How Much of a Threat Is Bird Influenza?

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.

Influenza virus

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 15 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.

Bird 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. The CDC is expecting the virus to appear in wild migratory Arctic birds in Alaska. From there it will spread south throughout the United States. Even if humans aren’t involved, infection may devastate the poultry industry in this country.

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 there have been more reported cases. H5N1 causes severe illness in both birds and humans, and currently, about half of people who get this flu die.

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. The CDC and WHO have reported human cases of H5N1 in several countries: Thailand, Vietnam, Indonesia, and Azerbaijan. 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.

Planning for the next pandemic

The CDC and flu researchers around the world believe it is only 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. Vaccine manufacturers are working on a new vaccine that they hope will protect against the H5N1 strain. Getting a regular flu vaccine may provide some slight protection against bird flu.

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 urgently recommended increased monitoring.

The larger plan

Epidemiologists and flu experts believe that a pandemic can be prevented. Two separate studies concluded that a pandemic could be prevented, but they differed on the degree of intervention needed. Both agree that, should an outbreak occur, health care workers would converge on the outbreak location and begin distributing antiviral drugs (Tamiflu). Neither study takes into account immunization, which may or may not be effective. Both studies suggest that a very rapidly spreading infection might be unstoppable. 

The future

Nature is unpredictable. We have learned from history that predictions don’t always pan out. 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-Barré syndrome).

The future of bird flu is just as unpredictable. Everyone involved in flu research, epidemiology and medical care believes it is better to attempt to be prepared than to do nothing.