Flu Influenza

Influenza is a highly contagious infectious disease caused by three types of influenza virus: type A, B and C. Types A and B often cause widespread outbreaks that occur every year, while type C usually occurs as sporadic cases or localized outbreaks. From time to time, an influenza pandemic (a global outbreak) occurs when a new influenza A virus causes serious human illness and spreads easily from person to person.

Type A is characterized by the designations “H” and “N” (e.g., the recently identified swine H1N1 or the avian H5N1 viruses), which represent the proteins on the surface of the virus. These proteins allow the virus to enter into cells. Frequent mutations in these proteins permit the virus to change rapidly to avoid the immunity built up during the previous flu season. Three to four pandemics have occurred each century since the 1500s. The first pandemic of this century (1918) resulted from the rapid spread of the new H1N1 (swine flu) strain.       

There is some concern regarding the possible transmission of avian (bird) flu to people.  Although this strain of flu is widespread in poultry and wild aquatic birds, so far there have been only a few human infections worldwide, usually resulting from very close contact with sick poultry. 

Flu is a serious illness because it can rapidly develop into an epidemic and spread worldwide as a pandemic. In elderly persons greater than 65 years old who have chronic heart, lung, kidney or immune suppression diseases, there is a high mortality rate. 

Because flu results in both excess mortality and serious economic losses, many countries provide routine annual vaccination against flu.  The vaccine is recommended especially for elderly persons and persons with underlying chronic illness.  Due to cost and limited production globally, however, the vaccine is generally not available in developing countries.

When a new influenza virus appears, humans are generally susceptible to influenza virus infection. Infection results in immunity to the specific infecting virus, but that immunity is limited by the tendency of the virus to mutate each year to produce a slightly different version to which a person might be susceptible. Thus, it is possible to be infected with influenza year after year. When a recombination appears, as is the case with the new H1N1 (swine flu) strain, susceptibility is global.

The symptoms are similar for all types and include fever, headache, muscle and joint aches and pains, fatigue and cough. Vomiting and diarrhoea may accompany the respiratory symptoms. Symptoms will vary from mild to moderately severe depending on the virus type and strain that circulates in the community.

The symptoms of influenza may be very similar to many respiratory infections that occur in the winter season, such as the common cold. Only a laboratory test can confirm that the symptoms are truly the result of an infection with the influenza virus.

While infection with the avian H5N1 rarely occurs, it causes a severe respiratory illness with difficulty breathing and a high mortality rate (as high as 70 percent in some areas).

Supportive measures and treatment of symptoms to reduce fever and maintain adequate hydration are generally available.  There are several antiviral drugs (e.g., oseltamivir, known as Tamiflu, and zanamivir, known as Relenza) that can shorten the duration of illness and its severity. To be effective they should be taken at the first sign of symptoms in an exposed person.  Because the virus may rapidly develop resistance to these drugs, they are generally reserved for the treatment of seriously ill persons, especially those with underlying chronic illness.

These drugs can also prevent illness if they are being taken prior to exposure to the virus.  In most cases, the prophylactic use of these drugs is recommended for persons, such as health care workers, who might be repeatedly exposed during an epidemic.

Influenza is a seasonal disease and usually occurs in countries all over the world as outbreaks or epidemics, and usually during the winter season.