In natural herds, Coxiella burnetii circulates between wild mammals, birds, and ticks. Domestic and farm animals become infected through excreta containing the pathogen, such as aborted material or milk, but tick feces can also play a role. Coxiellen can be transmitted from animals to humans by direct contact or through the respiratory tract by inhalation of dust or droplets containing Coxiella.
Q fever was first described in Queensland, Australia in 1937. The causative agent of Q fever is an obligate intracellular, pleomorphic gram-negative bacterium called Coxiella burnetii. The bacterium is highly resistant to chemical (e.g., formaldehyde) and physical agents (UV radiation, elevated temperature, desiccation). The ability to form permanent forms in the form of spores and the high resistance to desiccation allow it to survive outside organisms in dust, hay, wool, etc. for years.
The survival time of C. burnetii (Runge & Ganter 2008) is:
- in dust and wool at 20 °C storage 7-9 months, at 4 °C 1-2 years.
- in butter and soft cheese at 20 °C 42 days
- in meat at 4 °C one month
- in raw milk at 4-6 °C 90-273 days
- in dry milk at room temperature 40 months, at -20 °C 2 years, at +70 °C 15 sec, in 70 % ethanol 30 minutes
Coxiellen occur in 3 different forms: small cells (small cell variant, SCV), which are highly infectious; large cells (large cell variant, LCV), which form in cell cultures and are less infectious; spore-like particles (SLP), which are infectious and very robust to environmental conditions (Gürtler et al. 2013).
C. burnetii has a broad host range - predominantly infection has been diagnosed in sheep, goats, cattle, and wild ruminants. Course: mostly inapparent colonization of genitalia and udder, occurrence of herd-specific clustered abortions, postpartum behavior, udder inflammation. In addition, cats, dogs, rabbits, and birds may also be reservoir hosts.
In natural herds, C. burnetii circulates between wild mammals, birds, and ticks. Ticks are reservoirs and an important vector, but not the main source of infection for livestock. Infected afterbirths following abortions in sheep and goats are considered the main source. Ticks remain infected for life and can also pass the pathogen on to their offspring. Two independent cycles of infection are distinguished: the first is a natural hearth infection involving ticks and wildlife, in which infections of domestic animals may occur. This infection cycle is linked to the presence of specific tick species. A cycle develops between larvae, nymphs, and rodents that, when adult ticks occur, affects larger host animals (e.g., deer, foxes, sheep, goats, and cattle). The spring and summer-autumn seasons are known as seasonal risk periods because of the host transitions of adult ticks. The second cycle is an arthropod-independent domestic cycle. The infectious chain among warm-blooded animals is maintained in this cycle without an intermediate host. Transmission occurs through abortive material, excreta, and feces; respiratory transmission through dust and droplet aerosols is also possible. In domestic animals, coxial spread can also occur passively via pathogen-containing tick feces (in fleece, identifiable by dark discoloration).
Coxiellen travel through the bloodstream to the uterus and mammary gland, where they can persist for a long time without the infected animal showing symptoms. During gravidity, the infection can be reactivated, and the uterus and mammary glands in particular can harbor the pathogen for years. Significant amounts of pathogen are produced and excreted in both organs. Coxiellen are also excreted intermittently with milk. Birth products (e.g., afterbirth, amniotic fluid, lochia) and the newborns contaminated with them are particularly highly infectious. Dried-up fruit skins left on pasture can result in contamination of the premises for months.
Transmission to humans
Q fever can be transmitted from animals to humans by direct contact or via the respiratory tract by inhalation of dust or droplets containing Coxiella (= aerogenic transmission). Another route of infection for humans is transmission through tick feces, e.g. in sheep fleece. Occupational groups that come into contact with infected animals, such as farmers, veterinary staff, slaughterhouse staff, sheep herders, shearers, but also visitors to farms are at risk. Visitors to farms should therefore be prohibited from entering premises with infected animals to minimize the risk of inhaling infected dust. Dust containing C. burnetii can be carried by wind and endanger people at a distance of several kilometers. In this case, 1 to 10 coxiells already form a human infectious dose (HID) and are sufficient for infection.
Coxiella burnetii has not only been found in herds affected by Q fever, but has also been detected in raw milk and raw milk products (e.g., soft cheese, butter) and in the muscle meat and organs of infected animals. The risk to humans of contracting the disease through a foodborne infection has been documented in individual cases; however, transmission through food plays a rather minor role in the incidence of infection. For precautionary reasons, raw milk from infected herds should always be subjected to heat treatment; raw milk and raw milk products from C. burnetii-positive herds must not be sold to consumers. Pasteurization reliably destroys the pathogens.
Infected animals usually have only subclinical disease. In sheep, goats, and cattle, infection causes abortions or the newborns are often weak and barely viable; in cattle in particular, coxials are also the cause of fertility problems.
Since it is a bacterial pathogen, treatment with antibiotics is generally possible. However, due to the long survival time of Coxiellen in the environment, the probability is very high that new infections of previously healthy animals will occur again and again.
There is an approved vaccine for active immunization for cattle and goats, which can also be used for vaccination of sheep in Austria (Tierimpfstoff-Umwidmungsverordnung 2015). With vaccination, there is an opportunity to reduce the infection cycle and minimize the use of antibiotics in the long term.
Control of Q fever in livestock also focuses on preventive measures. A prerequisite for Q fever prevention and control measures is timely identification of infections. Some important recommendations for identifying Q fever outbreaks are to look for the possible sources of infection, the presence of lambing/calving, examination for tick infestation (tick feces in the fleece - dark discoloration in deeper fleece, skin is inflammatory changed or encrusted, charcoal dust-like particles are present in the densely wooly region between the head, neck and withers) and serological and molecular biological examinations to clarify the infection event; immunohistochemical procedures and microbiological examinations.
Measures to reduce the spread of the pathogen:
- Checking new purchases for the presence of coxial infection.
- Move the pregnant animals to the barn for lambing or calving: lambing or calving should take place in closed barns at a sufficient distance from residential areas.
- Disposal of afterbirths: contamination of the environment with birth products from infected animals should be minimized to prevent airborne transmission of the pathogens. Afterbirths and stillbirths should be collected in closed, liquid-impermeable containers and disposed of properly. The containers must then be properly disinfected
- Professional disinfection of the stalls and equipment affected by the infection. Regular cleaning of the stables should not be carried out with high-pressure jets or steam jets in the first phase, as in this case the spread of pathogens is promoted by aerosols
- Dogs and cats should stay away from the stables with infected animals
- Do not spread contaminated bedding as well as manure from stables affected by the infection on agricultural land. Spores can be spread over long distances
- Controlled acaricide treatment of the herd. Tick control in affected herds annually at the beginning of the tick season can substantially reduce pathogen spread if ticks play a role in transmission
- Persons who are in barns with infected animals for shearing or other activities should follow standard hygiene procedures (e.g., wash hands several times, wear protective gloves, own work clothes) and wear protective masks. Indoor shearing. Disposal of contaminated wool
The diagnosis of Coxiella burnetii is possible both directly (pathogen detection) and indirectly (antibody detection).
Sample material for pathogen detection by PCR:
- Milk (single milking)
- Abortion material
- Vaginal swab
Sample material for antibody detection by ELISA:
- Blood (plasma/serum)
- Milk (single and tank milk)
Last updated: 21.07.2023