Causes Of Influenza. Influenza, or in short flu, is caused by several viruses. The influenza virus can easily be transmitted from one person to another by the cough or sneeze of an infected person.
Besides, influenza virus belongs to the type of virus that is constantly changing. Owing to this reason, every year, a new influenza vaccine needs to be prepared against the specific year’s existing viral strains.
During the flu season, lasting between November and March (winter), the virus is capable of infecting millions of people simultaneously, through its contagious nature, thus finally resulting in an epidemic.
Features of the Flu Virus
The flu virus is normally round in shape, but it may also be elongated or irregular in shape sometimes, due to its changeability. The flu virus has eight segments of single-stranded Ribonucleic acid (RNA), which possesses the genetic instructions for the virus to make new copies.
A remarkable feature of the flu virus is the layer of spikes propelling from the surface. The spikes are of two different types. One type is the protein called HA (hemagglutinin) allowing the virus to adhere to a cell surface to start the infection process. The other type is the NA (neuraminidase) helping the newly produced viruses to break the host cell and exit.
Depending upon this protein composition, the flu viruses are divided into three main types namely type A, B and C. All the types are capable of infecting and causing flu. Type A is found in ducks, pigs, chickens, whales and humans, type B in humans alone and type C in dogs, pigs and humans.
Type C is responsible for mild respiratory infections and do not result in epidemics. Type A is the most dangerous and has induced global outbreaks in 1918, 1957 and in 1968.
Type A alone has subtypes and type B and C do not. Based on HA and NA on its surface, type A has been found to have different combinations of 16 HA and 9 NA. a subtype is named as A/Chicken/Hong Kong/G9/97 (H9N2), in which A is the type, chicken is the animal infected, Hong Kong is the place of discovery, G9 is the lab identification number, 97 is the time of discovery and it has 9 HA and 2 NA proteins. If it is in humans, the chicken part is omitted.
The flu virus is normally present in wild aquatic birds, from where it can get transmitted to domesticated birds. Though it does not cause infection in wild birds, it can induce an infection in the domesticated ones.
In a farm, where all animals and birds are living together, the virus can spread from the domesticated birds to pigs. Pigs are responsible for the transmission to humans. It can act as a mixing bowl. Pigs can get the infection from both human and avian flu virus.
When both the viruses affects simultaneously, the virus can use its changeability nature and produce a new strain. Likewise, it can produce a new strain and make it essential to prepare new vaccines.
There has been evidence for the virus infecting humans directly from birds in 1997 without the pig as an intermediary. Luckily, the virus was unable to spread between humans, and thus an epidemic was prevented.
The flu virus involves genetic changes ranging from small and continuous to large and abrupt. These small and continuous changes can occur in both type A and B when new viruses are produced.
Such a process is called antigenic drift. This occurs so frequently that the new viral strain cannot be recognized by our immune system. This is the main reason behind the preparation of a new vaccine.
Type A flu viruses undergo another process called antigenic shift. This is infrequent but sudden. This can occur when two infections by different flu strains occur in the same cell. This leads to exchange of genetic material.
This new variety can result in forming a new A subtype involving a different mixture of HA and NA proteins. Since people possess only a little immunity against this subtype, it results in severe flu epidemics and sometimes pandemics also.