Anaplasma phagocytophilum

Background

• Causes human granulocytotropic anaplasmosis

Microbiology

• Small, obligate intracellular Gram-negative bacterium
• Related to Ehrlichia and Rickettsiae
• Tropism for neutrophils

Epidemiology

• Transmitted by Ixodes scapularis and Ixodes pacificus ticks
• Rare case reports in Ontario
• Transmission may take as little as 24 hours of tick attachment

Clinical Manifestations

• Incubation period of 1 to 2 weeks
• Usually an acute undifferentiated fever
• Can be mild or severe
• Fever, headache, malaise, and myalgias are common
• Can also cause nausea, vomiting, diarrhea, cough, arthralgias, nuchal rigidity, and confusion
• Less than 10% have rash, most of which is concurrent Lyme disease
• 36% of cases require hospitalization, and it is severe in 3 to 7%
• Severe complications include respiratory failure, septic shock, rhabdomyolysis, hemorrhage, and opportunistic infections
  • Rare meningoencephalitis
• Higher case-fatality rate in Shandong Province in China, for no clear reason (16% versus 2.6%)
• Thrombocytopenia, leukopenia, and mild anemia are common
  • Return to normal range within 14 days, but with persistent left shift
• Abnormal liver enzymes are common in the first 7 days

Diagnosis

• 20-80% of patients will have circulating neutrophils with detectable morulae on blood film (in neutrophils/granulocytes)
• Serology used to diagnose, by measuring IgG levels ≥1:64 or a fourfold rise
  • IgM testing less sensitive and specific
• Culture not done routinely
• PCR possible

Management

• Doxycycline 100 mg po bid until at least 72 days after defervescence
  • Possibly 10 days if risk of concurrent Lyme disease