A 36-year-old man presents to his primary care physician's office complaining of fever and headache. On examination, he has leucopenia, increased liver enzymes, and inclusion bodies are seen in his monocytes. History reveals that he is outdoorsman and that he remembers removing a tick from his leg. Which of the following is the most likely diagnosis?

All the listed diseases except Q fever are tick-borne. Two human forms of ehrlichiosis can occur: HME, caused by E. chaffeensis, and HGE, caused by E. ewingii. Ehrlichiosis was previously recognized only as a veterinary pathogen. HME infection is transmitted by the brown dog tick and A. americanum. HGE infection is transmitted by I. scapularis, the same tick that transmits Lyme disease. Both infections cause fever and leukopenia. A rash rarely occurs. E. chaffeensis infects monocytes, and HGE infects granulocytes; both organisms produce inclusion bodies called morulae. Francisella tularensis is a small, gram-negative, nonmotile coccobacillus. Humans most commonly acquire the organism after contact with the tissues or body fluid of an infected mammal or the bite of an infected tick.The Rickettsia C. burnetii causes Q fever, and humans are usually infected by aerosol of a spore-like form shed in milk, urine, feces, or placenta of infected sheep, cattle, or goats. Lyme disease is caused by a spirochete, B. burgdorferi, and produces the characteristic lesion erythema chronicum migrans (ECM). The etiologic agent of RMSF is R. rickettsii. It usually produces a rash that begins in the extremities and then involves the trunk.