Differential Diagnosis: 30-Year-Old Male with Fever, Cough, and Generalized Body Ache (Myalgia)

MOST COMMON (Top Priority)

1. Influenza (Flu)

• Classic presentation: Sudden-onset high fever, nonproductive cough, diffuse myalgia, headache, chills, malaise
• Key: abrupt onset distinguishes it from most other causes
• Diagnosis: Rapid influenza antigen test, PCR
• Red Book 2021, Influenza section

2. COVID-19 (SARS-CoV-2)

• Fever, dry cough, myalgia, fatigue - nearly indistinguishable from influenza initially
• May add anosmia/ageusia, shortness of breath
• Diagnosis: RT-PCR nasopharyngeal swab, rapid antigen test
• Still circulating in 2026

3. Dengue Fever

• Mosquito-borne (Aedes); sudden high fever, severe myalgia and bone pain ("breakbone fever"), headache, retro-orbital pain
• Rash (maculopapular) may appear on day 3-5
• Look for: thrombocytopenia, leukopenia, raised hematocrit
• Diagnosis: NS1 antigen, dengue IgM/IgG serology, PCR
• Andrews' Diseases of the Skin

4. Malaria (Plasmodium spp.)

• Episodic fever and chills (classic cyclical pattern), myalgia, anemia, headache
• "Malaria must be considered in any person returning from or living in the tropics with fever" - Tintinalli's Emergency Medicine
• P. falciparum can be severe and life-threatening
• Diagnosis: Peripheral blood smear (gold standard), RDT, PCR

5. Typhoid Fever (Salmonella typhi)

• Stepladder rising fever over days, relative bradycardia, myalgia, dry cough (up to 30% of patients), abdominal discomfort
• Rose spots (faint maculopapular rash on trunk) in some
• Diagnosis: Blood culture (gold standard), Widal test (limited utility), PCR

IMPORTANT BACTERIAL CAUSES

6. Community-Acquired Pneumonia (CAP)

• Fever, productive or nonproductive cough, myalgia, dyspnea
• May have pleuritic chest pain, dullness to percussion
• Organisms: S. pneumoniae, Mycoplasma, Legionella, Chlamydia
• Diagnosis: CXR, sputum culture, CBC

7. Rickettsial Infections (e.g., Scrub Typhus, Spotted Fever)

• Fever, myalgia, headache, sometimes cough and rash
• Look for: eschar (painless black lesion at mite/tick bite site), lymphadenopathy
• Common in rural/forested areas; often underdiagnosed
• Diagnosis: Serology (Weil-Felix, specific IFA), PCR

8. Leptospirosis

• Fever, severe myalgia (especially calves), cough, conjunctival suffusion
• Exposure to contaminated water/animals is key history
• Weil disease (severe form): jaundice + renal failure + hemorrhage
• Diagnosis: MAT serology, PCR, urine culture

9. Ehrlichia / Anaplasma Infection

• Acute febrile illness with fever, headache, chills, myalgia, cough, nausea
• Tick-borne; variable thrombocytopenia and leukopenia
• Goldman-Cecil Medicine

LESS COMMON BUT IMPORTANT

10. Infectious Mononucleosis (EBV)

• Fever, pharyngitis, lymphadenopathy, malaise, myalgia
• Splenomegaly - risk of rupture
• Diagnosis: Monospot test, EBV-specific antibodies, CBC (atypical lymphocytes)

11. COVID-19 Variants / Other Viral URTIs (RSV, Adenovirus, Parainfluenza)

• Similar flu-like presentation
• Diagnosis by viral panel or PCR

12. Tuberculosis (TB) - Primary or Reactivation

• Chronic cough (>2 weeks), fever (usually low-grade/evening), night sweats, weight loss, myalgia
• Less typical for acute presentation but must consider in endemic areas
• Diagnosis: CXR, sputum AFB smear/culture, GeneXpert

13. Chikungunya

• Similar to dengue: fever, severe polyarthralgia/arthritis, myalgia, rash
• Arthralgia is more prominent and can persist for months
• Aedes mosquito vector (same as dengue)

14. Acute HIV Seroconversion

• 2-4 weeks after exposure: fever, pharyngitis, myalgia, rash, lymphadenopathy
• Ask about sexual history/risk behaviors
• Diagnosis: HIV RNA PCR, 4th-gen Ag/Ab combo test

15. Acute Hepatitis (A, B, E)

• Pre-icteric phase: fever, myalgia, malaise, nausea, anorexia - before jaundice appears
• Diagnosis: LFTs, hepatitis serology

16. Sepsis (Bacteremia)

• Any bacterial infection can cause systemic fever, myalgia, and cough if lungs involved
• Consider if toxic appearance, hypotension, tachycardia

Rare but Do Not Miss

Recommended Initial Workup

Clinical Approach Summary

The geographic location, travel history, season, and exposure history (mosquitoes, water, animals, sick contacts) are the most important factors to narrow this differential quickly.

• Urban + no travel: Influenza, COVID-19, CAP most likely
• Tropical/endemic area: Add dengue, malaria, typhoid, rickettsiae
• Rural/water/animal exposure: Leptospirosis, Q fever, brucellosis
• Sexual risk history: Acute HIV
• Cough >2 weeks + weight loss: TB

Updated Differential with Normal LFTs

What Normal LFTs Rule Out / Make Less Likely

Revised Priority Differential

TOP PRIORITIES (Most Likely)

• Normal LFTs fully consistent
• Key features: abrupt onset, high fever, severe myalgia, dry cough, headache
• No hepatic involvement expected
• Next test: Rapid influenza antigen test or PCR

• Normal LFTs consistent (unless severe disease)
• Fever + cough + myalgia = classic triad
• Next test: COVID-19 rapid antigen or PCR

• Normal LFTs possible in early or mild dengue (LFTs may rise in later days)
• Key features: severe retro-orbital headache, bone/joint pain, thrombocytopenia
• Next test: NS1 antigen (positive in first 5 days), CBC - look for platelet drop + leukopenia

• Normal LFTs common, especially in first week
• Key features: stepladder fever, relative bradycardia, dry cough (~30%), constipation or loose stools
• Next test: Blood culture x2 (gold standard), Widal test

• Normal LFTs consistent with uncomplicated P. vivax or early P. falciparum
• Key features: cyclical fever + chills, rigors, anemia
• Next test: Peripheral blood smear + RDT

• Normal LFTs possible in early disease
• Key features: eschar at bite site, lymphadenopathy, headache, rash
• Easy to miss - look for eschar actively
• Next test: Weil-Felix reaction, specific IgM serology

• Normal LFTs expected
• Key differentiator from dengue: severe joint pain/arthritis rather than bone pain
• Aedes mosquito vector

What to Focus on Next

• Any mosquito bites / travel to endemic area? → Dengue, malaria, chikungunya
• Retro-orbital headache or bone pain? → Dengue
• Cyclical fever with chills/rigors? → Malaria
• Any rash or eschar? → Rickettsiae, dengue
• Any sick contacts / seasonal flu? → Influenza
• Duration of symptoms? (>1 week + stepladder fever → Typhoid more likely)

Updated Differential: Fever + Cough + Myalgia + Vomiting + Normal LFTs

How Vomiting Reshapes the Differential

🔴 NOW TOP PRIORITY - Dengue Fever (with Warning Signs)

• Persistent vomiting ← this patient has this
• Abdominal pain/tenderness
• Fluid accumulation (ascites, pleural effusion)
• Mucosal bleeding
• Lethargy/restlessness
• Liver enlargement >2 cm

Persistent vomiting in dengue = warning sign that can progress to severe dengue (plasma leakage, shock, organ impairment). This patient needs close monitoring.

Priority List (Revised)

Conditions Made Less Likely by Normal LFTs + Vomiting Pattern

• Acute hepatitis - ruled out by normal LFTs
• Leptospirosis severe form - less likely
• Gastroenteritis alone - cough doesn't fit

Immediate Action Plan

1. CBC urgently - platelet count + WBC
   • Platelets <100,000 + leukopenia = dengue very likely
2. Dengue NS1 antigen (days 1-5 of fever) - most sensitive early
3. Dengue IgM/IgG if fever >5 days
4. Malaria smear + RDT - do not miss
5. Blood culture x2 for typhoid
6. COVID rapid test
7. IV fluid assessment - is the patient tolerating oral fluids?
8. Monitor for warning signs progression: abdominal pain, bleeding, restlessness, rapid breathing

• If vomiting is persistent and patient cannot tolerate oral fluids → IV fluid resuscitation (especially critical if dengue confirmed)
• Avoid NSAIDs (especially if dengue suspected - bleeding risk)
• Avoid aspirin

Bottom Line

Dengue with warning signs is now the #1 diagnosis to rule in or out urgently. Persistent vomiting in the context of fever + myalgia + cough in a young male significantly elevates the dengue probability. Malaria must be excluded simultaneously. Normal LFTs make hepatitis and severe leptospirosis unlikely.