Flu, the short form of Influenza (the virus that causes the flu), is one of the most common infections of the respiratory tract (nose, throat, and lungs). There are different types of flu, and some variants like swine flu and bird flu have been featured prominently in the media due to their sudden outbreaks.
The symptoms of flu include cough, sore throat, and fever, and the condition can spread easily from one person to another through air. The severity of the disease may range from mild to severe, and the illness sometimes might even lead to the death of the individual.
The Different Types Of Flu – How It Is Classified
Flu, swine flu, bird flu, or seasonal flu are the variations of this disease that you are sure to be familiar with. The Influenza virus can be classified based on the nucleic acid (RNA) and protein (on its outer coat) composition as Type A, B, and C.
Influenza has two protein spikes haemagglutinin (HA) and neuraminidase (NA) in its outer coat and these form the basis for its further classification (into subtypes) as H1N1 or H3N2 etc.
Given below are the three common types of influenza, and how they can distinguished from one another.
Influenza Type A Virus
Type A influenza viruses are the most virulent (extremely infectious) among the three and cause severe disease. Wild aquatic birds are the natural hosts. The virus can also be found in humans, ducks, chickens, pigs, and whales. It causes global outbreaks and was responsible for the 1918, 1957, 1968, and 2009 pandemics (outbreak of global proportions).
Type A viruses have 16 HA (H1-H16) and 9 NA (N1–N9) subtypes that can occur in different combinations and evolve into new strains. For example, the influenza virus that caused the recent 2009 pandemic was H1N1 (Swine Flu). H stands for HA and N for NA and H1N1 is a subtype of the Influenza Type A virus.
Other examples of Type A Influenza viruses are:
• H1N1–1918 1918 (Spanish Flu)
• H2N2 – 1957 (Asian Flu)
• H3N2 – 1968 (Hong Kong Flu)
• H5N1 – 2004 (Bird Flu)
H7N7, H1N2, H9N2, H7N2, H7N3, H10N7 are other Type A viruses. Only H1, H2, H3, N1, and N2 combinations have infected humans till date, but studies have shown that even H7, H9, and H5 combinations can affect humans.
All possible combinations of surface proteins can infect birds. When a subtype that causes an outbreak in birds emerges, it can colloquially obtain the name Bird Flu. Similarly, the types of flu that target swine are commonly christened swine flu.
Influenza type A viruses are unique because they can change their structure very rapidly (high rate of mutation) and can emerge into new viral strains (same but slightly different organisms), bypassing the immune system’s defence mechanism and causing devastating pandemics.
In short, humans are not entirely protected against the new emerging subtypes of Influenza A viruses, and the chances of acquiring an infection are still high.
Influenza Type B Virus
Type B viruses are mostly found in humans, but are less common than Type A viruses. These viruses can also infect seal and ferrets.
Among the types of flu, type B viruses cause local epidemics but not pandemics. They also don’t have subtypes like type A viruses. Although these viruses can sometimes cause severe illness, their mutation rate is slow and for this reason, immunity against this type is usually acquired in early life itself. However, there is no long lasting protection against such infections.
Influenza Type C Virus
Influenza Type C viruses are found in humans, pigs, and dogs. These viruses are less common and cause mild cold-like illness.
Structural Differences Between The Types Of Flu
Basically, the three types of flu have almost similar structures – so much so that it’s hard to distinguish between type A and B viruses even with an electron microscope. Some notable differences between the types have been given below.
Type A: Has three membrane proteins, namely HA, NA, and M2, a matrix protein M1, ribonucleoprotein core (with PA, PB1 and PB2), and NEP/ NS2 proteins.
Type B: Has four envelope proteins HA, NA, NB, and BM2.
Type C: Has HEF protein (haemagglutinin-esterase-fusion protein), which has the functions of HA and NA. It has CM2 as the viral envelop protein.
Ribonucleic Acid (RNA)
Type A: RNA has 8 segments
Type B: RNA has 8 segments
Type C: RNA has 7 segments
People often get confused and use the term “flu” for any respiratory illness that is caused by a virus. By learning about the types of flu, you’ll find it easier to identify the type of infection and choose the relevant treatment.