Influenza, with its fever, aches, fatigue and threat of complications, seems a uniquely human illness. But the flu, caused by a virus, can infect animals and birds, as well. The good news is that many strains of the flu virus don't cross over to humans. The sobering news is that the few strains that do make the crossover tend to be virulent, with the potential to cause widespread illness.
That's what happened in 1918, when the "Spanish flu" swept around the world, killing at least 40 million people. That was the worst death toll of any cause in history. And it was caused by a virulent strain of the flu virus that infected both birds and humans.
Nearly 30 years later, in 1946, Thomas Francis and Jonas Salk (who later developed the polio vaccine) produced the first vaccine against the flu. Despite the vaccine's availability, influenza pandemics occurred again in 1957-58 (Asian flu), 1968-69 (Hong Kong flu) and 1977 (Russian flu). Many people worldwide died in these pandemics, but none of these outbreaks matched the Spanish flu in deadliness.
Today, researchers have a better understanding of influenza viruses. They know there are three influenza virus types: A, B and C. Type A can infect humans, birds, pigs, and other animals and is responsible for the most severe illness. Type B appears to cause disease only in humans and the infection is milder than type A. Type C seldom causes illness in humans, animals, or birds.
Type A virus is further divided into "subtypes." These subtypes are designated as H for the protein hemagglutinin and N for the protein neuraminidase; these proteins are found on the surface of the flu virus. There are 16 different hemagglutinins and nine neuraminidases, and are designated by H(number)N(number).
Researchers have been able to isolate the Spanish flu virus subtype from people who died of the flu and were buried in the permafrost in Alaska. The Spanish flu subtype was H1N1. (The Asian flu was H2N2, the Hong Kong flu was H3N2 and the Russian flu was H1N1.) The bird flu that now threatens the world is designated as type A, H5N1. This designation is what news stories refer to when discussing the flu.
This subtype —H5N1—was first discovered in birds in China in 1986. It wasn't until 2003, however, that it began spreading rapidly through the bird population there. Because many wild birds migrate, scientists fear that this disease will spread worldwide in the bird population.
Wild birds are the natural host for all subtypes of influenza type A virus. Typically, wild birds do not get sick when they are infected with influenza virus. Domestic poultry, such as turkeys and chickens, can get very sick and die from avian influenza, and some avian viruses also can cause serious disease and death in wild birds. Infected birds can pass on the virus through their saliva, nasal secretions, and feces. In an agricultural setting, animal manure containing the influenza virus can contaminate dust and soil, causing infection when the contaminated dust is inhaled. Contaminated farm equipment, feed, cages, or shoes can carry the virus from farm to farm. The virus can also be carried on the bodies and feet of animals, such as rodents.
Bird flu viruses do not usually infect humans, but some of them do. In 1997, H5N1 infected people in Hong Kong who had close contact with poultry, and six people died. Since that time, there have been more reported cases. H5N1 causes severe illness in both birds and humans, and about 60 percent of the people who have gotten this form of flu have died. Since 2003, 371 people worldwide infected with H5N1 have died.
At this point, people appear to be infected by close contact with birds and poultry. In only a few cases has this flu virus been passed from one person to another, and that was through very close contact. Researchers at the CDC and the World Health Organization (WHO) are worried that the virus may change just enough to allow for easy transmission from human to human. In October 2005, WHO reported human cases of H5N1 in four countries: Cambodia, Indonesia, Thailand, and Vietnam. In June 2005, WHO reported evidence of human-to-human spread in Indonesia. In this situation, eight people in one family were infected. The first family member is thought to have become ill through contact with infected poultry. This person then infected six family members. One of those six people (a child) then infected another family member (his father). No further spread outside of the exposed family was indentified. Although intensive treatment can reduce the death rate, even if the death rate drops to 5 percent, hundreds of millions of people worldwide could die in a bird flu pandemic.
A new strain—H7N9—has been reported in more than 130 people in China, according to the WHO in 2013. About 20 percent of people infected have died. But there have been no reported cases of H7N9 in the U.S., and there is a low risk of being infected by H7N9 at this time, according to the CDC.
The CDC and flu researchers around the world believe it may only be a matter of time until the virus changes enough to allow for easy human-to-human spread. That change may increase or decrease the virulence of the virus, something no one can predict. When this happens, the pandemic will begin, but epidemiological studies suggest that a pandemic can be prevented.
First line of defense: a flu vaccination. In April 2007, the U.S. Food and Drug Administration (FDA) announced its approval of the first vaccine to prevent human infection with one strain of avian influenza (bird flu) H5N1 virus. The vaccine has been purchased by the federal government for the U.S. Strategic National Stockpile; it will be distributed by public health officials if needed. This vaccine will not be made commercially available to the general public. Other H5N1 vaccines are being developed by other companies against different H5N1 strains.
Second line of defense: the antiviral drug Oseltamivir (Tamiflu). Oseltamivir appears to reduce the severity of the flu if taken within 24 to 48 hours of symptoms. Hope was high for this drug until viral resistance to the drug was discovered in northern Vietnam. Viral resistance may be spreading, and WHO has recommended increased monitoring.
Nature is unpredictable. We have learned from history that predictions don’t always work. In 1976, the CDC predicted the swine flu pandemic. A new vaccine was rapidly made available. The pandemic never appeared, but some people who got the vaccine developed a devastating neurological reaction (Guillain-Barre syndrome).
The future of bird flu is just as unpredictable. Everyone involved in flu research, epidemiology and medical care believes it is better to be prepared than to do nothing.