Biological and Cultural Properties
The bacterium Coxiella burnetii belongs to the class Gammaproteobacteria and the family Coxiellaceae. This microorganism was first isolated in Australia in 1937 by researchers F. Burnet and M. Freeman.
Morphologically, they are small gram-negative coccobacilli prone to pleomorphism (shape variability). Using special staining methods such as Gimenez or Ziehl-Neelsen, the cells acquire a deep red color.
Coxiellae are obligate intracellular parasites — they are completely unable to grow on artificial cell-free nutrient media. In laboratory practice, cell cultures, guinea pigs, or the yolk sacs of chicken embryos are used for their cultivation. Upon entering the target cell, the pathogen localizes and actively replicates within cytoplasmic vacuoles and phagolysosomes.
Antigenic Structure and Survival
A unique feature of C. burnetii is its ability to undergo phase variations, which alter the morphology and antigenic composition of the bacterium:
- Phase I (natural, virulent): A specific structural polysaccharide is present in the cell wall. These bacteria are hydrophilic, highly immunogenic, and reliably protected from phagocytosis (macrophages cannot engulf them unless specific antibodies are present in the blood).
- Phase II: Characterized by reduced virulence and high susceptibility to phagocytosis.
Inside infected cells, coxiellae form spore-like (endospore-like) forms. These are most pathogenic for humans and provide the bacteria with extreme environmental resistance. The pathogen easily withstands exposure to phenol, formalin, low pH, high temperatures, and prolonged drying. In contaminated water and on objects, the bacteria survive for months, and in dry animal feces, they can remain viable for up to two years.
Epidemiology of the Infection
In the wild, the reservoirs of infection are rodents and birds, while in agriculture, they are cattle, sheep, goats, and horses. The pathogen circulates among animals via vectors such as ixodid and argasid ticks.
The main source of human infection is infected animals. The disease in animals is often asymptomatic, but microorganisms are shed in huge quantities in milk, urine, excrement, and especially massively in amniotic fluid.
There are three main routes of human infection:
- Airborne (dust-aerosol): Inhalation of dust or infected aerosols. Due to high volatility, aerosols can be carried by wind for kilometers from the source, which is why C. burnetii is considered a potential bioterrorism agent.
- Alimentary: Consumption of raw infected milk or contaminated water.
- Contact: Dust entering the conjunctiva of the eyes or contact of damaged skin areas with infected secretions (e.g., during veterinary examinations).
Clinical Presentation, Diagnosis, and Treatment
The incubation period of coxiellosis ranges from 18 to 21 days. The disease manifests acutely: the patient is troubled by severe headache and fever. In most cases, a respiratory syndrome resembling "atypical pneumonia" develops, frequently accompanied by dyspeptic disorders (nausea, vomiting).
The infection is often accompanied by internal organ involvement: half of the patients with pneumonia exhibit hepatosplenomegaly (enlargement of the liver and spleen), and in rare cases, the heart is affected (endocarditis). Despite pronounced symptoms, the prognosis is favorable — mortality does not exceed 1%.
The basis of microbiological diagnosis is serological methods (ELISA, complement fixation test [CFT], indirect immunofluorescence assay [IFA]). The disease is characterized by antibody dynamics: antibodies against Phase II antigens are produced early in the infection, while antibodies against Phase I antigens appear during the peak and convalescence periods. The disease must be differentiated from other atypical pneumonias (caused by legionellae, mycoplasmas, chlamydiae).
Aetiotropic therapy includes tetracycline antibiotics and fluoroquinolones. Strictly according to epidemiological indications, specific prophylaxis may be used — a live vaccine.