Ecological and epidemiological classification of infections

Classification principles and the concept of endemicity

The main principle in constructing the ecological-epidemiological classification of infectious diseases is identifying the main habitat of the pathogen in nature. This environment, also called the reservoir, provides the microorganism with conditions for continuous existence and preservation of its species.

Three main environments are distinguished in which pathogens are able to stably maintain their population:

1. The human body.
2. The animal body (including mammals, birds, and arthropods).
3. The external environment, i.e., abiotic factors (primarily water and soil).

When discussing the association of reservoirs with territories, it is essential to understand the term endemicity. While the concept of an epidemic reflects the scale and intensity of the spread of an infectious process, endemicity indicates exclusively its localization within a specific geographic zone. The maintenance of endemic foci is ensured by the continuous circulation of the pathogen among specific animal species inhabiting the area. The stability of such foci is directly influenced by the region's climatic and geographic features, as well as various socio-economic factors.

Anthroponoses: humans as the sole source

The class of anthroponoses unites a group of infectious diseases in which the sole natural reservoir and source of infection is exclusively humans. Within this group, transmission of the pathogen is strictly limited to the human population.

Within the class of anthroponoses, infections are classified in detail based on where the pathogen localizes in the body and the mechanism of transmission involved. The main subgroups include:

• Intestinal anthroponoses: classic examples include typhoid fever, viral hepatitis A, and poliomyelitis.
• Respiratory anthroponoses: the pathogen localizes in the respiratory tract (measles, rubella, diphtheria, varicella).
• Blood-borne infections: epidemic typhus, louse-borne relapsing fever.
• Infections of the integumentary tissues: transmitted through close contact (syphilis, gonorrhea).
• «Vertical» infections: distinguished as a separate group because the transmission mechanism involves direct infection of the fetus from the mother via the transplacental route.

Zoonoses: animals and arthropods

The class of zoonoses includes diseases in which the primary reservoir of infection in nature is animals (primarily mammals) as well as various arthropods.

Ecologically and epidemiologically, zoonoses are generally divided into two large groups depending on the type of animals serving as sources:

1. Diseases of agricultural, domestic, and synanthropic animals. This group emphasizes pets, livestock, and rodents living in proximity to humans. This category includes brucellosis, psittacosis, foot-and-mouth disease, Q fever, and trichophytosis.
2. Diseases of wild animals. These are so-called natural focal infections, for which wildlife serves as the reservoir. This group includes tularemia, tick-borne encephalitis, rabies, and plague.

Sapronoses and the transitional form (saprozoonoses)

Sapronoses are a class of infections whose reservoirs are environmental substrates (soil, water). These environments provide microorganisms with the ability to survive autonomously and persistently outside a living organism. As components of natural ecosystems, such pathogens form natural focal diseases.

The most important epidemiological feature of true sapronoses is that humans and warm-blooded animals become a biologic dead end for the pathogen. This means there is no chain transmission of infection from one sick individual to another. Sapronoses are divided into:

• Water-borne: autonomous existence has been proven in water (legionellosis, cholera, melioidosis).
• Soil-borne: the reservoir is the soil (clostridial infections such as gas gangrene and tetanus, as well as causative agents of deep mycoses—blastomycosis, aspergillosis).

There are also diseases combining two reservoirs — these are saprozoonoses (zoophilic sapronoses). In this case, there is a smooth transition from the properties of sapronoses to zoonoses, with the role of animals in the pathogen's life cycle increasing significantly. The pathogen combines a saprophytic lifestyle in the abiotic environment with a parasitic one in the host organism. The connection with animals is consistent but often non-specific, encompassing a wide range of hosts. Human infection occurs both from substrates (water, soil, plants) and directly from animals. Typical examples of saprozoonoses include anthrax, listeriosis, leptospirosis, and yersiniosis.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of transmission of anthroponotic infections within the population
• The role of climatic and geographic factors in maintaining endemic foci
• Features of the existence of saprozoonotic pathogens in soil and water
• Classification of natural foci of zoonotic infections

Other topics in the section "Study of Infection"

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