In the last months of 2020 and early 2021 we have suffered an unusually low incidence of seasonal flu due to social distancing and prevention measures against covid-19. Interestingly, the opposite has happened with bird flu, which has been causing serious outbreaks in birds almost all over the planet for months.
Avian influenza is one of the infectious diseases that most severely affect poultry farming, as it causes high mortality and enormous economic losses. Furthermore, outbreaks caused by this virus have serious repercussions on food safety. In many countries, poultry meat and eggs are the main source of protein, and their scarcity can lead to malnutrition of the population.
To all this must be added the risk that this animal disease represents for public health, since some strains of the virus are zoonotic.
It is important to note that there are two types of avian flu strains (or “patotypes”): those with low pathogenicity and those with high pathogenicity. The latter produce a lethal infection in a high percentage of infected birds. With rare exceptions, only two subtypes (H5 and H7) are capable of generating highly pathogenic strains for birds. Some of them also have zoonotic potential, but so far only two (H5N1 and H7N9) have caused serious infections in humans.
From wild to domestic birds
The natural reservoir of the virus is made up of wild birds, mainly those related to aquatic environments such as ducks, geese, swans and seagulls. As a general rule, influenza viruses circulate in these birds without causing any illness.
However, when one of these viruses of subtypes H5 or H7 is introduced into a poultry farm, it undergoes a process of adaptation to domestic species. Then a series of mutations are produced that can transform a low pathogenic strain into a highly pathogenic one.
This is what has been happening in recent months in many European countries and also in Africa, Asia and Australia. An H5N8 strain has wreaked havoc in the poultry industry and killed hundreds of thousands of poultry, either as a direct effect of infection or as a control measure to contain the spread of the disease.
Interestingly, this strain has also seriously affected some species of wild birds. This is exceptional: as I mentioned earlier, the infection rarely causes clinical symptoms in wild birds. In fact, only on two other occasions had bird flu had a serious impact on wildlife : in the early 2000s with the famous zoonotic H5N1 flu and in 2016-2017 with another virus of the same subtype as the current one that caused almost a thousand outbreaks in wild birds in Europe.
Since 2017, hardly any new cases have been reported, but in 2020 there was a new wave. The first were reported in August in Russia and since then, continuous outbreaks have occurred in Europe, affecting 25 countries, including Spain, which has declared three outbreaks in wild birds in Cantabria, Zamora and Gerona.
The main route of entry of the virus into a poultry farm is direct or indirect contact (through contaminated water) with infected wild birds. In fact, on many occasions a very clear overlap can be established between the migratory routes of the birds and the appearance of the cases on the farms. For this reason, the veterinary authorities have placed so much emphasis these months on the obligation to keep domestic birds in closed spaces to avoid contact with wildlife.
Once the virus is introduced into a farm, transmission between birds is very fast and it is practically impossible to stop it, so increasing biosecurity measures is one of the main keys to prevention.
First cases in humans
Since this subtype of bird flu was first detected in 2014, there have never been cases of infection in people. This despite the fact that there has been a high exposure, especially in professionals related to the management and control of outbreaks in birds such as farm and slaughterhouse workers and veterinarians.
However, in February 2021, Russia reported the first cases of human infection with the H5N8 strain. These are seven workers from a large poultry farm (with 900,000 birds) that suffered a serious outbreak of the disease. None of them developed symptoms and person-to-person transmission has not occurred.
The ECDC recently conducted a study to evaluate the risk that this strain poses to human health and concluded that the zoonotic risk is very low for the general population and low for professionals in the poultry sector. Even so, this finding has raised alarms because it once again demonstrates the enormous adaptability of avian flu viruses and the constant threat they pose to human health.
Many virologists were surprised when a coronavirus was shown to be behind the current health crisis, as all bets were on a flu virus. Despite covid-19, the threat of influenza is still present and these increasingly frequent and virulent outbreaks in birds confirm this. The interdisciplinary study of zoonotic viruses at the interface between wildlife, domestic and humans must be an absolute priority if we are to be prepared for the next pandemic.
Elisa Pérez Ramírez, Veterinary Virologist at the Animal Health Research Center (CISA), National Institute for Agricultural and Food Research and Technology (INIA)
This article was originally published on The Conversation. Read the original.
