Though India is one of the six countries where the Highly Pathogenic Avian Influenza A(H5N1) is considered endemic in poultry and several places in Kerala are favourite destinations for migratory birds, the State remained outbreak-free until recently. But on November 20, Kerala was robbed of that status when two outbreaks in ducks occurred in Alappuzha and Kottayam districts, with the virus killing over 20,000 birds. Incidentally, this is just the second instance of H5N1 outbreak in South India; the first outbreak occurred in October 2012 in the Central Poultry Development Organisation near Bengaluru. Most of the outbreaks since 2006 have been in West Bengal and the northeastern States, primarily due to cross-border transport of infected birds from Bangladesh, a hot-spot for H5N1 outbreaks. The H5N1 virus has infected seven people and killed one in Bangladesh between 2003 and 2013. For now, culling of nearly 260,000 birds in the villages where the outbreaks occurred, together with intensive surveillance in a 10-km radius around the epicentre of the outbreak have prevented the spread of the virus. But there is an overwhelming need to continue the intensive surveillance as ducks have been infected. Domestic ducks, which have long been recognised as one of the primary reservoirs of the virus, are responsible for the spread and outbreaks of H5N1.
According to two studies published in the journal Veterinary Research in June 2013 and November this year, unlike in the case of chicken, disease presentation in ducks depends on the H5N1 subtype and the bird species; the way the immune system responds to the virus infection in the two birds is vastly different. As a result, while most subtypes of H5N1 cause severe disease in chicken and kill nearly all of them, even clinical manifestation of infection is absent when certain species of ducks are infected with particular virus subtypes. Unlike chicken, which die, ducks not only turn out to be perfect hosts for the virus to survive but also provide an ideal environment for diversity to emerge through genetic reassortment of the virus. As of now, H5N1 infection in humans is “sporadic” and human-to-human transmission has not been reported. But a lethal reassortment of the virus can change all that. Hence, the death of thousands of diseased ducks and the prompt culling are reassuring steps. Now that the spread of the infection has been stopped, at least temporarily, concerted efforts should be directed at finding out the virus subtype and the duck species. The need to investigate if other duck species have been infected and for continued surveillance cannot be overstated, especially since duck-rearing is widespread in Alappuzha district.