Influenza is a highly infectious respiratory illness caused by the influenza virus. Common symptoms include fatigue, fever, chills, a hacking cough, and body aches which can self-resolve in 1-2 weeks. However, complications can arise including life-threatening secondary infections. Influenza is a serious disease, and approximately 1 in 1,000 cases result in death.
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There are three main types of influenza virus (Types A, B, and C) that cause infection in humans and these are further characterized into subtypes and strains. The continued emergence of new flu strains each year is due to the ability of the influenza virus to mutate slowly (through small genetic changes called antigenic drift) and quickly through a process called reassortment. Antigenic drift is responsible for the seasonal variations every year and reassortment is responsible for the development of new strains that can cause pandemics.
Influenza type A (Flu A) viruses are especially prone to reassortment due to their wide host range (humans, dogs, birds, pigs, horses, whales, seals, and other animals). Specifically, the Flu A genome is made up of eight loosely linked segments, each of which harbors at least one important gene. Those genes direct the expression of the major viral proteins such as hemagglutinin (HA) and neuraminidase (NA). In the process of viral reproduction, the linkages between the eight segments of the Flu A genome break apart. Since it is possible for two different Flu A strains to infect a cell simultaneously, some of the genetic segments from one strain can be swapped with another during reproduction. For instance, if a human flu virus and a bird flu virus infect a person, reassortment can intermingle genes from both viruses during replication and create a virus with a protein against which humans have little or no immunity, plus human influenza genes that are more likely to cause sustained human-to-human transmission. In contrast, Influenza B (Flu B) and C viruses do not cause pandemics, most likely due to their limited host range of only humans.
Flu A virus is the most common flu virus infecting humans, animals, and birds. It is divided into subtypes, based on the nature of their surface glycoproteins, HA and NA. There are 18 different HAs and 11 NAs that are distinguishable serologically (antibodies to one virus subtype do not react with another). In comparison, Flu B infection mostly occurs in humans and is divided into lineages and strains. Currently circulating influenza B viruses belong to one of the two lineages: B/Victoria and B/Yamagata. This virus is responsible for significant morbidity which is why the seasonal trivalent influenza vaccine contains Flu B as an integral component. Unlike Flu A or B, Influenza C viruses only cause a mild respiratory illness in humans and secondary complications are rare. Flu C is structurally different from Flu A and B viruses and contains a glycoprotein called HEF (hemagglutinin-esterase-fusion).
Influenza viruses are mostly spread by aerosolization made when an infected person coughs or sneezes. Complications usually arise from bacterial infections of the lower respiratory tract and signs of a secondary respiratory infection often appear just as the infected person seems to be recovering. The elderly and the chronically ill are at greater risk for secondary infection and other complications. Children can also experience a rare, but serious complication called Reye’s syndrome.
Diagnostic influenza tests help the identification of influenza types A and B and influenza A subtypes 2009 H1N1, H1, H3, H5, N1, and N2. Influenza tests include rapid influenza diagnostic tests (RIDTs), direct fluorescent antibody stains, viral cultures, and molecular assays.
RIDTs have become routine influenza tests since their initial FDA approval in 1999, and they typically detect both Type A and B influenza. They are easy to use, relatively inexpensive, and provide rapid results in 10-30 minutes, allowing physicians to prescribe antivirals in the relatively small window of effectiveness (1-2 days after onset of symptoms). The performance of RIDTs is highly dependent on the quality of reagents, proficiency of operation, transport and storage conditions, time from illness onset to sample collection, and the emergence of genomic variations and novel strains. Many RIDTs detect the nucleoprotein (NP), which is one of the more conserved proteins in the influenza virus and subsequently less likely to undergo mutations that lead to antigenic drift (which in turn can cause the functional components of an RIDT to not recognize a current influenza strain). The major limitation of currently available RIDTs is their low and variable sensitivity. To obtain a true increase in assay sensitivity, monoclonal antibodies capable of recognizing existing and emerging strains are critical.
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