Avian influenza natural history, complications and prognosis

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For more information about seasonal human influenza virus that is not associated with animal exposure, see Influenza

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

Following exposure to the avian influenza virus, an incubation period of 2 to 3 days for H5N1 and 2 to 8 days for H7N9 delays the onset of symptoms. The majority of patients present with with a high grade fever, cough, headache, sore throat, vomiting, diarrhea, and abdominal pain. Approximately 50% of patients with avian influenza develop severe complications such as acute respiratory distress syndrome, renal failure, sepsis, and multiple organ dysfunction syndrome. Of the reported cases, about 60% of patients have died, particularly those who develop early complications. Patients without complicated disease usually recover within 3 to 5 days with no sequelae.

Natural History

• Following initial exposure to the virus, the usual incubation period for the H5N1 avian influenza is approximately 2 to 3 days, but possibly as long as 17 days in some individuals. For the H7N9 avian influenza, the incubation period ranges from 2 to 8 days, with an average of 5 days.
• The majority of patients present with a high grade fever, cough, headache, sore throat, vomiting, diarrhea, abdominal pain.
• The illness is has an very aggressive clinical course, often with rapid deterioration.
• Most patients develop significant lower respiratory tract involvement, with aggressive viral pneumonia, pleural effusions, and ARDS.
• The rate of complications is very high with approximately 50% of patients suffering from life-threatening complications.

Complications

Approximately 50% of patients with avian influenza develop one or more of the following complications:

• Pneumonia
• Acute pulmonary hemorrhage
• Pleural effusion
• Acute respiratory distress syndrome
• Acute renal failure
• Multiple organ dysfunction syndrome
• Sepsis / Septic shock
• Disseminated intravascular coagulopathy
• Reye's syndrome

Prognosis

Of the human cases associated with the H5N1 outbreaks in poultry and wild birds in Asia and parts of Europe, the Near East, and Africa, about 60% of people reported to be infected with the virus have died. Most cases have occurred in previously healthy children and young adults. Patients with early complications are at a much higher risk of death, often from multiple organ dysfunction syndrome. Patients with no complications usually recover within 3 to 5 days with no sequelae.