Ehrlichia chaffeensis is causative agent of ?
**Core Concept**
Ehrlichia chaffeensis is a gram-negative bacterium that belongs to the family Anaplasmataceae. It is an obligate intracellular pathogen that primarily infects mononuclear phagocytes, such as macrophages and monocytes.
**Why the Correct Answer is Right**
Ehrlichia chaffeensis is transmitted to humans through the bite of an infected tick, specifically the lone star tick (Amblyomma americanum). Once inside the host, the bacteria evade the host's immune system by residing within the cytoplasm of infected cells. This intracellular location allows Ehrlichia to manipulate the host cell's functions and replicate within it. The bacteria cause a condition known as human monocytic ehrlichiosis (HME), characterized by symptoms such as fever, headache, and fatigue.
**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect because Anaplasma phagocytophilum is the causative agent of human granulocytic anaplasmosis, a different tick-borne disease.
**Option B:**
This option is incorrect because Rickettsia rickettsii is the causative agent of Rocky Mountain spotted fever, a tick-borne disease that primarily affects the endothelial cells.
**Option C:**
This option is incorrect because Coxiella burnetii is the causative agent of Q fever, a zoonotic disease that can be transmitted through inhalation of contaminated dust or through tick bites.
**Clinical Pearl / High-Yield Fact**
Ehrlichia chaffeensis is an obligate intracellular pathogen that primarily infects mononuclear phagocytes, making it difficult to diagnose using standard blood cultures.
**Correct Answer:** C.