Avian influenza: why such an outbreak?

Duck breeding in Mugron (Landes), December 29, 2020. GAIZKA IROZ / AFP

Two hundred and sixty-four outbreaks recorded in France on January 14 (including 224 in the Landes) and 700,000 poultry slaughtered. The avian influenza epizootic continues to spread rapidly in the south-west of France. For the Minister of Agriculture, Julien Denormandie, the priority is to “Limit the spread and support breeders with social and financial support”, in his words of January 12. A down payment of up to 75% of compensation was to begin this week.

Read also: “In the countryside, anger is mounting”: in the Landes, farmers faced with the bird flu epizootic

But the questions are erupting to try to understand why the region, spearhead of the breeding of ducks intended for foie gras, is struck for the third time in a few years. In 2015-2016, the virus that had spread was of the H5N1 strain, and the ducks were healthy carriers. Only the chickens were affected. But in 2016-2017, H5N8 had already wreaked havoc on bird farms. Since the appearance of the virus, in mid-November 2020 in Corsica, the decisions taken have therefore been taking into account this previous experience.

For Gilles Salvat, director of animal health and welfare at the National Agency for Food, Environmental and Occupational Health Safety (ANSES), “The determining factor is the encounter between a highly pathogenic virus and an area densely populated with susceptible animals”. This strain of H5N8 appears to be particularly virulent among palmipeds, in which it multiplies rapidly and is excreted in large quantities. In addition, ducks develop symptoms four to five days after the onset of contagiousness, allowing the virus to spread quietly.

Under the wheels

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According to ANSES, the initial introduction into the Landes is attributable to migratory birds from northern Europe, where an exceptional mortality of wild avifauna has been observed. But once the pathogen has been introduced, it is rather human movements and environmental contamination that have facilitated its movement between farms. “We have a contamination step by step, by the environment, the exchanges of personnel and materials, the transfers of animals and the circulation of vehicles between farms”, specifies Gilles Salvat. The virus can thus nestle under the wheels of a van or in straw brought into a building.

Read also Avian flu: “The agricultural industry has created a cozy nest for pathogens”

Another factor played: the weather. “H5N8 can survive about sixty days at 4 ° C in a humid environment”, details the head of ANSES. The role of sedentary birds in the Landes remains to be clarified: the veterinary services have to date not observed an increased mortality of this avifauna in the South-West, without excluding that it is a passive vector of H5N8. For the epidemiologist Mathilde Paul, professor at the National Veterinary School of Toulouse, “The sectors have made considerable efforts since 2017 in terms of biosecurity, in particular to secure transport, but faced with an extremely contagious virus, that is not enough”.

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