Sources of pathogenic microorganisms

Forms of symbiosis: who uses the macroorganism

The relationships between microbes and the macroorganism can take three main forms.

Parasitism is a classic form of antagonism characteristic of pathogenic microorganisms. In this case, the microbe uses the host's body as a permanent or temporary habitat, as well as the main source of nutrition. Parasites cause direct damage to the macroorganism, which is often accompanied by the production of specific toxins.

Mutualism is a mutually beneficial coexistence. An example is the bacteria of the normal intestinal microflora, which are classified as non-pathogenic.

Commensalism is an intermediate type of symbiosis typical of opportunistic microbes. They are localized as part of the normal intestinal microflora, on the skin, and mucous membranes. Such microorganisms actively multiply in the macroorganism, but under normal conditions do not cause it any harm.

Animals as a reservoir of pathogenic microorganisms

Animals often serve as a source of pathogenic microorganisms. A classic example of such an interaction is brucellosis (Bang's disease), an acute anthropozoonosis with a pronounced tendency to become chronic. The disease is widespread globally, including in the territory of the Russian Federation, although the incidence rate varies greatly depending on the specific region.

The causative agents of the infection are small bacteria of the genus Brucella. The main reservoir and source of infection for humans are sick animals: cattle, small ruminants, and pigs.

Among the various species of the pathogen, B. melitensis poses the greatest danger to humans. Other species, such as B. abortus and B. suis, are more widely distributed specifically among livestock and are considered less pathogenic to humans. These microorganisms are highly resilient: they can persist for a long time in the environment and food products, especially dairy products. In addition, Brucella species are capable of forming L-forms.

Routes of shedding and mechanisms of transmission of infection

For a pathogenic microorganism to find a new host, it must be shed from the primary source. Sick animals shed the pathogen into the external environment in numerous ways: with the aborted fetus, amniotic fluid, milk, as well as urine and feces.

Transmission of the infection to humans occurs via several mechanisms:

• Alimentary route — infection occurs very frequently through food, in particular by consuming unpasteurized milk, cheese, sour milk, and meat from sick animals.
• Contact route — the pathogen penetrates through microtraumas and damaged skin upon direct contact with the source.
• Aerogenic route — infection through the respiratory tract.

The portals of entry for the infection are damaged skin, the respiratory tract, and mucous membranes (furthermore, Brucella can penetrate even through undamaged mucous membranes). The epidemiology of the disease has a clear seasonality: the peak of infections occurs in the winter-spring period, which directly coincides with the season of mass lambing and calving of livestock.

Pathogenesis and intracellular parasitism

After penetration through the portal of entry, the pathogen spreads via lymphatic pathways. Moderate lymphadenitis develops, while a distinct primary affect at the site of penetration is absent. Brucella species exhibit a pronounced tropism for cells of the reticuloendothelial system (RES).

Next comes the generalization stage: dissemination of the pathogen occurs, its entry into the bloodstream, and seeding into the RES organs (lymph nodes, spleen, liver, bone marrow). There, the microbes live and actively multiply intracellularly.

The course of the infectious process is wave-like. Approximately every 2–3 weeks, exacerbations occur due to the repeated release of Brucella into the blood and sensitization of the body. The increase in brucelloremia is accompanied by the development of delayed-type hypersensitivity (DTH). It is this allergic restructuring that determines the clinical manifestations of the disease and its chronicity. The immunity formed in this disease is non-sterile.

Clinical presentation and pathomorphological changes

Typical clinical symptoms are most often observed in unvaccinated patients. In the initial (acute) stage, which lasts 1–2 months, there is a wave-like high fever, severe chills, increased sweating (hyperhidrosis), and hepatosplenomegaly (enlargement of the liver and spleen). An interesting feature is that despite such pronounced symptoms, the patient's general well-being may remain relatively satisfactory.

At the tissue level, the morphological picture is characterized by the appearance of small (1–2 mm) foci of necrosis or abscesses. Large foci up to 1 cm in size are quite rare. Specific changes resembling granulomas form in the brain and liver.

If a secondary infection joins the process, suppuration of the foci is possible. Complications are also most often caused by secondary flora, which can lead to the development of focal or lobar bronchopneumonia. Nevertheless, with timely and intensive therapy, the prognosis of the disease generally remains favorable.

Class and exam questions

What else to study on this topic

• Morphological properties and ability to form L-forms in bacteria of the genus Brucella
• Features of the formation of non-sterile immunity in zoonotic infections
• The role of delayed-type hypersensitivity (DTH) in the chronicity of the infectious process
• Clinical picture of the acute, subacute, and chronic stages of brucellosis
• Pathomorphology of liver and brain lesions in intracellular parasitism

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