When a person’s body temperature temporarily increases, this is known as fever (also called controlled hyperthermia, pyrexia or elevated temperature). Often as a result of an illness, a fever is a symptom that something abnormal is happening in the body. How serious a fever is can be influenced by a person’s age, the underlying cause (which can sometimes be non-infectious in nature) and the temperature increase degree. A slightly elevated temperature may not always be as serious for an adult as it is likely to be for a young child (infant or toddler).
Fever is typically accompanied by a wave of fatigue or chills, and is often a response to an infection (viral or bacterial) or inflammation (due to tissue injury or illness). Non-infection causing fevers can be as a result of poisons, drugs, injuries to or abnormalities of the brain, heat exposure or endocrine diseases (problems with the glandular or hormonal system).
A typical fever (mild) should clear within a matter of days and can be treated with an assortment of available over-the-counter medications (or antipyretics). Sometimes a mild fever can clear on its own without the need for any medicinal treatment.
It has been noted that fever can also play a key role in helping the body’s immune system to naturally fend off a variety of infections (i.e. body temperature increases as a natural immune response to infection). The symptom of fever is one of the immune system’s natural attempts to get the better of an infection that is apparent in the body. The immune system alerts to the presence of an infection and ‘uses elevated body temperature’ to help resolve it. If body temperature (fever) is too high, the body cannot help to resolve whatever is wrong (i.e. neutralising some kind of bacterium or virus in the body) and the fever may result in serious side effects including dehydration, hallucinations and even convulsions.
A higher than normal increase in temperature can affect anyone of any age. A fever, alone, is not an illness, but does indicate that something not working as it should in the body. An infection of some kind is usually the underlying condition causing the symptom of fever. When severe, a fever is often a warning sign that the body is seriously unwell (a person may have a serious health condition) and needs prompt medical attention.
What is a normal body temperature?
A normal body temperature does vary from one person to the next, but is typically around 36 to 37 degrees Celsius (or 98.6 degrees Fahrenheit). Body temperature also tends to fluctuate at different times of the day – it is lower in the mornings and increases towards late afternoon and evening. Our body temperature is normally at its highest at around 6pm and at its lowest at around 3 in the morning. Some other factors which influence fluctuations in body temperature are intense exercise and for women, in particular, a menstrual cycle.
As a person’s body temperature increases, he or she may experience a temporary sensation of cold until the temperature plateaus (reaches its peak). A person experiencing a fever rarely does so without the presence of other symptoms. Most fevers are accompanied by a set of very specific symptoms. Together, a set of symptoms, including fever, can help a medical professional to be able to distinguish an underlying cause and diagnose a condition. The correct course of treatment can then be implemented so that a person can return to optimum health.
How does body temperature work?
The hypothalamus (pronounced hi-poe-THAL-uh-muhs) is a part of the brain which usually controls temperature in the body. The hypothalamus links the endocrine and nervous systems via the pituitary gland (hypophysis). This part of the brain is, in effect, the body’s own thermostat, which maintains a normal temperature through heating (such as increased metabolism) and cooling (such as sweating) mechanisms. The hypothalamus is effectively in the driver’s seat using key temperature sensors to control heating.
As skin temperature rises above its baseline, sweating begins and can increase rapidly. If body temperature drops below baseline, the control mechanisms initiate activity to conserve heat in the body and increase production of heat or warmth. This is done by means of the cessation of sweating, shivering (to increase the production of heat in the body’s muscles), the secretion of norepinephrine, thyroxine and epinephrine (hormones and neurotransmitters that regulate fight or flight, thyroid function and metabolism), and vasoconstriction (decreasing the flow of heat to the skin).
Fever-producing substances, or pyrogens, can enter the body and upset the natural flow of temperature, thus causing fever. Pyrogens are foreign to the body and typically come from a source outside of it (such as viruses, fungi, bacteria, illicit drugs or toxins), and stimulate additional pyrogens once inside. These effectively send a ‘message’ to the hypothalamus (known as neural feedback mechanisms) to increase body temperature. The effect is that the body’s normal heating mechanisms react by shivering and constricting blood vessels. The body is then primed to reach a new temperature that is higher than its normal baseline in order to eliminate temperature sensitive viruses and bacteria.
The body also produces its own pyrogens in response to inflammation causing fever (this is a natural response). These are known as endogenous pyrogens or cytokines.
Temperature devices (thermometers) can be used to determine a body temperature measurement. These devices can be inserted into a person’s mouth, rectum, axilla (under the armpit), skin or ear. Other devices which a medical professional may use have temperature-sensing probes that can record temperature while in use. These include laryngoscopes, rectal probes and bronchoscopes. A mercury thermometer is the most common device used, but digital thermometers with disposable probe covers are also regularly used. Rectal temperature measurements are generally more accurate (reflecting the core temperature) than those taken orally.
What temperature classifies as a fever?
- For babies or infants, a young child or teenager: When the body temperature exceeds 37.5 degrees Celsius (99.5 degrees Fahrenheit).
- For an adult: When the body temperature exceeds between 37.2 and 37.5 degrees Celsius (99 to 99.5 degrees Fahrenheit).
Fevers can be classified as:
- Low-grade: Temperatures range between 37.7 and 38.3 degrees Celsius (100 and 101 degrees Fahrenheit). If a low-grade fever doesn’t clear within 4 to 7 days, medical treatment should be sought out. If any fever persists, you must consult with you doctor for a thorough check-up.
- Intermediate: Temperatures of 38.8 degrees Celsius (102 degrees Fahrenheit) for an adult and ranging between 39.4 to 40 degrees Celsius (103 to 104 degrees Fahrenheit) for an infant (0-6 months of age). If a fever reaches this stage, it is best to seek assistance from a medical professional as soon as possible. The more prompt you are at addressing the fever and having it checked, the better.
- High-grade: Temperatures range between 39.4 and 41.6 degrees Celsius (103 and 107 degrees Fahrenheit) or higher. Extremely high fever is known as hyperpyrexia and can be very dangerous or life-threatening,
What are some of the different fever types?
Terms used to characterise fever types include:
- Persistent fever (or prolonged fever): A fever that lasts between 10 and 14 days or longer. These fevers are typically low-grade.
- Constant fever (or continuous fever): A fever that appears to change by 1 degree within a 24-hour period. These fevers are also commonly low-grade.
- Chronic fever: A fever that recurs for several months or years and lasts between 3 and 4 days at time. These fevers are usually of the intermittent variety.
- Intermittent fever: Temperature fluctuates between baseline (normal) and at fever levels over the course of a single day. A fever may recur between 1 and 3 days after a previous one (which may only last a day).
- Remittent fever: A fever may come and go, with fluctuating temperature at regular intervals.
- Hyperpyrexia fever: A dangerously high fever (at least 41 degrees Celsius / 106 degrees Fahrenheit or higher) is considered a medical emergency and requires immediate assistance.
What are some of the most common causes of fever?
Body temperature can elevate for a variety of reasons. Some of the most common reasons why include:
- Infections – due to a bacteria or virus coming in contact with the body (these can include colds and flu, gastroenteritis, chickenpox or infections of the ear, skin, throat, lungs, bladder or kidneys).
- Heat exhaustion or heat stroke (due to exposure to high temperatures or prolonged strenuous exercise)
- Sunburn (extreme)
- Teething (infants)
- An inflammatory condition, such as rheumatoid arthritis, lupus or inflammatory bowel disease (IBD)
- Hormone disorders, such as hyperthyroidism.
- Use of medications, such as antibiotics or those used to treat seizures or high blood pressure (hypertension).
- A malignant tumour
- Blood clots
- Food poisoning
- A side-effect of a vaccination, such as those for tetanus, diphtheria, pneumococcal vaccine or acellular pertussis (DTaAp).
- The use of illicit drugs (cocaine or amphetamines) or alcohol withdrawal
- Unidentified or ‘fever of unknown origin’ or FUO (sometimes a cause of a fever may not be able to be identified following extensive evaluation).
Common types of fever
Based on the variety of underlying causes, some of the most common fever types include:
- Viral fever: Most viral fevers caused by airborne viruses may cause accompanying symptoms of nausea and vomiting, an upset stomach and diarrhoea along with fever. These fevers usually clear with time, with or without the aid of relieving medications (over-the-counter).
- Bacterial fever: A bacterial infection can affect almost any and every organ system in the body – namely the central nervous system (the brain and spinal cord), lower respiratory system, upper respiratory system, the genitourinary system (affecting the bladder, kidneys and urinary tract), the reproductive system, gastrointestinal system (digestive system), circulatory system (heart and lungs) and the skin. Antibiotics are necessary to treat any of these affected areas.
- Fungal fever: A infection can also affect any organ system in the body. A physical exam and testing can determine the underlying cause of an infection. These types of related infections are typically treated with antifungal medications.
- Animal exposure fever: A rare bacteria can be contracted by those who work closely with animals (livestock). Exposure (which can include contact with the urine of an infected animal or unpasteurised dairy products) can cause headache, muscle and joint aches, as well as fever and chills.
- Traveller’s fever: Exposure to new foods, insects, toxins or some vaccine-preventable illnesses (such as yellow fever). Low-grade fevers can occur following the consumption of contaminated water, or uncooked fruits or vegetables, and unpasteurised dairy products. Antibiotics can be used to treat symptoms of fever, nausea, vomiting, headache and abdominal bloating.
- Blood clot fever: Blood clots can develop in the leg (calf) and cause pain and swelling. When a portion of the clot breaks and travels through the bloodstream to the lungs causing inflammation in the blood vessels (pulmonary embolus), chest pain and breathing problems occur. Inflammation can bring on fever as well.
- Drug (medication) fever: Starting a course of new medication can bring about fever as a side-effect. Fever typically clears with the discontinuation of the medication. In some cases, a fever following the start of a new medication can be as a result of an allergic reaction. In this instance, the development of fever is usually immediate.
- Environmental fever: Overheating and very high body temperatures can lead to hyperthermia. This can be caused by prolonged strenuous exercise of hot and humid weather conditions. Hyperthermia (environmental fever) can result in a person experiencing lethargy, confusion and even becoming comatose. This type is considered a medical emergency and a person must be immediately taken to hospital.
- Tumour fever: Pyrogens and chemicals can produce fever in those battling cancer, especially when tumours become infected. Medications used to treat cancer can also bring about fever side-effects as well as lead to weakened immune systems.
- Special medical conditions: This type of fever is as a result of certain medical conditions, such as diabetes or HIV, that prevent a person’s immune system from working effectively in the body. These conditions make individuals especially vulnerable to fever-causing infections. In some instances, it may be difficult to find a cause of fever, depending on the type of disease. When a person has a condition that limits the body’s ability (immune system) to fend off infection, the symptom of fever can be dangerous or life-threatening. Fever in combination with an existing condition should be seen and treated by a medical professional as soon as possible.
Signs and symptoms associated with fever
Fever is usually accompanied with a variety of other symptoms, and depending on the underlying cause, the combination can be useful in diagnosing a particular condition, allergy or infection. Some associated signs and symptoms include:
- Shivering or feeling cold (unexplained)
- Joint and muscle aches
- Body weakness
- Loss of appetite
- Confusion and concentration problems
- Hallucinations or delirium
- Hyperalgesia (pain sensitivity)
When should you worry about a fever?
When to worry depends on a person’s age and level of risk. Infants and young children, for instance, are at particularly high-risk of serious fever-related complications. You should consult a medical professional as soon as possible in the following scenarios:
- Infants and babies: If an infant has a rectal temperature of at least 37.5 degrees Celsius (99.5 degrees Fahrenheit), you should consult with your healthcare professional immediatly so that a thorough examination can be conducted and treatment administered. It is important to seek medical assistance immediately so that the fever can be lowered before it causes any complications.
- Children and teenagers: If a child has a fever, but is responsive (i.e. making eye contact and responding with facial expressions and speech or conversation), there is generally no major cause for alarm. If a child is not drinking fluids, is irritable or listless, has a severe headache or abdominal pain, poor eye contact and is repeatedly vomiting along with fever, seek medical attention. Other warning signs whereby medical attention should be sought include if a child develops fever after being inside a hot car for a period of time or has a fever that lasts for at least 2 days or longer.
- Adults: If an adult experiences a fever that is at least 37.5 to 38 degrees Celsius (99.5 to 100.4 degrees Fahrenheit) or higher, for 2 days or longer, a consultation with a healthcare provider should be booked. Immediate medical attention is necessary if a person experiences other symptoms along with fever, including severe swelling in the throat, difficulties with swallowing, severe headache, a sensitivity to bright light, unusual skin rash (or skin that appears red, swollen and feels warm or hot), a stiff neck, persistent vomiting, chest pain, breathing difficulties, mental confusion, abdominal pain, pain while urinating, blood in stool (faeces), swelling in the legs, sensory changes, muscle weakness, extreme irritability or seizure.
Diagnosing and treating fever
Determining the presence of fever is not too difficult and can be easily measured. Finding and diagnosing an underlying cause can sometimes be somewhat tricky, depending on the nature of what brought on the fever in the first place (i.e. a fungus or bacteria).
How is a diagnosis made?
At a consultation, your doctor will likely ask a series of questions to try and determine possible causes or rule out any potential reasons for elevated body temperature. It is important that you answer any and all questions as openly and honestly as possible, so as to assist your doctor in being able to make an accurate diagnosis. This ultimately ensures that you receive the most effective treatment plan.
Questions you may be asked can include:
- When did you first notice symptoms, including fever? (how long ago did you start feeling unwell?)
- Are you experiencing any other symptoms?
- Would you describe all of your or your baby / child’s symptoms as mild or severe?
- Have you or your baby / child recently been around anyone who has been unwell or ill?
- Have you tried to measure your or your child / baby’s temperature?
- If yes, how did you go about this?
- Do you know what the temperature of the environment was where you measured this?
- Did you take any known fever-relieving medications or give any to your baby or child?
- Have you taken anything or given your baby / child something that has seem to improve or worsen symptoms?
- Do you or your baby / child have any other known medical conditions?
- Are you or your baby / child currently taking any medications or supplements?
- Have you or your baby / child recently undergone a surgery?
- Have you or your baby / child recently travelled (locally or abroad)?
- Have you or your baby / child recently been exposed to animals, such as livestock?
- Do you or your baby / child have any known allergies?
From there a doctor will request a physical exam to better assess your symptoms, as well as check your overall condition and note any relevant findings. Your doctor will take your temperature and note its classification type. If necessary, he or she will recommend tests to help rule out or diagnose a suspected underlying cause of fever. Tests can include blood tests (to measure white blood cell count) or a chest X-ray.
Other tests which can be recommended include:
- A throat swab for strep culture
- Blood culture
- Sputum (mucus) sample
- Urine culture or analysis
- Stool (faeces) sample
- A lumbar puncture or spinal tap
- A CT scan
- Liver function test
- Thyroid function test
If a baby, especially a new-born that is less than a month old, is being evaluated, it is highly likely that your doctor will admit the little one to hospital for careful monitoring, testing and treatment.
Treatment for fever
The nature of treatment will depend on the determined cause of fever. A general low-grade fever may not be given a recommended treatment plan. In some instances, treating a low-grade fever can lead to prolonging an illness or masking other symptoms. In this way, a cause may not be easily determined or treated.
Intermediate and high-grade fevers may be treated in the following ways, depending on the nature of the cause:
- Medications (over-the-counter or prescription - NSAIDS and paracetamol): Your doctor may recommend medications (over-the-counter at the pharmacy or prescribed non-steroidal anti-inflammatory or paracetamol medications) to help alleviate fever and reduce body temperature to its normal baseline state. Your doctor will give you very specific doses and use procedures, which should be followed exactly as prescribed so as to minimise risk of side-effects (some side-effects and overdoses can lead to serious health complications and even death). Aspirin-based medications will only be recommended for adults. Aspirin should never be given to children or teenagers (under 18 years of age) suffering from viral infections as it can lead to a rare, but potentially life-threatening condition known as Reye’s syndrome. Antibiotics may be prescribed if the underlying cause of fever is found to be bacterial. Antibiotics prescribed will be specific to the type of bacterium in the body. If the cause is due to a virus in the body, antiviral medications may likely be prescribed, along with recommendations of plenty of rest and fluids. If the underlying cause is a fungus, antifungal medication will be prescribed. If an infant is being diagnosed, it is likely that a little one will be given intravenous medications (through an IV line) and round-the-clock care in hospital. Fever that is found to have occurred as a result of medication or drug use will be treated with a discontinuation of the particular substance.
- Fluids: A doctor will also likely recommend plenty of fluids and rest. The intake of liquids is important to prevent the occurrence of dehydration. If you or your child are unconscious or unable to receive fluids orally, a doctor will administer an IV line.
- Hospitalisation: Some bacterial infections, existing medical conditions or immune-resisting illnesses or a blot clot causing fever may likely require admission to hospital and careful monitoring. A person with fever due to environmental heat exposure will also require urgent hospitalisation to be treated with aggressive cooling methods. In this instance, vital signs and organs will be meticulously monitored to control a hyperthermia fever, and lower body temperature safely.
Home care for treating a fever
A low-grade fever, accompanied by other mild symptoms can easily be resolved with a little bed rest (avoiding activity that elevates body temperature) and plenty of cool fluids (water, juices or broth).
Hydration solutions containing proportioned water and salts can also help to replenish a dehydrated body. If your doctor sends you home, you can help things along and treat elevated body temperature by:
- Sponging yourself with lukewarm water or use a cool compress on the body’s pulse points (chest, arms, forehead, torso and legs) every 10 to 15 minutes (or when you feel that the cloth you are using is warming up).
- Taking baths with cool or lukewarm water – It is best to take care not to make yourself feel physically uncomfortable. If baths make you shiver, this will work against you and cause your body temperature to start increasing.
- Ensuring that the room you are resting in is at a comfortable temperature.
- Sleep with a light blanket or sheet.
- Dress in light clothing.
Is a follow-up appointment with the doctor necessary?
Low-grade fevers with mild symptoms typically clear with appropriate treatment over the course of a few days. A doctor may request a follow-up appointment to ensure that the underlying cause is completely cleared. Depending on the nature of the underlying cause, a doctor may request that you return for a check-up within a few days or weeks following your initial consultation.
During treatment, you may find that symptoms and fever don’t clear. If your fever persists with treatment, for longer than 3 days, or if you still have a fever for longer than a week without medicinal treatment, consult your doctor again.
A follow-up will be most important for those with existing conditions, such as cancer, or those who are experiencing fever due to an infectious or medication-induced cause, or hormone problem. Relapses are common and repeat treatment, or even hospitalisation, may be required.
What are some of the most common complications?
Who is most at risk?
Those with chronic conditions are generally most at risk of fever, particularly low-grade fever. Conditions can include:
- Allergic rhinitis (hay fever)
- Rheumatoid arthritis
Intermediate and high-grade fevers are at risk for health complications if not properly treated. A high fever that occurs suddenly or for an extended period of time can be dangerous or life-threatening. In extreme circumstances a severe fever can result in brain damage, coma or death.
When body temperature increases too high, for a long period of time, the hypothalamus effectively ‘works harder to rid the body of the cause’. This overheats the body. For an infant, this is especially dangerous as little ones do not yet have a hypothalamus which adequately regulates their body temperature (this develops later during childhood, into adulthood). A high fever for infants can become uncontrollable very quickly and cause serious complications, such as brain damage.
If the fever is as a result of a medical condition, fever can also be life-threatening in severe circumstances. A person with a weakened immune system due to illness, such as pneumonia, is at risk for complications if fever isn’t adequately controlled.
Severe fever can cause serious damage to the body’s tissues and organs if not diagnosed and treated timeously.
Can fever be prevented?
Fever as a result of an infectious condition can be dramatically reduced in the following ways:
- Hygiene habits: Regular handwashing before eating, or after petting animals or using the toilet is vitally important when it comes to viral and bacterial infections. Handwashing throughout the day in between using public transportation or touching various surfaces is also a good idea, especially if you are around someone who is unwell. It is also very important to teach children healthy handwashing habits. You can also get into the habit of carrying hand sanitisers or antibacterial wipes with you if areas you frequent are not effectively hygienic or clean.
- Be mindful of virus transmission: If you are around someone who is unwell with an infectious condition, it is a good idea to be mindful of touching your nose, mouth or eyes. Infections are typically spread this way. If you, yourself are down with an infection, you can be mindful of spreading it when you cough or sneeze. You can also be mindful of sharing eating utensils, hygiene products, towels, clothing, water bottles or cups.
- Ensure you are properly immunised: Ensuring that you are immune to various diseases or infectious conditions can also go a long way in preventing fevers which can be complex or become serious. If you are travelling, appropriate vaccinations and preventative medications will help reduce any risk.
- Be mindful of strenuous exercise: If you’re active and enjoy a little strenuous exercise activity, ensure that you are sufficiently hydrated, take frequent breaks, wear cool clothing that is not restrictive or that encourages your body to overheat, as well as cool down effectively after your workout.
- Refrain from excessive use of alcohol and illicit drugs
What is a ‘fever of an unknown origin’?
It can happen that a distinctive cause of fever is unknown, even after extensive testing. When fever peaks at 38.3 degrees Celsius (101 degree Fahrenheit) and persists for more than 3 weeks or occurs frequently without any apparent explanation, a doctor may classify this as a ‘fever of unknown origin’ (FUO).
An FUO can be classified as:
- Classic: An unexplained fever that lasts 3 weeks can be classified as a classic FUO. This type of fever is commonly seen in previously healthy individuals as a result of conditions such as leukaemia or diseases that affect connective tissue in the body.
- Nosocomial: This type of FUO is typically seen with individuals who have been hospitalised for something unrelated. The fever that develops is not likely caused by what they were admitted for and appears to be unexplained.
- Immune-deficient: This classification type generally affects those with already compromised or weakened immune systems and whom are at increased risk of infection. This can affect individuals who are already treating a condition affecting the immune system or going through a treatment process, such as chemotherapy.
- HIV-associated: The condition of HIV itself can cause fevers in a sufferer. A weakened immune system can make a person more susceptible to other infections and fevers which may be unexplained, however.
A doctor may diagnose a fever as an FUO, but is likely to continue trying to determine an underlying cause. This is one reason why a doctor will look for specific elements (along with other symptoms which may provide clues) so as to classify a FUO as they go through steps to try and determine a cause. Once a doctor recognises a classified FUO type, he or she may look for signs that help narrow down possible causes. These categories can possibly point to:
- Infections (such as tuberculosis, Lyme disease, or cat scratch fever)
- Inflammation (such as lupus or rheumatoid arthritis)
- Malignancy (such as pancreatic carcinoma or lymphoma)
- Miscellaneous (as a result of illicit drug use or conditions such as hepatitis, hyperthyroidism or others that don’t relate to any other category classification)
Clinical tests are likely to happen as a process of elimination to rule out possibilities as well as to try and determine a cause. It can sometimes happen that an otherwise undiagnosed condition is discovered through a testing process for a FUO and ultimately produces a clear result as to why the fever occurred in the first place. Tests can include culture tests (urine, sputum and blood), and imaging tests (X-rays or endocardiograms).
Any recommended treatment will largely depend on the cause (if found). Sometimes a FUO resolves itself eventually. Your doctor will guide you as to the best way to handle the fever and help to effectively treat any other red flag symptoms that may occur. Medications, such as NSAIDS (non-steroidal anti-inflammatory) and antihistamines may be prescribed to help reduce a fever, even if an underlying cause is not determined. Antibiotics may be prescribed where a FUO is suspected as having originated from an immune-deficient related cause.
Sometimes a cause is never diagnosed. In all cases, a person will be carefully monitored and treated by medical professionals so as to effectively clear a fever.
What is ‘febrile seizure’ and how does an elevated temperature in children cause it?
A complication for young children (often between the ages of 6 months to 6 years), a febrile seizure can occur when a child’s body temperature increases rapidly (rising too quickly or spikes). Febrile seizure is also known as a febrile fit or febrile convulsion.
Children who develop ear infections or respiratory viruses such as the common cold can sometimes experience febrile seizures. It can also happen that children who develop more serious conditions, such as pneumonia and kidney infections can also experience rapid body temperature increases. Seizures can occur on the same day a fever develops.
Seizures which happen as a result can be classified as:
- Simple febrile seizures: These last between 5 and 15 minutes and typically don’t occur again within a 24-hour period. A seizure (a generalised tonic-clonic seizure) affects the entire body, whereby stiffening occurs and limbs twitch. A child may lose consciousness but their eyes can remain open during a seizure. Breathing can also become irregular and vomiting and defecation can occur.
- Complex febrile seizures: A seizure can last for a longer period of time and recur. These typically only affect a portion of the body, but can be more serious than a simple febrile seizure.
As frightening as it can be to witness a child experiencing a seizure, this fever-induced complication does not typically result in an ongoing problem. With prompt medical assistance, a child can be effectively treated and their body temperature sufficiently controlled until well again.