What is the likely diagnosis and appropriate management for a 46-year-old patient presenting with headache, fever, chills, muscle pain, dehydration, vomiting, and a rash, following a recent Caribbean cruise, where a relative was also sick?

Likely Diagnosis: Dengue Fever

This patient most likely has dengue fever, and immediate diagnostic testing with dengue PCR/NAAT should be performed given the 6-day symptom duration, along with supportive management including acetaminophen for fever, aggressive oral hydration, and daily complete blood count monitoring for warning signs of progression to severe disease. 1, 2

Clinical Reasoning

The constellation of findings strongly suggests dengue fever:

• Recent Caribbean cruise exposure where dengue is endemic, transmitted by day-biting Aedes aegypti mosquitoes 3, 2
• Classic symptom timeline: 6-day history fitting the typical 4-8 day incubation period 3, 1, 4
• Characteristic clinical features: frontal headache, fever, chills, myalgia (muscle pain), and a generalized flat non-itchy (maculopapular) rash 3, 1, 5
• Sick relative: clustering of dengue cases among family members or travel companions is well-recognized 3
• Rash timing: appearing on day 6 is consistent with dengue, which typically develops during the febrile phase 1, 5

Immediate Diagnostic Workup

Priority Testing (Within Hours)

• Dengue PCR/NAAT on serum - this is the preferred test since symptoms are within 1-7 days of onset 1, 2, 4
• Complete blood count (CBC) - to assess for thrombocytopenia and hematocrit changes that predict progression to dengue hemorrhagic fever 1, 2
• Comprehensive metabolic panel - to evaluate for dehydration and organ dysfunction 3
• Three malaria thick films/rapid diagnostic tests - malaria must be excluded in any febrile traveler from tropical regions, even though Caribbean risk is lower 3

Additional Testing

• Blood cultures (two sets) - to exclude bacterial causes like typhoid fever, which can present similarly 3
• Liver function tests - dengue commonly causes transaminitis 3, 2
• NS1 antigen test - excellent alternative if PCR unavailable, detectable up to 10 days after symptom onset 4

Immediate Management

Supportive Care

• Acetaminophen only for fever and pain control at standard doses 1, 2
• Absolutely avoid aspirin and NSAIDs - these increase bleeding risk with thrombocytopenia that occurs in dengue 1, 2
• Aggressive oral hydration - aim for >2500 mL daily with oral rehydration solutions given the vomiting and dehydration 2

Monitoring Strategy

• Daily CBC monitoring to track platelet counts and hematocrit - rising hematocrit (>20% increase) with falling platelets signals progression to severe disease 1, 2
• Watch for warning signs requiring immediate hospitalization: persistent vomiting preventing oral intake, severe abdominal pain, lethargy, restlessness, mucosal bleeding, rapidly declining platelets 1, 2

Hospitalization Criteria

Admit immediately if any of the following develop:

• Persistent vomiting preventing adequate oral hydration 1, 2
• Rising hematocrit (>20% increase from baseline) with falling platelets 1, 2
• Platelet count ≤100,000/mm³ with rapid decline 2
• Signs of plasma leakage or shock (narrow pulse pressure ≤20 mmHg, hypotension) 2
• Severe abdominal pain, altered mental status, or mucosal bleeding 1, 2

Differential Diagnosis Considerations

While dengue is most likely, also consider:

• Chikungunya - also transmitted by Aedes mosquitoes in the Caribbean, presents with similar fever/rash but typically more severe joint pain 3, 5
• Zika virus - same vector and geographic distribution, though typically milder symptoms 1, 4
• Rickettsial infections (African tick bite fever, murine typhus) - can present with fever, headache, myalgia and rash, though eschar is more characteristic 3
• Typhoid fever - can cause fever and rash, but rash is typically rose spots on trunk, not generalized 3

Critical Pitfalls to Avoid

• Never prescribe NSAIDs or aspirin when dengue cannot be excluded - this dramatically increases bleeding risk 2
• Do not delay fluid resuscitation if signs of shock develop - initial bolus should be 20 mL/kg over 5-10 minutes 1, 2
• Do not empirically prescribe antibiotics (like azithromycin) without evidence of bacterial co-infection - this contributes to antimicrobial resistance without benefit 2
• Do not rely on single negative malaria test - three tests over 72 hours are needed to confidently exclude malaria 3

Special Considerations

If the patient were pregnant, both dengue and Zika NAAT testing would be mandatory regardless of outbreak patterns due to risks of maternal death, hemorrhage, preeclampsia, and vertical transmission 1, 2, 4

Expected Clinical Course

Most dengue cases resolve within 5-7 days with supportive care 2. The patient can be discharged when afebrile for ≥48 hours without antipyretics, with stable vital signs, adequate oral intake, and normalizing laboratory parameters 2. Post-discharge, temperature should be monitored twice daily with immediate return if fever recurs or warning signs develop 2.