Fever: A Comprehensive Overview

1. Definition and Normal Temperature

• Adults: ≥38.3°C (≥101°F) is the most widely used clinical threshold
• Infants: ≥38°C (100.4°F) warrants concern; in neonates <2-3 months, hypothermia (<36°C / 96.8°F) is equally alarming
• Hyperpyrexia: >41°C (105.8°F) - associated with increased risk of serious bacterial infection and tissue damage

Body temperatures under different conditions - Guyton & Hall, Textbook of Medical Physiology

2. Pathophysiology

The Hypothalamic Set-Point Mechanism

1. Exogenous pyrogens (bacteria, bacterial endotoxins/LPS, viruses, fungi, toxins, tumor antigens) enter the body
2. They are phagocytized by leukocytes, tissue macrophages, and large granular killer lymphocytes
3. These cells digest the bacterial products and release cytokines (endogenous pyrogens):
   • Interleukin-1 (IL-1) - the most important; also called leukocyte pyrogen or endogenous pyrogen; can raise temperature within 8-10 minutes; as little as 1/10,000,000 gram of bacterial LPS (acting with leukocytes) can trigger it
   • Interleukin-6 (IL-6)
   • Tumor necrosis factor (TNF)
4. These cytokines act on the anterior hypothalamus → stimulate local prostaglandin E2 (PGE2) synthesis via cyclooxygenase (COX)
5. PGE2 raises the hypothalamic set-point temperature

Chills and the "Crisis" (Flush)

• Core temp is now below the new set-point
• The body activates heat-conservation mechanisms: cutaneous vasoconstriction (cold skin), shivering (heat production)
• The patient feels intensely cold ("chills" or "rigors") even though body temperature may already be elevated

• The set-point drops back to normal
• Core temp is now above the set-point
• The body activates heat-loss mechanisms: vasodilation (flushed skin) and sweating
• This is the "crisis" or "flush" - historically a good prognostic sign

How Antipyretics Work

3. Causes and Differential Diagnosis

Infectious (Most Common)

• Bacterial: pneumonia, UTI, meningitis, septicemia, endocarditis, osteomyelitis, intra-abdominal abscess, tuberculosis, typhoid
• Viral: influenza, COVID-19, EBV, CMV, HIV, hepatitis viruses, dengue
• Parasitic: malaria, leishmaniasis, toxoplasmosis
• Fungal: candidiasis, histoplasmosis, cryptococcosis (especially in immunocompromised)
• Other: Lyme disease, rickettsial infections, zoonoses

Inflammatory / Autoimmune

• Rheumatoid arthritis, SLE, Adult-onset Still's disease (AOSD)
• Vasculitides (giant cell arteritis, polyarteritis nodosa)
• Polymyalgia rheumatica
• Inflammatory bowel disease (Crohn's, UC)
• Sarcoidosis, Reactive arthritis, Familial Mediterranean fever (FMF)

Malignant

• Lymphoma (both Hodgkin's and non-Hodgkin's), leukemia
• Solid tumors (renal cell carcinoma - "internist's tumor," hepatocellular carcinoma)
• Hemophagocytic lymphohistiocytosis (HLH)

Drug Fever

• Many drugs can cause fever through hypersensitivity mechanisms
• Common offenders: beta-lactam antibiotics, sulfonamides, phenytoin, allopurinol, procainamide, quinidine

Other / Miscellaneous

• DVT/pulmonary embolism
• Transfusion reactions
• Factitious fever
• Brain lesions (hypothalamic)
• Endocrine: thyroid storm, adrenal insufficiency
• Postoperative fever (see below)

Postoperative Fever - The "Five Ws"

4. Clinical Evaluation and Workup

History

• Onset, duration, pattern - sustained, remittent, intermittent, relapsing
• Associated symptoms - localizing symptoms (cough, dysuria, headache, rash, arthralgia)
• Travel history - malaria, typhoid, leishmaniasis, arboviral infections
• Animal / occupational exposure - zoonoses
• Immunosuppression - medications, HIV, transplant status
• Social history - IV drug use (endocarditis risk), sexual history

Physical Examination

• Vital signs (temperature pattern, heart rate, blood pressure)
• Lymphadenopathy, hepatosplenomegaly
• Skin: rashes, petechiae, Janeway lesions, Osler nodes
• Joints: swelling, warmth, range of motion
• Fundoscopy, ENT, dental (occult abscesses)
• Heart: new murmurs (endocarditis)

Initial Investigations

• CBC with differential - leukocytosis (bacterial), lymphocytosis (viral), pancytopenia (HLH, marrow involvement, SLE)
• CRP, ESR, procalcitonin - infection vs. inflammation markers
• Blood cultures (x2-3 sets before antibiotics)
• Urinalysis + urine culture
• Chest X-ray
• LFTs, renal function, electrolytes
• HIV serology if indicated
• Specific serology based on clinical clues

In Children (Special Considerations)

5. Management and Treatment

Antipyretic Therapy

• Fever in itself is generally not dangerous at levels below ~41°C (105.8°F)
• However, fever causes discomfort, increases metabolic demand, and may lower the seizure threshold in susceptible children (febrile seizures)
• Treating fever does not clear the underlying infection faster - it provides symptomatic relief

Causal Treatment

• Antibiotics for bacterial infections (guided by cultures and sensitivities)
• Antivirals (e.g., aciclovir, oseltamivir) for specific viral infections
• Antifungals for fungal infections, especially in immunocompromised patients
• Antimalarials, antiparasitics as appropriate
• Immunosuppressants/DMARDs for inflammatory/autoimmune causes

Supportive Care

• Adequate hydration (fever increases insensible losses)
• Nutrition (prolonged fever increases metabolic rate significantly)
• Cooling measures for hyperpyrexia (>41°C): tepid sponging, cooling blankets, cool IV fluids

6. Fever of Unknown Origin (FUO)

Definition (Harrison's 22nd Ed., 2025)

1. Fever ≥38.3°C (≥101°F) on at least two occasions
2. Illness duration of at least 3 weeks
3. No known immunocompromised state
4. Diagnosis uncertain despite a minimum standard diagnostic workup (originally: 1 week of inpatient evaluation; now increasingly outpatient-based)

Etiology (by broad category across large studies)

• Intra-abdominal abscess (liver, spleen, intraperitoneal) - most common in industrialized world
• Tuberculosis (especially extrapulmonary, miliary, or in immunocompromised)
• Culture-negative endocarditis (HACEK organisms, Bartonella, Coxiella/Q fever, Brucella, fungi)
• Osteomyelitis (vertebral osteomyelitis is the most common site in FUO)
• CMV, EBV (most common viral causes)
• Zoonoses: cat-scratch disease (Bartonella) - consider in young patients with pet exposure and tender lymphadenopathy

FUO Workup Approach

1. First line: CBC, ESR, CRP, LFTs, LDH, ferritin, blood cultures x3, urinalysis/culture, CXR, HIV, CMV/EBV serology, ANA, RF, ANCA
2. CT chest/abdomen/pelvis - high yield; early identification of lymphadenopathy, masses, abscesses
3. Echocardiography - if endocarditis suspected
4. Bone marrow biopsy - if hematologic malignancy or miliary TB suspected
5. PET-CT - increasingly used to identify occult foci of infection or malignancy
6. Molecular testing - PCR panels for blood/body fluids have greatly expanded pathogen detection
7. Tissue biopsy - of accessible lymph nodes, liver, skin lesions

FUO Treatment

• Rational treatment is based on final diagnosis
• Empirical trials should generally be avoided - they can obscure the diagnosis (antibiotics destroy cultures; glucocorticoids mask lymphoma)
• Exceptions for empirical therapy:
  • Hemodynamic instability or neutropenia → empirical broad-spectrum antibiotics
  • Positive TST/IGRA, or endemic area + clinical picture → empirical antituberculars (after collecting mycobacterial cultures/PCR)
• Supportive: acetaminophen or NSAIDs for symptomatic relief
• Colchicine: highly effective prophylaxis for FMF; may be tried in suspected FMF
• IL-1 inhibitors (anakinra): effective for autoinflammatory syndromes (FMF, CAPS, TRAPS, AOSD) once infection and malignancy are excluded
• Glucocorticoids: dramatic effect in giant cell arteritis/PMR, but avoid until infection and lymphoma are excluded
• Prognosis: ~40-50% of FUO in developed countries remain undiagnosed; the majority of these resolve spontaneously with a favorable long-term outcome

7. Dangerous Temperature Thresholds

• Guyton and Hall, Textbook of Medical Physiology - Chapters 73-74 (thermoregulation)
• Costanzo, Physiology, 7th Ed., p. 184
• Harrison's Principles of Internal Medicine, 22nd Ed. (2025), Chapter 22 (FUO)
• Goldman-Cecil Medicine, Chapter 259 (fever approach and FUO)
• Frameworks for Internal Medicine - FUO categories and causes
• Tintinalli's Emergency Medicine - fever in children, postoperative fever
• Bailey & Love's Short Practice of Surgery, 28th Ed. - postoperative fever