What is onset fever?

Onset fever, also known as biphasic fever or saddleback fever, is a pattern of fever in which the temperature rises rapidly to a high level, then falls to normal or below normal, and then rises again. The pattern is often seen in certain infectious diseases, such as dengue fever, malaria, and yellow fever, and is characterized by two distinct phases:

1. Initial high fever: The temperature rises rapidly, often reaching 104°F (40°C) or higher within a few hours. This phase may last for several days.

2. Defervescence: The temperature drops rapidly, sometimes reaching normal or below normal levels. This phase may also last for several days.

3. Second high fever: The temperature rises again, often reaching the same or higher levels as the initial high fever. This phase may last for a few days before gradually returning to normal.

The exact mechanism of onset fever is not fully understood but it is thought to be related to the body's immune response to the infection. It is believed that the initial high fever is caused by the release of inflammatory mediators, such as cytokines, in response to the infection. The defervescence phase occurs when the body's immune response begins to control the infection, leading to a decrease in inflammation and fever. The second high fever may be due to a secondary immune response or a resurgence of the infection.

Onset fever is a common symptom of several infectious diseases, and the specific pattern of fever can help healthcare professionals in making a diagnosis. It is important for individuals experiencing onset fever to seek medical attention promptly, especially if accompanied by other symptoms, to receive appropriate treatment and monitoring.