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Discovering the Secrets of Flu Surveillance: Why Sometimes You Don't Get Detected?
Influenza-like illness (ILI) is a medical diagnosis that may be caused by the flu or other conditions and presents with a range of common symptoms such as fever, shivering, chills, general malaise, dry cough, decreased appetite, body aches, and nausea and sneezing, etc., often accompanying a sudden onset of significant illness. Most of these symptoms are caused by cytokines released by the activation of the immune system, so their specificity is relatively low. Although the flu may be rare, its symptoms are usually more severe than the common cold.
The World Health Organization defines ILI as: If the patient's body temperature reaches or exceeds 38°C and a cough occurs, and the cough begins within the past 10 days, then the illness can be classified as an influenza-like illness.
Multiple causes of ILI
Influenza-like illnesses can have a variety of causes, ranging from self-limiting, benign gastroenteritis to viral colds and even potentially serious illnesses such as meningitis and sepsis. For example, influenza is considered ILI in clinical diagnosis and does not necessarily refer to the influenza infection itself, which makes the diagnosis of ILI more flexible.
The process of influenza surveillance
The United States has a global surveillance network of more than 110 national influenza centers responsible for clinical surveillance of ILI cases. These centers take samples from patients diagnosed with ILI and test them for the presence of influenza viruses. It is important to note that not all patients with ILI are tested, and not all test results are reported. Testing is usually based on ILI severity, routine sampling, or at participating surveillance clinics.
In most years, influenza viruses are not detected at high rates in the samples tested. During the 2008-2009 flu season, the CDC reported that only 14.1% of 183,839 samples tested positive.
Other possible causes
Other infectious diseases that cause influenza-like illness include respiratory syncytial virus, falciparum malaria, and acute HIV infection. If a patient is found to have characteristics of an influenza-like illness during testing, the doctor may conduct a comprehensive assessment and necessary laboratory tests to diagnose the cause based on a series of epidemiological and clinical data.
Differential diagnosis of influenza-like illness
Because of the non-specific characteristics of ILI, doctors need to carefully conduct differential diagnosis when making a diagnosis. For example, during the 2009 influenza pandemic, a large number of ILI cases were reported as suspected swine flu, but most were actually misdiagnosed.
During the 2009 influenza pandemic, the CDC recommended that physicians consider swine flu infection in the differential diagnosis, especially if the patient had recent contact with a confirmed swine flu case.
In addition, if the patient has a history of exposure to anthrax, the differential diagnosis of ILI needs to include anthrax. Although ILI is common, not all cases appear to be influenza, which may result in potentially severe cases being missed.
ILI in horses
In some cases, ILI can develop in horses after vaccination. Mild exercise for these horses will help relieve symptoms.
Research on ILI is ongoing in all medical circles, aiming to better screen and identify various diseases and protect public health. However, as influenza viruses change and new diseases emerge, surveillance systems may not be able to cover all patients. This has led many people to start thinking, in this high-tech era, why sometimes influenza patients cannot be effectively detected?
