Fever = Temperature ≥100.6° F (38.0° C) orally. If we find the temperature is only 100.0, but the patient just got sick and feels lousy, we don’t argue and assume there’s a fever.
This topic will discuss the most common case: the first few days of acute fever. Fevers lasting a week or more are rare, and quite complex, so we’ll address them very briefly.
IMPORTANT NOTE: Fever is NOT dangerous. It doesn’t cause seizures, fry the brain, or anything like that (unless the temperature is >107°, which only happens from environmental heat stroke, or some psychiatric medications). What MAY BE dangerous is the disease that’s causing a fever. So 102° from meningitis is much worse than 104° from the Flu.
During the Covid Pandemic: Every setting has its own way to deal with “fever” without letting patients mingle inside, & doing a Covid test. But we can’t send a patient off for this & let them die. ≤ Such patients might include those with:
- Septic Shock — low blood pressure, very rapid pulse, clammy skin
- Meningitis — fever + severe headache & stiff neck, or simply confused
- Cyanosis — looks bluish
- Short of Breath —
- oxygen saturation <90%;
- breathing 30 times / min.;
- respiratory retractions (skin sucks in with each breath: over collarbones, bottom of trachea, or between ribs)
Shock: We note their pulse & blood pressure, but especially observe that they’re acting & thinking reasonably normally, and that they don’t get dizzy every time they stand up. If for some reason we’re in doubt, we check postural vital signs:
- They lie down for 5 min.
- We take their pulse & blood pressure
- They stand right up for 2 minutes
- We recheck pulse & blood pressure. A pulse increase of 20% is abnormal, as is a significant blood pressure drop
If a child is overly irritable, or an adult seems unduly weary or unable to concentrate, it’s because either:
- The fever makes them feel lousy; OR
- They have a life-threatening illness.
So we give a single large dose of acetaminophen (Tylenol®): 1300 mg for adults, 10 mg/lb. for children. This is more than labels say, but it’s a one-time diagnostic trial, & way lower than anything dangerous. In 45 minutes, if the patient is smiling, joking, running around the waiting room, or complaining about the wait, we’ve avoided a trip to the ER or a huge (& expensive) work-up.
I’ve seen 3 kids with usually-lethal Meningococcal Meningitis — fevers dropped nicely with acetaminophen, but behavior remained unchanged. They got their spinal taps, & IV antibiotics while waiting for results.
Meningitis in Adults: Fever, severe headache, & stiff neck (can’t bend the chin down at all). We send them to an ER for spinal tap & IV antibiotics; transport by ambulance. If this will be delayed, we draw a blood culture to send with patient, & give a dose of antibiotic in the office (intramuscularly).
For the rest of our topic “Fever,” let’s say our patient only looks uncomfortable at the very worst (and if so, acetaminophen makes them feel better). As always, the issue is not “What to do?”, but rather, What’s the Diagnosis?
Fever for 3 Days or Less
For a list of possible causes of fever, we need merely to look at a human body: virtually any organ can harbor infection. There’s the Brain (Meningitis, Encephalitis, Abscess); Ear (Otitis); Nose (Common Cold, Sinusitis); Mouth (tooth abscess, Stomatitis); throat… and on down. We touch on each through history &/or physical exam.
Most causes are obvious. Nobody has an infected ingrown toenail without mentioning toe pain. Ear infections & sore throat aren’t subtle. But Hepatitis can begin with mere fever and loss of appetite. Important rashes (Measles, Chickenpox) may go unnoticed.
Kidney Infections needn’t give urinary symptoms (those come from the bladder), but patients look somewhat uncomfortable, are nauseated or vomiting, and have some degree of tenderness in the flank (the side of the abdomen) or mid-back. If suspicious, we order a simple Urinalysis, done rapidly on the spot.
Pneumonia often causes a cough with thick green phlegm, and maybe chest pain with breathing (not just with coughing). We get a chest x-ray. Shortness of breath while walking is a serious sign.
But many/most patients with fever simply just feel lousy (general malaise), ache all over, but have no signs or symptoms pointing to a specific organ. First thing we think of is Covid-19, and obtain a Covid test (after reading down a few more paragraphs). I the patient to self-isolate, and repeat the test in 2-3 days.
If not Covid, from late Nov. to May it’s Influenza (“the Flu”), which we may test for (40% false-negatives), or simply treat. Other months, we diagnose it as a simple Virus. They require no work-up. Time cures. Reassurance relaxes. We advise fluids, normal-dose acetaminophen etc. for fever, and rest. We give a school/work excuse as needed, and instructions to return if not improving in 1-2 days. Done!
Almost!!! Some uncommon diseases look like a simple virus during the first 2-3 days, then turn rapidly lethal. So we do need to think about them (before sending patients away for Covid tests). They include:
** Malaria — Specifically the species Plasmodium falciparum. So we always inquire about foreign travel. If a patient just returned from a tropical country within the last 40 days, unless they’d only stayed in a major city, we assume it’s malaria until proved otherwise. Even if they took preventive medicine (prophylaxis).
The every-other-day fever pattern doesn’t begin for a week, and P. falciparum kills by then. Highest risk for death are healthy tourists without immunity from prior infection; also immigrants who’d returned to visit, but had been away long enough (a few years) for their prior immunity to fade. We order blood tests for malaria and consult an Infectious Disease specialist. Illness from its cousin species P. vivax can be delayed up to 3 years from travel, but P. vivax malaria is much milder, not fatal. We worry most about travelers back ≤40 days.
Other bad viruses can be imported from the tropics, like Ebola (bleeding), Dengue (bleeding; “bone pain”), Chikungunya (severe joint pains), and more. We consult a specialist as needed, but Malaria is still the most common, quickly lethal, & easily treatable.
Rocky Mountain Spotted Fever (RMSF) — Caused by a rickettsial bacteria and transmitted by certain ticks, infection occurs in tick season, April to September. It’s almost only in high-risk areas — you’ll never guess. North Carolina usually leads the list, followed by OK, TN, AR, MO, northern counties of MS and AL, southeast VA, & maybe the Chesapeake Peninsula (every county in the country has its own statistics). There’s a new focus on Arizona tribal lands, & northern Mexico. RMSF is very rare in CO & the Rockies. 50% of patients have noticed a tick bite, which means 50% haven’t.
Symptoms are mild at first, but include fever & headache. No tests are useful. We treat based on symptoms alone, in the right place at the right time.
The “spots,” come later, sometimes too late. They’re called “petechiae,” — clusters of tiny reddish-purple spots which by definition don’t blanche (“blanche” = turn white for a split second when you push on them, then resume their color). However, in their first few hours, soon-to-be-petechiae actually do blanche. We must never dismiss new, discrete, blanching spots in a patient with fever.
** Meningococcemia — This is the same germ (Neisseria meningitidis) which causes a type of meningitis, but if it’s only in the blood stream & not the brain, the patient may not look sick at first (until full-blown shock rapidly ensues). The telltale sign are petechiae, which don’t blanche, unless, as noted above, they’re just starting out. We must never dismiss… (see prior paragraph).
** Endocarditis — A heart infection caused by bacteria. The major risk factors include injection drug use, artificial heart valve, diseased heart valve, and a history of having had endocarditis. Heart exam may be normal at the onset. These patients usually require hospitalization for IV antibiotics while blood culture results are pending. This expense might be avoided in flu season, if a nasal swab for influenza is positive (ruling-in an alternative diagnosis for the fever).
** Diabetic Ketoacidosis (DKA) — Infections in Type-1 Diabetes can cause sudden onset of accumulation of acids in the blood (due to inability of the body to regulate its sugar). The diagnosis is tricky the very first time it happens, before a person even knows they have diabetes. This primarily occurs in children, but is possible at any age.
Patients may complain of excessive thirst, and drinking & urinating a lot. They’re often vomiting. Our main clue may be deep rapid respirations, over-and-above that expected for fever. Their breath may smell fruity, like nail polish remover. A simple urine test makes the diagnosis.
** AIDS-Related Opportunistic Infections — This is also tricky to identify in patients who don’t know they have HIV. If HIV isn’t treated, AIDS develops in an average of 8 years, but some people progress faster. If a patient with fever hasn’t been tested in the last 2 years, and has had any risks for HIV (past or present), a rapid oral test is key.
** Stevens-Johnson / Toxic Epidermal Necrolysis / Toxic Shock (Strep/Staph) — These horrible skin conditions may begin with a 1-2 days of fever and discomfort. Unusual skin pain, or muscle pain out of proportion to the aches of a virus, should alert us. There may be early signs of painful rash: blisters or sunburn-like reddening (not itching). The inside of the mouth and eyes may be involved with blisters; women may have vaginal burning.
** Peritonitis — This comes from a ruptured bowel; rarely, in other patients with advanced liver disease (cirrhosis). Appendicitis is a classic example. If a patient has abdominal pain which began before the fever, and is getting worse, this would be a major thought.
** Immunocompromised Patients with Fever — People with weak immune systems may look deceptively well, & can go south fast. Any new fever makes me nervous. They have as much right to get a simple virus as anyone, but may deserve at least a urinalysis, blood count, or chest x-ray, and especially lots of thought and consideration. They include:
- Elderly : Defined by biologic age more than physical, especially if they have numerous medical conditions. We define “elderly” as “over 65,” but once you yourself are almost there, you’ll probably ratch it up a bit.
- Debilitated, Malnourished, bad Alcoholics.
- Active Cancer, especially if on Chemotherapy.
- Renal Failure, Liver Failure, Adrenal Failure, bad Heart Failure, etc.
- Immunosuppressive Medications, (after Organ Transplant, for Lupus, etc).
- No spleen (removed, usually for trauma)
- Chronic Rheumatologic disorders; Sickle Cell Anemia
- AIDS and not taking medications (well-controlled HIV isn’t a risk).
** Acute (Primary) HIV Infection — Up to 80% of people who were just infected with HIV develop an acute illness 9-30 days later. Most have fever and tender lymph nodes (“swollen glands”). Symptoms last 1-3 weeks & go away on their own. AIDS develops an average of 8 years later.
I never used to worry about Acute HIV until a fever had gone on at least 4 days, by which time most common viruses are over & done. There’s obviously plenty of time to diagnose and treat HIV. But now I worry more, because our “fever-for-2-days” patient may come for care this one time only, and missing Acute HIV has major consequences:
- An estimated 50% of all HIV transmission is due to Acute HIV, when patients carry enormous amounts of virus. This level of virus may persist 4 months. So even though the fever goes away, people are out and around infecting others through sex.
- If our Acute HIV patient was unlucky enough to have been infected with a drug-resistant viral strain, it’s best identified as early as possible. If they get diagnosed a few years later, the test for drug-resistance may not show it. Treat HIV with a wrong drug, & the virus becomes resistant to even more medications.
So even at the very beginning of a fever, I may mention it & urge certain patients to return if symptoms last a few more days. These cases include:
- People with major risk factors for getting HIV.
- Presence of tender lymph nodes in atypical locations, like behind the ears, back of neck, back of head (not under the jawbone, where they’re common).
- A funny rash, often subtle, not itchy, usually on the abdomen or back, maybe elsewhere.
Fever PLUS Cough in Influenza Season (Dec. to late April) — We worry about Pneumonia, but can’t order a chest x-ray on everyone. If our exam is normal, we assume it’s just the Flu (or a similar virus) if:
- Otherwise healthy
- Body aches, but no one-sided chest pain with every breath
- Phlegm is not thick green
- No unusual shortness of breath when walking
- Normal lung exam by stethoscope (Pneumonia may have “rales” in just one spot)
And of course we do a Covid test.
CONCLUSION — Fever for 3 Days or Less
If the patient looks well, without any specific symptoms except feeling lousy, generalized achiness, and maybe a headache (especially when the fever goes up), we diagnose “Flu” or “Virus,” unless there’s reason to consider the serious conditions above.
Fever Going On 4 Days or More
At this point, if symptoms haven’t begun to improve in any way whatsoever, we can’t easily call it a simple “Virus” any more (even though it’s still possible). Localized bacterial infections are much more likely at this point. Even without specific symptoms, we order a:
Pneumonia and Pyelonephritis (kidney infection) are too serious to miss. A high white cell count on the CBC makes us search for serious bacterial disease. Low white cells & low platelets may suggest unusual infections. The CBC will also identify the unexpected case of leukemia.
Mononucleosis is a form of mild hepatitis that can cause a fever lasting over 4 days. There’s usually a sore throat at first, that turns into extreme fatigue. Swollen glands (really called “lymph nodes”) occur in atypical locations like back of the neck, back of the head, arm pits, inside of the elbow. It usually occurs in teens and young adults.
The CBC can show “atypical lymphocytes,” and abnormal Liver tests are very common. The “Monospot” test (heterophile antibodies) makes the diagnosis, but is often false-negative in the first or even second week of illness. Definitive testing sooner requires the specific Epstein-Barr IgM antibody (expensive, & unlikely if standard Liver Tests are normal).
Anytime we ever think “mono,” also think Acute (Primary) HIV Infection. It looks & feels exactly the same. This is the “window period,” when the common diagnostic HIV test may be false-negative, and the patient is extremely infectious.
Diagnosis is made by an HIV viral load test (a.k.a. PCR for RNA); the result must be very high (& often pretty expensive). HIV can cause false-positive Monospot tests. If the patient has any risk factors for having gotten infected in the past month, we advise them to abstain from sex until results are back. If viral load testing isn’t feasible, the now standard “4th Generation HIV Test” can be positive by 18 days. See also our topic Sore Throat.
Viral Hepatitis (A, B, C, D, E, & more) usually causes loss of appetite and nausea, but may begin with fever alone. Liver Function Tests make the diagnosis easily; more tests are required to distinguish among the different types of viruses. None are immediately dangerous (except for people with other chronic liver diseases, or rare Hepatitis E during pregnancy).
Childhood rashes, which might also occur in adulthood, all begin with just general fever and malaise for 2-5 days before the rash appears. These include Rubella (“German Measles,” eliminated by vaccination), Roseola (HHV-6), Rubeola (Measles), Varicella (Chickenpox), and B19 Parvovirus (“5th Disease,” “Slapped-Cheek”). Measles is the most serious, causes outbreaks, & can be suspected before the rash appears by “3 C’s” triad — Cold [runny nose], Conjunctivitis (red eyes), Cough, and white Koplik’s spots on inside of cheeks opposite the molars (“grains of salt on a red sea.”). Even though there’s no treatment, it’s critical to order blood tests for suspected measles, for public health reasons (it’s extremely contagious). B19 Parvovirus can be fatal in the extremely immunocompromised.
Some simple viruses can last 1-2 weeks and go away on their own. The key here, is that the patient progressively feels better and better. In El Salvador, I had a 2-week fever. It began at 104º, dropped to 103º in 2 days, then to 102º in 2 more days, and so on until it disappeared. I hardly felt ill (kept on working). Tests were all negative.
Fever >1 Week
A patient with fever, whose overall feeling of sickness continues completely unchanged for a week, or is getting worse sooner, should receive an aggressive work-up. We’d start by ordering blood cultures and CT scan of abdomen and pelvis seeking hard-to-diagnose infections of the heart, bone, & deep abscesses. We’d also order a simple test for Syphilis.
We might also order general very-non-specific tests for inflammation: the Sed Rate (ESR) and CRP. They would only be helpful if extremely high, which wouldn’t make a diagnosis, but would clue us in to the need to be more aggressive with our diagnostic search (see link for examples).
Then there’s a whole host of other infections, some quite serious & equally rare, often due to specific exposures. They include Typhoid (food-contamination), Ehrlichiosis (ticks), Bartonella (outdoor kittens), Typhus (body lice), Brucella (sheep; raw milk), Listeriosis (raw milk, Mexican cheese), Tuberculosis (exposure any time in life in a poor country, jail / institution) … the list is endless.
There are also non-infectious causes of fever. Drug fever (a special kind of allergy), blood clots, inflammatory arthritis (without the joint pain!), and unidentified cancer are the biggies.
We let Infectious Disease specialists tailor a work-up based on risks. Fortunately, fevers lasting 1-2 weeks without starting to get better are quite rare.
And that’s all for this clearly hot topic.
See also Fever for the clinician’s condensed thought-process when face-to-face with a patient.