The causative agent of Q fever

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 (variability in shape). When using special staining methods according to Zdrodovsky or Romanowsky-Giemsa, the cells acquire an intense red color.

Coxiellas are obligate intracellular parasites — they are completely incapable of growing on artificial cell-free nutrient media. In laboratory practice, cell cultures, infected 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 inside 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): The cell wall contains a specific structural polysaccharide. Such bacteria are hydrophilic, highly immunogenic, and reliably protected against phagocytosis (macrophages are unable to engulf them unless specific antibodies are present in the blood).
• Phase II: Characterized by reduced virulence and high susceptibility to phagocytosis.

Inside infected cells, coxiellas form spore-like (endospore-like) forms. These are the most pathogenic for humans and provide the bacterium with extreme resistance to the environment. The pathogen easily withstands exposure to phenol, formalin, low pH levels, and endures 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.

Infection epidemiology

In the wild, the reservoir of infection consists of rodents and birds, while in agriculture, it includes large and small ruminants and horses. The pathogen circulates among animals via vectors: ixodid and argasid ticks.

For humans, the main source of infection is infected animals. The disease in animals is often asymptomatic, but microorganisms are released in huge quantities with milk, urine, excreta, and most massively with amniotic fluid.

There are three main routes of human infection:

1. Airborne (dust-borne): Inhalation of dust or infected aerosols. Due to their high volatility, aerosols can be carried by the wind for kilometers from the source, which is why C. burnetii is considered a potential bioterrorism agent.
2. Alimentary: Consumption of raw infected milk or contaminated water.
3. Contact: Dust getting onto the conjunctiva of the eyes or contact of damaged skin areas with infected secretions (e.g., during veterinary examination).

Clinical presentation, diagnostics, and treatment

The incubation period of coxiellosis ranges from 18 to 21 days. The disease has an acute onset: the patient is troubled by severe headache and fever. In most cases, a respiratory syndrome resembling "atypical pneumonia" develops, often accompanied by dyspeptic disorders (nausea, vomiting).

The infection is frequently accompanied by internal organ involvement: in half of the patients with pneumonia, hepatosplenomegaly (enlargement of the liver and spleen) is recorded, 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 diagnostics is serological methods (ELISA, CFT, indirect IFA). The disease is characterized by antibody dynamics: antibodies to Phase II antigens are produced early in the infection, while antibodies to Phase I antigens are produced during the peak and convalescence periods. The disease must be differentiated from other atypical pneumonias (caused by legionellae, mycoplasmas, chlamydiae).

Etiotropic therapy includes tetracycline antibiotics and fluoroquinolones. Strictly according to epidemic indications, specific prophylaxis may be used — a live vaccine (M-44 strain, developed by P.F. Zdrodovsky and V.A. Genig).

Class and exam questions

What else to study on this topic

• Features of cultivating obligate intracellular bacteria (chicken embryos, cell cultures).
• Mechanism of coxiella survival within macrophage phagolysosomes.
• Principles of differential diagnostics for pathogens of "atypical pneumonias".
• Interpretation of serological reaction results (CFT, ELISA, IFA) over time.
• Live vaccine of strain M-44: composition, mechanism of action, and epidemiological indications.

Other topics in the section «Gram-negative aerobic rods (Pertussis, Brucella, Tularemia, Legionella, Coxiella/Q fever, Bartonella, Pseudomonas, Burkholderia, Acinetobacter, etc.)»

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.