What are the different types of fever and examples of each?

Types of Fever and Clinical Examples

Fever Definition and Measurement

Fever is defined as a single temperature measurement ≥38.3°C (101°F) in adults, or >38.0°C (100.4°F) in children, though specific populations have modified thresholds. 1

• In elderly patients (>65 years) in long-term care facilities, fever is defined as a single oral temperature >37.8°C or repeated measurements >37.2°C (oral) or >37.5°C (rectal), or an increase from baseline >1.1°C 1
• In neutropenic patients receiving chemotherapy, fever is defined as a single oral temperature ≥38.3°C or >38.0°C sustained over at least 1 hour 1
• Core temperature monitoring using pulmonary artery thermistor, bladder catheter, or esophageal balloon thermistor is preferred when precise measurement is critical; oral or rectal temperatures are acceptable alternatives 1, 2

Classification by Etiology

Infectious Causes (Most Common)

Infectious etiologies account for approximately 70-75% of fever cases in most clinical settings. 1

Tropical/Travel-Related Infections

• Malaria: Accounts for 22.2% of tropical disease-related fevers in returning travelers, with incubation period of 7-30 days depending on species 1
• Dengue fever: Represents 5.2% of febrile illnesses in travelers, with incubation period of 4-8 days (range 3-14 days); presents with fever, headache, retro-orbital pain, myalgia, arthralgia without joint swelling, and rash 1, 3
• Chikungunya: Incubation period of 2-3 days (range 1-12 days); distinguished from dengue by prominent joint swelling, leukocyte count ≥5,000 cells/mm³, and absence of thrombocytopenia 1, 3
• Enteric (typhoid) fever: Accounts for 2.3% of tropical fevers, caused by Salmonella typhi 1
• Rickettsial infections: Include African tick bite fever (R. africae), Mediterranean spotted fever (R. conorii), murine typhus (R. typhi), and scrub typhus (O. tsutsugamushi); incubation period 5-7 days (up to 10 days); characterized by fever, headache, myalgia, eschar, rash, and lymphadenitis 1

Common Non-Tropical Infections

• Respiratory tract infections: Account for 13.5% of febrile illnesses, including upper respiratory infections (6.0%), pneumonia (2.7%), influenza (2.5%), and bronchitis (0.8%) 1
• Acute diarrheal disease: Represents 13.6% of cases (bacterial or unspecified etiology) 1
• Genitourinary tract infections: Account for 2.7% of febrile cases 1
• Skin and soft tissue infections: Represent 2.5% of cases 1

Non-Infectious Causes (Critical to Recognize)

Non-infectious etiologies should be actively considered in every febrile patient, especially when fever persists despite appropriate antimicrobial therapy or when no clear infectious source is identified. 2

Drug-Induced Fever

• Beta-lactam antibiotics: Most common medication cause, typically emerging 7-21 days after initiation (median 8 days), resolving within 1-7 days after discontinuation 2
• Malignant hyperthermia: Develops up to 24 hours after exposure to succinylcholine or halogenated anesthetics; produces intense muscle contraction, fever, and elevated creatine kinase; requires immediate dantrolene administration 2
• Neuroleptic malignant syndrome: Associated with antipsychotics (phenothiazines, haloperidol); presents with muscle rigidity, fever, elevated creatine kinase; requires immediate drug discontinuation, benzodiazepines, external cooling, and IV fluids 2
• Serotonin syndrome: Results from excessive 5-HT₁A receptor stimulation (SSRIs, linezolid); manifests with autonomic instability and neuromuscular hyperactivity 2
• Drug withdrawal: Alcohol, opioids, barbiturates, or benzodiazepines can produce fever with tachycardia, diaphoresis, and hyperreflexia days after ICU admission 2

Cardiovascular and Thromboembolic

• Acute myocardial infarction: Can present with fever as part of inflammatory response 2
• Dressler syndrome: Post-infarction pericardial injury causing fever 2
• Venous thrombosis and pulmonary infarction: Recognized fever-inducing conditions 2
• Fat embolism: May cause fever in acute setting 2

Neurological

• Intracranial hemorrhage and ischemic stroke: Can be associated with fever 2
• Non-convulsive status epilepticus: May manifest as unexplained fever 2

Endocrine and Metabolic

• Adrenal insufficiency: Can present with fever 2
• Thyroid storm: Hypermetabolic state frequently including high fever 2
• Acute gout: Attacks may be accompanied by fever 2

Iatrogenic and Procedural

• Blood product transfusion reactions: Documented febrile reactions 2
• Cytokine release syndrome: Fever following certain immunotherapies 2
• Immune reconstitution inflammatory syndrome: Fever after initiation of antiretroviral therapy 2
• Transplant rejection: Episodes frequently febrile 2
• Tumor lysis syndrome: Metabolic emergency commonly including fever 2

Pulmonary

• Atelectasis: May be source of fever in postoperative or ventilated patients 2
• Fibroproliferative phase of ARDS: Can generate fever 2
• Non-infectious pneumonitis: Drug-induced or radiation-induced may present with fever 2

Fever of Unknown Origin (FUO)

FUO represents 17.8% of febrile cases when no diagnosis is established after appropriate investigation. 1, 4

• Defined historically as fever >38.3°C on multiple occasions, lasting >3 weeks, with no diagnosis after 1 week of investigation 4
• Modern approach recognizes subgroups: classic FUO, nosocomial FUO, neutropenic FUO, and HIV-associated FUO 4

Classification by Temporal Pattern

Classical fever patterns based on recordings every 8-12 hours have limited diagnostic utility, though continuous temperature monitoring may reveal unique patterns for specific etiologies. 5, 6, 7

Temporal Fever Patterns

• Continuous fever: Persistent elevation with minimal variation (<1°C) 5
• Remittent fever: Daily fluctuations >1°C but temperature never returns to normal 5
• Intermittent fever: Temperature returns to normal between fever spikes 5
• Hectic fever: Large fluctuations with significant temperature swings 5

Critical Diagnostic Considerations

When evaluating fever, leukocyte count ≥5,000 cells/mm³ strongly favors chikungunya over dengue (positive likelihood ratio 3.3-6), while absence of thrombocytopenia markedly lowers dengue probability (negative likelihood ratio 0.2). 3

• Procalcitonin or C-reactive protein measurements are useful to guide antimicrobial discontinuation when pre-test probability of bacterial infection is low-to-intermediate 1, 2
• Daily complete blood count monitoring is essential in dengue to track platelet trends and hematocrit changes 3, 8
• PCR testing should be performed within 7 days of symptom onset for dengue and within 5 days for chikungunya; IgM testing is appropriate after 5-7 days 3, 8

Common Pitfalls

Delayed initiation of effective antimicrobial therapy increases mortality in sepsis; antibiotics must be administered within 1 hour when infection is suspected, even if non-infectious causes are being considered. 2

• Never rechallenge patients who experienced anaphylaxis or toxic epidermal necrolysis with the offending drug 2
• Avoid aspirin and NSAIDs in dengue due to increased bleeding risk with thrombocytopenia 3, 8
• Drug fever is a diagnosis of exclusion; consider when fever persists despite appropriate antibiotic therapy 2
• Not all patients with infection manifest fever; absence of fever in infected patients is associated with worse outcomes 1