What could cause fever for 3-5 days, followed by a rash moving from arms to back, with loss of taste and smell, and chills?

Differential Diagnosis: COVID-19 vs Rocky Mountain Spotted Fever

Based on the clinical presentation of 3-5 days of fever followed by rash starting on the arms and moving to the back, combined with loss of taste and smell, COVID-19 is the most likely diagnosis, though Rocky Mountain Spotted Fever must be urgently excluded due to its high mortality risk. 1

Primary Diagnostic Consideration: COVID-19

The loss of taste and smell is the critical distinguishing feature in this presentation:

• Loss of smell and taste occurred in 59-68% of COVID-19 patients, with onset typically occurring around day 3 (median 3.3 days) after disease onset 1
• Anosmia presented before other symptoms in 11.9% of cases, at the same time in 22.8%, or after other symptoms in 26.7% 1
• The combination of fever, chills, and subsequent anosmia/dysgeusia is highly specific for COVID-19 (specificity 98.7%) 1
• Recovery of smell typically occurs within 7-14 days in most patients 1

Critical Life-Threatening Alternative: Rocky Mountain Spotted Fever

You must immediately exclude RMSF because delayed treatment carries 5-10% mortality risk 1, 2:

RMSF Clinical Pattern:

• Incubation period of 3-12 days after tick exposure, followed by fever, headache, and chills 1
• Rash appears 2-4 days after fever onset (matching this patient's timeline) 1
• Rash classically begins on ankles, wrists, or forearms (this patient has arms involved) 1
• Rash spreads centrally to trunk and back (matching this patient's progression) 1
• Critical: Up to 20% of RMSF cases never develop a rash, and <50% have rash in first 3 days 1, 2

Why RMSF is Less Likely Here:

• Loss of taste and smell is NOT a feature of RMSF 1
• RMSF rash typically involves palms and soles by day 5-6, which is not mentioned 1
• The presence of anosmia strongly suggests viral etiology over rickettsial disease 1

Immediate Action Required

Do NOT wait for definitive testing—initiate empiric doxycycline 100 mg twice daily immediately if ANY of the following are present 2:

• Tick exposure history or residence in endemic area
• Severe headache (nearly universal in RMSF) 1
• Thrombocytopenia or hyponatremia on labs 2

Urgent Laboratory Workup:

• Complete blood count (looking for thrombocytopenia in RMSF, lymphopenia in COVID-19) 1, 2
• Comprehensive metabolic panel (hyponatremia and elevated transaminases suggest RMSF) 1, 2
• COVID-19 PCR testing 1
• Acute serology for Rickettsia rickettsii if any epidemiologic risk factors present 2

Other Differential Diagnoses to Consider

Ehrlichiosis (Human Monocytic Ehrlichiosis):

• Rash occurs in only 30% of adults (60% in children), appearing median 5 days after illness onset 1, 2
• Rash pattern varies from petechial to maculopapular to diffuse erythema 1
• 3% case-fatality rate 1
• Loss of taste/smell is NOT a feature 1

Infectious Mononucleosis (EBV):

• Can present with fever, myalgia, and loss of taste and smell 3
• Maculopapular rash occurs, especially if patient received ampicillin or amoxicillin 2
• Less likely given the specific rash progression pattern described 3

Enteroviral Infections:

• Most common cause of viral maculopapular rashes with trunk and extremity involvement 2
• Typically spare palms, soles, face, and scalp 2
• Loss of taste/smell is not a typical feature 2

Clinical Decision Algorithm

Step 1: Assess for RMSF red flags:

• Severe headache? 1
• Tick exposure or endemic area residence? 1, 2
• Rash involving palms/soles? 1

Step 2: If ANY red flags present → Start doxycycline immediately 2

Step 3: Obtain urgent labs (CBC, CMP) 2:

• Thrombocytopenia + hyponatremia → strongly suggests RMSF, continue doxycycline 1, 2
• Normal labs → RMSF less likely but not excluded 2

Step 4: Test for COVID-19 given prominent anosmia 1

Step 5: If doxycycline started, expect clinical improvement within 24-48 hours if RMSF 2

Critical Pitfalls to Avoid

• Never wait for the classic triad of fever, rash, and tick bite—it is present in only a minority of RMSF patients at initial presentation 1, 2
• Absence of headache does NOT exclude RMSF, though it is nearly universal 1
• In darker-skinned patients, rashes may be difficult to recognize, increasing risk of delayed diagnosis 1, 4
• Do not assume COVID-19 based solely on anosmia—other viruses including EBV can cause this symptom 3
• Rash on arms and back is NOT pathognomonic—consider RMSF, ehrlichiosis, drug reactions, and viral exanthems 1, 2, 4