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Did you know there are so many different causes of flu-like illness?
Influenza-like illness (ILI), also known as influenza-like syndrome or flu-like illness, is a medical diagnosis for a group of common symptoms caused by a variety of potential causes. According to the World Health Organization, symptoms of influenza-like illness include fever, shivering, general weakness, dry cough, loss of appetite, body aches, nausea, and sudden onset of sneezing. Most of these symptoms result from the release of cytokines as a result of immune system activation and therefore tend to be relatively nonspecific in nature.
Common causes of ILI include the common cold and influenza, although the common cold is more common, the influenza is usually more severe.
The causes of influenza-like illness are not limited to influenza. Its origin may come from a variety of aggressive pathogens, ranging from relatively mild self-limited diseases (such as gastroenteritis and rhinovirus infection) to severe life-threatening diseases (such as meningitis, sepsis and leukemia). This complicates the diagnosis and management of influenza-like illness.
Influenza and its epidemiology
Technically, any clinical diagnosis of influenza could be classified as an ILI, not just influenza itself. It is important to note that even at the peak of the flu epidemic, most cases of ILI were not caused by the influenza virus. It is estimated that in the United States, the average adult will experience 1 to 3 influenza-like illnesses per year, while children may experience 3 to 6. Influenza virus surveillance is carried out by a network of more than 110 National Influenza Centres around the world, which test for the presence of influenza viruses based on samples provided by patients with influenza-like illness.
In the United States, during the 2008-2009 flu season, only 14.1% of reported samples tested positive.
Influenza test samples mainly come from the patient's respiratory tract, usually collected by a doctor or nurse and sent to a hospital laboratory for screening. There are several ways to obtain a respiratory sample, the simplest of which is to swab the nose with a dry cotton swab.
Other possible reasons
Infectious diseases that cause influenza-like illness include respiratory syncytial virus, malaria, acute HIV/AIDS infection, herpes simplex, hepatitis C, Lyme disease, rabies, myocarditis, Q fever, dengue fever, polio, pneumonia, Measles, SARS, COVID-19, etc. Certain medications may also cause an ILI, including many biologics (such as interferons and monoclonal antibodies) and chemotherapy drugs. Not only that, flu-like symptoms can occur after influenza and other vaccinations, and physically dependent individuals can experience similar symptoms after stopping opioids.
Influenza-like illness is a nonspecific respiratory illness characterized by symptoms such as fever, fatigue, and cough that usually disappear on their own within a few days.
Diagnosing influenza-like illness requires doctors to use a combination of epidemiological and clinical data, and if necessary, perform laboratory and imaging tests to confirm the cause. When influenza outbreaks occur, many cases are mistaken for swine flu, which requires a careful review of medical records and contact history to make the identification.
Influenza-like illness in horses
Influenza-like illness has also occurred in some horses, often following vaccination. For these horses, light exercise can help speed recovery. Nonsteroidal anti-inflammatory drugs (NSAIDs) may also be used with the vaccine to reduce symptoms.
Influenza-like illness is a common clinical manifestation, but it hides a variety of different potential causes. The flu as we know it isn't always to blame; in fact, many infections can cause similar symptoms. In such a complex situation, how can we effectively identify the root cause of influenza-like illness?
