Q fever is a zoonotic bacterial infection associated primarily
with parturient ruminants, although domestic animals such as
cats and a variety of wild animals have been identified as
sources of human infection. Q fever is considered a potential
agent of bioterrorism due to its high rate of infectivity,
stability in the environment, and potential for aerosol
dispersion. Etiology, Epidemiology, and Transmission Q fever is
caused by the gram-negative coccobacillus Coxiella burnetii.
Although classically considered a rickettsial agent, recent
phylogenetic analyses suggest that C burnetii is more closely
related to Legionella and Francisella than to the genus
Rickettsia. It resides and reproduces in phagolysosomes of host
monocytes and macrophages. Two forms exist—the large cell
variant is a vegetative form found in infected cells, and the
small cell variant is the extracellular infectious form that is
shed in milk, urine, and feces and found in high concentration
(109 ID50/g) in placental tissue and amniotic flui