Migratory wild birds, asymptomatic carriers of the virus, can infect domestic birds such as chickens, ducks, and turkeys. Infection makes the domestic birds sick, and most die.
Humans acquire avian influenza viruses primarily through direct contact of the mucous membranes with infectious secretions and excreta from infected birds or contaminated poultry products. The major portal of entry appears to be the upper respiratory tract. Although human-to-human transmission has been suggested in several household clusters, so far, there has been no sustained human-to-human transmission. Much remains to be learned about the exact mode of transmission. Most human illness from avian influenza has resulted from infection with Asian lineage H7N9 and H5N1 viruses. A man in China was hospitalized in May 2021 with what is believed to be the first ever human case of H10N3 bird flu, an uncommon and low pathogenic strain.
The incubation period is generally between 2-5 days, but an upper limit of 8 days is possible. Most patients have headache, malaise, high fever, sore throat, cough, shortness of breath, and myalgia. Conjunctivitis, watery diarrhea, abdominal pain, vomiting, pleuritic pain, and bleeding from the nose have also been reported. Respiratory distress, tachypnea, and inspiratory crackles are present on physical examination. Lymphopenia and thrombocytopenia are commonly present. The frequency of milder illnesses, subclinical infections, and atypical presentations such as encephalopathy is not known. Pregnant individuals are at increased risk for severe illness from influenza.
Atypical presentations of avian influenza have been reported. Patients have developed nausea, vomiting, and diarrhea preceding acute respiratory failure. Progression to acute respiratory distress syndrome (ARDS) and respiratory failure is common. Complications have included bacterial sepsis, pulmonary hemorrhage, and multi-organ failure. The mortality rate of hospitalized patients has been high due to progressive respiratory failure.
J09.X2 – Influenza due to identified novel influenza A virus with other respiratory manifestations
442438000 – Influenza caused by Influenza A virus
- Typical influenza infection (not avian influenza)
- Other causes of viral pneumonia (respiratory syncytial virus, adenovirus, or human metapneumovirus, among others)
- Bacterial causes of community-acquired pneumonia – Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis, and Legionella pneumophila
- Endemic fungal infection (histoplasmosis, coccidioidomycosis)
- Other pathogens can be seen in immunocompromised patients, including Pneumocystis jirovecii or Toxoplasma gondii
- ARDS due to any cause (pancreatitis, aspiration, etc)
- Other noninfectious causes, such as diffuse alveolar hemorrhage and graft-versus-host disease