Final answer:
Ticks are responsible for transmitting Ehrlichia spp., which causes diseases like human monocytotropic ehrlichiosis (HME). The lone star tick is the primary vector for HME in the United States.
Step-by-step explanation:
The external parasite responsible for the transmission of Ehrlichia spp. is the tick. These organisms are vectors for diseases such as human monocytotropic ehrlichiosis (HME), which is caused by the bacterium Ehrlichia chaffeensis. The lone star tick (Amblyomma americanum) has been identified as the primary vector for HME in the United States. Lyme disease and ehrlichiosis share the common vector of ticks, although different species are involved. The first-line treatment for Ehrlichiosis in both adults and children is doxycycline, an antibiotic effective against this obligate intracellular pathogen.