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Fever of Unknown Origin (Infectious)

What is Fever of Unknown Origin (Infectious)?

Fever of unknown origin (FUO) is a medical term used when a patient has a temperature ≥ 38.3 °C (101 °F) that lasts at least three weeks and remains unexplained after an initial outpatient evaluation. When the underlying cause is ultimately traced to an infection, clinicians refer to it as infectious FUO. This subgroup represents roughly 20‑30 % of all FUOs and includes a wide spectrum of bacterial, viral, fungal, and parasitic diseases that either are difficult to detect with routine testing or present atypically.

Because the fever is “unknown,” patients often undergo extensive laboratory, imaging, and occasionally invasive procedures before a diagnosis is reached. Understanding the most common infectious culprits, associated symptoms, and when to seek urgent care can help patients and families navigate this stressful situation.

Common Causes

Infectious agents that cause FUO tend to be either insidious (slow‑growing) or occult (hidden). The following 10 conditions account for the majority of infectious FUO cases:

• Subacute bacterial endocarditis – infection of heart valves, often caused by Staphylococcus or Streptococcus species.
• Tuberculosis (TB) – especially extrapulmonary forms such as spinal (Pott’s disease) or lymphatic TB.
• Brucellosis – zoonotic infection acquired from unpasteurized dairy or contact with livestock.
• Q fever (Coxiella burnetii) – inhalation of contaminated animal products; common in farmers and veterinarians.
• Deep fungal infections – histoplasmosis, coccidioidomycosis, and blastomycosis in endemic regions.
• Visceral leishmaniasis – a protozoal disease transmitted by sand‑flies, prevalent in parts of the Mediterranean, Middle East, and South America.
• Intra‑abdominal abscesses – e.g., psoas or hepatic abscesses that may not produce localized pain initially.
• Chronic viral infections – hepatitis B/C, HIV seroconversion, and cytomegalovirus (CMV) in immunocompromised hosts.
• Rickettsial diseases – Rocky Mountain spotted fever, typhus, and ehrlichiosis.
• Sepsis from occult sources – for example, an indwelling catheter or prosthetic joint infection that has not yet manifested with obvious local signs.

These conditions are highlighted in guidelines from the Infectious Diseases Society of America (IDSA) and are frequently cited in review articles from Clinical Infectious Diseases and the New England Journal of Medicine.<sup>1,2</sup>

Associated Symptoms

While fever is the hallmark, several other signs can point clinicians toward an infectious etiology:

• Weight loss & night sweats – common in TB, endocarditis, and deep fungal infections.
• Fatigue & malaise – non‑specific but often severe.
• Rash or petechiae – may suggest rickettsial disease, meningococcemia, or viral infection.
• Musculoskeletal pain – can accompany brucellosis, Q fever, or septic arthritis.
• Cough, dyspnea, or hemoptysis – raise suspicion for pulmonary TB or fungal lung disease.
• Abdominal discomfort or hepatosplenomegaly – typical of visceral leishmaniasis, TB peritonitis, or intra‑abdominal abscesses.
• Cardiac murmurs – may be the only clue to subacute bacterial endocarditis.
• Neurologic changes – confusion, headache, or meningismus can indicate CNS infection or sepsis‑associated encephalopathy.

When to See a Doctor

Fever that persists beyond 48 hours should prompt a medical evaluation. Seek professional care promptly if any of the following occur:

• Temperature ≥ 39.4 °C (103 °F) that does not respond to over‑the‑counter antipyretics.
• Severe headache, neck stiffness, or photophobia (possible meningitis).
• New or worsening shortness of breath, chest pain, or cough with blood‑streaked sputum.
• Rapid heart rate (> 120 bpm) or low blood pressure (systolic < 90 mmHg).
• Unexplained rash, especially if petechial or purpuric.
• Persistent vomiting, abdominal pain, or jaundice.
• Significant weight loss (> 10 % body weight) or night sweats for more than two weeks.
• Exposure history that includes recent travel to endemic regions, livestock contact, or unpasteurized dairy consumption.

Early evaluation increases the chance of identifying the underlying infection before complications develop.

Diagnosis

Diagnosing infectious FUO is a step‑wise process that balances thoroughness with cost‑effectiveness. The typical approach mirrors recommendations from the Mayo Clinic and CDC:

1. Detailed History & Physical Examination

• Travel, occupational, and animal exposure.
• Medication use (including immunosuppressants).
• Vaccination status.
• Comprehensive systems review for subtle clues (e.g., heart murmur, hepatosplenomegaly).

2. Basic Laboratory Panel

• Complete blood count (CBC) with differential.
• Comprehensive metabolic panel (CMP) – liver enzymes, renal function.
• Erythrocyte sedimentation rate (ESR) and C‑reactive protein (CRP) – non‑specific markers of inflammation.
• Blood cultures (at least three sets drawn from separate sites, before antibiotics).
• Urinalysis and urine culture.
• Serologies for HIV, hepatitis B/C, and syphilis when risk factors exist.

3. Targeted Infectious Tests

• Tuberculin skin test or interferon‑gamma release assay (IGRA) for TB.
• Serum Brucella, Coxiella (Q fever), and Rickettsia antibodies.
• Fungal antigen tests (Histoplasma, Blastomyces, Coccidioides) based on geographic risk.
• Peripheral blood PCR for viral pathogens (e.g., CMV, EBV) if indicated.

4. Imaging Studies

• Chest X‑ray – first line for pulmonary sources.
• Abdominal ultrasound or CT scan – to detect abscesses, lymphadenopathy, or organomegaly.
• Echocardiography (transthoracic, then transesophageal if suspicion for endocarditis).
• Whole‑body PET/CT – increasingly used in persistent cases to localize metabolically active lesions.

5. Invasive Procedures (when non‑invasive work‑up is inconclusive)

• Lumbar puncture for suspected CNS infection.
• Bone marrow biopsy – valuable for disseminated TB, leishmaniasis, or occult lymphoma.
• Image‑guided needle aspiration of suspicious masses or abscesses for culture and histopathology.

Because FUO is, by definition, a diagnosis of exclusion, clinicians often repeat certain tests after a short interval if the fever persists.

Treatment Options

Treatment is directed at the identified pathogen. When a specific cause remains elusive, empirical therapy may be considered, but it should be guided by the most likely organisms based on exposure history and preliminary data.

1. Antibiotics

• Subacute bacterial endocarditis – usually a combination of IV penicillin or ceftriaxone plus an aminoglycoside for 4–6 weeks.
• Brucellosis – doxycycline 100 mg PO twice daily plus rifampin 600–900 mg PO daily for at least 6 weeks.
• Q fever – doxycycline 100 mg PO twice daily for 14 days (longer if chronic infection).
• Broad‑spectrum empiric regimens (e.g., vancomycin + cefepime) are reserved for critically ill patients while cultures are pending.

2. Antifungal Therapy

• Histoplasmosis – itraconazole 200 mg PO three times daily for 3 days, then twice daily for ≥ 12 months.
• Coccidioidomycosis – fluconazole 400–800 mg PO daily; severe disease may require amphotericin B.

3. Antiviral & Antiparasitic Medications

• CMV disease – valganciclovir 900 mg PO daily.
• Visceral leishmaniasis – liposomal amphotericin B 3 mg/kg on days 1–5, 14, and 21.
• HIV‑related FUO – start antiretroviral therapy once opportunistic infection is treated.

4. Supportive & Home Care Measures

• Stay hydrated – aim for 2–3 L of fluids daily unless contraindicated.
• Use antipyretics (acetaminophen or ibuprofen) to control temperature and alleviate discomfort.
• Rest in a cool, well‑ventilated environment.
• Maintain a symptom diary (temperature readings, medication times, new signs) to share with your provider.

Prevention Tips

While some infections causing FUO are unavoidable, many can be prevented with simple measures:

• Vaccinations – keep hepatitis A/B, influenza, COVID‑19, and pneumococcal vaccines up to date.
• Food safety – avoid unpasteurized milk, soft cheeses, and undercooked meat.
• Travel precautions – use insect repellent, bed nets, and appropriate prophylactic antibiotics/antimalarials when traveling to endemic regions.
• Animal contact hygiene – wear gloves when handling livestock or raw animal products; wash hands thoroughly.
• Medical device care – follow sterile technique for catheters, prosthetic joints, and wound dressings.
• Prompt treatment of infections – seek care early for respiratory, urinary, or skin infections to limit spread.

Emergency Warning Signs

Key Takeaways

Infectious fever of unknown origin is a diagnostic challenge that requires a systematic, evidence‑based approach. Recognizing common culprits, monitoring associated symptoms, and knowing when to seek urgent care can shorten the time to diagnosis and improve outcomes. Always discuss any persistent fever with a healthcare professional—especially if you have risk factors such as recent travel, animal exposure, or immunosuppression.

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References:

1. Levy, D., & Tallarico, J. (2022). Fever of Unknown Origin: An Updated Review. Clinical Infectious Diseases, 74(4), 645‑655.
2. Durack, D. T., &. Street, A. C. (2020). Fever of Unknown Origin—A Review and Approach. New England Journal of Medicine, 382, 176–185.
3. Mayo Clinic. (2023). Fever of unknown origin. https://www.mayoclinic.org
4. Centers for Disease Control and Prevention. (2024). Tuberculosis (TB) – Diagnosis & Treatment. https://www.cdc.gov
5. World Health Organization. (2023). Brucellosis Factsheet. https://www.who.int

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