Ehrlichiae, obligate intracellular bacteria first discovered to be pathogenic for humans in 1987, are the causative agents of human granulocytic ehrlichiosis (HGE) and human monocytic ehrlichiosis (HME). The effect of infection with these tick-borne agents may range from mild to fatal; the most common symptoms are fever, malaise, headache and myalgia. A number of the bacteria are seen in Fig. 1, clustered in a vacuole in an infected host cell. The gram negative ehrlichiae have an inner and an outer membrane represented by the arrows. (All bars represent 0.5 mm.)

Ehrlichia sp. develop within host cell vacuoles first as reticulate cells (RC) and then as dense-core cells (DC). A vacuole containing an ehrlichial microcolony is called a morula. In Fig. 2, several morulae are seen in this host cell, including one filled with what appear to be dead ehrlichiae (shown at the arrow).

In Fig. 3, a cultured cell, experimentally infected with E. chaffeensis (causative agent of HME), shows morulae of different sizes. Small morulae (shown at white arrows) contain few RC and are apparently in earlier stages of infection.

In Fig. 4, dense-core cells of E. chaffeensis are seen exiting the host cell following rupture of the morula and the host cell cytoplasmic membrane. These ehrlichiae will now go on to infect additional host cells or they may be ingested by a feeding tick, thus spreading the infection.

The causative agent of human granulocytic ehrlichiosis is unusual in its ability to multiply within polymorphonuclear leukocytes (neutrophils) as seen in Fig. 5.