OVERVIEW
Highly pathogenic human avian influenza virus pneumonia is caused by avian influenza viruses of the genus Influenza A of the family Orthomyxoviridae. It can be categorized into 16 HA (outer membrane hemagglutinin) subtypes and 9 NA (neuraminidase) subtypes. Human avian influenza is an acute respiratory infection caused by some strains of certain subtypes of avian influenza A viruses, which can cause pneumonia and multiple organ dysfunction (MODS). In recent years, evidence has been obtained that avian influenza viruses of subtypes H9N2, H7N2, and H7N3 have infected humans, and WHO has warned that this disease may be one of the greatest potential threats to human health.
Causes
Avian influenza viruses belong to the genus Influenza A of the family Orthomyxoviridae, which can be divided into 16 HA subtypes and 9 NA subtypes. The subtypes of avian influenza viruses that infect humans are H5N1, H9N2, H7N7, H7N2, H7N3, etc. Among them, patients infected with H5N1 have a serious condition and a high case fatality rate, which makes them known as highly pathogenic avian influenza viruses. Human infection with H5N1 after the onset of 1 to 16 days, can be detected from the patient’s nasopharyngeal part of the isolates of the virus. Viral RNA has been detected in the serum and feces of most patients and in the cerebrospinal fluid of a few patients, while urine specimens have been negative. It is unclear whether feces or blood can be a vector for transmission of infection. Human infection with H5N1 is consistent with avian-to-human transmission, and environmental-to-human transmission may exist.
Symptoms
The incubation period ranges from 1 to 7 days, with the majority being between 2 and 4 days. The main symptom is fever, with most of the body temperature persisting above 39℃, which may be accompanied by runny nose, nasal congestion, cough, sore throat, headache, muscle aches and general malaise. Some patients may have nausea, abdominal pain, diarrhea, dilute watery stools and other gastrointestinal symptoms.
Severe patients may have high fever, rapid progression of the disease, almost all patients have clinically significant pneumonia, often acute lung injury, acute respiratory distress syndrome (ARDS), pulmonary hemorrhage, pleural effusion, pancytopenia, multiple organ failure, shock and Reye’s (Reye) syndrome and other complications. Sepsis may occur secondary to bacterial infection.
Examination
Peripheral blood leukocyte count is low or decreased, especially lymphocytes; thrombocytopenia is also present. Viral antigen and genetic testing can detect influenza A virus nucleoprotein antigen (NP) or matrix protein (M1), avian influenza virus subtype H antigen. RT-PCR can also be used to detect the avian influenza virus subtype-specific H antigen gene. Avian influenza viruses can be isolated from patients’ respiratory specimens (e.g., nasopharyngeal secretions, oral gargles, tracheal aspirates, or respiratory epithelial cells), especially from upper respiratory tract specimens. Diagnosis is aided by a 4-fold or greater elevation in antibody titers to subtyped strains of avian influenza viruses in duplicate sera during the early stages of illness and recovery.
Chest imaging may show intrapulmonary lamellar shadows. In severe cases, the intrapulmonary lesions progress rapidly, showing large patchy gross glassy shadows and solid images of the lungs, and in the later stages of the disease, diffuse solid shadows in both lungs, which may be combined with pleural effusion.
Diagnosis
Diagnosis can be made on the basis of etiology, clinical manifestations and laboratory tests.
Treatment
All patients with suspected or confirmed H5N1 infection should be hospitalized and isolated for clinical observation and antiviral treatment. In addition to symptomatic treatment, oral oseltamivir is given as early as possible (within 48 hours of onset).
Patients with severe, highly pathogenic human avian influenza virus pneumonia often require ventilatory support and intensive monitoring to prevent and treat multiple organ dysfunction. Corticosteroids can also be used, but their efficacy has not been confirmed. alpha interferon, which has both antiviral and immunomodulatory activity, can also be tried. Serum from recovered patients can be used to significantly reduce the titer of the virus in the patient’s blood.