Influenza Caused by Other Identified Influenza Viruses

Description of the Disease

Influenza is a highly contagious viral infection of the respiratory tract characterized by the sudden onset of fever, dry cough, sore throat, headache, muscle aches, fatigue, and general weakness.

Causative Agent

Influenza is caused by influenza viruses, which are classified into three main types: A, B, and C. Influenza A and B viruses are responsible for the majority of seasonal outbreaks. These viruses undergo frequent antigenic changes, producing new strains to which the population has little or no pre-existing immunity. This constant evolution is responsible for annual influenza epidemics.

Epidemiology

Humans are the primary reservoir for seasonal influenza viruses. However, mammals and birds also serve as important reservoirs, particularly for influenza A viruses, contributing to the emergence of new viral strains.

The source of infection is an infected person, including asymptomatic carriers during outbreaks. Individuals are most contagious during the first few days of illness. Influenza spreads mainly through respiratory droplets produced by coughing, sneezing, or talking.

Susceptibility to influenza is high, especially when a new viral subtype emerges. Although influenza can affect people of all ages, children are infected more frequently, while adults over 65 years of age, children younger than 2 years, pregnant women, and individuals with chronic medical conditions are at the highest risk of severe illness and complications.

Natural infection provides immunity only against the specific viral strain encountered. Immunity following vaccination generally lasts for one influenza season. Influenza has a marked seasonal pattern, with epidemics in Lithuania typically occurring between January and March.

Symptoms

Influenza usually begins abruptly. Symptoms typically develop 1–4 days after exposure to the virus and commonly include:

• Fever above 38°C.
• Chills.
• Headache.
• Muscle and back pain.
• Dry cough.
• Sore throat.
• Nasal congestion or runny nose.
• Loss of appetite.
• Fatigue and weakness.

Children, particularly those between 3 and 5 years of age, may also experience gastrointestinal symptoms such as abdominal pain, diarrhea, and vomiting.

Most symptoms resolve within one week, although fatigue and a dry cough may persist for one or two additional weeks. Common complications include secondary bacterial infections and worsening of pre-existing chronic diseases.

Diagnosis

Diagnosis is based on the patient’s symptoms, physical examination, and epidemiological circumstances, such as the presence of seasonal influenza activity or local outbreaks.

Laboratory confirmation may be obtained by one or more of the following methods:

• Detection of influenza viral antigens.
• Detection of influenza viral RNA using molecular tests (such as PCR).
• Isolation of the influenza virus in culture.
• Detection of specific antibodies against influenza A or B viruses.

Treatment

Treatment focuses on relieving symptoms and, when appropriate, reducing viral replication.

Specific antiviral medications include neuraminidase inhibitors such as oseltamivir and zanamivir. Older antiviral drugs, including amantadine and rimantadine, have limited use because many circulating influenza A viruses are resistant to these medications.

Supportive treatment includes:

• Bed rest.
• Adequate fluid intake.
• Paracetamol (acetaminophen) to reduce fever and relieve pain.
• Symptomatic medications, such as antihistamines or mucolytics, when appropriate.

Antibiotics are not effective against influenza and should be prescribed only if a secondary bacterial infection develops.

Some countries also use non-specific antiviral products intended to reduce viral replication and relieve common cold and influenza symptoms, although their effectiveness may vary.

Prevention

Annual influenza vaccination is the most effective method of preventing influenza and its complications. Vaccination is especially recommended for older adults, young children, pregnant women, people with chronic medical conditions, healthcare workers, and others at increased risk of exposure.

Influenza vaccines are updated each year to match the strains expected to circulate during the upcoming season. Vaccination is ideally performed in October or November. Protective immunity develops approximately two weeks after vaccination and generally lasts for 6–12 months.

For individuals at high risk who have been exposed to influenza, antiviral medications may be used for post-exposure prophylaxis when indicated.

Additional preventive measures include frequent hand washing, good respiratory hygiene, avoiding close contact with infected individuals, staying home when ill, and maintaining adequate ventilation in indoor spaces.

Source | Author Doctor Nikas Samuolis, reviewed by Prof. Virginijus Šapoka | Vilnius University | Faculty of Medicine | Head of the Department of Internal Medicine, Family Medicine, and Oncology