Avian influenza history and symptoms

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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

Highly pathogenic avian influenza (HPAI) Asian H5N1 and low pathogenic avian influenza (LPAI) H7N9 have been responsible for most human illness worldwide to date, including the most serious illnesses and deaths. H5N1 virus infection is associated with a wide range of illness from conjunctivitis only, to influenza-like illness, to severe respiratory illness (e.g., shortness of breath, difficulty breathing, pneumonia, acute respiratory distress, respiratory failure) with multi-organ disease, sometimes accompanied by nausea, abdominal pain, diarrhea, vomiting, and sometimes neurologic changes (altered mental status, seizures). The reported signs and symptoms of H7N9 virus infection in humans range from conjunctivitis to influenza-like illness (e.g., fever, cough, sore throat, muscle aches) to lower respiratory disease (pneumonia) requiring hospitalization.

History and Symptoms

Most avian influenza viruses do not cause disease in humans. However, some are zoonotic, meaning that they can infect humans and cause disease. The most well known example is the avian influenza subtype H5N1 viruses currently circulating in poultry in parts of Asia and northeast Africa, which have caused human disease and deaths since 1997. Other avian influenza subtypes, including H7N7 and H9N2, have also infected humans. Some of these infections have been very severe and some have resulted in deaths, but the majority of infections have been mild or even subclinical.

H5N1

The disease caused by the influenza A(H5N1) virus usually follows an aggressive clinical course, with rapid deterioration and high fatality. The incubation period for A(H5N1) avian influenza may be longer than that for normal seasonal influenza, which is around 2 to 3 days. Current data for A(H5N1) infection indicate an incubation period ranging from 2 to 8 days and possibly as long as 17 days.

Initial symptoms include high fever, usually with a temperature higher than 38°C, and other influenza-like symptoms (cough or sore throat). Diarrhea, vomiting, abdominal pain, chest pain, and bleeding from the nose and gums have also been reported as early symptoms in some patients. One feature seen in many patients is the development of lower respiratory tract early in the illness. Respiratory distress, hoarseness, and a crackling sound on inhalation are commonly observed. Sputum production is variable and sometimes bloody. The course of A(H5N1) infection may be complicated by hypoxemia, multiple organ dysfunction, and secondary bacterial and fungal infections.

H7N9

A small number of clinically mild H7N9 virus infections with uncomplicated influenza (febrile upper respiratory tract illness) have been identified in children and adults. Severe illness is more likely to occur in older persons with underlying chronic conditions.

Current data for A(H7N9) infection indicate an incubation period ranging from 2 to 8 days, with an average of five days. Therefore it is recommended that an incubation period of 7 days be used for field investigations and the monitoring of patient contacts. The median time from onset to hospital admission is approximately 4.5 days, and a high proportion of patients with confirmed H7N9 infection have been admitted to intensive care. The median time from illness onset to death is approximately 11 days, ranging from 7 to 20 days.

The disease caused by the virus is characterized by rapidly progressing severe pneumonia. Common symptoms are not disease specific and those of typical acute respiratory infection, such as fever, cough, and shortness of breath. Complications of H7N9 virus infection include the refractory hypoxemia, respiratory failure, acute respiratory distress syndrome (ARDS), septic shock, acute renal failure, rhabdomyolysis, encephalopathy, bacterial and fungal infections such as ventilator-associated pneumonia and bloodstream infection, and multi-organ failure requiring intensive care and mechanical ventilation.

See Also

• CDC – Information on Avian Influenza
• WHO – Avian influenza in humans