2*Parasitism is a pervasive phenomenon in nature involving, either as hosts or as parasites, virtually all species on
earth. By definition, parasites are costly for their hosts as they divert resources for their growth, reproduction and
survival with no rewards for the hosts. Parasitism is not restricted to a few taxonomic groups since a parasitic life
style has evolved in viruses, bacteria, protozoa, invertebrate and vertebrate metazoan. Given the cost of parasitism,
hosts are expected to evolve defense mechanisms aiming at limiting the negative effect of parasitism on their
fitness. In agreement with this view, hosts have evolved a series of morphological, physiological, behavioral
adaptations to fight off parasitic attacks. On their side, parasites have responded to the selection pressures exerted
by their hosts by evolving counteradaptations to overcome host defenses. These cycles of host–parasite
adaptations–counteradaptations define the so-called coevolutionary process, one of the most prominent
characteristics of host–parasite interactions.
Criteria of parasitism:
1) special relations with the host;
2) feeding at the cost of the host;
3) pathogenic action on the host (inflicted harm).
The «parasite–host» system. This system includes one individual of the host and one or a group of parasites of a
definite species. For the formation of this system, the following conditions are necessary:
a) a contact between the parasite and the host;
b) providing conditions for the development of the parasite by the host;
c) the ability of the parasite to withstand the host’s reactions.
The basic direction of evolution is to achieve equilibrium, smoothing the antagonism between partners and
improving the reliability of the system.
Smoothing of the antagonism is achieved due to co-adaptation:
– in the parasite — morphologic and biologic adaptations;
– in the host — complication of defense mechanisms.
Directions of evolution are also different (co-evolution):
– in the parasite — complication of adaptation mechanisms to the host;
– in the host — improving defense reactions at all levels (for destroying
the parasite)
3*Classification of parasites:
1. According to relation with the host:
The forms of parasitism are extremely diverse. There are different classifications of parasites. From the point of
view of the obligatory parasitic way of life for a given species, one distinguishes between true and false, as well
as obligate and facultative parasites.
In true parasitism, the relationship between the parasite and the host is regular and has an evolutionary basis.
Parasitology studies mainly the phenomenon of true parasitism.
False parasitism is a random phenomenon for the species. Under normal conditions, representatives of this
species lead a free lifestyle. When it enters the host organism, the false parasite can remain viable for some time
and disrupt the host's vital functions. Examples of false parasitism are cases of detection of leeches in the nasal
cavity and nasopharynx of a person.
Obligate parasites are species that can lead an exclusively parasitic lifestyle (roundworm, pinworms). Facultative
parasites are able to lead a free lifestyle, but when they enter the host organism, they pass part of their
development cycle in it and disrupt its vital activity. Such are many types of synanthropic flies, the larvae of
which can normally develop either in human food products or in their intestines, causing intestinal myiasis.
– true — a parasitic way of life is a species character (ascarids, lice);
– false (pseudo-parasites) — free living, but when they get into a living organism, they may exist there and produce
harm (larvae of the filth fly);
– hyper-parasites or super-parasites — are parasites of parasites (bacteria in parasitizing protists).
4*
2. According to localization in the host:
– ectoparasites inhabit body coverings of the host (lice, fleas);
– endoparasites live inside the host’s organism:
a) intracellular (toxoplasm);
b) intracavital (ascarids);
c) tissue (liver sucker);
d) intradermal (scabby tick).
5*
According to duration of the relation with the host:
– constant — they spend the whole life cycle in the host (an ascarids);
– temporal — they spend a part of their life cycle in the host: larval parasitism (larvae
of the horse fly); immarginal parasitism — sexually mature individuals parasitize
(mosquitoes, fleas).
6*
The causative agent of parasite
Parasitic infections can be caused by three types of organisms: protozoa. helminths. ectoparasites.
A vector is a living organism that transmits an infectious agent from an infected animal to a human or another
animal. Vectors are frequently arthropods, such as mosquitoes, ticks, flies, fleas and lice.
Direct life cycles proceed without changing hosts. In addition to ectoparasites, direct life cycles are characteristic of
protozoa living in the intestinal cavities (dysentery amoeba, Giardia, Trichomonas, balantidia, etc.), for geohelminths
spreading through the soil (ascaris, whipworm, pinworm, etc.)
A much larger number of parasites have indirect life cycles that include one or more intermediate hosts. If one of the
pair of hosts participating in the indirect cycle is smaller and more active and motile, then it is designated as a carrier
(vector) of invasions.
Parasites that parasitize and develop in the body of one host are called monoxenous or single-host. For example,
dwarf tapeworm, pinworm parasitize only in humans. Most monoxenous helminths (roundworm, whipworm,
hookworm, etc.) necessarily need the release of a fertilized egg from the host into the external environment to
complete the development cycle.
Endoparasites that need 2 or more different hosts to complete their life cycle are called heteroxenous or multi-host
(malarial plasmodium, porcine and bovine tapeworms, flukes, etc.)
7*
Classification of hosts:
1. According the parasite’s development stage:
a) definitive or final — the parasite reaches its sexual maturation and undergoes its sexual
reproduction in his organism (the human for tenias);
b) alternate or intermediate — parasite’s larvae inhabit his organism, here their asexual
reproduction occurs (the human for malaria plasmodia);
c) supplementary (additionally) or secondary (predatory fish for larvae of
Diphyllobothrium).
d) When reproduction of parasite is absent and parasite only accumulates in host organism
without further development, it is called as paratenic (reservoir) host.
The reservoir host is an organism in which the development of the parasite does not occur, but only its accumulation
in the invasive stage is observed. The reservoir host accumulates the pathogen and retains the invasion, which
contributes to a greater infestation of the final host. For example, pike, eating an additional host of a wide tapeworm
(cyprinids), accumulates larval stages (plerocercoids) of the parasite in its tissues and thus performs the function of a
reservoir host.
8* Human diseases caused by pathogenic protozoa, helminths or arthropods are called invasive, in contrast to
infectious diseases caused by pathogenic microbes, spirochetes or viruses.
Invasion is the infection of the body by any parasite of animal nature. Re-infection of a person or animal with
parasites that he had previously infested and had been ill with is called reinvasion. Sometimes a person himself,
infected with some kind of parasite, can serve as a source of infection not only for others, but also for himself. This
phenomenon is called autoreinvasion. It is often found in enterobiasis, when children bite their nails, under which
there are parasite eggs that got there when combing the perianal region, and thereby re-infect themselves with
pinworms.
The source of invasions are carriers of parasites - sick animals, humans. For example, a person with ascariasis
releases parasite eggs into the environment, and thereby contributes to the spread of this helminthiasis.
Depending on the development cycle of the pathogen, the disease is divided into zoonoses and anthroponoses.
Zoonoses are diseases whose pathogens can parasitize both vertebrates and humans (leishmaniasis,
trypanosomiasis, balantidiasis, taiga encephalitis, plague, etc.).
Anthroponoses are diseases whose pathogens can parasitize only in humans (malaria, amoebiasis, ascariasis,
enterobiasis, etc.).
In the case of facultative zoonoses (amphixenoses), transmission is reciprocal
9*
Ways of infecting the human with parasites.
Permeation ways into the host organism:
1) alimentary — with food and water orally (helminthes eggs, protists cysts);
2) air-drop (respiratory) — through the respiratory tract (cysts of soil amoebas, some
viruses and bacteria);
3) percutant — through the intact skin (larvae of suckers);
4) transplacental — intrauterally from mother to fetus (toxoplasm, malaria plasmodia);
5) transfusional — in transfusion of infected blood (trypanosomes, malaria plasmodia);
6) contact-household — in contact with a sick person or animal, through utensils (scabby
tick);
7) transmissive — with participation of an arthropod (trypanosomes, malaria plasmodia);
8) sexual — in sexual contacts (vaginal trichomonade)
10*
Natural focal diseases are called diseases associated with a complex of natural conditions.
Obligate-transmissible diseases - transmitted from one host to another only through a carrier.
For example, a person can become infected with malaria or typhus only through an insect
bite, since the pathogen must enter the bloodstream.
Facultative transmissible diseases - can be transmitted both through a carrier and without it
in other ways, i.e. the participation of the carrier is not required. Examples of such diseases
are tularemia and plague.
Non-transmissible diseases - any method of transmission, except for an insect bite
(opisthorchiasis, trichinosis; are natural focal non-transmissible).
Diseases, the causative agents of which are transmitted to humans with the help of blood-sucking arthropods, are
called transmissible. These include not only parasitic (malaria, leishmaniasis, trypanosomiasis, filariasis), but also
infectious (typhus and relapsing typhus, plague, etc.) diseases.