CDC Influenza Case Definition: Symptoms & Guidelines

The Centers for Disease Control and Prevention (CDC) have developed guidelines for defining a case of influenza, also referred to as influenza-like illness (ILI). Updated case definitions are prepared as new influenza strains emerge.

Influenza Definition

• Human influenza, or ILI, is defined by a temperature in excess of 100 degrees F and the presence of a cough and/or a sore throat in a patient.

Misconceptions

• Contrary to popular belief, patients who experience cold chills, muscle aches, runny nose and nasal congestion are not necessarily suffering from influenza, unless there is also a fever and a sore throat or cough present.

Exceptions

• It is possible for a patient to experience both a fever in excess of 100 degrees F and a sore throat, but not have influenza. Further testing of the patient can rule out other illnesses, such as strep throat.

Novel Influenza A

• The CDC states that infection with novel influenza A virus is compatible, yet different, from the more commonly circulating human influenza H1 and H3 viruses. As of 2009, novel influenza types include avian flu, swine flu, H1N1, and other mammal-originated flu viruses.

Novel Influenza Classification

• Novel influenza cases can be confirmed via lab tests perform by the CDC, and suspected cases have been tested and are awaiting confirmation from the CDC. Probable cases are those that have not been tested, but the patient has defined symptoms and contact with a confirmed case.