Epidemiology and Etiology

Coxiellae differ from other rickettsiae in not needing an arthropod vector for transmission. Two cycles of natural infection have been recognized. In the wildlife cycle, transmission between wild animals and birds occurs from tick bites, from inhalation of dust, and, possibly in carnivores, from ingestion of infected placentas and meat. Ticks are infected from the blood of an infected host, but transovarial spread has also been documented. Coxiellae multiply in the tick gut and salivary gland, and transmission occurs by biting or when tick feces contaminate broken skin. In the domestic animal cycle and in human infection, tick-borne infection is considered less important, and transmission occurs usually from inhalation of contaminated dust and possibly from direct contact with infected animals. Cows, sheep, and goats are the main reservoirs for human infection. In these animals, coxiellae localize in the genital tract and udder and are excreted in vast numbers in milk, birth fluids, and placentas without usually causing disease or a drop in milk yield in cows, although abortion may occur in sheep and goats. Recent investigations have implicated rabbits and parturient cats as possible sources of human infection.

Coxiellae are very resistant to environmental conditions, surviving for months or years in dust or animal litter. Experiments have shown that these organisms can survive in 1 percent formalin for 24 hours, on wool for 7 to 9 months at 20°C, in dried blood for at least 6 months at room temperature, and in tick feces for at least 18 months at room temperature; they can also survive temperatures of 63°C for up to 30 minutes. The latter observation is important since it places coxiellae at the border of resistance to high-temperature, brief-interval milk pasteurization. However, there is still dispute about whether symptomatic Q fever can be acquired from drinking contaminated milk, although the prevalence of antibodies is raised in raw milk drinkers. Person-to-person spread has been documented but is very unusual. Laboratory accidents are a serious hazard when working with live organisms, and for this reason routine diagnostic laboratories seldom attempt isolation.

The vast number of organisms excreted by infected animals, the dissemination of the organisms in dust by wind, the hardy nature of the organism, and the low infective dose for humans (said to be one organism) explain a characteristic feature of Q fever-the occurrence of localized, explosive outbreaks often without an obvious source of infection. Moreover, even when the source of infection is identified, such as sheep used in medical research institutions, human cases occur in people only indirectly exposed by being in the vicinity of the animals.

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