Rocky Mountain spotted fever vs Ehrlichiosis and anaplasmosis
How to tell Rocky Mountain spotted fever and Ehrlichiosis and anaplasmosis apart when a board question makes them look alike.
RMSF is endothelial vasculitis with classic rash; ehrlichia/anaplasma are cytopenic tick illnesses with morulae and rash less common.
Side by side
RMSF causes fever, severe headache, myalgias, GI symptoms, hyponatremia, thrombocytopenia, and rash that often begins wrists/ankles then spreads to palms/soles and trunk.
Ehrlichiosis and anaplasmosis cause fever, headache, myalgias, leukopenia, thrombocytopenia, transaminitis, and rash less often; ehrlichia infects monocytes and anaplasma infects granulocytes.
Rickettsia rickettsii infects vascular endothelium, causing vasculitis and rash.
Ehrlichia chaffeensis infects monocytes; Anaplasma phagocytophilum infects neutrophils, producing intracellular morulae and cytopenias.
Tick exposure, fever/headache, rash including wrists/ankles/palms/soles, thrombocytopenia, hyponatremia, and empiric doxycycline support RMSF.
Leukopenia, thrombocytopenia, elevated AST/ALT, morulae in monocytes or granulocytes, and absent/uncommon rash support ehrlichiosis/anaplasmosis.
The trap
Do not wait for confirmatory tests before doxycycline when RMSF is suspected; early rash may be absent.
The repair move
For tick fever, use rash pattern, leukopenia/transaminitis, morula cell type, and urgency of doxycycline.
Try a free question
CoreA patient has fever, severe headache, myalgias, thrombocytopenia, hyponatremia, and a rash that began on wrists and ankles and spread to palms and soles after a tick exposure. Which diagnosis is most likely?
Show answer and explanation
Answer: A. Rocky Mountain spotted fever. Tick exposure with severe headache, thrombocytopenia, hyponatremia, and wrists/ankles-to-palms/soles rash supports RMSF.
Why the other choices are tempting
- B. Ehrlichiosis more often has leukopenia/transaminitis and rash is less common.
- C. Anaplasmosis often has granulocyte morulae and rash is uncommon.
- D. Lyme disease classically causes erythema migrans, facial palsy/carditis, or arthritis by stage.
- E. Typhoid fever is Salmonella Typhi systemic illness.
Flashcards
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What rash pattern supports RMSF?
Rash beginning on wrists/ankles and spreading to palms/soles and trunk supports RMSF.
What organism causes RMSF?
Rickettsia rickettsii causes Rocky Mountain spotted fever.
Quick answers
What is the fastest way to tell Rocky Mountain spotted fever from Ehrlichiosis and anaplasmosis?
RMSF is endothelial vasculitis with classic rash; ehrlichia/anaplasma are cytopenic tick illnesses with morulae and rash less common.
What is the key clue for Rocky Mountain spotted fever?
RMSF causes fever, severe headache, myalgias, GI symptoms, hyponatremia, thrombocytopenia, and rash that often begins wrists/ankles then spreads to palms/soles and trunk.
What is the key clue for Ehrlichiosis and anaplasmosis?
Ehrlichiosis and anaplasmosis cause fever, headache, myalgias, leukopenia, thrombocytopenia, transaminitis, and rash less often; ehrlichia infects monocytes and anaplasma infects granulocytes.
What is the common trap with Rocky Mountain spotted fever vs Ehrlichiosis and anaplasmosis?
Do not wait for confirmatory tests before doxycycline when RMSF is suspected; early rash may be absent.