Influenza Virus: Symptoms, Spread, and Prevention
Influenza: Causes and Terminology
The flu, or influenza, is an infectious disease affecting birds and mammals, caused by an RNA virus belonging to the Orthomyxoviridae family. The word “flu” originates from the French grippe (from Swiss-German groupie, meaning ‘curl’), while “influenza” comes from Italian.
Symptoms in Humans
In humans, influenza primarily affects the airways. Initial symptoms may resemble a common cold but often include systemic signs such as:
- Fever
- Sore throat
- Weakness
- Muscle pain (myalgia)
- Joint pain (arthralgia)
- Headache
- Cough (generally dry and non-mucous)
- General malaise
In more serious instances, complications like pneumonia can occur, which may be fatal, especially for young children and the elderly. Although often confused with the common cold, influenza is a more severe illness caused by a different virus. In children, influenza can also frequently cause nausea and vomiting; this presentation is sometimes termed “stomach flu” or abdominal gastroenteritis.
Transmission and Viral Stability
The flu spreads primarily through droplets containing the virus, emitted via coughing, sneezing, or even talking by infected persons (aerosolized secretions from saliva, nasal, and bronchial discharge). More rarely, transmission can occur through the feces of infected birds. It is also transmissible via blood and contaminated surfaces or objects, known as fomites.
Viral Survival
The influenza virus is more resistant in cold and dry conditions. It can retain infectivity for:
- One week at human body temperature.
- 30 days at 0°C.
- Much longer at lower temperatures.
The virus can be easily inactivated by detergents or disinfectants.
Epidemics and Pandemics
Influenza is distributed in seasonal epidemics, causing hundreds of thousands of deaths globally, escalating to millions during a pandemic (a global epidemic). During the twentieth century, five flu pandemics occurred due to the appearance of new strains through mutation. Often, these new strains emerge from the transfer of typical animal strains to humans, known as species jump or heterocontagion.
Historical Strains
A deadly strain of avian influenza virus, H5N1, was considered a leading candidate for the next human pandemic after it crossed the species barrier in 1990, causing dozens of deaths in Asia, preceding the emergence of novel flu A (H1N1) in 2009. Fortunately, that H5N1 variant did not mutate to spread easily from person to person; its infection required very specific conditions, primarily involving contact with infected birds.
Prevention and Treatment
Vaccination Programs
In developed countries, annual vaccination campaigns are established for those at the highest risk of contracting the disease or those most vulnerable to its complications. Strict controls are also placed on poultry vaccination alongside the usual human vaccination.
The standard vaccine is trivalent, containing purified and inactivated proteins from the three strains considered most common for the upcoming epidemic: two subtypes of influenza A viruses and one influenza B virus. A vaccine effective one year may not be effective the next due to the virus’s frequent and rapid antigenic changes and the variable dominance of different strains.
Medical Intervention
Treatment is primarily symptomatic. In severe cases requiring hospitalization, constant maintenance is necessary because antiviral drugs have limited efficacy (neuraminidase inhibitors are the most effective) and are not without toxicity. Antibiotics are only useful if a secondary bacterial infection is present.