What is the diagnosis for this patient?

Diagnosis: Tickborne Rickettsial Disease (Human Monocytic Ehrlichiosis or Anaplasmosis)

Based on the clinical presentation of fever, thrombocytopenia, leukopenia, and the epidemiologic context (mid-May in upstate New York with a dog that died from similar illness), this patient most likely has a tickborne rickettsial disease, specifically Human Monocytic Ehrlichiosis (HME) or Human Granulocytic Anaplasmosis (HGA). 1

Key Diagnostic Features Present

Laboratory Findings Strongly Suggestive of Ehrlichiosis/Anaplasmosis:

• Thrombocytopenia (99 x 10⁹ cells/L; normal: 150-350) 1
• Leukopenia (3.8 x 10⁹ cells/L; normal: 4.5-11.0) 1
• Fever (100.4°F/38°C) with nonspecific symptoms (nausea, vomiting, fatigue) 1
• Mental status changes (confusion, lethargy) developing on day 2 1

Critical Epidemiologic Clues:

• Timing: Mid-May presentation (peak tick season) 1
• Geographic location: Upstate New York (endemic area for both HME and HGA) 1
• Zoonotic exposure: Dog died 2 days earlier with similar illness (suggests shared tick exposure) 1

Diagnostic Reasoning Algorithm

Step 1: Recognition of Classic Triad

The combination of fever + thrombocytopenia + leukopenia in a patient from an endemic area during tick season should immediately trigger consideration of ehrlichiosis/anaplasmosis, even without documented tick bite or rash. 1

Important caveat: Rash is uncommon in HME adults (unlike Rocky Mountain Spotted Fever), so its absence does not exclude the diagnosis. 1

Step 2: Differential Diagnosis Considerations

While the initial presentation mimicked gastroenteritis, the laboratory findings and clinical deterioration with mental status changes expanded the differential to include: 1

• Tickborne rickettsial diseases (HME, HGA) - MOST LIKELY
• Viral encephalitis
• Bacterial sepsis
• Lyme disease (though less likely with this presentation)

Step 3: Confirmatory Testing Required

Immediate diagnostic tests needed: 1

• Peripheral blood smear examination for morulae (though only visible in 1-20% of HME cases and slightly higher in HGA)
• PCR testing for E. chaffeensis (HME) and A. phagocytophilum (HGA) on whole blood
• Acute serology for IgG and IgM antibodies (though often negative early in disease)
• Liver function tests (typically show elevated transaminases: ALT and AST)

Critical point: Early serologic testing is often negative, so negative serology does NOT exclude the diagnosis. 1 PCR and blood smear should be obtained before starting treatment but treatment should not be delayed waiting for results.

Treatment Recommendation

Empiric doxycycline 100 mg IV every 12 hours should be initiated immediately based on clinical suspicion, without waiting for confirmatory test results. 1

Rationale for Immediate Treatment:

• HME/HGA can rapidly progress to severe disease with multi-organ failure, as demonstrated in the case examples where patients developed pneumonia, renal failure, and death despite treatment. 1
• Mental status changes indicate potential CNS involvement and disease severity. 1
• The combination of thrombocytopenia and leukopenia with appropriate epidemiology provides sufficient clinical suspicion to warrant empiric therapy. 1

Common Diagnostic Pitfalls to Avoid

Pitfall #1: Waiting for Tick Bite History

Many patients with ehrlichiosis/anaplasmosis do not recall a tick bite. The absence of documented tick exposure should not delay diagnosis or treatment. 1

Pitfall #2: Expecting to See Rash

Unlike Rocky Mountain Spotted Fever, HME rarely causes rash in adults. Do not exclude ehrlichiosis based on absence of rash. 1

Pitfall #3: Relying on Early Serology

Antibodies are typically negative in the first 5-7 days of illness. A negative acute serology does not rule out disease. 1

Pitfall #4: Overlooking Epidemiologic Context

The death of the patient's dog with similar symptoms is a critical clue suggesting shared environmental exposure to infected ticks. This zoonotic context should raise suspicion immediately. 1

Pitfall #5: Misdiagnosing as Simple Gastroenteritis

The initial nonspecific symptoms (nausea, vomiting, fever) can mimic viral gastroenteritis, but the CBC findings of cytopenias distinguish tickborne disease and should prompt immediate reassessment. 1

Follow-Up Confirmation Strategy

• Convalescent serology (2-4 weeks after acute illness) showing 4-fold rise in antibody titer confirms diagnosis retrospectively 1
• PCR results from acute blood samples provide rapid confirmation 1
• Blood culture for Ehrlichia or Anaplasma (though rarely performed in clinical practice) 1

The diagnosis should be confirmed by PCR, serology, immunostaining, or culture isolation, but treatment must begin empirically based on clinical and laboratory suspicion. 1