1.1.1. Assessment of pathogenicity
The term HPAI relates to the assessment of pathogenicity in chickens and implies the
involvement of highly pathogenic strains of virus. It is used to describe a disease of fully
susceptible chickens with clinical signs that may include one or more of the following: ocular
and nasal discharges, coughing, snicking and dyspnoea, swelling of the sinuses and/or head,
listlessness, reduced vocalisation, marked reduction in feed and water intake, cyanosis of the
unfeathered skin, wattles and comb, incoordination, nervous signs and diarrhoea. In laying
birds, additional clinical features include a marked drop in egg production usually accompanied
by an increase in numbers of poor quality eggs. Typically, high morbidity is accompanied by
high and rapidly escalating unexplained mortality. However, none of these signs can be
considered pathognomonic and high mortality may occur in their absence. In addition, low
pathogenicity influenza A viruses that normally cause only mild or no clinical disease, may
cause a much more severe disease if exacerbating infections or adverse environmental factors
are present and, in certain circumstances, the spectrum of clinical signs may mimic HPAI.