Zoonotic and Vector-Borne Infections — Written Clinical Reasoning

SAQ 1 — Integrated Diagnostic Approach to the Patient with Culture-Negative Prosthetic Valve Endocarditis (20 marks, 30 minutes)

Prompt: Outline your integrated diagnostic approach to Mr Baxter's presentation, addressing: (a) the prioritised differential diagnosis for culture-negative prosthetic valve endocarditis and the key exposure; (b) the specific diagnostic tests you would send and their interpretation; (c) the principles of treatment for the most likely diagnosis, including the drug regimen, the duration, and the rationale for the combination; (d) the follow-up and surveillance plan; (e) the occupational and public health implications; and (f) the common exam trap in this presentation. [1]

Model Answer

(a) Prioritised differential diagnosis and the key exposure (4 marks): [1]

Mr Baxter has culture-negative endocarditis on a prosthetic valve. The critical exposure is his occupation as an abattoir worker — the quintessential risk factor for Q fever (Coxiella burnetii), which is the leading cause of culture-negative endocarditis in this occupational group. The prosthetic valve is the major risk factor for chronic Q fever: C. burnetii preferentially infects previously abnormal or prosthetic valves, and chronic Q fever endocarditis develops months to years after an often-unrecognised acute infection. The prioritised differential is: [1]

1. Chronic Q fever endocarditis (the leading diagnosis) — abattoir worker, prosthetic valve, culture-negative, small vegetation, subacute course with night sweats and weight loss, splenomegaly and splinter haemorrhages.
2. Bartonella endocarditis — less likely without cat exposure or homelessness/alcohol misuse, but part of the culture-negative endocarditis differential.
3. Brucella endocarditis — less likely without unpasteurised dairy or direct animal handling exposure.
4. HACEK endocarditis — fastidious Gram-negatives that are culture-negative on routine media but grow on enriched media with prolonged incubation.
5. Fungal endocarditis — Candida or Aspergillus, usually in immunocompromised patients or long-term IV catheter users; less likely here. [1]

(b) The specific diagnostic tests and their interpretation (4 marks): [1]

The single most important diagnostic test is Coxiella burnetii phase I and phase II serology by immunofluorescence. The serological pattern differentiates acute from chronic infection:

• Phase I IgG high titre (above 1:800 or 1:1024) is diagnostic of chronic Q fever and is a MAJOR criterion in the modified Duke criteria for infective endocarditis.
• Phase II IgM would indicate acute Q fever.
• Phase II IgG elevated with low phase I suggests past resolved infection. [1]

Additional tests: Bartonella henselae and B. quintana serology (titre above 1:800 is suggestive), Brucella serology (Rose Bengal screening, tube agglutination), and 16S rRNA PCR on blood or resected valve tissue (a broad-range bacterial PCR that can identify any bacterial species). Repeat blood cultures with prolonged incubation (up to 21 days) should be sent, but are expected to be negative. Transoesophageal echocardiography (TOE) should be performed to better characterise the prosthetic valve and look for perivalvular complications (abscess, dehiscence). A PCR for C. burnetii on blood can be sent but is often negative in chronic disease. [1]

(c) The principles of treatment (5 marks): [1]

The treatment of chronic Q fever endocarditis is doxycycline 100 mg twice daily PLUS hydroxychloroquine 200 mg three times daily [1]. The rationale for the combination is fundamental to the pathophysiology: C. burnetii is an obligate intracellular pathogen that survives within the acidic phagolysosome. The acidic environment protects the organism from doxycycline, which is only bacteriostatic at low pH. Hydroxychloroquine is a weak base that accumulates in the phagolysosome and raises the pH, alkalinising the compartment and restoring the bactericidal activity of doxycycline. This combination was pioneered by Raoult and colleagues, who demonstrated a lower relapse rate and shorter treatment duration than the previous doxycycline-ofloxacin regimen [1].

The duration is at least 18 months for native valve endocarditis and at least 24 months for prosthetic valve endocarditis. Mr Baxter has a prosthetic valve and therefore requires at least 24 months of combination therapy. Doxycycline plasma levels should be monitored (target above 5 micrograms per mL) because sub-therapeutic levels risk treatment failure. Hydroxychloroquine requires baseline and annual ophthalmic review (screening for bull's-eye maculopathy). Valve surgery may be required for heart failure, perivalvular abscess, or failure of medical therapy, but must be accompanied by prolonged antibiotic therapy before and after surgery. [1]

(d) The follow-up and surveillance plan (3 marks): [1]

Mr Baxter requires serial serological monitoring — phase I and phase II serology every 3 to 6 months during and after treatment, with the goal of a declining phase I IgG titre. The duration of treatment is guided by the serological response (the phase I IgG titre should fall below 1:800 before considering cessation). Serial echocardiography monitors the prosthetic valve and detects complications. After treatment completion, lifelong clinical follow-up is appropriate, as late relapse occurs. The Q fever fatigue syndrome may persist and require supportive management. [1]

(e) The occupational and public health implications (2 marks): [1]

Q fever is a notifiable disease in Australia. The public health unit should be notified. The workplace (the abattoir) should be assessed — colleagues may also be at risk, and Q-VAX vaccination should be offered to seronegative workers (with mandatory pre-vaccination screening with serology and skin testing). The occupational health and safety implications include workplace risk assessment and the employer's duty of care. [1]

(f) The common exam trap (2 marks): [1]

The trap is treating chronic Q fever endocarditis with doxycycline ALONE, without hydroxychloroquine. Doxycycline monotherapy is bacteriostatic against C. burnetii and has a high failure rate; the combination with hydroxychloroquine is required for bactericidal activity. The second trap is failing to send Coxiella serology in a case of culture-negative endocarditis — the organism does not grow on routine blood cultures, and the diagnosis will be missed without specific serological testing. [1]

SAQ 2 — The Empiric Doxycycline Principle in the Febrile Patient with an Eschar (10 marks)

Prompt: A registrar asks you to explain the principle of starting doxycycline empirically in a patient with fever and an eschar before the serology returns. (a) What is the reasoning? (b) What are the alternative diagnoses to consider? (c) What is the regimen and its modification in pregnancy? [1]

Model Answer

(a) The reasoning behind empiric treatment (4 marks): [1]

The principle is based on three facts. First, the clinical picture of fever, headache, myalgia and an eschar in a patient with the appropriate exposure (northern Australia or the Asia-Pacific for scrub typhus, eastern coastal Australia for Queensland tick typhus) is highly suggestive of a rickettsial infection. Second, rickettsial serology requires a fourfold or greater rise in titre between paired acute and convalescent sera drawn two weeks apart — it is unhelpful in the acute illness because the antibody titres have not yet risen. Third, untreated rickettsial disease has a significant mortality (up to 30 per cent for severe scrub typhus, and higher for some spotted fevers). Therefore, the risk of a short course of doxycycline (a safe, well-tolerated antibiotic) is far less than the risk of the untreated disease. The defervescence within 24 to 48 hours of starting treatment provides retrospective clinical confirmation. The INTREST trial confirmed that combination IV doxycycline plus azithromycin is superior to monotherapy for severe scrub typhus [2].

(b) The alternative diagnoses (3 marks): [1]

The differential for fever with an eschar includes scrub typhus, Queensland tick typhus (Rickettsia australis), Flinders Island spotted fever (Rickettsia honei), other spotted fever group rickettsiae (e.g., Mediterranean spotted fever in a returned traveller), cutaneous anthrax (the black eschar, but with marked surrounding oedema and a different epidemiology), and a spider bite (the necrotic lesion can mimic an eschar). The exposure history and geographical setting narrow the differential: northern Australia or the Asia-Pacific points to scrub typhus; eastern coastal Australia points to Queensland tick typhus; Flinders Island points to Rickettsia honei. [1]

(c) The regimen and its modification in pregnancy (3 marks): [1]

The standard regimen is doxycycline 100 mg orally twice daily for 7 days. For severe scrub typhus with multi-organ involvement, combination IV doxycycline plus azithromycin is recommended per the INTREST trial. In pregnancy, doxycycline is contraindicated (it causes dental discolouration and bone growth inhibition in the fetus), and azithromycin 500 mg daily for 5 to 7 days is the alternative. In children under 8 years, doxycycline is generally avoided for the same reason, though short courses are increasingly accepted for life-threatening rickettsial disease given the severity of untreated infection. [1]