Respiratory Diseases

Antigen Detection Assays Influenza

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 and cause life-threatening secondary infections. Overall influenza is a serious disease, and approximately 1 in 1,000 cases result in death.

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 annual emergence of new flu strains 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 range of potential hosts (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. In contrast, influenza B (Flu B) and influenza C predominately only infect humans and do not cause pandemics. Flu A virus is the most common flu virus to infect 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 which can be distinguished serologically (antibodies to one virus subtype do not react with another). In comparison, Flu B

Source: J Clin Microbiol. 2007 Sep; 45(9): 3109–3110.

infection is divided into lineages and strains and current circulating strains only belong to one of two lineages: B/Victoria and B/Yamagata. Flu B is responsible for significant morbidity which is why the seasonal trivalent influenza vaccine contains the main circulating strain for Flu B as an integral component. Influenza C viruses, unlike Flu A or B, only cause a mild respiratory illness in humans and secondary complications are rare. Flu C is structurally different to Flu A and B viruses and contains a glycoprotein called HEF (hemagglutinin-esterase-fusion). Influenza viruses are mostly spread by aerosol droplets created when an infected person coughs or sneezes. Secondary bacterial infections are a significant risk for the elderly and chronically ill. Children can also experience a rare, but serious complication called Reye’s syndrome.