For influenza viruses, imperfection is strength. They constantly mutate, producing new strains that challenge immune systems primed to fight earlier varieties. That’s what makes flu a life-long threat to humans and the animal species, birds mainly, that are vulnerable to it. People often think of the flu as a bad cold. But it can lead to complications such as pneumonia and worsen underlying conditions like asthma. Influenza kills as many as 500,000 people in a normal year. A virulent swine flu or the increasingly common avian variety can devastate farms, raising egg and meat prices. Such an outbreak also increases the odds of a flu virus emerging that people can easily catch and to which they have little or no immunity. In that case, a pandemic can occur, putting millions of lives at risk.
A bird flu epidemic swept across Asia and Europe beginning in late 2016. In March, a strain of the disease was detected at a poultry farm in Tennessee that supplies Tyson Foods Inc., marking the first case at a U.S. commercial operation in 2017. In 2016, serious outbreaks of bird flu occurred in, among other places, Nigeria, Japan and France, where cases at duck farms led to halts in foie gras production. Bird flu originating in China in late 2014 spread to dozens of countries. It resulted in 2015 in the worst animal disease outbreak in the history of the U.S., the world’s second largest chicken exporter. It killed 48 million birds, diminished poultry-product exports and cost the federal government almost $1 billion, mostly to dispose of infected birds and compensate farmers. The flu strains responsible for the U.S. outbreaks are not known to have sickened humans. However, a strain called H5N1 has killed 452 people around the world, mostly in Indonesia and Egypt, since 2003. Another variant, H7N9, has caused repeated epidemics in China, killing about 40 percent of humans infected, including 79 people in January.
Horses, ferrets, dogs and even sea otters are susceptible to flu, but birds and pigs are the main worry for humans. The possibility of a pandemic arises when flu is passed from a wild bird -- migratory waterfowl and seabirds are the main sources -- to a human, usually via a domesticated bird or pig. Sometimes the domesticated animal is also infected by a human flu strain, producing a mutant mix like the swine flu that killed an estimated 284,000 people in 2009. People have no immunity to new strains and existing vaccines don’t protect against them, so they spread easily. Flu pandemics have occurred four times in the last 100 years. In 1918, the most devastating of them killed as many as 50 million people. Among humans, flu is transmitted mainly via tiny droplets that the ill emit when they cough, sneeze or talk, although airborne transmission is thought to be possible.
Public-health experts advocate vaccination as the best protection against flu. However, its efficacy varies widely depending on the closeness of the match between that season’s viruses and the vaccine, which is usually reformulated each year. Another factor is the age and health of the person immunized. Studies in the U.S. suggest average vaccine effectiveness each season has varied from 10 percent to 60 percent over the past decade. Of the two types of vaccines available in the U.S., there is little evidence that one protects the elderly and a lack of evidence that the other protects people aged eight to 59, according to a review of such studies by researchers at the Center for Infectious Disease Research and Policy. The researchers recommended greater government support for development of a so-called universal flu vaccine, which would protect against all strains. A number of experimental vaccines with that potential are being tested in trials. Meanwhile highly virulent bird flu, which was relatively rare until 1997, causes about 100 times more bird deaths than it did in the 1950s. That raises questions about a link to modern farming methods. Global meat production more than doubled between 1980 and 2014, making animal protein available to more of the world’s poor. The density of animals in modern livestock facilities, however, leaves them vulnerable to mass casualties in the event of disease outbreaks.
The Reference Shelf
- The U.S. Department of Agriculture mapped the movement of H5 bird flu viruses in 2014 and 2015.
- The U.S. Geological Survey chronicles avian flu outbreaks.
- The Food and Agriculture Organization answers frequently asked questions on avian influenza and gives biosecurity and risk-management recommendations.
- A report by the Center for Infectious Disease Research and Policy recommended development of game-changing human flu vaccines.
First published Dec. 2, 2013
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