The most likely diagnosis for this patient, who presents with fever, lymphocytic meningitis, and petechial rash following tick exposure, is Rocky Mountain spotted fever (RMSF). Headache is common in RMSF, and meningoencephalitis is seen in as many as 20% of cases. A rash is present in 85% to 90% of cases of RMSF, although the onset of skin findings may be delayed several days after the initial fever. Early on, the rash is often macular and localized to the wrists and ankles. Over time, this evolves to a diffuse petechial eruption, usually sparing the face. Involvement of palms and soles is characteristic of RMSF, although it is not universal. The low platelet counts and elevated liver enzyme levels also suggest RMSF. Finally, the rapid clinical response to doxycycline is characteristic of RMSF and would make viral entities less likely. The negativity for RMSF antibodies should not discount this diagnosis. During the acute presentation, antibodies to Rickettsia rickettsii are often undetectable because of insufficient time to mount a serologic response to the infection. Testing a convalescent serum specimen obtained more than 14 days after symptom onset allows for retrospective confirmation of RMSF infection. Skin biopsy frequently reveals evidence of vasculitis, and the diagnosis may be confirmed by immunohistochemistry; however, this test is not widely available. A clinically compelling presentation for RMSF should prompt initiation of doxycycline treatment, and this should be continued despite the absence of detectable antibodies.

Rash caused by enterovirus is distinguishable from RMSF because it appears morbilliform rather than petechial. Hematologic abnormalities are uncommon with enterovirus infection.

Heartland virus is a newly identified phlebovirus transmitted by the same tick that causes ehrlichiosis. Although this infection is associated with thrombocytopenia and elevated liver enzyme levels, it can be distinguished from RMSF by lack of a rash or response to doxycycline treatment.

Lyme disease can cause neurologic manifestations, including meningitis, but is associated with erythema migrans rather than a petechial rash.

West Nile virus causes neuroinvasive disease in less than 1% of infected patients. It may be associated with a nonspecific rash; however, symptoms would not improve with doxycycline.