Avian flu is making the news again with new human cases in China reported in January. What does "avian flu" mean to you—and how dangerous is it?
Each year you, or someone you know, probably gets a flu shot. The seasonal flu shot changes each year to reflect circulating influenza viruses. This year, the flu shot protects against two influenza A viruses, and one influenza B virus. Influenza is a respiratory virus that causes fatigue, fever, aches, headache, cold-type symptoms, and more.
In addition to humans, influenza A viruses also afflict dogs, pigs, seals, horses, and even whales, but doesn't always sicken its host. Wild waterfowl also get influenza A, or avian influenza, and are primary reservoirs for the frequently mutating virus, taking it with them along migratory pathways and into bird markets.
The wet markets of China are marketplaces crowded with animals and humans together in conditions ripe for microbial transfer. These markets are a dangerous mixing pot for seasonal, human-adapted influenzas and wild ones to which humans aren't able to fight off.
Many avian flus come from conditions like this and can be mild or dangerous, depending on how well they spread between humans and how sick they make us. For wild birds and wet markets, there is no vaccine and no way to stop of spread of infection between birds, other than killing them en masse.
Influenza is a complicated virus and keeping its strains, or names, straight can be confusing. Different strains of the virus have slightly different genetic code and can infect different hosts, in different ways, with varying degrees of danger. The January 2017 alert from the Centers for Disease Control and Prevention is specifically an avian strain of influenza A called H7N9, which has low pathogenicity and can infect both birds and humans.
Most influenza virus strains include H and N numbers. You might also see something like HPAI or LPAI. These terms simply mean "high pathogenic avian influenza" and "low pathogenic avian influenza," and refer to the capability of the virus strain to cause disease. A highly pathogenic strain of influenza results in higher infection rates.
For example, the influenza epidemic of 1918 was caused by a strain of influenza A, type H1N1. By the time the Spanish flu subsided, one-third of the population of the planet had been infected, and between 50 and 100 million people had died. Later in the century, researchers used modern tools and data analysis to discover the virulent flu strain had mutated in pigs, and then transformed to infect humans.
The Spanish flu pandemic is called the "mother of all pandemics," for its modern-day severity, and because its genetic material drifted forward, becoming incorporated in avian and other flu strains present today.
Because of some probable remaining immunity in humans to the H1N1 strain that caused the 1918 pandemic, the CDC does not believe the same flu strain will cause another pandemic. The danger exists from emerging influenza A types that borrow the ability to move from animal, like a pig or bird, to human.
We saw this back in 1996 when the highly pathogenic avian flu strain H5N1 was isolated in geese farmed in southern China. Given the tight confines of Chinese wet markets, the strain successfully jumped to humans, with approximately 900 reported infections and more than 400 deaths.
Spreading across Asia, Europe, and Africa, these highly pathogenic avian flu strains gained a wealth of genetic diversity, making it easier for influenza A to adapt to new conditions—and new hosts.
In wet markets, wild bush animals and birds are packed side by side with domestic poultry, reptiles, fish in buckets, caged rats, bagged, living cats, dogs intended for cooking, and more. These markets are believed to play a part in the transfer of avian influenza strains to humans through crowded conditions, accumulation of animal waste, and chronic contamination of cages and other equipment. Consumers crowd into market stalls packed and stacked with these live animals in cages.
A 2014 study of wet markets found "that poultry workers and the general population are constantly exposed to H7N9 virus at these markets." Of the ongoing outbreaks and mutations of avian influenza strains in China, the World Health Organization notes:
[I]t can be assumed that interspecies transmission of influenza A viruses occurs more frequently than we think, mainly from birds to mammalian species. Although the outbreaks in poultry have weakened economies and jeopardized food security, the greatest concern for human health is the risk that present conditions could give rise to an influenza pandemic."
In August 2016, China reported an outbreak of highly pathogenic avian influenza called H5N6. By January 2017, Chinese health authorities reported that another 110 people are confirmed with infections from influenza A type H7N9 from an outbreak that began in September 2016. As expected, many of these were individuals who worked with, or had visited, wet markets.
Those numbers are just the top of the stack. Since 2013, more than 900 confirmed cases of H7N9 have been reported in China, with a mortality rate of about a third of those infected. A few cases of human-to-human infection, without transmission through a bird market, have occurred, but they remain rare—at present.
In southern China, it is not unusual for humans to share their space with their food animals, mixing microbes, breathing the same air, swiping waste from a sleeve, or sorting diseased and dead animals from the living. With influenza A strains mixing and mutating in these conditions right now, it is only a matter of time before H7N9, or other virulent flu type, figures out how to share itself by a sneeze or cough and perhaps causing the next global pandemic.