Influenza

Influenza (Grippus, Influenza) is an acute infectious disease with an aerosol mechanism of pathogen transmission, characterized by mass spread, short-term fever, intoxication and damage to the airways, as well as a high frequency of complications.

Influenza is a specific acute respiratory viral disease of the respiratory tract with high fever, runny nose, cough, headache, and malaise. It occurs mainly in the form of epidemics in winter. Death is possible during epidemics, especially among high-risk patients (for example, those in organized groups, the elderly, with pulmonary heart failure, in late pregnancy). In severe cases, severe weakness, hemorrhagic bronchitis, and pneumonia are observed. Influenza is usually diagnosed clinically. Influenza can be prevented with annual vaccination. It is given to patients at high risk of the disease, medical personnel, people with a lot of contacts, and children from 6 to 24 months. Influenza types A and B are treated with zanamivir (a neuraminidase inhibitor) and aceltamivir; influenza A is treated with amantadine and rimantadine.

ICD-10 code

• J10. Influenza due to identified influenza virus.
  • J10.0. Influenza with pneumonia, influenza virus identified.
  • J10.1. Influenza with other respiratory manifestations, influenza virus identified.
  • J10.8. Influenza with other manifestations, influenza virus identified.
• J11. Influenza, virus not identified.
  • J11.0. Influenza with pneumonia, virus not identified.
  • J11.1 Influenza with other respiratory manifestations, virus unidentified.
  • J11.8. Influenza with other manifestations, virus not identified.

Influenza: epidemiology

Every year in late fall - early winter, the influenza virus causes a sporadic increase in the incidence of the disease. Large epidemics in the United States occur approximately every 2-3 years. Influenza A virus causes acute influenza. Influenza B virus causes mild influenza. But it can cause epidemics that have a 3-5 year cycle. Usually, an epidemic is caused by one serotype, although in one region there may be different viruses and cause disease simultaneously or replace each other; and one may dominate.

Seasonal flu often has two waves: the first among schoolchildren and those who come into contact with them (usually young people), and the second among people from closed groups and those who are constantly at home (especially the elderly).

Influenza is spread from person to person through the air (the most significant route); in addition, droplets containing the virus can settle on objects and also cause infection.

Influenza is severe in people with cardiovascular and pulmonary diseases, metabolic diseases (diabetes mellitus) requiring constant medical supervision, renal failure, hemoglobinopathies and immunodeficiency. Also, severe flu with a fatal outcome occurs in pregnant women in the 2nd and 3rd trimesters, small children (less than 24 months), the elderly (over 65 years old) and bedridden patients.

What causes the flu?

Influenza is a disease caused by the influenza virus, and the use of this term for diseases caused by other respiratory viruses is inappropriate. Influenza viruses are classified according to their nucleoproteins and protein matrix into types A, B, and C. Influenza C virus does not cause typical influenza and is not discussed here.

The nucleocapsid is covered by a membrane containing two major glycoproteins, one of which has hemagglutinin (HA) activity and the other has neuraminidase (NA) enzyme activity. Hemagglutinin enables the virus to bind to the cell. The virus is taken up by the cell by endocytosis, its membrane fuses with the endosome membrane, and the genetic material is released into the cytoplasm. Replication occurs inside the cell, and new virions are assembled from the resulting viral components on the cell surface, which bud off with the participation of viral neuraminidase (removes sialic acids from the host cell surface). Small mutations in these agglutinins lead to a high frequency of formation of new viral serotypes (antigenic drift). The consequence of this is a decrease in the protective effect of antibodies that were formed upon contact with previous serotypes. In contrast to antigenic drift, large mutations of influenza A virus glycoproteins (antigenic shift) are longer lasting (10-40 years over the last 100 years); thus, there is no immunity to the new virus in the population, which is the cause of pandemics.

What are the symptoms of flu?

Influenza has an incubation period of 1-4 days (48 hours on average). In mild cases, the symptoms resemble those of a cold (sore throat, runny nose), mild conjunctivitis. Influenza begins suddenly with chills and a rise in temperature to 39-39.5 C, severe weakness and generalized pain (most pronounced in the back and legs) appear. But the patient is especially bothered by headaches, often combined with photophobia and retrobulbar pain. At first, the symptoms of influenza from the respiratory tract may be mild, limited to a sore throat, burning behind the breastbone, dry cough and sometimes a runny nose. Later, the symptoms of influenza, reflecting damage to the lower respiratory tract, become predominant; the cough intensifies and becomes productive. Children may have nausea and vomiting. Usually after 2-3 days acute flu symptoms disappear and the temperature drops, but it can last up to 5 days without complications. Usually bronchociliary drainage and bronchial resistance are impaired. Weakness, sweating and fatigue do not go away for several days, sometimes weeks.

Signs of pneumonia include shortness of breath, the appearance of purulent or bloody sputum, cyanosis, hemoptysis, wheezing, and a secondary increase in temperature or relapse.

Sometimes, usually during the recovery period, influenza can be complicated by diseases such as encephalitis, myocarditis, and myoglobinuria. The causes are unclear, but such complications are more common with influenza A. Reye's syndrome, characterized by encephalopathy, fatty liver, hypoglycemia, and lipidemia, is associated with influenza A epidemics, especially in children who have taken aspirin.

How is flu diagnosed?

Influenza is diagnosed based on the clinical presentation of the disease and the epidemiological situation in the community. Although many diagnostic tests are available, their sensitivity and specificity vary widely across studies. The use of such tests in a specific group of patients has yielded conflicting results. A more specific diagnosis of influenza is made by cell culture of nasopharyngeal scrapings and determination of antibody titers in paired sera. These tests require 2 days or more and are necessary to assess the epidemic situation and determine the serotype of the virus.

When symptoms of lower respiratory tract damage are detected, such as dyspnea, hypoxia, wheezing in the lungs, X-ray examinations are performed to exclude pneumonia, which often accompanies influenza. Typical primary influenza pneumonia is detected as diffuse interstitial infiltrates or manifests as acute respiratory distress syndrome. Secondary bacterial pneumonia is more often focal or lobar.

How is flu treated?

Uncomplicated influenza usually resolves, although this may take 1–2 weeks. In some patients, especially those in the high-risk groups mentioned above, viral pneumonia and other complications may lead to death. Antiviral treatment for influenza in these cases is unknown. Specific antimicrobial chemotherapy reduces mortality from severe secondary pneumonia.

In most cases, flu is treated symptomatically, bed rest and rest, plenty of fluids, antipyretics are indicated, however, in children, aspirin should be avoided.

Antiviral drugs given within 1-2 days of the onset of symptoms can reduce their duration. Influenza is also treated with antiviral drugs, which are recommended for high-risk patients who develop flu-like symptoms, but there is no evidence that this treatment is effective.

When influenza is treated, resistance to amantadine and rimantadine often develops, and resistance to either renders both ineffective. Resistance that develops during treatment does not affect the effectiveness of treatment in other patients, but may result in transmission of resistant viruses. Resistance to aceltamivir and zanamivir is not clinically significant. Aceltamivir has been shown to reduce the incidence of otitis media in children, but there is no other evidence that treating influenza prevents complications.

Influenza A is treated with amantadine and rimantadine; they inhibit viral penetration into the cell. Influenza treatment is stopped after 3-5 days or 1-2 days after symptoms have stopped. For both drugs, 100 mg twice daily. To eliminate side effects due to drug accumulation, the dose is reduced for children (2.5 mg/kg twice daily, but not more than 150 mg daily for children under 10 years of age or 200 mg daily for children over 10 years of age). In patients with impaired renal function, the dose is calculated based on creatinine clearance. In case of impaired liver function, the dose of rimantadine should not exceed 100 mg daily. Dose-dependent effects on the central nervous system occur in 10% of individuals receiving amantadine (causes increased excitability, insomnia) and in 2% of those receiving rimantadine. These effects may be observed within 48 hours after the start of treatment, are most pronounced in the elderly and in individuals with CNS pathology or impaired renal function, and often disappear with continued use. Anorexia, nausea, and constipation may also be observed.

Influenza A and B are also treated with the neuraminidase inhibitors oseltamivir and zanamivir. The dosage of zanavir is 10 mg (2 inhalations) 2 times a day, oseltamivir - 75 mg 2 times a day for patients over 12 years old. The dosage is reduced in younger patients. These drugs have relatively minor side effects. Zanamivir should not be given to patients with bronchial hyperreactivity, as it causes bronchospasm when inhaled. Oseltamivir can cause nausea and vomiting.

Flu: Antiviral Treatment

Influenza can be effectively prevented by vaccination, but some antiviral drugs are also effective. Antiviral treatment of influenza is indicated for individuals who were vaccinated less than 2 weeks ago, patients for whom vaccination is contraindicated, and immunocompromised patients whose immune response to the vaccine may be insufficient. Taking medications does not affect the development of specific immunity. Antiviral drugs can be discontinued 2 weeks after vaccination; in the absence of vaccination, they should be taken for the duration of the epidemic.

Amantadine and rimantadine are used as preventive measures against influenza A virus. Neuraminidase inhibitors oseltamivir and zanamivir are effective against influenza A and B. The dosage of these drugs is the same as for treatment, with the exception of oseltamivir - 75 mg once a day.

Flu vaccines

Influenza vaccines are modified annually to include the most common serotypes (usually serotype 2 of influenza A and 1 of influenza B). If the influenza vaccine contains the serotype of the virus that circulates in the population, the incidence of the disease in adults can be reduced by 70-90%. In elderly people in nursing homes, the effectiveness of vaccination is somewhat lower, but it can reduce the mortality rate from pneumonia by 60-80%. If the antigenic composition of the virus changes significantly (antigenic drift), the vaccine provides only weak immunity.

Vaccination is especially important for the elderly; for those with heart, lung, and other chronic diseases; for caregivers at home or in health care facilities; for pregnant women whose 2nd and 3rd trimesters of pregnancy fall in the winter. Vaccination by intramuscular injections is best done in the fall so that antibody titers are high by the time of peak influenza incidence (November through March in the United States). Vaccination of all children aged 6–24 months and their contacts is recommended. Regardless of changes in the vaccine strain, immunization should be done annually to maintain high antibody titers.

The inactivated influenza vaccine is administered intramuscularly. Adults are given 0.5 ml. Few children have already had the flu, and if there has been no previous immunization, both primary and revaccination are required (at the age of 6 months to 3 years, 0.25 ml, from 3 to 10 years - 0.5 ml) with an interval of 1 month. Side effects are rare and minor - there may be pain at the injection site, occasionally - fever, myalgia. Vaccination is contraindicated for people with a history of anaphylactic reactions to chicken meat or egg whites.

A live attenuated influenza vaccine is now available in the United States for use in healthy individuals aged 5 to 50 years. Influenza vaccine is contraindicated in high-risk individuals, pregnant women, health care personnel caring for immunocompromised individuals, and children receiving aspirin therapy. Influenza vaccine is given intranasally, 0.25 ml in each nostril. Children aged 5 to 8 years who have not previously been vaccinated with an attenuated vaccine should receive a second dose of the vaccine, no more than 6 weeks after the first dose. Side effects are mild, with mild rhinorrhea common.

How to prevent flu?

Influenza can be prevented by annual vaccination. Antiviral chemoprophylaxis is useful in certain situations. Prophylaxis is indicated for all patients, but is especially important for high-risk individuals and health care personnel.