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Flu -viral infections

Information Flu -viral infections

Flu is one thing and the common cold you usually get is another. The flu is often confused with the common cold due to its symptoms, but the flu presents stronger symptoms. The flu is an infection caused by the influenza virus and much stronger than a common cold. The Influenza virus can be spread through air or by touch (sometimes a simple handshake will do). This virus may infect the nose, throat or lungs. Regarding the cold, this is a contagious viral disease that infects the airways and is less aggressive and more common. Recent studies demonstrate that within 1 year, one in ten people get the flu, while the common cold can range from 2-5 per year. Flu epidemic is when the disease spreads quickly through a population. This hits the news several times as such usually leads to many people getting infected at the same time and quite quickly. This is characterized by a sudden onset of fever, cold tremors and body aches.

Influenza is a viral infection that causes fever, acute rhinitis, cough, headache, and malaise. Fatalities are possible during seasonal epidemics, especially among patients at risk (for example, patients in long-term care facilities, people of extremely advanced age, with cardiopulmonary insufficiency or in the last stage of pregnancy); even healthy young patients may die during pandemics. The diagnosis is usually clinical and is established taking into account the epidemiological situation. All children over 6 months of age should receive an annual flu vaccination. Antiviral treatment reduces the duration of the disease by approximately 1 day and can be used in patients at risk.

Influenza refers to diseases caused by influenza viruses, but the term is usually misused to refer to similar diseases caused by other viral respiratory pathogens. Influenza viruses, according to their nucleoproteins and matrix proteins, are classified into types A, B and C. Influenza caused by a type C virus infection does not cause typical influenza and is not discussed here.

Flu virus antigens

Hemagglutinin (H) is the surface glycoprotein of the influenza virus, which allows the virus to bind to the cell sialic acid and bind to the host cell membrane. Neuraminidase (NA), another surface glycoprotein, enzymatically removes sialic acid, helping to release the virus from the host cell. There are 18 types of H and 11 types of NA, giving 198 possible combinations, but only a few of them are pathogenic for humans.

Antigenic drift refers to relatively slow, long-term mutations in existing combinations of H and NA antigens, which lead to the frequent emergence of new strains of the virus. These new strains can cause seasonal epidemics because the protection of antibodies that were produced during contact with the previous strain is reduced.

Antigenic shift refers to the relatively rare formation of new combinations of H and/or NA antigens that result from the recombination of subunits of the viral genome. Because of the antigenic shift, pandemics can occur, because antibodies against other strains (as a result of vaccination or real infection) practically do not provide protection against the new strain.


Every year in autumn and winter, influenza causes widespread sporadic disease in a temperate climate (seasonal epidemics). Seasonal epidemics are caused by influenza viruses type A and B and often occur in 2 waves - 1 among schoolchildren and those with whom they contact at home (mainly young children), and 2 mainly among people who are constantly at home or living in institutions long-term stay, especially among the elderly. Type B influenza viruses can cause mild illness, but more often cause epidemics with moderate or severe forms of the disease, usually with a 3-5 year cycle. Most influenza epidemics are caused by the prevailing serotype, however, different influenza viruses can appear sequentially in one place or can appear simultaneously, with one virus prevailing in one place.

Influenza viruses can spread by airborne droplets, direct human-to-human contact, or contact with contaminated objects. The airborne spread is the most important mechanism.

Risk groups

Certain patients are at high risk for influenza complications and need special precautionary measures:

  • Children 4 years

  • Adults > 65 years old

  • People with chronic illnesses (e.g., cardiopulmonary disease, diabetes mellitus, renal or hepatic insufficiency, hemoglobinopathy, immunodeficiency)

  • Women in the 2nd or 3rd trimester of pregnancy

  • Patients with impaired respiratory secretion (e.g., cognitive dysfunction, neuromuscular disorders, seizures, epilepsy)

  • Patients  18 years old taking aspirin (due to the risk of Reye's syndrome)

Morbidity and mortality in these patients may be due to worsening of the underlying disease, acute cardiorespiratory insufficiency syndrome, primary influenza or secondary bacterial pneumonia.

Clinical manifestations

The incubation period ranges from 1 to 4 days with an average of approximately 48 hours. In mild cases, many symptoms resemble cold symptoms (eg, sore throat, rhinorrhea); there may be slightly expressed conjunctivitis. Typical adult influenza is characterized by a sudden onset with chills, high fever, severe general weakness, coughing, and generalized myalgia (especially in the back and legs). Headache is clear, often with pain in the eyeball and photophobia. Symptoms of airway damage may initially be mild, in the form of pharyngitis, a burning sensation behind the sternum, an unproductive cough, and sometimes acute rhinitis. Later, damage to the lower respiratory tract becomes dominant; coughing can be persistent, hoarse, and productive.

After 2–3 days, acute symptoms quickly subside, although the fever can last up to 5 days. Coughing, weakness, sweating, and fatigue may persist for several days or sometimes for weeks.


Pneumonia can be suspected with increasing cough, sputum with blood, shortness of breath and wheezing. Secondary bacterial pneumonia is likely with persistent or recurring fever and cough after the primary disease has subsided.

Encephalitis, myocarditis, and myoglobinuria, sometimes with kidney failure, can develop after influenza A or B. Reye's syndrome ( Reye's syndrome ) - characterized by encephalopathy, fatty liver, increased levels of liver enzymes and/or ammonia; hypoglycemia and lipidemia - often occurs during influenza B epidemics, especially among children taking aspirin.


The diagnosis is usually made clinically in patients with typical symptoms during an influenza epidemic. Although a large number of quick diagnostic tests are available and most are quite specific, their sensitivity varies greatly, and they usually provide little information for patient management. 

  • Clinical assessment

  • Sometimes express diagnostics

  • Pulse oxigemometry and chest x-ray for patients in serious condition (suspected pneumonia)

Diagnostic tests should be performed when the results are related to clinical decision making. Reverse transcription PCR (RT-PCR) is a sensitive and specific study that helps different types and subtypes of influenza. If this test is available, then its results can be used to select appropriate antiviral therapy. These tests are also useful to determine if the current outbreaks are caused by the flu itself.

If patients have signs of lower respiratory tract damage (e.g., shortness of breath, wheezing in the lungs), pulse oximetry should be performed to detect hypoxemia, and a chest x-ray to diagnose pneumonia. Primary influenza pneumonia appears as focal or diffuse interstitial infiltrates or as acute respiratory distress syndrome. Secondary bacterial pneumonia is more likely to be lobar or segmental.


Most patients recover completely, although recovery often requires 1–2 weeks. However, influenza and influenza-associated pneumonia are important causes of increased morbidity or mortality among high-risk patients. The use of antiviral treatment in these patients reduces the likelihood of damage to the lower respiratory organs and the level of hospitalization. Appropriate antibiotic therapy reduces mortality caused by secondary bacterial pneumonia.


There are two types of treatments- (1) Symptomatic treatment, and (2) Sometimes antiviral drugs.

The treatment of most patients is symptomatic, including bed rest, hydration and, if necessary, antipyretic drugs, but aspirin is avoided in patients  18 years of age. Complications of bacterial infections require the use of antibiotics.

Anti-flu drugs

Antiviral drugs prescribed within 1–2 days after the onset of the disease reduce the duration of the fever, the severity of symptoms, and the time it takes to return to normal activity. Antiviral treatment is recommended for those at-risk patients who develop flu-like symptoms; this recommendation is based on data suggesting that early treatment can prevent complications in these patients. Flu drugs include the following:

  • Oseltamivir and zanamivir (neuraminidase inhibitors)

  • Amantadine and rimantadine (adamantanes)

Neuraminidase inhibitors prevent the introduction of influenza virus into cells and thus stop the spread of infection in the body.

Adamantanes block the channel of the M2 ion and thus affect the reproduction of the virus in the cell. They are effective only against influenza A viruses (in influenza B viruses lack a protein M2).

The choice of an antiviral drug is complicated by the resistance of various types and subtypes of influenza to different drugs (the sensitivity of the drug to different strains of influenza). If reverse PCR analysis is available, then its results can be used in the choice of treatment options. Otherwise, patients can only be treated with zanamivir or rimantadine plus oseltamivir.