Based on studies of patients with the highly pathogenic AI H5N1 virus, the most common presenting symptoms are high fever (>38°C) and an influenza-like illness (headache, vomiting, cough) followed by lower respiratory symptoms and signs (shortness of breath or worsening cough). Upper respiratory symptoms (rhinorrhea, sore throat) are occasionally present. Diarrhea is more common with AI than with influenza due to human viruses, and there have been rare reported cases of severe AI with vomiting and diarrhea in the absence of lower respiratory symptoms.

Conjunctivitis is prevalent in avian influenza H7 virus, but not with the HPAI H5N1 strain. The incubation period is likely between two and eight days.

In most cases, lower respiratory tract findings are prevalent, including respiratory distress, tachypnea, crackles and chest x-ray findings of pulmonary infiltrates (interstitial or lobar). Pleural effusions are generally not seen. In severe cases, acute respiratory distress syndrome (ARDS) follows and often leads to multi-organ failure (with manifestations such as lymphopenia, thrombocytopenia and abnormal liver and renal function) and death. Mortality is 50 per cent of reported HPAI H5N1 cases.

While the initial symptoms of AI are non-specific, and the clinical course is consistent with other severe respiratory viral syndromes, the key diagnostic point is exposure to sick or dead domestic fowl in an endemic area for avian influenza.

While there are rare reports of limited, non-sustained human-to-human transmission, these are very isolated and associated with prolonged, close contact with an infected person.

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