A 29-year-old woman presents to the hospital with fever, headache, and cough. She developed these symptoms 2 days ago, and now also endorses some shortness of breath. She denies any rash, neck stiffness, recent travel, or sick contacts, and she has received all of her immunizations including her annual influenza vaccine. She lives in Connecticut and has several dogs. She is febrile to 38.7°C, but the rest of her examination is unremarkable. Her laboratory values are shown below:
Which of the following is the most likely causative organism?A) Borrelia burgdorferi
Ehrlichia chaffeensis. Nonspecific flu-like symptoms associated with leukopenia, thrombocytopenia, transaminitis, anemia, and an elevated creatinine are strongly suggestive of ehrlichiosis/anaplasmosis. Ehrlichia chaffeensis is carried by the Dermacentor variabilis tick (which can also carry Rickettsia rickettsii) or the Amblyomma americanum tick and causes human monocytic ehrlichiosis; Anaplasma phagocytophilum is carried by the Ixodes tick (which may also carry Borrelia burgdorferi and Babesia microti, and coinfection can occur), and causes human granulocytic anaplasmosis. Both are gramnegative obligate intracellular bacteria that can produce the above clinical findings, and therefore they are often described together. The diagnosis is typically made with PCR, since the finding of intraleukocytic morulae on peripheral blood smear may be seen but is not a sensitive test. Treatment is with doxycycline. Although many of the other answer choices can produce the nonspecific symptoms seen in this patient, the laboratory abnormalities point to the diagnosis of E. chaffeensis/A. phagocytophilum infection. (A, E) These answer choices are other important tick-borne illnesses. B. burgdorferi causes Lyme disease, which presents with the rash of erythema migrans. F. tularensis can occur through contact with ticks or animal tissues and will produce a necrotic eschar at the site of infection and lymphadenopathy. B. microti can cause a severe hemolytic anemia, and a peripheral blood smear shows the parasite within red blood cells (RBCs). R. rickettsii causes Rocky Mountain spotted fever, which presents with a rash that starts peripherally and spreads centrally. (C) This patient had her annual influenza vaccine (though not 100% protective), and influenza would not produce the laboratory abnormalities seen in this patient. (E) S. pneumoniae is the most common cause of CAP and meningitis. (D) Yersinia pestis is carried by fleas and causes bubonic, pneumonic, or septicemic plague. Neisseria meningitidis is a cause of meningitis and sepsis; the incidence has decreased since the introduction of the meningococcal vaccine. Parvovirus B19 produces erythema infectiosum (fifth disease with the classic “slapped cheek” rash) in children, but in adults can produce arthralgias, aplastic crisis in patients with chronic hemolytic disease, miscarriage in pregnant patients, and chronic pure RBC aplasia in immunocompromised patients.