Classification of natural foci. Vector-borne diseases Arthropods as components of natural foci

Mechanical contamination - transmission of the pathogen, carried out by crushing it (mechanical transfer). The causative agent only persists for some time on the surface of the body, in the gastrointestinal tract, for example, intestinal infections.

Mechanical inoculation is the introduction of the pathogen at the time of bloodsucking or injecting it with saliva, for example, tularemia, anthrax.

Improvement of the transmission mechanism: Mechanical contamination - Mechanical inoculation - Specific contamination - Specific inoculation.

67. Diptera blood-sucking insects as carriers of infections. The concept of "vile". Measures to combat it.Gnus- a set of blood-sucking Diptera insects. An abomination is considered: mosquitoes, midges, biting midges, mosquitoes, horseflies, tsetse flies. The gnat is especially dangerous in those regions of the country where insects take part in the circulation of pathogens of various infectious diseases of humans - malaria, tularemia, Japanese encephalitis. Transmission method Blood-sucking insects, piercing the skin, infect. In this case, a person and flying bloodsuckers are links in the same food chain. The causative agent of the disease lives in the body of an insect, then in the blood and organs of a person, completing its life cycle in the body of one of the "hosts". Usually the natural focus of such infections is wild mammals or birds. And insects are just vehicles for moving from the body of one warm-blooded organism to another. Examples: tularemia, malaria, borreliosis, encephalitis, plague. Mosquitoes- these Diptera insects inhabit all continents except Antarctica. In total, there are about 3,000 known species. Mosquitoes live wherever breeding conditions exist. The larva and mobile pupa of the mosquito require a fresh reservoir of warm water: puddles, wetlands, garden reservoirs with water, damp basements of buildings. Only females attack a person. They need blood to ripen their eggs. Males feed on flower nectar. When bitten, they pierce the skin, insert their proboscis into the smallest capillaries, secrete a portion of saliva containing an anesthetic and a substance that prevents blood clotting. Malaria- transmissible anthroponous protozoal infection, occurring with attacks of fever, anemia, enlargement of the liver and spleen. The causative agents of malaria are protozoa of the genus Plasmodium, which are transmitted from sick to healthy through mosquitoes of the genus Anopheles.Go blind. They live near water bodies, as they need to constantly drink. Eggs are laid on coastal vegetation. Horseflies have a complex mouth apparatus. Attacking the victim, they gnaw through the skin and insert the proboscis. With saliva, toxins enter the tissues, which cause pain and swelling. Horseflies can be a source of anthrax, tularemia Tsetse flies distributed in Africa, live mainly near water bodies. This fly differs from the usual domestic one by the presence of a sharp proboscis and the manner of folding its wings together. When bitten, it infects a person with sleeping sickness. Midges and biting midges... These small insects, one of the types of gnats, are also representatives of the order Diptera. Midges swarm around a person, crawl under clothes. When attacking a victim, they bite off a piece of skin and drink blood from the resulting wound. The components of saliva have an analgesic effect, cause itching and allergic reactions... Midges live and breed near fresh flowing water bodies. They attack mainly during daylight hours. At night, their activity is significantly reduced.To eliminate the itching from a gnat bite, the skin must be wiped with a mixture ammonia and water in equal parts or a solution of baking soda (1/2 teaspoon in 1 glass of water) Personal protective measures include the use of repellents, capes impregnated with repellents, specially designed protective clothing or special fabrics, curtains, etc. Community prevention measures are: measures for the radical improvement of the territory and 2) extermination measures aimed at combating larvae or adults. A protective cape made of mesh or gauze can be impregnated with birch or pine tar or a liquid composition based on tar (10 parts of tar and 5 parts are added to 100 parts of water caustic soda and mix thoroughly). The resulting composition slightly moisten the cape and dry in a shady, windless place. After processing, the cape scares off the gnat for 10-12 days. It should be borne in mind that sweaty skin attracts blood-sucking insects 2-3 times more than clean and dry. fight against blood-sucking Diptera. It is necessary to use insecticides and acaricides - agents that destroy insects.

Vector-borne diseases are infectious diseases carried by blood-sucking insects and representatives of the arthropod type. Infection occurs when a person or animal bites an infected insect or tick.

There are about two hundred official diseases known to have a transmissible route of transmission. They can be caused by various infectious agents: bacteria and viruses, protozoa and rickettsia, and even helminths. Some of them are transmitted through the bite of blood-sucking arthropods (malaria, typhus, yellow fever), some of them are transmitted indirectly, when cutting the carcass of an infected animal, in turn, bitten by an insect carrier (plague, tularemia, anthrax).

Carriers

The pathogen passes through the mechanical carrier in transit (without development and reproduction). It can persist for some time on the proboscis, body surface, or in the digestive tract of an arthropod animal. If at this time a bite occurs or contact with the wound surface occurs, then human infection will occur. A typical representative of the mechanical carrier is the fly this. Muscidae. This insect carries a variety of pathogens: bacteria, viruses, protozoa.

As already mentioned, according to the method of transmission of the pathogen by the arthropod carrier from an infected vertebrate donor to a vertebral recipient, natural focal diseases are divided into 2 types:

obligate-transmission, in which the transfer of the pathogen from the vertebrate-donor to the vertebrate-recipient is carried out only through the blood-sucking arthropod during bloodsucking;

optional transmission natural focal diseases, in which the participation of a blood-sucking arthropod (carrier) in the transmission of the pathogen is possible, but not necessary. In other words, along with the transmissible (through the bloodsucker), there are other ways of transmitting the pathogen from a vertebrate donor to a vertebrate recipient and a person (for example, oral, alimentary, contact, etc.).

According to E. N. Pavlovsky (Fig.1.1), the phenomenon natural focus vector-borne diseases is that, independently of a person, on the territory of certain geographical landscapes there may be foci diseases to the causative agents of which a person is susceptible.

Such foci were formed in the process of long-term evolution of biocenoses with the inclusion of three main links in their composition:

Populations pathogens illness;

Wildlife populations - natural reservoir hosts(donors and recipients);

Populations of blood-sucking arthropods - carriers of pathogens disease.

It should be borne in mind that each population of both natural reservoirs (wild animals) and vectors (arthropods) occupies a certain territory with a specific geographic landscape, due to which each focus of infection (invasion) occupies a certain territory.

In this regard, for the existence of a natural focus of the disease, along with the three above-mentioned links (pathogen, natural reservoir and carrier), the fourth link is also of great importance:

natural landscape(taiga, mixed forests, steppes, semi-deserts, deserts, various reservoirs, etc.).

Within the same geographical landscape, natural foci of several diseases may exist, which are called conjugate. This is important to know when vaccinating.

Under favorable environmental conditions, the circulation of pathogens between vectors and animals - natural reservoirs can occur indefinitely long time... In some cases, infection of animals leads to their disease, in others asymptomatic carriage is noted.

By origin natural focal diseases are typical zoonoses, i.e., the circulation of the pathogen occurs only between wild vertebrates, but the existence of foci for anthropozoonotic infections.

According to E.N. Pavlovsky, natural foci of vector-borne diseases are mono-vector, if in

the transmission of the pathogen involves one type of vector (lice relapsing fever and typhus), and polyvector, if the transmission of the same type of pathogen occurs through carriers of two, three or more species of arthropods. The foci of such diseases are the majority (encephalitis - taiga, or early spring, and Japanese, or summer-autumn; spirochetosis - tick-borne relapsing fever; rickettsiosis - North Asian tick-borne typhus, etc.).

The doctrine of natural focus indicates the unequal epidemiological significance of the entire territory of the natural focus of the disease due to the concentration of infected vectors only in certain microstations. Such a focus becomes diffuse.

In connection with the general economic or purposeful human activity and the expansion of urbanized territories, mankind has created conditions for the mass distribution of the so-called synanthropic animals (cockroaches, bedbugs, rats, house mice, some ticks and other arthropods). As a result, humanity is faced with an unprecedented phenomenon of the formation anthropogenic foci of diseases, which sometimes can become even more dangerous than natural foci.

By virtue of economic activity In humans, irradiation (spread) of the old focus of the disease to new places is possible if there are favorable conditions for the habitation of vectors and animals - pathogen donors (construction of reservoirs, rice fields, etc.).

Meanwhile, it is not excluded destruction(destruction) of natural foci during the loss of its joints from the biocenosis, which take part in the circulation of the pathogen (when draining swamps and lakes, deforestation).

In some natural foci, ecological succession(replacement of some biocenoses by others) when new components of the biocenosis appear in them, capable of being included in the circulation chain of the pathogen. For example, the acclimatization of the muskrat in natural foci of tularemia led to the inclusion of this animal in the circulation chain of the causative agent of the disease.

E.N. Pavlovsky (1946) distinguishes a special group of foci - anthropurgic foci, the emergence and existence of which is associated with any type of human activity and also with the ability of many species of arthropods - inoculators (mosquitoes, ticks, mosquitoes that carry viruses, rickettsia, spirochetes and other pathogens) to synanthropic lifestyle. Such arthropod carriers live and breed in settlements of both rural and urban types. Anthropurgic foci arose a second time; in addition to wild animals, domestic animals, including birds, and humans are included in the circulation of the causative agent of the disease, therefore, such foci often become very tense. Thus, large outbreaks of Japanese encephalitis were noted in Tokyo, Seoul, Singapore and other large settlements in Southeast Asia.

The foci of tick-borne relapsing fever, cutaneous leishmaniasis, trypanosomiasis, etc. can also acquire an anthropurgic character.

The stability of natural foci of some diseases is explained primarily by the continuous exchange of pathogens between vectors and animals - natural reservoirs (donors and recipients), but the circulation of pathogens (viruses, rickettsia, spirochetes, protozoa) in the peripheral blood of warm-blooded animals - natural reservoirs are most often limited in time and lasts for several days.

Meanwhile, the causative agents of diseases such as tick-borne encephalitis, tick-borne relapsing fever, etc., in the intestines of tick carriers multiply intensively, make transcoelomic migration and are carried with hemolymph into various organs, including the ovaries and salivary glands. As a result, the infected female lays infected eggs, that is, transovarian transmission the pathogen to the progeny of the carrier, while the pathogens in the course of the further metamorphosis of the tick from the larva to the nymph and further to the imago are not lost, i.e. transphase transmission pathogen.

In addition, ticks retain pathogens in their bodies for a long time. E. N. Pavlovsky (1951) traced the duration of spirochete carriage in ornithodorin ticks up to 14 years and more.

Thus, in natural foci, ticks serve as the main link in the epidemic chain, being not only carriers, but also persistent natural keepers (reservoirs) of pathogens.

The doctrine of natural focus considers in detail the ways of transmission of pathogens by vectors, which is important for knowledge possible ways infecting a person with a particular disease and for its prevention.

Immunoprophylactic methods include immunization of the population. These methods are widely used for the prevention of infectious diseases. The development of immunoprophylaxis of invasions has a number of significant difficulties and is currently at the development stage. Measures for the prevention of natural focal diseases include measures to control the number of carriers of the disease (reservoir hosts) and arthropod vectors by influencing their habitat and the rate of their reproduction. in order to interrupt the circulation of the pathogen within the natural focus.

62. general characteristics protozoa (Protozoa) Overview of the structure of protozoa

This type is represented by unicellular organisms, the body of which consists of cytoplasm and one or more nuclei. A protozoan cell is an independent individual that exhibits all the basic properties of living matter. It performs the functions of the whole organism, while multicellular cells constitute only a part of the organism, each cell depends on many others.

It is generally accepted that unicellular creatures are more primitive than multicellular ones. However, since the entire body of unicellular organisms, by definition, consists of one cell, this cell must be able to do everything: eat, and move, and attack, and escape from enemies, and not worry favorable conditions environment, and multiply, and get rid of metabolic products, and protect against drying out and from excessive penetration of water into the cell.

A multicellular organism can do all this too, but each of its cells, taken separately, is good at doing only one thing. In this sense, the protozoan cell is by no means more primitive than the cells of a multicellular organism. Most members of the class have microscopic dimensions - 3-150 microns. Only the largest representatives of the species (shell rhizomes) reach 2–3 cm in diameter.

Digestive organelles are digestive vacuoles with digestive enzymes (similar in origin to lysosomes). Nutrition occurs by pino- or phagocytosis. Undigested residues are thrown out. Some protozoa have chloroplasts and feed on photosynthesis.

Freshwater protozoa have organs of osmoregulation - contractile vacuoles, which periodically release excess fluid and dissimilation products into the external environment.

Most of the protozoa have one nucleus, but there are representatives with several nuclei. The nuclei of some protozoa are polyploid.

The cytoplasm is heterogeneous. It is subdivided into a lighter and more homogeneous outer layer, or ectoplasm, and a granular inner layer, or endoplasm. The outer covers are represented either by the cytoplasmic membrane (in the amoeba) or by the pellicle (in the euglena). Foraminifers and sunflowers, inhabitants of the sea, have a mineral or organic shell.

Irritability is represented by taxis (motor reactions). There are phototaxis, chemotaxis, etc.

Reproduction of protozoa Asexual - by mitosis of the nucleus and cell division in two (in amoeba, euglena, ciliates), as well as by schizogony - multiple division (in sporozoans).

Sexual - copulation. The protozoan cell becomes a functional gamete; as a result of the fusion of gametes, a zygote is formed.

For ciliates, the sexual process is characteristic - conjugation. It lies in the fact that cells exchange genetic information, but there is no increase in the number of individuals. Many protozoa are able to exist in two forms - trophozoite (a vegetative form capable of active feeding and movement) and cysts, which forms under adverse conditions. The cell is immobilized, dehydrated, covered with a dense membrane, the metabolism slows down sharply. In this form, the protozoa are easily carried over long distances by animals, by the wind, and dispersed. When it enters favorable habitat conditions, excysting occurs, the cell begins to function in a state of trophozoite. Thus, encystation is not a method of reproduction, but helps the cell to survive adverse environmental conditions.

Many representatives of the Protozoa type are characterized by the presence of life cycle consisting in the regular alternation of life forms. As a rule, there is a generational change with asexual and sexual reproduction. Cyst formation is not part of a natural life cycle.

The generation time for protozoa is 6-24 hours. This means that once in the host's organism, the cells begin to multiply exponentially and theoretically can lead to its death. However, this does not happen, since the protective mechanisms of the host organism come into force.

Representatives of the protozoa belonging to the classes sarcodes, flagellates, ciliates and sporozoans are of medical importance.

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Introduction

Vector characteristics

Bibliography

Introduction

Over the past decade, there has been an increase in human and animal diseases of an infectious and invasive nature, which is associated with the high prevalence of pathogens of the infectious process in nature. The spread of infection is facilitated by vectors that live everywhere, including those adjacent to humans.

The spread of a number of infections carried by insects and ticks causes the need to draw the attention of a wide range of medical and veterinary workers, as well as the population, to these issues.

In their diversity, number of species, arthropods surpass all other groups of animals.

Arthropods are of the greatest epidemiological importance as specific carriers of pathogens of human infectious diseases. In the body of a specific carrier, the pathogen goes through a certain development cycle (plasmodium malaria in the body of a mosquito, Leishmania in mosquitoes) or only multiplies (the causative agent of plague in fleas, encephalitis virus in ticks). In mechanical vectors, pathogens are located on the surface of the body, in the proboscis, intestines (flies, horseflies, cockroaches). The transfer of the pathogen in such cases, as a rule, is possible for a short period of time, while it retains its viability. In some cases, the same species of arthropods can be a specific and mechanical carrier of some pathogens.

How infections spread

infectious invasive disease pathogen

The carriers of infectious agents can be divided into three groups:

Carriers of anthroponosis pathogens (malaria, typhus, etc.)

Carriers of pathogens of zooanthroponosis (plague, tularemia, boreliosis, etc.)

Carriers that ensure the circulation of the pathogen pathogen for humans among animals.

The mechanism of transmission of pathogens by vectors includes three phases: obtaining the pathogen; transfer of the pathogen by the vector from an infected person or animal to a healthy one; the introduction of the pathogen by the carrier into the human body (animal).

The transfer of infectious agents can be mechanical and specific. With mechanical transfer, the pathogens obtained by the vector

Only for a while do they remain viable and virulent on the surface of its body or in the digestive tract.

Sometimes the same vector can be mechanical for one infection and specific for another. For example, mosquitoes of the genus Anopheles, being specific carriers of malaria pathogens, can be mechanical carriers of tularemia and anthrax pathogens.

The introduction of pathogens into the body healthy person(animal) occurs either at the time of bloodsucking, when they are introduced using the oral apparatus of the carrier, or injected with its saliva.

This method of introduction is called inoculation. In another case, the carrier, upon contact with a person (animal), contaminates his skin, mucous membranes, wounds with its excrement or tissue fluid (for example, when the carrier is crushed), which contain pathogens, or transfers them from the surface of the body, legs, proboscis, contaminated with substrates containing pathogens on food and household items (for example, when transmitting pathogens of intestinal infections). This mode of transfer is called contamination.

Inoculation and contamination can be mechanical and specific. Mechanical contamination is most common in the transmission of most intestinal infections and infestations by flies and cockroaches.

Mechanical inoculation is observed during the transmission of tularemia pathogens by mosquitoes, biting midges, midges, anthrax - mosquitoes, flies, gadflies. An example of specific inoculation is the transmission of plague pathogens by fleas, encephalomyelitis, yellow fever, malaria by mosquitoes, leishmaniasis and phlebotomy fever by mosquitoes. Specific contamination is somewhat less common. Thus, trypanosomes (causative agents of Chagas disease) are transmitted by kissing bug, spirochetes and rickettsia (causative agents of lice relapsing and typhus) by lice, as well as spirochetes of endemic relapsing fever-argas mites.

A number of vectors are involved in the preservation of pathogens as a species by passing them on to their offspring (transovarial and transphase transmission). Transovarial transmission is the ability of female carriers to transmit the pathogens received to their offspring: they lay infected eggs, from which subsequent phases develop (larvae, pupae or nymphs and adults) that retain the pathogens.

Transphase transmission is the ability of the carrier to retain the pathogen during molting during the transformation of one phase into the next.

For example, an infected tick larva turns into an infected nymph, and the latter turns into an infected adult.

In the transmission of pathogens of a particular infectious disease, carriers of several species can sometimes participate, some of them are main, others are secondary.

The former are characterized by: large number populations, high activity of individuals, in particular regarding attacks on people, increased infection with pathogens in relation to them.

The most important in the transmission of infectious agents to humans are the so-called synanthropic species of vectors, i.e. species whose life is associated with humans. It is customary to divide synanthropic carriers into endophilic (endophiles), which spend most of their lives in human buildings, and exophilic (exophilic) inhabitants of open space.

Depending on the climate, landscape, economic and living conditions, the same type of vector may be the main one in one epidemic focus and the secondary one in another.

Vector characteristics

Argas mites are found mainly in the southern regions of the country. Revealed their infection with causative agents of viral, rickettsial, bacterial etiology. Argas mites are of paramount importance as specific carriers of Borrelia. Due to the long life cycle (according to some data - up to 25 years), foci of tick-borne spirochetoses steadily take root in nature. In the last 10 - 15 years, argas mites are increasingly common in urban-type buildings.

Diptera (mosquitoes, biting midges, midges, horseflies) are carriers of pathogens of many infectious diseases in humans and animals (tularemia, anthrax, etc.). Their role in the transmission of viruses is great. Mosquitoes are of the greatest epidemiological significance in this group of insects. They carry the causative agents of malaria, West Nile fever, yellow fever, Dengue fever, Sindbis fever, Japanese encephalitis and many others.

Wild, domestic and ornamental birds are the source of the causative agent of psittacosis. It should be noted the high degree of infection of pigeons, crows (up to 50%). The most important is the airborne and dusty transmission of infection, the lesser is the airborne and foodborne route.

Wild mammals (fox, wolf, jackal, raccoon, raccoon dog, the bats) in the population of which the rabies virus circulates are dangerous to humans. In addition to natural foci, secondary anthropurgic foci are formed, in which the virus circulates between dogs, cats and farm animals.

Goats, sheep, cows, pigs, deer are the main sources of the pathogen of brucellosis.

Thus, it is necessary to prevent the occurrence of infectious processes not only by active and passive immunization, but also to prevent the collision of infectious animals with humans.

On the basis of many years of scientific and experimental research, a system of protection against blood-sucking and non-blood-sucking arthropods has been developed, taking into account the climatic-geographical, ecological and epidemiological features of the geographical regions.

Currently, prophylactic and therapeutic vaccines have been developed against many infectious diseases, with which it is necessary to vaccinate the population in endemic areas. And also observe the sanitary and epidemiological regime at agricultural and food enterprises and food storage areas.

Bibliography

1.http: //www.gkb2.grodno.by/health/gkb2/ing8/

2.http: //46cge.rospotrebnadzor.ru/info/105628/

3.http: //nd-ek.ru/nas

4.http: //dic.academic.ru/dic.nsf/enc_medicine/22944/%D0%9F%D0%B5%D1%80%D0%B5%D0%BD%D0%BE%D1%81%D1 % 87% D0% B8% D0% BA% D0% B8

5. Infectious Diseases, ed. N.D. Yushchuka, Y. Ya. Vengerova

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Diseases whose pathogens are transmitted only from animals to animals are called zoonoses (plague chickens and pigs).

Diseases whose causative agents are transmitted only from person to person are called anthroponoses.(measles, diphtheria).

Diseases whose pathogens are transmitted from one organism to another by means of blood-sucking vectors (insects, ticks) are called transmissible (malaria, taiga encephalitis).

They are subdivided into:

1) obligate-transmissible, the pathogens of which are transmitted through specific vectors (malaria - by mosquitoes of the genus Anopheles, taiga encephalitis - taiga ticks);

2) facultatively transmissible, the pathogens of which can be transmitted both through vectors and
and in other ways (infection with tularemia and anthrax is possible through numerous carriers and when cutting the carcasses of sick animals).

Carriers of pathogens of vector-borne diseases can be specific and mechanical. In the body of a specific carrier, the pathogen goes through part of the life cycle (the plague bacillus multiplies in the digestive tract of a flea; malaria plasmodia undergo a sexual development cycle in mosquitoes of the genus Anopheles).

The causative agents of diseases in mechanical vectors (flies, cockroaches) are located on the integuments of the body, on the limbs and parts of the mouth apparatus.

The entrance gate of the pathogen is always the oral apparatus of a specific carrier. The exit of the pathogen from the carrier can occur through the anus or through the oral apparatus.

In the first case, the pathogen passes through the intestines (rickettsia of lousy typhus). Host infection occurs when the excrement of the carrier is rubbed into the skin while scratching the bite site. This method of infection is called contamination.

If the pathogen passes through the body cavity of the carrier and accumulates in the salivary glands (sporozoites of malaria plasmodia), then infection of the host occurs through the oral apparatus during bloodsucking. The method of infection is called inoculation.

The exciter exit gate may not be present. In this case, the pathogen accumulates in the body cavity of the carrier. Infection of the host occurs when the carrier is crushed and the hemolymph with the pathogen is rubbed into the skin during scratching - a variety contamination(transmission of spirochetes of relapsing fever by lice).

In the first and second cases, the carrier can transmit pathogens multiple times, in the third - only once, since the transmission of the pathogen is associated with the death of the carrier.

Many vectors are characterized by transovarian (through eggs) transmission of pathogens of vector-borne diseases. If a female taiga tick contains the encephalitis virus, then during sexual reproduction she will pass it on to subsequent generations.

Natural focal diseases are diseases associated with a complex of natural conditions. They exist in certain biogeocenoses independently of a person, and trophic connections are important for their maintenance. E. N. Pavlovsky gave the following definition of natural focal diseases: “ Natural focus of vector-borne diseases- this is a phenomenon when the pathogen, its specific carrier and animal reservoirs of the pathogen during the change of their generations exist for an unlimited time in natural conditions regardless of the person, both in the course of his already past evolution, and in its present period. "

Natural hearth- this is the smallest area of ​​one or several landscapes where circulation takes place without bringing it from the outside for an indefinitely long period.

Components of the natural focus of the disease:

1) the causative agent of the disease;

2) organisms susceptible to this pathogen;

3) carriers of the pathogen;

4) certain conditions environment (biotope)

For example: a diagram of a natural plague focus

Specific carrier

rodents man

In the outbreak, the pathogen circulates from sick animals (pathogen donors) through the carrier to healthy ones (recipients), which later become donors of the pathogen. Carriers are blood-sucking arthropods, and rodents and birds can be donors and recipients. If a person enters the natural focus of the disease, then he first becomes a recipient, and then a donor of the pathogen. Natural foci exist long time, but they acquire epidemiological significance when a person gets into them and becomes infected

Carriers (blood-sucking arthropods)

Donors Recipients

(wild animals, (wild animals)

more often rodents)

Classification of natural foci:

By origin emit foci:

1) natural (tick-borne encephalitis);

2) synanthropic - exist in locality where the circulation of the pathogen is carried out by synanthropic animals (scabies);

3) anthropurgic - arise as a result of transformation natural environment human (opisthorchiasis in places of artificially created reservoirs);

4) mixed (trichinosis).

The area of ​​natural foci is determined by the area of ​​natural hosts of the pathogen and the area of ​​the vector.

Extension (area) foci can be:

1) narrowly limited (a rodent's hole, a bird's nest - a focus of tick-borne relapsing fever);

2) diffuse (taiga is the focus of taiga encephalitis);

3) conjugate, if pathogens of several vector-borne diseases (tularemia and plague) circulate in the focus.

The result of infection of the recipient in a natural focus may be his death (in the case of high virulence of the pathogen), illness with subsequent recovery or vaccination (the formation of immune protective bodies without pronounced clinical signs of the disease - with a weak virulence of the pathogen).

The outcome of infection of the recipient in the outbreak is also influenced by the following factors:

1) pathogenicity of the pathogen for a given recipient;

2) "aggressiveness" of the carrier (frequency of bloodsucking);

3) the dose of the pathogen introduced into the recipient's body;

4) the severity of nonspecific and specific immune responses of the recipient.

Some natural focal diseases are characterized by endemism, i.e. occurrence in strictly limited areas. This is due to the fact that the causative agents of the corresponding diseases, their intermediate hosts, animal reservoirs or carriers are found only in certain biogeocenoses. So, only in certain regions of Japan are four species of pulmonary flukes from p. Paragonimus. Their dispersal is hampered by their narrow specificity in relation to intermediate hosts, which live only in some water bodies of Japan, and such endemic species of animals as the Japanese meadow mouse or the Japanese marten are a natural reservoir.

Viruses of some forms of hemorrhagic fever are found only in certain zones of East Africa, because the area of ​​their specific carriers is located here - ticks from the r. Ambliomma.

Not a large number of natural focal diseases are found almost everywhere. These are diseases, the causative agents of which, as a rule, are not associated in the cycle of their development with the external environment and affect a wide variety of hosts. Diseases of this kind include, for example, toxoplasmosis and trichinosis. A person can become infected with these natural focal diseases in any natural and climatic zone and in any ecological system.

The vast majority of natural focal diseases affects a person only if he gets into the appropriate focus (hunting, fishing, hiking, in geological parties, etc.) under the conditions of his susceptibility to them. So, a person becomes infected with taiga encephalitis when bitten by an infected tick, and opisthorchiasis - by eating insufficiently heat-treated fish with larvae of a cat fluke.

Prevention of natural focal diseases presents particular difficulties. Due to the fact that a large number of hosts, and often carriers, are involved in the circulation of the pathogen, the destruction of entire biogeocenotic complexes that have arisen as a result of the evolutionary process is ecologically unreasonable, harmful and even technically impossible. Only in those cases, if the foci are small and well studied, is it possible to carry out a complex transformation of such biogeocenoses in a direction that excludes the circulation of the pathogen. Thus, the reclamation of desertified landscapes with the creation of irrigated horticultural farms in their place, carried out against the background of the fight against desert rodents and mosquitoes, can dramatically reduce the incidence of leishmaniasis in the population. In most cases of natural focal diseases, their prevention should be aimed primarily at individual protection(prevention from bites by blood-sucking arthropods, heat treatment of food, etc.) in accordance with the pathways of circulation in nature of specific pathogens.

Medical protistology

1. Morphophysiological characteristics of the subkingdom Protozoa

2. Subtype Sarcode

3. Flagellate subtype

4. Type of ciliates

5. Class Sporozoa