Fever or pyrexia is when the body’s temperature is above the normal range. Normal body temperature is 36.5–37.5 °C (97.7–99.5 °F) and most would consider a fever being present at around 38 °C (100.4 °F). A fever is probably one of the most common clinical signs and is considered the body’s natural response to an infection.
A fever is usually accompanied by feeling lethargic and run down, headache, loss of appetite, tiredness, hyperalgesia, dehydration, irritability and maybe confusion.
The fever response may have evolved to give an advantage to fight infections and help the host defense. The higher temperature may inhibit bacterial reproduction and mobility. Some immunological functions and reactions are increased in higher temperatures.
Body temperature is a balance between heat produced by tissues (mainly the the liver and muscles) and heat that is lost from the periphery. The hypothalamus is the thermoregulatory center which maintains the internal temperatures. A fever will result when the hypothalamus set point is raised. This will initiate a vasoconstriction and shunting of blood from the periphery to decrease heat loss. This will continue until the temperature of the blood passing through hypothalamus reaches that new set point. Endogenous pyrogens are the substances that cause fever and are usually microbes or their products (eg endotoxins) and they induce the release of endogenous pyrogens (eg interleukin-1, tumor necrosis factor and other cytokines). These endogenous pyrogens are transported to the hypothalamus thermoregulatory centre where they trigger the synthesis of prostaglandins which raise the thermostatic set point to initiate the fever response.
The hypothalamic set point can be lowered by, for example antipyretic drugs, and this will initiate a heat loss through increased sweating and vasodilation.
Causes of a Fever:
A fever can be caused by a wide range of medical conditions that vary from infectious to non-infectious and range from the not serious to life-threatening.
The infectious causes can be viral, bacterial or parasitic infections causing problems such as influenza, common colds, meningitis, ebola, urinary tract infections, appendicitis, osteomyelitis, Lyme disease, HIV, Rocky Mountain spotted fever, COVID-19 and malaria.
The non-infectious causes include conditions like vasculitis, deep vein thrombosis, gout, systemic lupus erythematosus, malignancy, post immunization, connective tissue disorders and postoperative fever.
Differential diagnosis: hyperthermia (temperature over 37.5 or 38.3 °C (99.5 or 100.9 °F) due to too much heat production or inadequate heat loss; not considered a fever; doesn’t respond to antipyretic medications)
Fever of Unknown Origin (FUO):
This is generally when there is a fever of more than 3 weeks and no explanation has been found for the raised temperature despite appropriate investigations. An infection is probably the most likely cause but could be due to neoplasia or connective tissue disease. There are generally considered four types of FUO: classic FUO, nosocomial FUO, neutropenic FUO and HIV-related FUO.
This is a life threatening emergency when the body temperature goes above 41.5 °C (106.7 °F). In addition to the high temperature they may also have extreme sweating, confusion, thirst, dizziness, headache, vomiting, nausea, muscle cramps and fatigue. If it is not treated it will lead to organ failure and be fatal.
Management of Fever:
Generally not specifically needed, but depends on the underlying cause.
Supportive care: rest; adequate hydration (especially if sweating a lot).
The use of a fan or air conditioning to cool may increase the comfort for some.
Regular monitoring of temperature and symptoms for changes, especially for signs of hyperpyrexia.
Antipyretics: ibuprofen, paracetamol (acetaminophen)
Hyperpyrexia will need more aggressive cooling eg ice packs.
Hyperthermia | Effects of Exercise in Hot Weather
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