Types of influenza viruses and the main clinical symptoms of the disease

For the first time the influenza virus loudly and tragically declared itself in 1918-1919, when it took the lives of more than 50 million people. The Spanish flu pandemic, also known as the "Spanish flu", is the largest flu pandemic in history. Relatively recently, humanity had to endure another pandemic in 2009, the so-called "swine flu".

Influenza – a highly contagious and dangerous disease that makes itself felt suddenly and "knocks down" people almost every winter.

This article on estet-portal.com describes influenza symptoms, the characteristics of the most clinically significant subtypes of the influenza virus, and outlines the principles of modern disease diagnosis.

Features of the most clinically significant influenza virus subtypes

Influenza viruses are classified into three types: A, B and C. Influenza A and B viruses are the most clinically significant.
Depending on the types of hemagglutinin (H1-H16) and neuraminidase (N1-N9), there are many subtypes influenza A.

Influenza A virus subtypes (also called serotypes) H1N1, H2N2, H3N2 and H3N8 are known to cause epidemics.

In addition, the H5N1, H7N7 and H7N9 virus subtypes can also provoke the onset of the disease. The following are the main features of the most clinically relevant types of influenza virus:

1.    Type A virus, subtype H1N1 - leads to seasonal outbreaks of influenza in the winter, most often affects children and adolescents;

2.    Virus type A, subtype H3N2 – occurs almost every winter, affects all age groups, especially dangerous for people over the age of 65;

3.    Virus type A, subtype H1N1 (2009) – caused a "pig" pandemic influenza in 2009-2010, after which it underwent some changes and continues to initiate seasonal outbreaks of influenza. Most often affects children, adolescents and pregnant women, rarely – people over the age of 65;

4.    Virus type A, subtypes H5N1, H7N9 – caused outbreaks of the so-called "bird" flu» in Egypt and the Far East in 2000. In total, about 600 people were ill with them, among whom about 60% died. The virus has a high pandemic potential, but so far its human-to-human transmission has not been fast enough. In the spring of 2013, cases of influenza caused by the H7N9 subtype were reported from China. A third of the patients died;

5.    Virus Type B – causes irregular outbreaks of influenza in winter, leads to influenza epidemics closer to spring.
The World Health Organization recommends annual influenza vaccination for people whose health and life can be seriously threatened by the infection.

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How to suspect a disease in time: typical flu symptoms

Influenza virus is transmitted by airborne droplets, as well as by direct contact with a sick person. The incubation period is from 1 to 7 days, most often - 2-3 days. Hyperthermia may precede other clinical symptoms of influenza by several days. The disease makes itself felt suddenly: there is a severe headache, malaise, myalgia. Fever may be accompanied by chills.

For influenza, a dry cough is typical, but rhinitis in adults in the early stages of the disease is not typical. This cannot be said about children: the symptoms of rhinitis from the first days of the disease are present in most of them along with fever. The duration of infection averages three to eight days.

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Influenza Specific Diagnosis: Rapid Diagnostic Tests and PCR

Flu-like symptoms such as a sudden rise in temperature and the appearance of a dry cough may be the only diagnostic criteria during an epidemic. In children, the clinical diagnosis of infection is somewhat difficult, since there are other types of viruses that can cause similar symptoms.

In doubtful cases, rapid influenza diagnostic tests should be used. There are several types of them: some are aimed at identifying both types of viruses (A and B), others determine only type A.

For the study, material from the nasopharyngeal mucosa is used. The result can be obtained in 10-30 minutes. The specificity of such diagnostic systems is quite high, but their sensitivity is inferior to diagnostic laboratory methods.

PCR (polymerase chain reaction) is a highly specific and sensitive laboratory diagnostic method for influenza. It can not only determine the type of virus, but also identify subtypes of the A virus. However, this diagnostic method is not widely used for primary health care in the population. PCR is used only when it is necessary to determine the epidemiological situation in the country, as well as in some severe cases of the disease.

Complication of influenza in adults and children: characteristic features

Influenza can cause serious complications. The most common of these in adults are pneumonia and maxillary sinusitis, as well as exacerbation of asthma, COPD, and chronic bronchitis. In most cases, pneumonia is caused by bacteria - pneumococci and Staphylococcus aureus - but there are also cases of primary viral pneumonia due to influenza.

The influenza virus can also lead to complications from the central nervous system (meningitis, encephalitis) and the cardiovascular system (myocarditis, pericarditis).

The most common complication of influenza among children under 3 years of age is acute otitis media, which occurs in about 40% of cases.

Thank you for staying with estet-portal.com. Read other interesting articles in the "Infectious Diseases" section. You may also be interested in Skin health and the role of the microbiome in preventing skin agingi.

Material written based on the EMB Guideline «Influenza» (2018).