By Bonny McClain
Whether the topic is seasonal influenza, bird flu or something called a pandemic, everyone seems to be searching for answers about how to avoid them all. One of your best defenses is to understand them.
The cooler evenings, changing of the leaves and return to school signify that autumn is upon us. They also mean that more people are thinking about the coming flu season. It is time to gear up for seasonal influenza and to make sure you understand all the talk about bird (avian) flu and pandemics.
The flu season in the United States typically ranges from November through March, but it is never too early to be prepared.
Influenza, or the flu as it is often called, is a contagious disease caused by the influenza virus. Influenza attacks the respiratory tract in humans (nose, throat and lungs), has the potential to cause mild to severe illness and at times can lead to death. The flu is different from a cold, and typically comes on suddenly and may include the following symptoms:
- fever (usually high)
- extreme tiredness
- dry cough
- sore throat
- runny or stuffy nose
- muscle aches
- stomach symptoms, such as nausea, vomiting and diarrhea, also possible; more common in children than adults
According to the Centers for Disease Control and Prevention (CDC), influenza accounts for approximately 36,000 deaths and more than 200,000 hospitalizations every year in the United States. It is among America's most lethal killers simply because the virus infects so many people—some 5 to 20 percent of the U.S. population every year. (For information on new influenza vaccines, see page 5 in this issue.)
There are three types of influenza viruses with the ability to infect humans. They are designated A, B and C. Only influenza types A and B cause epidemics of illness that typically occur every winter. Influenza type C usually does not cause symptoms or may cause a very mild respiratory illness. This type does not have the severe public health impact of types A and B. Efforts to control the impact of influenza are directed at types A and B.
Currently, different strains circulate globally: three type A viruses and one type B. Type A viruses are divided into subtypes based on differences in two viral proteins called hemagglutinin (H) and neuraminidase (N). The current subtypes of influenza A are designated A(H2N1) and A(H3N2).
Influenza type A viruses undergo two kinds of changes. One is a series of mutations that occur over time and cause a gradual evolution of the virus. This is called antigenic "drift." The other kind of change is an abrupt change in the hemagglutinin and/or the neuraminidase proteins. This is called antigenic "shift." In this case, a new subtype of the virus emerges suddenly. Type A viruses undergo both kinds of changes; influenza type B viruses change only by the more gradual process of antigenic drift.
This constant changing of the virus allows it to evade the immune system of its host, so that people are susceptible to influenza virus infection throughout their lifetime. For example, a person infected with influenza virus develops antibodies against that virus. An antibody is a protein produced by the body's immune system. The protein recognizes and helps fight infections, such as the influenza virus and other foreign substances in the body. As the virus changes, the "older" antibody no longer recognizes the "newer" virus, and infection can occur again. The older antibody can, however, provide partial protection against the virus and may result in a milder infection.
Influenza viruses can also emerge that have the ability to infect species other than their natural reservoirs, such as migratory birds. These viruses can infect domestic poultry or pigs, and rarely, humans. When animal viruses develop the ability to infect humans, these infections usually are not easily spread into the human population. However, the virus has the ability to mutate in a manner that may allow humanto- human transmission to occur more easily. The concern is that an animal and human influenza may infect a person or animal simultaneously, allowing the genes between the viruses to be exchanged. This would potentially yield a virus that would be readily transmitted between humans, but the human population would have no immunity. This example of antigenic shift is the likely source of a global influenza pandemic.
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- For a slideshow with sound and pictures: Click on Interactive Tutorials at upper right; on the next screen, under Diseases and Conditions, click on Influenza.
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What Is Bird Flu?
The National Institutes of Health (NIH) has increased influenza research at least five-fold, from $20.6 million in 2001 to approximately $119 million in 2005. Research efforts have discovered three known "A" subtypes of influenza viruses (H1N1, H1N2 and H3N2) currently circulating among humans. It is likely that some genetic component or components of current human influenza A viruses originally came from birds. The risk from avian influenza is generally low for the majority of people because the viruses do not usually infect humans. H5N1 is one of the few avian influenza viruses to have crossed the species barrier to infect humans, and it is the most deadly of those that have crossed the barrier.
Most cases of H5N1 influenza infection in humans have resulted from contact with infected poultry (e.g., domesticated chicken, ducks and turkeys) or surfaces contaminated with secretion/excretions from infected birds. Currently, the spread of H5N1 virus from person to person has been limited and has not continued beyond one person. Because all influenza viruses have the ability to change, scientists are concerned that H5N1 virus one day could be able to infect humans and spread easily from one person to another.
"The avian flu interacts in a way that human influenzas do not," says Richard Webby, Ph.D., a research scientist at St. Jude Children's Research Hospital in Memphis, Tennessee. He is part of a highly regarded research team, funded in part by the NIH's National Institute of Allergy and Infectious Diseases (NIAID), that is studying emerging flu viruses and the development of flu vaccines to stop them.
Currently, H5N1 is transmitted to humans through close contact with infected chickens. But if the virus acquires a mutation that enables it to pass directly from person to person like the annual human influenza does, the resulting pandemic could be disastrous, notes Robert G. Webster, Ph.D., a colleague of Dr. Webby's and co-author of a recent research paper on H5N1.
"Our findings tell us that the best way to prevent such an outbreak is to control H5N1 at its source—domestic poultry in southern China," Webster says. "Early detection of the virus and large-scale culling of infected poultry is one of the key strategies for controlling highly pathogenic strains of H5N1. Control measures have been less effective in China, Indonesia and Vietnam; and the problem that has allowed the virus to take root in those countries and repeatedly jump from poultry to other species, including humans, has not been solved."