The global spread of bird flu and the number of viral strains currently circulating and causing infections have reached unprecedented levels, raising the risk of a potential human outbreak, according to disease experts.
Multiple outbreaks have been reported in poultry farms and wild flocks across Europe, Africa, and Asia in the past three months. While most involve strains that are low risk for human health, the sheer number of different types, and their presence in so many parts of the world at the same time, increases the risk of viruses mixing and mutating — and possibly jumping to people.
“This is a fundamental change in the natural history of influenza viruses,” said Michael Osterholm, an infectious disease specialist at University of Minnesota, of the proliferation of bird flu in terms of geography and strains — a situation he described as unprecedented.
Global health officials are worried another strain could make a jump into humans, like H5N1 did in the late 1990s. It has since caused hundreds of human infections and deaths, but has not acquired the ability to transmit easily from person to person. The greatest fear is that a deadly strain of avian flu could then mutate into a pandemic form that can be passed easily between people — something not as yet seen.
While avian flu has been a prominent public health issue since the 1990s, ongoing outbreaks have never been so widely spread around the world — something infectious disease experts put down to greater resilience of strains currently circulating, rather than improved detection or reporting. While there would normally be around two or three bird flu strains recorded in birds at any one time, now there are at least half a dozen, including H5N1, H5N2, H5N8, and H7N8.
The Organisation for Animal Health says the recent outbreaks in birds are “a global public health concern”, and the World Health Organisation’s director-general warned this week the world “cannot afford to miss the early signals” of a possible human flu pandemic.
The precise reasons for the unusually large number and sustained nature of bird outbreaks in recent months, and the proliferation of strains, is unclear — although such developments compound the global spreading process.
Bird flu is usually spread through flocks by direct contact with an infected bird. However, Dr Osterholm said wild birds may be “shedding” more of the virus in droppings and other secretions, increasing infection risks. He said there now appears to be “aerosol transmission from one infected barn to others, in some cases many miles away”.
Ian MacKay, a virologist at Australia’s University of Queensland, said that the current rise of strains means that by definition, there is an increased risk to humans.
“You’ve got more exposures, to more farmers, more often, and in greater numbers, in more parts of the world — so there has to be an increased risk of spillover human cases,” said Dr McKay.
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