Natural focality of diseases

Obligate links of a natural focus

Such foci historically form within interspecific biocenoses. For the epidemic chain to function, the simultaneous presence of three populations is required:

1. Pathogen population (bacteria, viruses, rickettsiae, protozoa).
2. Wild animal population — these are natural reservoirs (donors and recipients of the infection).
3. Blood-sucking arthropod population — vectors of the disease.

The fourth uniting factor is a specific natural landscape. Since all participants of the chain inhabit certain conditions (for example, only in the taiga or only near water bodies), the focus itself has strict geographical boundaries. Under favorable conditions, the pathogen circulates between animals and vectors indefinitely. The infection of wild animals itself can proceed as a severe disease or latent carriage (a typical zoonosis).

Structure of parasitic systems and transmission vectors

Interaction systems are either simple (the pathogen exists at the expense of a single host species) or complex (many animal species are involved). Depending on how many arthropod species participate in transmission, natural foci are divided into two types:

• Monovector: the infection is transmitted by only one species of arthropod (for example, lice spread relapsing and epidemic typhus).
• Polyvector: the same pathogen travels in two or more vector species. Thus, encephalitis viruses can be transmitted by both ticks and mosquitoes.

Ticks play a crucial role in preserving the infection. Warm-blooded animals carry the pathogen in their peripheral blood for only a few days, whereas ticks are persistent natural reservoirs. The pathogen multiplies in their gut and is carried throughout the body with hemolymph. Ticks can transmit infection transovarially (from the female to laid eggs) and transphasically (retaining it during the transition from larva to nymph and adult).

Classification by the mode of infection transmission

According to the mode of pathogen transmission to the vertebrate host, diseases are divided into two major groups:

Obligate-vector-borne (transmissive) diseases Transmission occurs exclusively through blood-sucking via a bite. This group includes:

• Anthroponoses (source is humans only): malaria, louse-borne relapsing and epidemic typhus, pappataci fever.
• Anthropozoonoses (source is humans and animals): tick-borne encephalitis, yellow fever, tick-borne typhus, leishmaniases.

Facultatively transmissible diseases Here, an insect or tick bite is possible, but not mandatory. Infection can occur via alimentary, contact, or oral routes. All of these are anthropozoonoses. They include plague, tularemia, anthrax, rabies, brucellosis, and Lyme disease.

Human impact: anthropurgic foci

Human activity cardinally changes the epidemic picture. Deforestation or marsh draining leads to the destruction (degradation) of old foci. However, the construction of reservoirs or rice paddies causes irradiation — the spread of infection to new territories.

Academician Ye.N. Pavlovsky in 1946 introduced the concept of an anthropurgic focus. These are secondary foci arising due to economic activity. Vectors (mosquitoes, sandflies) become synanthropic, moving to live and reproduce directly in cities and villages. Domestic animals, rats, birds, and humans themselves become involved in the circulation of the infection. Examples of such high-intensity outbreaks are Japanese encephalitis in Tokyo and cutaneous leishmaniasis in populated areas.

Principles of natural focus elimination

To make a territory safe, it is necessary to break the continuous circulation of the pathogen. The set of measures includes:

1. Destruction of wild animals acting as natural reservoirs.
2. Direct control of vectors (taking their ecology into account).
3. Sanation of agricultural and domestic animals.
4. Economic measures that deprive arthropods of breeding sites.
5. Personal protection of humans (suits, repellents).
6. Specific vaccination of the population.

Class and exam questions

What else to study on this topic

• Etiological classification: arboviruses, rickettsiae, and spirochetes
• Vector microstations and the diffuse nature of the natural focus
• Global geography of viral encephalitis foci
• Pathogenesis of facultatively transmissible anthropozoonoses

Other topics in the section "General Parasitology Questions"

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