Classification Principles and the Concept of Endemicity
The main principle of constructing the ecological and epidemiological classification of infectious diseases is identifying the primary 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.
There are three main environments in which pathogens can stably maintain their population:
- The human body.
- The animal body (including mammals, birds, and arthropods).
- The external environment, i.e., abiotic factors (primarily water and soil).
In the context of linking reservoirs to territories, it is crucial to understand the term endemicity. While the concept of epidemic reflects the magnitude and intensity of the spread of an infectious process, endemicity indicates exclusively its localization in a specific geographic zone. The maintenance of endemic foci is ensured by the continuous circulation of the pathogen among specific animal species inhabiting the given area. The stability of such foci is directly influenced by the climatogeographic features of the region, as well as various socio-economic factors.
Anthroponoses: Humans as the Only Source
The class of anthroponoses unites a group of infectious diseases in which the sole natural reservoir and source of infection is exclusively humans. In this group, transmission of the pathogen is strictly limited to the human population.
Within the class of anthroponoses, infections are classified in detail considering where the pathogen is localized in the body and which transmission mechanism is implemented. 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 integument: transmitted through close contact (syphilis, gonorrhea).
- "Vertical" infections: classified into a separate group, as 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 consists of animals (primarily mammals), as well as various arthropods.
Zoonoses are ecologically and epidemiologically divided into two large groups depending on the type of animals serving as sources:
- Diseases of agricultural, domestic, and synanthropic animals. This group emphasizes pets, livestock, and rodents living close to humans. This category includes brucellosis, psittacosis, foot-and-mouth disease, Q fever, and trichophytosis.
- Diseases of wild animals. These are so-called zoonotic vector-borne and natural focal infections whose reservoir is wild fauna. 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 maintain prolonged and stable autonomous existence outside a living organism. Being components of natural ecosystems, such pathogens form natural focal diseases.
An essential epidemiological feature of true sapronoses is that humans and warm-blooded animals become a dead-end host for the pathogen. This means that chain transmission of infection from one sick individual to another is absent. Sapronoses are divided into:
- Water-borne: autonomous existence proven in water (legionellosis, cholera, melioidosis).
- Soil-borne: the reservoir is soil (clostridial infections such as gas gangrene and tetanus, as well as agents of deep mycoses — blastomycosis, aspergillosis).
There are also diseases combining two reservoirs — 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 organism. The association with animals is regular 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.