What Is Considered a Fever? Causes & Treatment

For body systems to function properly, billions of chemical processes occur to create hemostasis, which is the state of balance within our bodies. Most of those chemical reactions need the optimal temperature to occur. That is why temperature is under the very strict control of many nervous and hormonal mechanisms. Any alteration of those mechanisms can cause a rise or a drop in body temperature. Such change is never normal and can mean anything from a mild infection to a severe life-threatening condition.

What is a fever?

One of the common misconceptions regarding human body temperature is that it is a constant number – usually 37 Celsius or 98.6 Fahrenheit-. Generally speaking, there is never a single value for any vital measurement, be it the heart rate, temperature, blood pressure, or even laboratory values. The reason behind this is the wide variation of internal and external environments of the human body. To counter that, we usually place a normal range for such values, and for temperature, this value ranges from 36.5–37.5 °C or 97.7–99.5 °F.

There is no reason to worry if your temperature is within this range. Reasons as to why your temperature can “swing” within those values include:

Age: Age can be a factor owing to the difference in the level of activity, muscle mass, and intolerance to rapid external temperature change.
Sex: Women and men differ widely in their muscular build.
Female monthly cycle: Female menstrual cycle is a major contributor to monthly temperature changes (about half a degree difference). This results from the fluctuation of the levels of female sex hormones (estrogen and progesterone).
Level of activity
External temperature: No matter how effective your sweating or muscular structure are, your temperature is likely to be affected by outside cold or heat.

If your temperature rises above this normal range, it is called fever and is classified into three grades:

Low-grade fever: 99.5-100.5 °F.
Moderate fever: 100.6-103.5 °F.
High-grade fever: 103.5-105 °F.
Hyperpyrexia: >105 °F: Hyperpyrexia is the most severe and is considered life-threatening unless immediate action is taken to protect the brain against such a high temperature.

How is temperature measured?

Temperature is measured using a thermometer. Many types of thermometers are present including analog and digital ones. Analog thermometers use mercury inside a glass tube since it expands under heat. Mercury thermometers have to be handled with care since mercury is poisonous on ingestion or inhalation.

Thermometers can measure temperature from different body sites including:

- Mouth: The mouth is the most used site for temperature measurement. The thermometer is placed under the tongue, and the patient is asked to keep it for a few minutes. This method is not suitable for children or non-cooperative individuals since they can bite the thermometer bulb and ingest the mercury.
- Rectum: This is the most accurate measuring site for temperature. Children are usually the ones that need rectal temperature measurement, since their core temperature can differ from their skin surface temperature and measuring by mouth is not suitable for them.
- Ear: The ear is also a common site for quick temperature measurements using digital thermometers.
- The armpit: The armpit is the least accurate site for temperature measurement since it can easily be cooled without affecting the body’s real temperature.

Different values exist for different reading sites. In general, you can add half a degree to the recorded armpit temperature to get the correct reading.

Symptoms of fever

Fever affects different chemical processes, therefore its occurrence causes generalized rather than local symptoms. Other symptoms that may be present differ according to the underlying cause of fever rather than the fever itself.

Common symptoms of fever include:

Chills: When your body’s temperature rises, your brain misinterprets that into a fall of the room’s temperature, causing you to feel cold and to shiver.
Sweating: Sweating is our natural way of reducing our body’s temperature.
Fatigue: Fatigue can result as a response to the depletion of the body’s energy on controlling the fever due to an underlying infection.
Loss of appetite
Headache
Body aches
Dehydration: Dehydration is felt as an unquenchable thirst, fatigue, and weakness. It results from excessive sweating.
Delirium: Delirium can occur in high-grade fever or hyperpyrexia. The brain becomes unable to function properly at such high temperatures affecting awareness. If the temperature rises higher, there is a risk of permanent brain damage unless the temperature is brought down.

Causes of fever

There are several causes for fever including infectious causes, autoimmune conditions, medications, environmental conditions, and some more serious conditions like cancer.

Common infectious causes of fever include:

Upper respiratory tract infections: Respiratory tract infections are by far the commonest causes of fever in adults. Common cold and influenza are usually the culprits, especially in winter. Patients can have other symptoms like coughing, sneezing, muscle aches, and sometimes gastrointestinal manifestations like vomiting and diarrhea. Most upper respiratory infections are viral and cause a low-grade fever. If an upper respiratory infection is associated with a high-grade fever, it is likely to be of bacterial origin or a bacterial infection superimposed on a viral one.
Gastrointestinal infections: Bacterial and viral gastrointestinal infections are common, especially in children and infants. They are unlikely to cause a fever in adults and tend to be of a milder course. In infants, viral diarrhea can be severe and cause life-threatening dehydration. That is why rapid oral rehydration is needed in such cases. Doctors avoid using antibiotics unless there is a clear bacterial origin to diarrhea -evidenced by high fever, bloody diarrhea, or stool analysis- since antibiotics can worsen the condition is misused.
Tuberculosis: Tuberculosis is a bacterial infection that can cause night sweating and fever. It also presents by expectoration which can be bloody, loss of weight, and weakness.
Abscesses: An abscess anywhere in the body can trigger fever and cause severe manifestations depending on its size and location.
Meningitis: Meningitis is one of the most serious infections. Bacterial meningitis can cause a sharp rise in body temperature and rapid deterioration of the general condition that the patient can die within a few days. Other symptoms include severe headache, neck stiffness and pain, blurred vision, and photosensitivity.
Parasitic infestations: Parasites do not commonly trigger fever except if they migrate through the body like ascaris or infest the blood like malaria. Malaria has a special type of fever that comes in episodes every three or four days. It is also associated with profuse sweating.
Rheumatic fever: Rheumatic fever is a reaction to a special bacterial infection of the tonsils called streptococci. It is commonly seen in children and includes other symptoms like rash, joint pain, and chest pain if the heart is involved. It is an immunological reaction to the bacteria that can develop lifelong heart conditions unless treated.
Bacterial endocarditis: Bacterial endocarditis is the infection of the inner lining of the heart and heart valves by special bacteria. The condition is common among patients who had previous valve damage by rheumatic fever, those with prosthetic valves, and drug addicts. It can be life-threatening.

Autoimmune conditions: Autoimmune conditions cause systemic inflammation which stimulates white blood cells to secrete excessive amount of chemicals that raise body temperature. It can be used to detect flares of the condition. Common autoimmune diseases that may cause fever include rheumatoid arthritis, systemic lupus, systemic sclerosis, and dermatomyositis.

Drugs: Several drugs can cause fever due to hypersensitivity reactions towards the drug itself as in penicillin, anticonvulsants and some antigout drugs like allopurinol. Other drugs can cause fever through its action as chemotherapy, while others can cause fever by altering the body metabolism as thyroid drugs.

Prolonged exposure to the sun and heatstroke: Severe dehydration and loss of mineral-rich sweat in excess can cause a rapid rise of body temperature.

Cancer

Many cancers can cause an elevation of the body’s temperature due to:

Increased body metabolism owing to the rapid uncontrolled division of cancer cells.
As a result of cancer cell destruction by chemotherapy or radiotherapy which results in the release of chemical inflammatory mediators that induce fever.
Paraneoplastic syndrome: paraneoplastic syndrome comprises a group of symptoms not related to the cancer itself, rather the result of the release of hormones or chemical mediators by cancer cells. Among these hormones are those that raise body temperature, blood pressure, or cause gastrointestinal side effects like diarrhea and cramping.

Pyrexia of unknown origin (PUO)

Sometimes, fever can have no detectable cause either by clinical examination or laboratory investigations, and in such case, it is called a pyrexia of unknown origin. For fever to be named as PUO, it has to last for more than 3 weeks and to be more than 100.9 °F. Being of an unknown origin doesn’t mean that it doesn’t have one, however, and further, more detailed investigations would be needed.

Diagnosing a case of fever

Fever is not a standalone disease, and identifying its origin is the main key to successful treatment. The treatment of some causes of fever can irreversibly harm a patient of another cause. After hearing the patient’s complaint, the doctor will proceed to measure their temperature and perform a thorough physical examination including:

Assessing vital signs like pulse and blood pressure. Pulse is normally elevated in any case of fever by 10 beats per minute for every 1 degree rise of temperature. Some bacterial infections dull such rise including typhoid fever.
Examining the main site of complaint, which is commonly the throat in upper respiratory infections or the abdomen in case of gastrointestinal infections. Your doctor may also listen to your breathing sounds and examine your chest if they suspect pneumonia.
Full examination of joints or any other tender musculoskeletal site. Infections like osteomyelitis or arthritis can be missed or hidden. Any tender site can also mean an abscess.
In infants who present with persistent crying or vomiting, the doctor will exclude meningitis using special clinical tests.

Lab investigations

Lab investigations are not always needed, but they are ordered to detect the causing organism, confirm the diagnosis and assess the general condition. They include:

Complete blood count: White blood cell count is of particular importance. Patients who have infections tend to have an elevated white cell count. If a patient has all signs of infection and a low or normal white cell count, it can mean they are immunocompromised. Differential count of white blood cells can also be of significance in detecting the type of organism. Neutrophils rise in bacterial infections, lymphocytes in viral infections and autoimmune disorders, and eosinophils in parasitic infestations.
ESR and CRP: These two markers indicate of inflammation and a rise in their titers usually means that active inflammation is present.
Sputum culture and tests: In some cases of respiratory tract infection if bacteria are suspected. Sputum culture can identify it and its antibiotic susceptibility. Tuberculosis needs special testing and microscopic stains.
Blood culture: Blood culture is commonly needed in cases with bacterial endocarditis.
Urine analysis: Urine analysis can identify the presence of a urinary tract infection and the bacteria causing it. Culture can also be done on the bacteria to identify its antibiotic susceptibility.
Stool analysis and culture: Stool analysis can reveal the presence of parasitic ova and some bacteria. Stool culture can also help identify bacteria and their antibiotic susceptibility.
Lumbar puncture: Lumbar puncture and CSF analysis are done when meningitis is suspected by clinical symptoms or examination. The CSF will be examined both by eye and under the microscope and any bacteria will be cultured.
Immunological markers: Immunological markers can be used in case of suspicion of autoimmune diseases like systemic lupus. Commonly ordered serological tests include rheumatoid factor for rheumatoid arthritis, anti-ds DNA and anti-nuclear antibodies for lupus.

Imaging studies

Chest X-ray: Chest radiograph may be needed in cases suspected of having pneumonia.
Ultrasonography: Ultrasonography may be needed to visualize the kidney, the liver and the gall bladder. It is important to see the kidney in cases of urinary tract infection since there is likely to be an obstruction as well.
CT: CT scan is usually not routine except in cases of abscesses or bone infections.
Echocardiography: Echocardiography is needed in cases of bacterial endocarditis.

Treatment

Treating fever has two main aims, the first is to bring down the temperature to alleviate symptoms, and the second is to treat the underlying condition that caused the fever in the first place. In some mild viral infections, controlling symptoms is all that is needed, and the body takes care of the rest. That means that not all causes of fever are bacterial and not all causes of fever need antibiotics.

A) Symptomatic control:

- Drink water: Drinking water may be the best method to bring down a fever. Your body uses its water to control your fever, so it is at risk of dehydration.
- Rest: Exerting yourself causes your muscles to contract and produce heat, which worsens your fever.
- Take a shower: Taking a bath or a shower helps reduce temperature rapidly. Do not take a warm shower nor a very cold one, rather aim at a tap water temperature shower or bath.
- Sponge bath: Sponge bath is another option that helps reduce the temperature. Soak sponges in tap water and apply them under your armpits, on your neck, and groin. These regions have large blood vessels which makes contact more direct with the blood and helps reduce temperature faster.
- Eat plenty of protein: There is a myth regarding fish and its negative effect on fever which is scientifically false. It is preferred that you eat all sources of protein to help your body produce antibodies. The fastest digestible protein is poultry.
- Over-the-counter medications: Some over-the-counter medications are useful including Paracetamol which is the safest. Avoid using other analgesics like Diclofenac or Ibuprofen without your doctor’s advice since they come with their share of side effects on the stomach and kidneys.

B) Specific therapy

If you are feeling unwell, visit your doctor and do not self-medicate at home. If you are living somewhere where medications are easily accessible, do not try to self-medicate with antibiotics or other drugs as they can have severe side effects and may not help in your condition. Your doctor will order the necessary investigations and will prescribe the best treatment for your condition. If you do have a bacterial infection and you blindly take antibiotics, this can affect culture results and can give bacteria another antibiotic to be resistant to.

Viral infections don’t need specific drugs except in a few cases when the infection is severe. Autoimmune disorders need immunosuppressive drugs which if given to a patient with an underlying infection can worsen the condition. Such considerations can never be done at home and need various investigations and monitoring.