Infection could occur via skin abrasions and splashes of infected material into the eye. The consumption of unpasteurized milk from infected cows and goats has accounted for small numbers of Q fever cases yet, in some instances, drinking infected raw milk has had an immunising effect in the absence of a clinical disease. Human to human infection is very uncommon as is infection from tick bites.
Inhalation of the organism, as a result of direct or indirect exposure to contaminated aerosols, is the most common mechanism of human infection.
Fine mists, or very small droplets; liberated from the blood, milk, urine, faeces, when the young are born and especially from the placenta and birth fluids of an infected animal; directly exposes humans to infected aerosols. Air samples from the vicinity of parturient animals and areas contaminated with heavily infected placentas have shown the organism will continue to be released, contaminating the environment for up to 2 weeks following the birth of young.
During the slaughter and processing of infected animals, fine mists can also be released into the air from the blood and when handling the udder, bladder, intestines and the uterus, foetus and other products of conception. Infection may also occur with exposure to contaminated water droplets or fine mists, dispersed when using high pressure hoses to wash infected material, or dust, from stock, building structures, animal transport vehicles and personal protective clothing such as boots and plastic aprons.
The larger droplets and released infected matter can collect on the animal’s hide, hair or fleece and heavily contaminate the ground 1 floor, surrounding area, nearby structures or machinery and such materials as straw and clothing. The lighter smaller droplets freely disperse into the air, and may be disseminated for some distance, before settling. These contaminated droplets and matter then dry to form a highly infectious dust.
Humans may inhale infected dust, formed from contaminated droplets and the organism-laden products from an infected animal, when it is blown (possibly for a kilometre or more) in dry and windy weather. The organism can be released into the air when handling materials, working within areas or on structures, that have been contaminated by infective dust. This dust may have collected during wind borne dissemination or as the result of direct contamination with infected products that have dried to form a dust. Moving animals in the yards, pens or holding paddocks and stock transport trucks can also raise infective dusts.
As the organism can endure harsh conditions for many months in a dried state; either in the ground or attached to buildings, machinery, stock transport vehicles, straw, wool, hides or work clothing; it is a constant, and often hidden, source of infection. Infected dust and dried matter may also be transported on the above mentioned materials and later, released into the air, exposing individuals outside of the recognised risk environments to infection.
Contaminated aerosols, whether they be infected dust or droplets, are considered extremely infectious. It is estimated that very small numbers of C. burnetii (possibly between one to ten organisms) are sufficient to cause Q fever in humans.