Phylum Sarcomastigophora. Subphylum Sarcodina. Class Lobozea.
Phylum Apicomplexa. Class Sporozoa.
Phylum Sarcomastigophora. Subphylum Sarcodina. Class Lobozea.
1. The forms of association between organisms of different species.
2. Medical Parasitology as a science.
3. Classification of parasites and hosts.
4. Protozoan diseases.
5. Classification of Protozoa.
6. General characteristic of Protozoa.
7. Species of class Lobozea: Entamoeba histolytica, Entamoeba coli, Entamoeba gingivalis.
Medical Parasitology is the science about human’s parasites. Medical Parasitology consists of three parts: Medical Protozoology, Medical Helminthology and Medical Arachnoentomology. Medical Protozoology studies Protozoa as human’s parasites.
Parasite is an organism that lives upon or within another living organism (host) at whose expense it obtains some advantage. External parasite is an ectoparasite that lives on skin or hair of host. Internal parasite is an endoparasite that lives in body organs, body tissues, in cells or in cavity of host’s body.
Host is an organism that harbours or nourishes another organism (parasite). The hosts are divided into definitive hosts and intermediate hosts. Definitive host (final h.) harbours the adult or sexually mature parasite. Intermediate host harbours the immature or asexual stages of the parasite; usually designated first and second, if there is more than one.
Diseases that are caused by animals are known as invasive diseases. Protozoan diseases are caused by protozoa. Diseases which are characteristic for human, (e.g. amoebiasis) called Anthroponotic. Diseases which are characteristic for animals (e.g. malaria of birds) called Zoonotic. Anthropozoonotic diseases are characteristic for humans and animals, for example, leishmaniasis.
There are four ways of agent transmission of invasive diseases: 1) contagious (by skin contact, sexual contact); 2) alimentary or faecal-oral (ingestion of raw or undercooked food or use of drinking water containing the infective stage of the parasite); 3) by blood (by bite of vectors containing the infective stage, blood transfusion). Vector is an arthropod that carries a parasite to host (e.g. Anopheles mosquito); 4) congenital (transplacental).
Protozoa are considered to be a subkingdom of the kingdom Animalia. Three of Protozoa phyla: Sarcomastigophora, Apicomplexa and Ciliophora – contain the most important species causing human disease.
Morphology and Ultrastructure of Protozoa.
1) Protozoa (Gk. protos first, zoon animals) are unicellular animal organisms.
2) Each protozoon performs all functions of life.
3) Parasitic protozoa in humans are from 1 to 150 micro;m in size.
4) The protozoon is made of a mass of protoplasm differentiated into cytoplasm and nucleoplasm. The cytoplasm is divided into ectoplasm (the outer, transparent layer) and endoplasm (the inner layer containing organelles).
5) The ectoplasm functions are: protection, locomotion, ingestion, excretion and respiration. The plasma membrane encloses the cytoplasm, covers the locomotive structures (pseudopodia, cilia and flagella). Сontractile vacuoles are for osmoregulation (e.g. Balantidium coli). Food vacuoles contain food during digestion. The nutrition of all protozoa is holozoic; that is, they require organic materials, which may be particulate or in solution. Reproduction in the Protozoa may be asexual or sexual. The most common types of asexual multiplication are binary fission, endogony and shizogony (multiple fission).
6) Life Cycle Stages. During its life cycle, a protozoan generally passes through several stages that differ in structure and activity. Trophozoite (Greek for «animal that feeds») is a general term for the active, feeding, multiplying stage of most protozoa. Cyst is inactive.
Class Lobozea. These organisms move with the help of pseudopodia. Reproduction is by binary fission. The production of a cyst is one of the stages in the life cycle. Among the pathogenic species for man is Entamoeba histolytica, the causative agent of human intestinal amoebiasis, or amoebic disentery. Morphology. E. histolytica occurs in the human body in such forms: 1) Entamoeba histolytica forma magna – vegetative large tissue form which feeds on the erythrocytes and does not become encysted; 2) Entamoeba histolytica forma minuta – vegetative small commensal encysted form which lives in the lumen of the large intestine; 3) cysts which develop from the forma minuta.
E. histolytica penetrates into the tissues of the large intestine under the influence of a number of factors (lowered resistance of the human body due to various diseases, intoxications, overheating, overstrain, injuries, and wounds) and gives rise to deep changes there. It produces proteolytic substances that cause lysis of cells and tissues. The parasite grows to a size of 30-50 micro;m in diameter and becomes capable of phagocytizing erythrocytes. This vegetative form is known as the tissue form, Entamoeba histolytica forma magna. It is usually found in the mucosanguineous stools (faeces with blood and mucus) of patient with amebiasis. The amoeba ectoplasm is translucent, while its endoplasm is granular. The endoplasm contains the nucleus and food vacuoles. The nucleus has a central karyosome. Forma minuta is the main form of E.histolitica. Its size ranges from 12 to 25 micro;m. Phagocytized bacteria are found in small numbers in the endoplasm. Ectoplasm is poorly developed and accumulated in the pseudopodia, motility is decreased. Amoebas inhabit the upper part of the large intestine of a healthy man and known as luminal, cavity or commensal forms. The forma minuta transforms into a cyst. The cysts are spherical in shape with a diameter of 9 to 16 micro;m. They have thin-walled, double membranes. Fully mature cysts contain four nuclei. Immature cysts have one, two, and sometimes three nuclei. Cysts are discharged with the faeces for a long period of time and sometimes throughout life. They may re-enter the human body with foodstuffs or water and transform into luminal forms in the intestine.
Pathogenicity and disease. Faecal-oral transmission of cysts involves contaminated food or water. Ingested cysts of E.histolytica excyst in the small intestine. Trophozoites (forma minuta) are carried to the colon, where they mature and reproduce. Forma minuta may live in the lumen as being commensal. Successful colonization and trasformation of forma minuta into forma magna depends on factors such as intestinal motility, transit time, the presence or absence of specific intestinal flora, the host’s diet. If mucosal invasion occurs, it may be limited to a few simple superficial erosion or it may progress to total involvement of the colonic mucosa with ulceration, most often in the caecum or sigmoid colon. Patients have acute or chronic diarrhoea, which may progress to hemorrhagic dysentery. Invasion of blood vessels leads to secondary extraintestinal lesions. Extraintestinal disease may be present as a complication (peritonitis) or as metastasis (liver, lung, skin or brain abscess).
Laboratory diagnosis. Fresh stools are examined under the microscope. E. histolytica (forma magna and cysts with 4 nuclei) can be demonstrated in the stools.
Prophylaxys. All patients are transferred to a hospital and adequately treated. Preventive measures against amoebiasis also include protection of foodstuffs and water from flies and contamination with faeces. The staff of catering establishments must be examined for cysts carriage. Health education of the population.
Non-pathogenic amoebas can parasitize the human gastrointestinal tract. These include Entamoeba gingivalis (oral cavity), Entamoeba coli (large intestine). E. coli larger than E. histolytica. Their cytoplasm is granular and the vacuoles contain bacteria, leucocytes, food particles, and glycogen, but no erythrocytes. E.coli forms cysts with 8 nuclei. E. gingivalis doesn’t form cysts
Phylum Apicomplexa. Class Sporozoa.
1. General characteristic of phylum Apicomplexa.
2. General characteristic of class Sporozoa.
3. Malaria parasites of man: Plasmodium vivax, Plasmodium malaria, Plasmodium falciparum, Plasmodium ovale.
4. Toxoplasma gondii - the causative agent of toxoplasmosis
The over 5,500 described members of phylum Apicomplexa are all endoparasites in animals. At two stages in the life cycle, all individuals possess a structurally complicated grouping of microtubules and organelles at one end of the cell; the phylum name formally acknowledges this “apical complex”. Adult sporozoans are highly specialized for existence as endoparasites and completely lack locomotory organelles. Sporozoans life cycles are complicated affairs, including sexual reproduction by means of gamete fusion, asexual reproduction through fission, and the production of resistant or infective spores.
The malaria parasites of man include four species: Plasmodium vivax, the causative agent of tertian malaria, Plasmodium malariae, the causative agent of quartan malaria, Plasmodium falciparum, the causative agent of tropical (falciparum) malaria, Plasmodium ovale, the causative agent of tertian ovale-malaria.
Morphology of plasmodia. Blood stages of Plasmodium: 1) young trophozoites (ring forms); 2) growing trophozoites; 3) mature trophozoites; 4) mature schizonts; 5) macrogametocytes; 6) microgametocytes. The merozoite is the youngest form of the parasite, appearing as the result of the splitting of a mature schizont. It is spherical or oval and small in size (1-2 micro;m). The merozoites consist of cytoplasm and a nucleus. Merozoites penetrate into the erythrocytes and give rise to asexual forms of the parasites. The young trophozoite (ring-form stage) grows larger and a vacuole appears in its cytoplasm. At this stage the malarian parasite has irregular contours and resembles a ring. The semimature schizont is capable of amoeboid movement. As it grows, a pigment appears within it (a product of haemoglobin breakdown) in the form of dark-brown spots. The mature schizont becomes rounded and pulls in its pseudopodia by the time of complete merulation, occupying almost the entire erythrocyte. The nucleus and cytoplasm divide and form from 6 to 24 merozoites (it depends from the species of the parasite). The pigment accumulates in the centre in a compact clump. The erythrocytes are destroyed by merozoites. The merozoites release into the blood plasma. Some of them again penetrate into erythrocytes. The gametocytes are sex cells and are subdivided into female (macrogametocytes) and male (microgametocytes) cells. The macrogametocytes are 12-14 micro;m, their nuclei are small. The microgametocytes are smaller, their nuclei are large.
Life cycle involves sexual stage (sporogony) in the mosquito (Anopheles) and asexual stage (schizogony) in humans. Man is an intermediate host and mosquito is a definitive host. The life cycle passes 3 stages: two in man: “Exoerythrocytic schizogony (liver phase)” and “Erythrocytic schizogony (blood phase)”; one in mosquito: “Sporogony”.
Exoerythrocytic schizogony (liver phase)
1. Mosquito bites man, takes blood meal and injects sporozoites from its salivary gland into the blood.
2. Sporozoites travel through blood to the liver, multiply asexually to form merozoites, which upon liver cell rupture. Merozoites are released into the bloodstream and infect erythrocytes.
Erythrocytic schizogony (blood phase)
1. Merozoites enter the erythrocytes, forming a ring-like trophozoite. Mature trophozoites asexually divide to form schizonts.
2. Schizont develops into merozoites, then lyse the erythrocytes membrane, leading to periodic paroxysms of disease due to resultant parasitemia. P. ovale, P. vivax, P. falciparum—membrane lysis in 48 hours, P. malariae—membrane lysis in 72 hours.
3. Some merozoites are developed into macrogametocytes and microgametocytes.
Sporogony
1. Mosquito ingests gametocytes with blood meal.
2. Gametocytes enter mosquito gut.
3. Zygote, formed from fertilization, invades gut wall to form an oocyst within 24 hours following ingestion.
4. Sporozoites are formed, released into the stomach, migrate to salivary glands, then injected into human with blood meal.
Clinical manifestations. The incubatory period in tertian malaria lasts from 10 days to 11 months, in quartan malaria, from 21 to 42 days, and in tropical malaria, from 9 to 16 days. A malarial attack is produced by result of the body reaction to protein substances appearing in the blood due to erythrocyte dissociation. The most characteristic symptom of malaria is fever. Other common symptoms include anemia, splenomegaly, hepatomegaly.
Laboratory diagnosis comprises microscopy of thin and thick films blood smears stained with Romanowsky stain and recognition of the different stages of the parasite (trophozoites, schizonts, gametocytes).
Prophylaxis. Malaria may be prevented by chemoprophylaxis and personal protective measures against the mosquito vector (Anopheles).
Toxoplasma gondii is the causative agent of toxoplasmosis.
Morphology. 1) Pseudocysts (intracellular collection of trophozoites in cells in the acute stage of infection); 2) Trophozoites (intracellular parasites, about 6 C 2 micro;m, crescent-shaped with central nucleus and multiply by binary fission forming pseudocysts); 3) Cysts (collection of trophozoites enclosed in a true tissue cyst, in the chronic stage or latent infection); 4) Oocysts (oval, 10 C 12 micro; m, contains two sporocysts each containing four sporozoites, and found in stool of infected cats).
Life Cycle. The final host is the cat in which the oocyst-producing sexual stage of Toxoplasma can occur. Mammals, including humans, birds are in which parasite reproduces asexually are intermediate hosts.
Transmitted to humans
1. Oocysts pass from cat intestine to cat feces.
2. Oocysts sporulate in soil and are viable for longer than one year.
3. Humans ingest oocysts either from soil or cat raw tissue infected with cysts. The alimentary route of infection takes place on ingestion of meat, milk, and dairy products of animals sick with toxoplasmosis, uncooked eggs of affected birds, and water contaminated by sick animals.
4. Transmitted via placenta when mother develops infection during gestation-congenital infection.
5. Toxoplasma gondii invades intestinal wall after entering host (usually orally) and disseminate via lymphatics and bloodstream forming trophozoites. T. gondii can spread to many host cells (the histophagocytic system, nerve tissue, liver, placenta, etc).
Clinical manifestations. Typical manifestations are pneumonia, enterocolitis, nephritis, hepatitis, myocarditis, and a high or subfebrile temperature. Infection is often asymptomatic. Congenital toxoplasmosis is characterized by hydro- or microcephalus, lesions in the organs of the vision (chorioretinitis), cirrosis of the liver and enlargement of the spleen. Asymptomatic toxoplasmosis in the mother leads to infection of the foetus and this results in abortion or birth of a dead child.
Laboratory diagnosis comprises the following:
Microscopic examination of fluids and organs of sick man (presence of T. gondii).
Biological tests with susceptible animals (mice, white rats, guinea pigs).
Serology of blood and cerebrospinal fluid.
Prophylaxis: washing of hands before meals and after handling animals and animal products, and the prohibition of preparing food from insufficiently cooked meat products, in particular liver. All women with a history of spontaneous abortion must be examined by laboratory methods for prevention of congenital toxoplasmosis.