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Seasonal Influenza, Avian Influenza and Pandemic Influenza

Influenza is a respiratory infection caused by influenza viruses of the family Orthomyxoviridae. There are 3 types of influenza viruses, called influenza A, influenza B and influenza C. Influenza A and B are associated with seasonal influenza and most outbreaks and epidemics of influenza. Influenza C is relatively rare and does not usually cause epidemics or outbreaks. 

Current Seasonal Influenza Update

The reporting period for the 2024-2025 Influenza season began on August 25, 2024. For up to to date influenza activity refer to interactive Canadian Respiratory Virus Surveillance Report and to the FluWatch Report from the Public Health Agency of Canada. Provincial, territorial and international respiratory virus reports can be found here.

What is Influenza?

Influenza is a respiratory infection caused by influenza viruses of the family Orthomyxoviridae. These viruses contain a lipid envelope. It can be distinguished from the common cold (which is also caused by viruses) because it generally results in a more severe illness, with a sudden onset of headache, chills and cough followed rapidly by a fever, appetite loss, muscle aches, and tiredness. Cold symptoms, on the other hand, generally involve the upper respiratory tract with runny nose, sneezing, watery eyes, and throat irritation, and disappear in a few days. Influenza typically lasts a week to 10 days.

The incubation period for influenza is 24 to 72 hours. Adults with influenza remain infectious for 3 to 5 days after onset of symptoms, and children may remain infectious for up to a week after onset.

In some people, especially young children and those over 65 years of age or people with other systemic illnesses (e.g. heart disease, diabetes, cancer, respiratory illnesses) influenza can be very severe, requiring hospitalization.  People with these risk factors should receive immunization against influenza each year. Every year, worldwide seasonal influenza causes an estimated 1 billion infections and between 290,000 to 650,000 deaths. In Canada, influenza and pneumonia are ranked among the top 10 leading causes of death in Canada. It is estimated that influenza causes approximately 12,200 hospitalizations and 3,500 deaths. 

Types of Influenza Viruses

There are 3 types of influenza viruses, called influenza A, influenza B and influenza C. Influenza A and B are associated with seasonal influenza and most outbreaks and epidemics of influenza. Influenza C is relatively rare and does not usually cause epidemics or outbreaks. Influenza B and C are human pathogens. Influenza A can infect humans as well as birds, pigs and other animals.

Influenza A viruses are divided into subtypes based on differences in their surface glycoprotein antigens, hemagglutinin (HA) and neuraminidase (NA). There are 14 recognized HA subtypes and 9 recognized NA subtypes. All of these subtypes have been isolated in birds but only 3 different HA and two different NA subtypes have been isolated in humans.

The influenza viruses are unique among the respiratory viruses in that they undergo significant antigenic variation. Antigenic drift involves minor antigen changes from one season to the next and may result in epidemic spread of the new strain. Antigenic shift involves major antigenic changes of the HA and NA molecules and occurs only with Influenza A viruses. These changes can result in the appearance of pandemic viruses.

Influenza A causes a more severe illness than influenza B, often resulting in hospitalization and death in the elderly and those at risk. World epidemics and pandemics have been due to influenza A.

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 Transmission and Control of Influenza

Influenza is spread from person to person through droplets (e.g. saliva, sneezing) and by touching objects and surfaces that are contaminated with the virus (e.g. doorknobs, telephone receivers). The influenza virus may persist for hours in dried mucus and be transmitted by direct contact. It is spread very easily indoors, which is why it is so prevalent in the winter months in northern countries, when people spend more time together inside.

Measures to reduce the spread of influenza include:

  • Good hand hygiene practices, such as handwashing or use of an alcohol-based hand rub after contact with the eyes, mouth, nose or secretions
  • Avoid handling soiled tissues or objects used by an ill person
  • Cover coughs and sneezes 
  • Ill persons should stay at home

In health care settings, Routine Practices should be used consistently with all patients including:

  • Hand hygiene before and after all patient contact
  • Appropriate use of personal protective equipment (gloves, masks, eye protection) for contact with all patient secretions/excretions
  • Disinfection of all equipment which is shared between patients
  • Cleaning/disinfection of all patient contact surfaces after patient leaves an examining room or area

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 Treatment of Influenza

[Ref: World Health Organization]

Treatment of influenza usually involves making the person more comfortable – increasing fluid intake and getting plenty of rest. Antibiotics do not kill viruses and have no role in treating influenza in otherwise healthy people, although they may be used to treat complications, such as pneumonia.

Antiviral drugs for influenza are an important adjunct to influenza vaccine for the treatment and prevention of influenza. However, they are not a substitute for vaccination. When taken before infection or during early stage of the disease (within two days of illness onset), antivirals may help prevent infection, and if infection has already taken hold, their early administration may reduce the duration of symptoms by one to two days.

For several years, amantadine and rimantadine were the only antiviral drugs. However, whilst relatively inexpensive, these drugs are effective only against type A influenza, and may be associated with severe adverse effects (including delirium and seizures that occur mostly in elderly persons on higher doses). When used for prophylaxis of pandemic influenza at lower doses, such adverse events are far less likely. In addition, the virus tends to develop resistance to these drugs.

A new class of antivirals, the neuraminidase inhibitors, has been developed. Such drugs, zanamivir and oseltamivir, have fewer adverse side effects (although zanamivir may exacerbate asthma or other chronic lung diseases) and the virus less often develops resistance. However, these drugs are expensive and may not be available for use in many countries.

In severe influenza, admission to hospital, intensive care, antibiotic therapy to prevent secondary infection and breathing support may be required.

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Avian Influenza 

[ref: WHO, PHAC]

Avian influenza is one of a subgroup of influenza viruses that normally affect birds. Until recently, this virus had not been seen in people. Available information suggests that this virus does not have the ability to transmit easily among humans. Clusters that have been reported suggest that limited human-to-human transmission may occur where there was unprotected close contact with symptomatic cases, but no onward transmission has been detected.

As yet, there is limited information about the scope of the disease the virus causes and about the source of exposure. Most human infections are believed to have occurred after exposure to infected poultry or contaminated environments.

Symptoms of infection include fever, cough and shortness of breath, with most progressing to severe pneumonia. The disease is of concern because most patients have been severely ill.

What is Pandemic Influenza?

A pandemic is an epidemic that spans across many nations around the world. It is generally believed that every quarter century or so, a new strain of influenza virus appears, to which people have no immunity. Such a strain would be able to infect everyone, would travel swiftly around the globe, and would be capable of causing severe disease. The resulting pandemic would have a huge impact on today's global economy with large numbers of the workforce disabled and hospitals filled to capacity.

There have been 3 influenza pandemics in the 20th century, of varying degrees of severity – the Spanish Flu of 1918/1919, the Asian Flu of 1957/1958, and the Hong Kong Flu of 1968/1969. The 21st Century saw its first influenza pandemic in April 2009 (H1N1), originating in Mexico and spreading around the world a month later [more information]. A global network of laboratories and surveillance systems under the coordination of the World Health Organization provides early warning of new, virulent influenza strains, and the production of vaccine to this strain will take place as quickly as possible. Early notification and vaccination will be the best weapons against pandemic flu.

For more information on pandemic influenza, refer to links below.

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Influenza Vaccine

The influenza vaccine is composed of killed influenza virus strains that were in circulation in the previous year as well as those determined to be a risk for the current year. The virus is treated in the laboratory so that it will not cause disease, but the body will recognize it as a foreign "invader" and produce antibodies against it. By having antibodies build up before influenza actually appears, individuals are able to fight off the virus before it can cause disease.

Seasonal influenza vaccine is recommended for the following people:

  • adults and children with chronic heart and lung disease
  • persons residing in nursing homes or retirement facilities
  • persons over the age of 65 years
  • persons with chronic conditions, such as diabetes, cancer, kidney disease, immune system dysfunction
  • persons with HIV
  • children and adolescents on long term ASA (acetylsalicylic acid)
  • healthcare workers and household contacts of any of the above

For updated information regarding influenza vaccine in Canada, see the National Advisory Committee on Immunization (NACI) Statement on seasonal influenza vaccine for 2024-2025.

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Laboratory Testing for Influenza

A number of tests may be done to aid in the diagnosis of influenza. During an outbreak, testing for influenza can be helpful in determining if influenza is the cause of the outbreak. Samples for influenza testing include nasopharyngeal or throat swab, nasal wash, or nasal aspirate, as well as blood for antibodies.

Viral culture provides results in 3 to 10 days. During outbreaks, some of the samples should include culture so that influenza subtypes can be determined and for surveillance for new strains that my need to be included in the next year's influenza vaccine. Viral culture can also help identify other causes of illness if influenza is not the agent. Viral culture detects both Influenza A and B. Acceptable cultures are nasopharyngeal swab, throat swab, nasal wash, bronchial wash, nasal aspirate and sputum. Specimens should be collected during the early febrile stage of disease.

Rapid influenza tests provide results within 24 hours, however they only offer about 70% sensitivity and 90% specificity (ie. up to 30% of influenza cases would show a negative test result). Acceptable specimens for rapid testing include nasopharyngeal swabs, throat swabs, nasal wash, and nasal aspirate. Results are often available within 30 minutes. Some rapid tests detect both Influenza A and B, others only detect Influenza A.

Serological testing involves testing serum samples for influenza antibody to diagnose recent infection. Two samples of blood are collected, one sample within the first week of illness and the second sample 2 to 4 weeks later. If antibody levels are higher in the second sample than in the first, it is likely that influenza virus was present.

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Influenza Links

National Guidance

Provincial Information

British Columbia

Alberta

Saskatchewan

Manitoba

Ontario

Quebec

New Brunswick

Nova Scotia

Prince Edward Island

Newfoundland

World Health Organization (WHO)

Centers for Disease Control and Prevention (CDC) - U.S.

Other Information

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