While avian influenza virus crossing the species barrier and directly infecting humans happens only occasionally, human-to-human spread has been rare and not sustained. But mutations in the virus or genetic reassortment could likely result in sustained transmission between humans thus increasing the risk of a pandemic influenza.
Just three months after India declared itself to be free of the avian influenza outbreak, the highly pathogenic avian influenza subtypes H5N1 and H5N8 have been reported from a dozen epicentres in four States — Rajasthan, Madhya Pradesh, Himachal Pradesh and Kerala. In addition, thousands of poultry birds have died in Haryana, while Jharkhand and Gujarat, too, have sounded an alarm; the cause of deaths in these three States is not known.
The two avian influenza subtypes have targeted different birds in the four States — crows in Rajasthan and Madhya Pradesh, migratory birds in Himachal Pradesh, and poultry and duck in Kerala. While tests have confirmed H5N1 for the cause of death in over 2,000 migratory birds in Himachal Pradesh, H5N8 has been found to be responsible for the deaths of thousands of poultry in Kerala, and hundreds of crows in Rajasthan and Madhya Pradesh.
In a bid to stop the spread of the virus, as on Wednesday over 69,000 birds, including ducks and chickens, were culled in Alappuzha and Kottayam as per India’s 2015 National Avian Influenza Plan. Other States have been asked to be vigilant of any unusual deaths or disease outbreak signs amongst birds, particularly migratory birds.
Migratory birds are culprits
Migratory birds have been largely responsible for long distance transmission of the virus into India during winter. Once introduced, the virus spreads through local movement of residential birds and poultry. Movement of men and material from poultry farms too have in the past been responsible for further local spread of the virus.
It is for this reason that States have been asked to strengthen biosecurity of poultry farms, disinfection of affected areas and proper disposal of dead birds. With backyard rearing of poultry birds common, the spread of the virus into the community will make the task of elimination particularly difficult.
According to a December 2020 report of the European Food Safety Authority, 561 avian influenza detections were made between mid-August and early December in 15 European countries and the U.K. The virus was predominantly found in wild birds, and a few in poultry and captive birds. H5N1 and H5N8 were two of the three subtypes found in Europe. Genetic analysis helped confirm the spread of the virus Asia to west-central Europe, suggesting a “persistent circulation of this virus strain, likely in wild birds in Asia”.
H5N1 and H5N8 came from Europe
While avian influenza virus crossing the species barrier and directly infecting humans happens only occasionally, human-to-human spread has been rare and not sustained. But mutations in the virus or genetic reassortment of an avian influenza A virus and a human influenza A virus in a person can create a new influenza A virus that could likely result in sustained transmission between humans thus increasing the risk of a pandemic influenza.
It is for this reason that all efforts should be directed at quickly stamping out the outbreaks in the affected States and prevent the spread to humans. It is also important to undertake genome sequencing of virus samples to track the evolution of the virus.