Influenza virus belongs to the family Orthomyxoviridae, which is a common contagious pathogen that causes acute respiratory tract illness in humans. Influenza virus can be classified into three types (A, B and C) based on matrix and nucleoproteins, of which Type A and B virus are prominent among the causes of severe respiratory illness.
Influenza Detection
Infants, the elderly, and individuals with compromised cardiac, pulmonary, or immune systems are at greatest risk of serious complications from Influenza viruses. Current diagnostic methods include virus isolation, antigen detection, and serology. The availability and use of rapid influenza diagnostic tests to detect influenza viral antigens in respiratory tract specimens by laboratories and clinics have increased in recent years.
Table: Influenza Virus Testing Methods
| Method1 | Types Detected | Acceptable Specimens2 | Test Time | CLIA Waived3 |
| Rapid Influenza Diagnostic Tests4 (antigen detection) | A and B | NP5 swab, aspirate or wash, nasal swab, aspirate or wash, throat swab | <15 min. | Yes/No |
| Rapid Molecular Assay [influenza viral RNA or nucleic acid detection] | A and B | NP5 swab, nasal swab | <20 min | Yes/No6 |
| Immunofluorescence, Direct (DFA) or Indirect (IFA) Florescent Antibody Staining [antigen detection] | A and B | NP4 swab or wash, bronchial wash, nasal or endotracheal aspirate | 1-4 hours | No |
| RT-PCR7 (singleplex and multiplex; real-time and other RNA-based) and other molecular assays [influenza viral RNA or nucleic acid detection] | A and B | NP5 swab, throat swab, NP5 or bronchial wash, nasal or endotracheal aspirate, sputum | Varies (1 to 8 hours, varies by the assay) | No |
Types of Influenza Viruses
Influenza virus belongs to the family Orthomyxoviridae, which is a common contagious pathogen that causes acute respiratory tract illness in humans. Influenza virus can be classified into three types (A, B and C) based on matrix and nucleoproteins, of which Type A and B virus are prominent among the causes of severe respiratory illness.
Influenzavirus A: According to different antigenicity of two surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA), influenza A virus can be further divided into 19 HA and 11 NA subtypes. This genus has one species, influenza A virus. Wild aquatic birds are the natural hosts for a large variety of influenza A. The type A viruses are the most virulent human pathogens among the three influenza types and cause the severest disease. The influenza A virus can be subdivided into different serotypes based on the antibody response to these viruses.
| H1N1 | Caused spanish flu in 1918, and swine flu in 2009. |
| H2N2 | Caused Asian flu in 1957. |
| H3N2 | Caused Hong Kong flu in 1968. |
| H5N1 | Caused bird flu in 2004. |
| H7N7 | Has unusual zoonotic potential. |
| H1N2 | Endemic in humans, pigs and birds. |
| H9N2 | Caused illness in several children aged nine months to 5 years. |
| More | H7N2, H7N3, H10N7, H7N9. |
Learn more about influenzavirus A.
Influenzavirus B: Influenza B viruses cause the same spectrum of disease as influenza A. However, influenza B viruses do not cause pandemics. The only other animals known to be susceptible to influenza B infection are the seal and the ferret. This type of influenza mutates at a rate 2–3 times slower than type A and consequently is less genetically diverse, with only one influenza B serotype. As a result of this lack of antigenic diversity, a degree of immunity to influenza B is usually acquired at an early age. However, influenza B mutates enough that lasting immunity is not possible. This reduced rate of antigenic change, combined with its limited host range (inhibiting cross species antigenic shift), ensures that pandemics of influenza B do not occur.
Learn more about influenzavirus B.
Influenzavirus C: Influenza C virus, which infects humans, dogs and pigs, sometimes causing both severe illness and local epidemics. However, influenza C is less common than the other types and usually only causes mild disease in children.