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30 - 01 - 2017

Human flu, animal influenza: what links?

As the epidemic of seasonal influenza rages on in the hexagon, poultry farms in the southwest are confronted to avian flu. Is there a link between human and animal influenza? What differentiates them? Should we fear transmission from animals to humans? Here is the beginning of an answer. 

 Wednesday 18th of January, the surveillance network Sentinelles-INSERM (1) announced that more than one million French people had seen a doctor for influenza symptoms since the beginning of the epidemic of seasonal influenza that started about 5 weeks before. At the same time, an avian influenza virus has been detected in wild birds and farmed poultry, especially in South-Western France. As a precautionary principle, a “preventive slaughter” was organized in 337 communes (2). A case of low pathogenic avian flu has also been recorded in the Val d‘Oise, Ile-de-France (3). The World Health Organization (WHO) has asked its member states to monitor the evolution of avian influenza and report all cases of human forms of the disease (4). Here are a few key ideas to better understand the different influenza viruses and their potential links.    The flu, what is it?  Influenza is an infectious disease caused by various viruses of the family Orthomyxoviridae which attack mainly the upper respiratory tract (nose, throat, bronchi) and more rarely the lungs. Viruses that cause the epidemics of seasonal flu in humans belong to the groups A and B of this family of viruses.  They make up the vaccines. The viruses of group A are divided into subtypes according to the proteins present on their surface: hemagglutinin (H1 to H15) and neuraminidase (N1 to N9).Influenza also occurs in animals, particularly in poultry, pigs and horses. It may affect other animal species, including mammals, but it is generally asymptomatic.  Influenza in birds Avian influenza or bird flu, is caused by Influenza A viruses, including H5, H7 and H9 subtypes. Birds are considered as hosts or reservoirs of these viruses, that is to say that they host these viruses almost permanently. Generally asymptomatic in wild birds, bird flu can cause severe symptoms or even death in farmed birds if the virus is highly pathogenic. It spreads in the digestive tract (and is found in droppings) and in the respiratory tract. In the most severe cases, the virus invades many other organs and tissues, and can cause significant internal bleeding, leading to a very high mortality rate. Hence the massive preventive slaughtering of livestock.A different virus subtypes can circulate in different species of birds and come into contact with each other, it happens that a genetic evolution of the virus occurs. This is called a reassortment. This occurred in China in March 2013, when the duck’s H7N3 virus was brought into contact with the H7N9 virus of a wild bird and the H9N2 virus of domestic hens which resulted in the formation of a new H7N9 virus which raised fears of a pandemic.    Pigs are at the mercy of avian and human influenza Pigs are mainly affected by H1N1 and H3N2 subtypes of A viruses. However, swine are a particularly vulnerable species as they can be affected by both avian and human influenzas. In addition, recombinant viruses are sometimes created when pigs are in the presence of avian and human viruses.   Transmission to man: rare but not impossible Overall, the authorities and the scientific community agree that the transmission of avian or swine influenza from animals to humans remains rare and occurs especially when there is significant direct contact with affected animals (dead or alive) or with their droppings. Breeders or sellers of pigs or poultry are therefore the main affected. At present, there is no evidence of effective human-to-human transmission of these viruses unless there is close and prolonged contact with a patient.On the other hand, different recombinants of influenza viruses are feared by health authorities, as they could lead to the creation of a contagious influenza virus that could be transmitted between men. This was particularly the case with the so-called Spanish flu episode in 1918. Due to a very virulent strain of an A virus subtype H1N1, this epidemic resulted in the deaths of at least 20 million people worldwide. To prevent this from happening again, the fight against influenza in animals is paramount. Continuous monitoring of human and animal populations is essential to know more about the viruses, to measure the risk of an epidemic and act accordingly.   Hélène Bour Sources :(1)