In a patient with recent pig exposure presenting with respiratory or systemic symptoms, what diagnostic tests and initial management are indicated for possible zoonotic infections?

Diagnostic Testing for Zoonotic Infections Following Pig Exposure

Immediate Respiratory Pathogen Testing

In patients with pig exposure presenting with respiratory symptoms, obtain nose and throat swabs in viral transport medium for PCR testing to detect swine influenza (H1N1 and H3N2 variants), as these viruses can transmit from pigs to humans and require specific antiviral therapy within 7 days of symptom onset. 1

Influenza-Specific Testing

• Send respiratory specimens for real-time RT-PCR to detect and differentiate swine influenza virus subtypes (H1N1, H3N2, H3N1), as these assays can distinguish between human-origin and swine-origin influenza A viruses with high sensitivity 2, 3
• Collect samples within 7 days of visiting areas with pig exposure, particularly if there was direct contact with swine 1
• Follow local infection control procedures carefully during specimen collection 1

Complete Blood Count and Basic Laboratory Panel

Obtain a full blood count, urea and electrolytes, liver function tests, and C-reactive protein for all patients requiring hospital evaluation, as these tests help differentiate between viral infection, bacterial superinfection, and other zoonotic pathogens. 4

Key Laboratory Findings to Assess

• Leukocytosis with left shift suggests bacterial pneumonia or mixed viral-bacterial infection 4
• Lymphopenia may indicate severe viral infection 4
• Thrombocytopenia combined with leukopenia is particularly suggestive of rickettsial diseases or leptospirosis 5, 6
• Elevated liver enzymes with thrombocytopenia following livestock exposure should prompt consideration of Q fever or leptospirosis 5

Bacterial Pathogen Investigation

For Non-Severe Pneumonia (CURB-65 Score 0-2)

• Send sputum samples for Gram stain, culture, and antimicrobial susceptibility testing only if patients fail to respond to empirical antibiotic therapy 1

For Severe Pneumonia (CURB-65 Score 3-5)

• Obtain blood cultures before antibiotic administration 1, 4
• Collect pneumococcal urine antigen test (20 ml urine) 1
• Send sputum for Gram stain, culture, and susceptibility testing if patient can expectorate purulent samples and has not received prior antibiotics 1
• Consider testing for Staphylococcus aureus, as swine can harbor borderline oxacillin-resistant S. aureus ST398 strains that are potentially pathogenic to humans 7

Zoonotic Pathogen Serology

If fever persists beyond 10 days with livestock exposure, immediately start empirical doxycycline 100mg twice daily while awaiting serological confirmation for Q fever and leptospirosis, as each hour of delay increases mortality. 5

Specific Serological Tests

• Collect acute phase serum (5-10 ml clotted blood) immediately and convalescent sample after 7 days for Q fever serology 1, 5
• Obtain IgM titers for leptospirosis (diagnostic if >1:320) 5
• Send blood cultures within first 5 days for leptospirosis before antibiotics, as cultures become positive after the first week 1, 5
• Consider rickettsial serology with acute and 3-6 week convalescent samples if tick exposure in addition to pig contact 1, 6

Imaging Studies

• Obtain chest radiograph (posterior-anterior and lateral views preferred) in all hospitalized patients to assess for pneumonia, multilobar involvement, pleural effusions, or cavitation 4
• Consider CT scan if inadequate clinical response to therapy to evaluate for complications such as empyema or lung abscess 1

Critical Pitfalls to Avoid

• Do not wait for serological confirmation before initiating doxycycline if Q fever or leptospirosis is suspected, as serology is often negative in the first week and delay increases mortality 5
• Do not rely solely on clinical presentation for pneumonia diagnosis without chest radiograph, as this approach is overly sensitive 4
• Do not delay obtaining respiratory specimens before initiating or changing antibiotics, particularly for influenza PCR testing 4
• Do not mistake leptospirosis for viral hepatitis, as leptospirosis has only mild transaminase elevations despite significant jaundice, whereas viral hepatitis has marked transaminase elevations 5
• Do not assume mecA-negative S. aureus is methicillin-susceptible, as borderline oxacillin-resistant S. aureus (BORSA) phenotypes from swine can cause hard-to-treat infections and may be misidentified 7

Epidemiological History Requirements

Document specific exposures that guide testing priorities 1:

• Direct contact with pigs versus indirect exposure (swine-origin influenza can transmit without direct contact) 3
• Exposure to parturient animals (Q fever risk) 1
• Contact with unpasteurized milk or livestock (brucellosis, Q fever) 1
• Timeline of exposure relative to symptom onset (influenza typically within 7 days, Q fever 10+ days) 1, 5