depression, inappetence, and a very marked increase in mortality may be the first abnormality observed. Oedema of the face and neck and neurological signs such as torticollis and ataxia may also be seen.

HPAI in turkeys is similar to that seen in domestic fowl, but some HPAI viruses appear more virulent in turkeys while others appear less virulent.

In geese infected with the HPAI virus the signs of depression, inappetence and diarrhoea are similar to those in layers, though frequently with swollen sinuses. Younger birds may exhibit neurological signs.

Ducks may show no clinical signs when infected with HPAI viruses, but some strains have been reported to induce signs similar to those in geese with some mortality.

Clinical signs may be absent in HPAI and LPAI infections of ostriches. In HPAI outbreaks, such as those in Italy in 1999 and 2000, guinea fowl and Japanese quail were reported to be susceptible to infections with signs and mortality resembling the disease in chickens or turkeys. However, in some experimental studies quail have been reported to be resistant to some HPAI strains. For all birds, the presence of antibodies to the same H subtype whether by vaccination or natural infection may mean that infection with HPAI virus does not result in overt clinical signs.