Ehrlichiosis is a difficult infectious disease to diagnose because it manifests as an acute, undifferentiated, febrile, RMSF-like illness with few or no physical findings. Most patients who are diagnosed with RMSF without rash probably have ehrlichiosis. Co-infections of various tick-borne pathogens transmitted by the same vector are rare, but they do occur.
Ehrlichiosis has the same distribution as RMSF and is transmitted by the same tick species (eg, Amblyomma, Dermacentor). However, RMSF causes physical findings that ehrlichiosis does not, including bilateral periorbital edema, edema of the dorsum of the hands and feet, and conjunctival suffusion. The maculopapular/petechial rash of RMSF on the wrists/ankles is usually absent in ehrlichiosis.
Laboratory findings associated with RMSF and ehrlichiosis are similar (eg, thrombocytopenia, relative lymphopenia, increased levels of serum transaminases, atypical lymphocytes). However, neutropenia is more common in ehrlichiosis than in RMSF.
Most patients with ehrlichiosis present with fever and a severe headache but do not have nuchal rigidity, as opposed to patients with aseptic or bacterial meningitis. The cerebrospinal fluid (CSF) profile in patients with ehrlichiosis is normal, in contrast to patients with viral or bacterial meningitis.
Other differential diagnostic possibilities include typhoid fever, malaria, and babesiosis. All of these infectious diseases manifest as acute, undifferentiated, febrile illnesses with a paucity of physical signs. The diagnosis of typhoid fever and malaria are suggested by an appropriate epidemiologic profile and/or travel history. Exposure to large Dermacentor ticks would suggest RMSF, whereas exposure to small Ixodes ticks would suggest the possibility of babesiosis.
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