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.
Bird flu originating in China in late 2014 spread to dozens of countries. It resulted in 2015 in the worst animal disease outbreak in U.S. history, killing 48 million birds, diminishing poultry-product exports and costing the federal government almost $1 billion, mostly to dispose of infected birds and compensate farmers. In 2016, serious outbreaks of bird flu have occurred in, among other places, Nigeria, Japan and France, where cases at duck farms led to halts in foie gras production. The flu strains responsible for the U.S. outbreak are not known to have sickened humans. However, a strain called H5N1 has resulted since 2003 in 856 reported cases in humans and 452 deaths.
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 maps the movement of H5 bird flu viruses since 2014 and the U.S. Geological Survey chronicles major H5N8 events.
- 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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