[Explainer] Understanding the science behind avian influenza

  • The cases of avian influenza or bird flu reported from some states in India have been causing a health concern.
  • Influenza viruses are a broad category of viruses under the Orthomyxoviridae family, capable of causing flu. There are four types of influenza viruses: A, B, C, and D.
  • An epidemic or a pandemic arises when a new strain of the virus emerges. Since the population has no immunity against the new strain, it spreads quickly and widely.

Since the past few weeks, avian influenza (bird flu) cases have been reported from several states of India. Several thousands of ducks and chickens have been culled to contain the situation in Kerala, Gujarat, Haryana, and Bihar. There is rising panic among the public regarding the transmission of the flu to humans.

Awareness about bird flu will enable people to take necessary measures to contain it which in turn will help the cumulative efforts by the public health departments. Often, by the time people recognise that the spread of disease is turning into a pandemic or epidemic, it may be too late to call for action. However, if people are familiar with the science behind diseases and their spread, they can identify the severity sooner making things much easier for the concerned departments.

What are influenza viruses?

Influenza viruses are a broad category of viruses under the Orthomyxoviridae family, capable of causing flu, which is a contagious respiratory illness infecting the nose, throat, and lungs. There are four types of such viruses: A, B, C, and D. Of these, Influenza A takes the most severe form in humans. It spreads by zoonotic transfer i.e., from animals to humans. Influenza B on the other hand spreads only from human to human and is mostly characterised by mild infections. Influenza C causes mild infections and influenza D is present only in animals especially cattle. Influenza A and B are responsible for the flu season in India (October- March).

Influenza A virus has several subtypes based on the proteins in the outer envelope of the virus, i.e., Hemagglutinin (H) and Neuraminidase (N). This is where the H and N in the name of Influenza virus come from. Again, there are 18 different types of H and 11 different types of N – from H1 to H18 and from N1 to N11.  The various permutations and combinations give 198 different subtypes.  However, only 131 such combinations have been detected so far in nature.

Read more: [Interview] Taking One Health approach to tackle zoonoses crucial for India

How does an epidemic or a pandemic arise?

An epidemic or a pandemic arises whenever a new strain of the virus emerges. Since the population has no immunity against the new strain, it spreads quickly and widely. There are two reasons for the influenza viruses to cause an epidemic: antigenic shift and genetic reassortment. Viruses multiply by entering the host body through the epithelial cell and then release their viral genetic material (RNA/ DNA) into the host cytoplasm.

So when two different strains of viruses enter the same host, chances are high that their viral RNA might get intermixed resulting in a new strain of virus having surface antigens of both the strains thus capable of infecting both the hosts. Since the hosts do not have immunity against the new strain it spreads rapidly resulting in a new epidemic if it is confined with a community or a country and a pandemic if it spreads across countries. This is what happened in all the outbreaks of avian influenza.

Though the probability of human to human transmission of avian influenza is low, chances cannot be entirely ruled out. Major modes of transmission of avian influenza are direct, through inhalation of infectious droplets or indirect, through fomites. Once exposed to the virus, initial symptoms come in the form of high fever (more than 38 degrees Celsius).

Advanced symptoms include vomiting, diarrhoea, abdominal pain, pneumonia, etc. Initial symptoms appear 4-5 days following exposure. Children, adults of the age 65+, people with chronic health conditions like asthma, diabetes, kidney disorders, etc. are more vulnerable to this disease, says a 2005 study.

The recent outbreak of H5N8 started from migratory birds, especially the bar-headed geese, an aquatic duck. Several thousands of ducks were found dead near water bodies, which is when the issue was brought to the attention of the animal husbandry officials. Ducks are the natural hosts of this virus. These viruses are highly pathogenic to poultry, which is why rapid culling of chickens and ducks is done.

To date, 40,000 birds are culled in Kerala alone. Standard precautions suggested by WHO are to follow hygienic cooking practices and to properly cook poultry since cooking temperatures are sufficient enough to kill the Influenza A (H5N8) virus.


Chotpitayasunondh T., Ungchusak K., Hanshaoworakul W., Chunsuthiwat S., Sawanpanyalert P., Kijphati R., … & Dowell S.F. (2005). Human disease from influenza A (H5N1), Thailand, 2004. Emerging infectious diseases, 11(2), 201.

Hui, D.S.C. (2008). Review of clinical symptoms and spectrum in humans with influenza A/H5N1infection. Respirology, 13,S10-S13.

Writing Committee of the World Health Organization (WHO) Consultation on Human Influenza A/H5. (2005). Avian influenza A (H5N1) infection in humans. New England Journal of Medicine, 353(13), 1374-1385.


Banner image: Several birds have been culled in India during the present outbreak of avian influenza. Photo by Laura Gil Martinez (IAEA)/Flickr.

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