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INTRODUCTION

Q fever in humans is caused by infection with the gram-negative bacterium Coxiella burnetii. The organism grows intracellularly in a parasitophorous vacuole at acidic pH (4.75).1 C. burnetii is a member of the gammaproteobacteria and is classified in the order Legionellales, family Coxiellaceae. This bacterium infects a broad range of animal species, and is thought to be transmitted primarily by inhalation, but tick transmission could play a role in maintaining the bacteria in wildlife. Q fever is a zoonosis, and humans usually acquire the disease via inhalation of contaminated dust or dried material from animal waste, with a small number of organisms (<10) thought to be able to initiate an infection.2 Infected livestock, such as cattle, sheep, and goats are the primary reservoirs for human exposure. The organism can grow to very high densities in the placenta of infected livestock. Therefore, these animals shed the largest amounts of C. burnetii during parturition. The replicative form of C. burnetii has been described as the “large cell variant” (LCV), whereas nonreplicating C. burnetii will form a spore-like “small cell variant” (SCV).3 The SCV is resistant to heat and desiccation resulting in impressive stability in the environment.4 The environmental stability, transmission by inhalation, and low infectious dose have led to classification of C. burnetii as a potential bioweapon and inclusion on the list of select agents maintained by the U.S. Department of Health and Human Services.

HISTORY

Q fever was first described in Australian abattoir workers in the 1930s by Derrick, with characterization of the causative agent by Burnet and Freeman. Between 1936 and 1938, Davis and Cox discovered the Nine Mile agent in a pool of ticks while studying the ecology of Rocky Mountain spotted fever at the Rocky Mountain laboratories in Hamilton, MT.5,6 The agent was first described as a Rickettsia, and later determined to be the same agent that was causing Q fever in Australia. The designation as C. burnetii to honor the contributions of Cox and Burnet gained widespread use in the 1960s. More recent genomic analysis indicates that C. burnetii is not closely related to the Rickettsiales, but is a close relative of Legionella species.7 Q fever has primarily been considered an occupational disease, with outbreaks occurring among people that work with animals, and in research institutions that work with livestock.8 A series of large outbreaks occurred in the Netherlands between 2007 and 2010.9 These outbreaks were linked to infected goats. C. burnetii spread from these goats through the air to the surrounding population resulting in over 4000 symptomatic infections. The outbreaks were halted by culling of infected goats and vaccination of other goats in the affected areas.10

EPIDEMIOLOGY

C. burnetii is endemic in the United States, though outbreaks of coxiellosis in livestock species are sporadic. While C. burnetii has been ...

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