In a previously healthy 23‑year‑old woman with five days of upper‑respiratory viral symptoms, a worsening cough with post‑tussive vomiting, nausea, diarrhea, mild tachycardia, and negative COVID‑19 and influenza tests, what differential diagnoses should be considered?

Pertussis (Whooping Cough) Must Be Ruled Out Immediately

This patient's presentation of paroxysmal cough with post-tussive vomiting following an upper respiratory prodrome is highly suggestive of pertussis and should be presumed to be pertussis until proven otherwise. 1, 2

Key Clinical Features Present in This Case

Your patient demonstrates the classic triad that should trigger immediate consideration of Bordetella pertussis infection:

• Paroxysmal cough (described as "frequent coughing fits")
• Post-tussive vomiting (explicitly stated)
• Absence of fever (vital signs stable except tachycardia)
• Appropriate timeline (5 days duration, with initial URI prodrome followed by worsening cough)

According to CHEST guidelines, when cough is accompanied by paroxysms, post-tussive vomiting, and/or inspiratory whooping, the diagnosis of pertussis should be made unless another diagnosis is proven 2. The presence of post-tussive vomiting has 79.5% specificity for pertussis in adults, and absence of fever has 81.8% sensitivity 1.

Critical Differential Diagnoses to Consider

1. Pertussis (Primary Concern)

• The combination of paroxysmal cough + post-tussive emesis + absence of fever makes this the leading diagnosis 1
• Highly contagious; requires immediate isolation and testing
• PCR testing of nasopharyngeal swab should be obtained urgently
• Treatment: Macrolide antibiotic (azithromycin, clarithromycin, or erythromycin) is effective when started early in disease course 2
• Even if testing is delayed, empiric treatment should be strongly considered given the clinical presentation and public health implications

2. Post-Infectious Cough Syndrome

• Common after viral URI, lasting 3-8 weeks 2
• Due to epithelial disruption and airway inflammation
• However, the severity of symptoms (post-tussive vomiting, GI symptoms) and early timing (only 5 days) make this less likely as the primary diagnosis

3. Mycoplasma pneumoniae

• Can cause severe respiratory symptoms in young adults 3, 4
• May present with extrapulmonary manifestations including GI symptoms
• However, your patient has a normal physical exam including normal lung exam, making pneumonia less likely
• Consider if symptoms worsen or chest imaging shows infiltrates

4. Upper Airway Cough Syndrome (UACS)

• Previously called post-nasal drip syndrome
• Can develop from persistent sinus inflammation after viral URI 2
• Less likely given the severity of paroxysmal cough and vomiting

5. Gastroesophageal Reflux Disease (GERD)

• Can be triggered or exacerbated by vigorous coughing 2
• May explain the nausea and could contribute to cough perpetuation
• Consider as secondary phenomenon rather than primary cause

Immediate Action Plan

Diagnostic Testing

1. Nasopharyngeal PCR for Bordetella pertussis (most sensitive in first 3 weeks)
2. Pertussis culture (if PCR unavailable, though less sensitive)
3. Consider chest X-ray only if lung exam becomes abnormal or patient deteriorates

Treatment Considerations

• If pertussis is suspected clinically (as in this case), do not wait for test results to initiate treatment 2
• Start azithromycin 500 mg day 1, then 250 mg daily for 4 days
• Alternative: clarithromycin 500 mg BID for 7 days
• Timing is critical: Antibiotics are most effective when started within the first 1-2 weeks of cough onset 2

Public Health Measures

• Immediate respiratory isolation until 5 days of appropriate antibiotic therapy completed
• Identify and notify close contacts for prophylaxis
• Report to local health department

Important Caveats

The mild tachycardia (104 bpm) warrants attention: While likely related to coughing paroxysms and possible dehydration from vomiting/diarrhea, ensure adequate hydration and monitor for deterioration. The tachycardia combined with GI symptoms could represent systemic inflammatory response.

The absence of "whooping" does not rule out pertussis—inspiratory whooping has only 32.5% sensitivity in adults 1. Many adults with pertussis never develop the classic whoop.

Normal lung exam is reassuring but does not exclude pertussis, which primarily affects the airways rather than causing parenchymal disease in most adults.

The diarrhea and nausea may represent: (1) GI manifestations of pertussis toxin, (2) concurrent viral gastroenteritis, or (3) GERD from vigorous coughing. These symptoms do not exclude pertussis as the primary diagnosis.

What NOT to Miss

Do not dismiss this as "just a viral cough" based on negative COVID-19 and influenza testing. Pertussis is a re-emerging infection in adults, often unrecognized because clinicians don't maintain high suspicion. The classic presentation is present in your patient, and delayed diagnosis leads to continued transmission and potential exposure of high-risk individuals (infants, pregnant women, immunocompromised).