Understanding Influenza

In recent weeks, we have heard and read a lot about avian influenza, or bird flu, and pandemic influenza. To put recent developments in perspective, it is important to understand the differences between these viruses.

• What is human influenza?
• What is avian influenza or "bird flu"?
• Why is bird flu a concern for people?

What is human influenza?

Three types of influenza are currently in the news: human influenza, avian influenza and pandemic influenza.

Human influenza, or the flu, is a respiratory infection caused by the influenza virus. Strains circulate every year, making people sick. Influenza typically starts with a headache, chills and cough, followed rapidly by fever, loss of appetite, muscle aches and fatigue, running nose, sneezing, watery eyes and throat irritation. Nausea, vomiting and diarrhea may also occur, especially in children.

Most people will recover from influenza within a week or ten days, but some - including those over 65 and adults and children with chronic conditions, such as diabetes and cancer - are at greater risk of more severe complications, such as pneumonia. Between 4 000 and 8 000 Canadians can die of influenza and its complications annually, depending on the severity of the season.

What is avian influenza or "bird flu"?

Birds and other animals, including pigs, also contract and transmit influenza. Wild birds, in particular, are natural carriers of influenza A viruses. They have carried animal influenza viruses, with no apparent harm, for centuries. Migratory waterfowl (ducks, geese) are known to carry viruses of the H5 and H7 strains or subtypes. These viruses are usually in the low pathogenic form - in other words, they aren't as deadly to birds as highly pathogenic strains.

Currently, avian influenza H5N1 is circulating in Asia, Europe, and Africa, infecting many poultry populations and some humans. This strain is highly pathogenic, or highly deadly to birds, and has infected a limited number of people. There is no evidence this virus is transmitted from person to person.

Why is bird flu a concern for people?

People are exposed to different strains of influenza many times during their lives. Even though the virus changes, their previous bouts of influenza may offer some protection against similar strains of the virus. However, three to four times each century, for unknown reasons, a radical change takes place in the influenza A virus causing a new strain to emerge.

One way this radical change can happen is that a person sick with a human influenza virus also becomes infected with the avian influenza virus and the two viruses re-assort or "mix." This means that the avian influenza virus acquires some of the human influenza genes, potentially creating a new subtype of the influenza A virus that people would have no immunity against. If the virus was easily passed to and among people, this would create the conditions for an influenza pandemic.

There is no pandemic influenza at this time anywhere in the world. However, there were three influenza pandemics in the last century and scientists recognize that another is inevitable. That is why governments are planning to prepare to respond to a possible influenza pandemic.