BASICS OF MEDICAL BIOLOGY - 2012

Phylum Apicomplexa. Class Sporozoa

The Class Sporozoa comprises exclusively parasitic Protozoa that have adapted to living in Body Cavities or within the Cells of humans and animals. Due to their parasitic lifestyle, the Organization of sporozoites is extremely simplified: Organelles of locomotion, Respiration, excretion, and feeding (digestive and contractile vacuoles) are absent. These processes occur via osmosis. During reproduction, most species form spores (embryos enclosed in a tough wall), hence the name of the class. Their life cycle is characterized by alternation of hosts, alternation of asexual reproduction (schizogony, endodyogeny), sexual reproduction, and sporogony (formation of spores and sporozoites).

Individuals of the asexual generation are called trophozoites or schizonts. They reproduce via schizogony, giving rise to small mononuclear bodies known as merozoites.

Individuals of the sexual generation—gamonts—give rise to Gametes. The sexual process culminates in gamete fusion and The formation of a zygote, termed an oocyst. Mature oocysts possess a thick wall and contain one or several small embryos called sporozoites. A host becomes infected by ingesting an oocyst containing sporozoites. In Blood-dwelling Sporozoans, the zygote never enters the external environment, and parasite transmission is mediated by a vector.

Human parasites belong to the order Haemosporidia and the order Coccidia.

Order Haemosporidia

At a certain stage of their development, they inhabit the erythrocytes of vertebrates and humans. The spore stage is absent, as haemosporidians are transmitted from one host to another without entering the external environment. Malaria parasites are human pathogens belonging to this group.

Malarial Plasmodia. Malarial plasmodia are the causative agents of malaria. Over 100 species of malarial plasmodia are known, parasitizing reptiles, birds, and mammals. Four species parasitize humans:

Plasmodium vivax — the CAUSATIVE AGENT OF tertian malaria.

Plasmodium falciparum — the causative agent of malignant tertian (falciparum) malaria.

Plasmodium malariae — the causative agent of quartan malaria.

Plasmodium ovale — the causative agent of ovale malaria (similar to tertian malaria).

Geographical distribution: found in all countries of Africa and the Middle East, Southeast Asia, Pacific islands, Central and South America (between 40⁰ south latitude and 60⁰ north latitude).

Localization: intracellular parasites; in humans, they inhabit Liver cells and erythrocytes.

Morphology: the malaria parasite undergoes a complex life cycle with several developmental stages. The following stages are detected in The Human Body:

sporozoite — spindle-shaped, measuring 1x15 µm;

tissue (pre-erythrocytic) schizont — spherical, measuring 50-70 µm;

tissue merozoite — round or oval, with an eccentrically positioned Nucleus, approximately 0.7 µm in diameter.

Erythrocytic trophozoites undergo the following developmental stages:

- ring-form trophozoite — occupies no more than 1/3 to 1/5 of the erythrocyte diameter; when stained using the Romanowsky-Giemsa method, a colorless vacuole is visible in the center of the trophozoite, the Cytoplasm forms a delicate blue rim, and The Nucleus is dark red;

- amoeboid trophozoite — occupies more than half of the erythrocyte, irregular in shape due to the formation of pseudopodia, motile; the vacuole decreases in size, and the cytoplasm contains dark brown pigment granules resulting from Hemoglobin degradation;

- mature trophozoite occupies nearly the entire erythrocyte and is spherical in shape; the vacuole is small or absent; the nucleus is large, and the number of pigment granules increases.

- schizont is characterized by a divided nucleus; cytoplasm segregates around each daughter nucleus to form merozoites. The pigment is pushed out of the cytoplasm and gathers in a clump to the side of the erythrocyte center. This stage is referred to as a morula.

- erythrocytic merozoite — structurally resembles the tissue merozoite, measuring about 1.5 µm.

- Female and male gametocytes (macro- and microgametocytes) are immature, rounded Germ Cells (crescent-shaped in P. falciparum). Female gametocytes outwardly resemble mature trophozoites, but are larger and take on a bluish tint. Male gametocytes are generally smaller than female ones, grayish-blue, with a large, loose, pale pink nucleus located in the center of The Cell.

The life cycle of malaria parasites is typical of sporozoans, encompassing asexual reproduction via schizogony, a sexual phase, and sporogony. The definitive host of the parasites is a female mosquito of the genus Anopheles, whereas humans serve as the sole intermediate host. The mosquito also acts as the vector, making malaria a classic anthroponotic vector-borne disease. During a bite, along with saliva, the mosquito introduces the malaria parasite into human blood at the sporozoite stage. The sporozoite is the infective stage for humans. The Development of parasites within the human body occurs synchronously. Carried by the bloodstream, the sporozoites invade liver cells, where they undergo exoerythrocytic (tissue) schizogony. Each schizont gives rise to A large number (from 1,000 to 5,000) of tissue merozoites. The exoerythrocytic cycle occurs only once. In P. falciparum, it lasts 6 days; in P. vivax, 8 days; in P. ovale, 9 days; and in P. malariae, 15 days. It has been proven that in quartan and tropical malaria, upon completion of tissue schizogony, the merozoites exit the liver entirely and enter the bloodstream. In tertian malaria, due to sporozoite heterogeneity (comprising both tachy- and bradysporozoites), tissue schizogony can occur either directly after the mosquito bite (via tachysporozoites) or 1.5–2 years later (via bradysporozoites), which accounts for the prolonged incubation period and delayed relapses caused by the parasite's so-called "dormant" stages. The incubation period is the time elapsed from the pathogen's entry into the human body until the appearance of clinical symptoms.

Tissue merozoites enter the bloodstream and invade erythrocytes, marking the initiation of the erythrocytic phase of the malaria parasite's life cycle (erythrocytic schizogony). Due to a potential difference, the infected erythrocyte carries a negative charge, while the merozoite is positively charged. They mutually attract one another, causing the erythrocyte cytoplasm to invaginate, allowing the merozoite to enter the cell. Other researchers note the presence of specific receptors on erythrocytes; when present, tissue merozoites successfully invade the cells. For Plasmodium vivax, the Duffy blood group isoantigens serve as such receptors. Duffy-negative individuals are common among Black populations in Africa, who are resistant to tertian malaria in 90% of cases, while among Black populations in the Americas, 70% are Duffy-negative.

Merozoites that have invaded erythrocytes increase in size; a vacuole forms near the merozoite's nucleus, pushing the nucleus to the periphery and giving the parasite a ring-like appearance (ring stage). Subsequently, the vacuole shrinks and pseudopodia develop (amoeboid trophozoite stage). Gradually, the plasmodium occupies the entire erythrocyte, assumes a spherical shape, and the nucleus shifts to the center (mature trophozoite stage). During the schizogony stage, the nucleus divides, resulting in a trophozoite containing 6 to 24 nuclei. Cytoplasmic fragments separate around these nuclei, forming erythrocytic merozoites. The erythrocyte membrane then ruptures, releasing the merozoites and toxic metabolic byproducts of the parasites into the bloodstream, a process that coincides with malaria paroxysms. Merozoites released into the plasma invade new erythrocytes, and the cycle repeats, generating a new crop of merozoites. This process recurs multiple times. The erythrocytic schizogony cycle lasts 48 hours in Plasmodium vivax, P. falciparum, and P. ovale, and 72 hours in P. malariae. This duration precisely dictates the intervals between malaria attacks.

Following a series of erythrocytic schizogonies, a subset of the merozoites that have invaded erythrocytes develops not into schizonts, but into immature sexual forms (gametocytes)—namely, macrogametocytes (immature female sexual forms) and microgametocytes (immature male sexual forms). For further development, the gametocytes must reach The Stomach of the definitive host—a female mosquito of the genus Anopheles—where sexual reproduction and sporogony take place. Gametocytes represent the infective stage of the malaria parasite for the malaria mosquito. They enter the stomach of the female mosquito during her blood meal on an infected human (male mosquitoes do not feed on blood).

Fig. 49. Life Cycle of Plasmodium vivax and P. ovale:

1 - release of a sporozoite from the salivary gland duct and its invasion of a liver cell; 2 - trophozoite within a liver cell: a - trophozoite, b - liver Cell Nucleus; 3 - schizont within a liver cell: a - schizont, b - cell nucleus; 4 - release of tissue merozoites from the liver cell into Blood Plasma; 5 - attachment of a merozoite to an erythrocyte; 6 - invasion of the erythrocyte by a merozoite; 7 - ring-stage trophozoite; 8 - young trophozoite in an erythrocyte; 9 - immature erythrocytic schizont; 10 - mature erythrocytic schizont; 11 - erythrocytic merozoites; 12 - male gametocyte (microgametocyte); 13 - female gametocyte (macrogametocyte); 14a - formation of male gametes; 14b - male gamete; 15 - female gamete; 16-17 - Fertilization; 18 - ookinete; 19-20 - oocyst formation and development of sporozoites; 21 - release of sporozoites from the oocyst into the mosquito's body cavity; 22 - sporozoites in the mosquito's salivary gland

Sexual reproduction and sporogony occur within the stomach of the female malaria mosquito. Mature female gametes, or macrogametes, develop from macrogametocytes. Upon maturation, microgametocytes undergo multiple divisions to produce mature male gametes, or microgametes. The micro- and macrogametes fuse (fertilization) to form a zygote. Because it is motile, it is termed an ookinete. The ookinete penetrates beneath the epithelium of the mosquito's midgut, increases substantially in size, and transforms into an oocyst. Inside the oocyst, repeated nuclear and cytoplasmic divisions take place (sporogony), generating a vast number (up to 10,000) of sporozoites. The oocyst wall eventually ruptures, releasing the sporozoites into the hemolymph, from which they migrate to the mosquito's Salivary Glands. During a bite, the mosquito injects these sporozoites into human blood along with its saliva, after which they invade liver cells.

All species of malaria parasites can be transmitted to humans via blood transfusion (hemotransfusion). In such cases, none of the parasites form an exoerythrocytic stage; consequently, late relapses do not occur. Hemotransfusional transmission is most commonly associated with quartan malaria because, in this form of the disease, schizonts are present in very low numbers within erythrocytes and may be overlooked during donor blood screening.

Occasionally, a person may be infected simultaneously with two or three species of plasmodia. Under such circumstances, malaria paroxysms lack a clear periodicity, making clinical Diagnosis challenging.

Pathogenic effect. Malaria is a severe disease accompanied by periodic fever attacks associated with the simultaneous release of large numbers of merozoites and their toxic metabolic byproducts from erythrocytes. Each attack comprises chills and a Temperature spike up to 40°C, lasting between 6 and 12 hours. Enlargement of The Liver and Spleen, as well as anemia, are characteristic features. Fatal outcomes are possible.

In tropical malaria, attacks initially occur at irregular intervals, but later establish a 24-hour periodicity. Complications involving the Central Nervous system or Kidneys can lead to the patient's death. Schizonts do not persist in liver cells, yet the disease can linger for up to 18 months.

Laboratory Diagnostics. Specimen: blood. Research Methods: Microscopy of blood smears and thick blood films. Blood is collected during or immediately after a paroxysm, prior to the initiation of specific Treatment. Schizonts and gametocytes are identified.

Prophylaxis. Personal: protection against mosquito bites, administration of prophylactic medications. Public: detection and treatment of infected individuals and parasite carriers, eradication of adult mosquitoes and their larvae using insecticides, Introduction of biological predators of mosquitoes, and environmental sanitation via land reclamation.

Order Coccidia (Coccidia). Toxoplasma is of the greatest medical significance.

Toxoplasma gondii is the causative agent of Toxoplasmosis. Toxoplasmosis is an anthropozoonotic infection. Toxoplasma is an obligate intracellular parasite.

Geographical distribution: ubiquitous.

Localization: Lymph Nodes, liver, spleen, Lungs, Brain, Uterus, eyes, skeletal Muscles, myocardium.

Morphology. Toxoplasmas localized within host cells are called endozoites. An endozoite is crescent-shaped, with one end pointed and the other rounded (4–7 × 2–4 µm). At the pointed end, Toxoplasma features an apparatus for host cell invasion (the apical complex)—comprising a conoid (for anchoring the parasite to the cell surface) and rhoptries containing Enzymes that dissolve The cell membrane. The nucleus is located in the center or at the posterior pole of the cell (Fig.). Aggregations of toxoplasmas beneath the cell membrane are referred to as pseudocysts. True (tissue) cysts are spherical or oval formations measuring 50–200 µm, representing clusters of several hundred endozoites enclosed within a dense protective wall.

Cytology/practical/136.html">Differential diagnosis OF human malaria parasites in blood smears (after: Sh.D. Moshkovsky, N.A. Demina)

Fig. 49. Ultrastructure of Toxoplasma.

The life cycle involves alternation of hosts as well as alternation of Selection/8.html">Asexual and sexual reproduction. Definitive hosts include the domestic cat and other members of the felid family; intermediate hosts include birds and mammals (approximately 350 species in total), humans, and more rarely, reptiles. Asexual reproduction occurs within the intermediate host via longitudinal fission and endodyogeny (internal budding). As a result of repeated divisions, a large number of toxoplasmas (12–32 daughter cells) accumulate within the cell. The aggregation of toxoplasmas beneath the cell membrane is termed a pseudocyst. The cell membrane ruptures, and the endozoites escape to invade neighboring cells. During this period, the parasite is shed in bodily excretions (saliva, milk, tears, etc.). As the host's Immune Response intensifies, the toxoplasmas begin to form true cysts. These persist throughout the host's life and can trigger disease relapses when Immunity declines. In chronic toxoplasmosis, both pseudocysts and true cysts are formed. Parasites enter the definitive host (the cat) via the meat of intermediate hosts containing pseudocysts with endozoites. Asexual reproduction of the parasite takes place in the cat's Internal Organs, whereas sexual reproduction occurs in the epithelium of the Small Intestine (meaning the cat acts as both definitive and intermediate host for Toxoplasma). Within the small intestinal epithelial cells, some endozoites develop first into macrogametocytes and then into macrogametes, while others develop into microgametocytes and then microgametes. Following copulation, a zygote is formed, which becomes enveloped in a thick wall. This form is called an oocyst and is shed into the external environment via the cat's feces. At temperatures exceeding +20°C, sporogony occurs inside the oocyst, producing two spores, each containing four sporozoites.

Fig. 50. Life cycle of Toxoplasma:

a - cat (definitive host); b, c, d - stages of oocyst development in the external environment (a mature oocyst contains two sporocysts with four sporozoites each); d, e - mouse (intermediate host); g - newborn mouse infected transplacentally with the causative agent of toxoplasmosis.

A distinctive feature of the toxoplasma developmental cycle is that intermediate hosts can become infected not only from the primary host, but also by preying on one another. Thus, pigs may become infected by eating the carcasses of rodents that died of toxoplasmosis, and rodents can become infected through cannibalism. Intrauterine transmission of the fetus from an infected pregnant female is also possible, with parasites penetrating the placental barrier. This route of transmission ensures the sustained persistence of natural foci of toxoplasmosis even among small rodents that are not prone to cannibalism.

Mechanisms of transmission of the causative agent of toxoplasmosis: 1) alimentary (via the oral route, with transmission factors including undercooked meat, dairy products, eggs containing endozoites and true cysts, and hands contaminated with oocysts); 2) percutaneous (through the Skin and mucous membranes, occurring in meat Processing plant workers, obstetricians, and laboratory physicians during laboratory and clinical Procedures); 3) transplacental (intrauterine infection of the fetus through the Placenta). When the parasite enters orally via dirty hands, unwashed vegetables and fruits, or cat fur, the infective stage is the oocyst; in other cases, it is endozoites and true cysts.

Pathogenic effect. Acquired toxoplasmosis manifests with fever, lymphadenopathy, and lesions of the central nervous system, eyes, myocardium, and skeletal muscles. Diagnosis is often challenging due to the wide variety of clinical presentations. Congenital toxoplasmosis, resulting from transplacental transmission when a woman is infected during Pregnancy, is characterized by fetal developmental anomalies, central nervous system damage, fever, jaundice, and rash. Miscarriages or stillbirths are also possible.

Laboratory diagnosis. Test Materials include tissue biopsy specimens, organ and lymph node punctates, CEREBROSPINAL FLUID, and blood. In the acute stage of the disease, the causative agent can be detected in both internal organs and the bloodstream.

Methods of laboratory diagnosis: 1) serological tests (detection of Antibodies against Toxoplasma); 2) microscopy of blood smears, cerebrospinal fluid, lymph node punctates, Amniotic Fluid, and fetal membranes; 3) biological assay (intraperitoneal inoculation of white mice with an emulsion of the studied organs); 4) intradermal skin test with toxoplasmin (an ether extract of peritoneal exudate from white mice infected with Toxoplasma). A definitive diagnosis is established based on a comprehensive evaluation of clinical and laboratory findings.

Prevention. Personal prevention involves consuming only thoroughly cooked meat, boiling milk, and washing hands before meals. Public health measures include health education and awareness campaigns, particularly targeting pregnant women.

Phylum Ciliophora (Infusoria). Class Litostomatea.

The organelles of locomotion in Ciliates are cilia. Another distinct feature is the presence of Two Types of nuclei: a large one (macronucleus) and a small one (micronucleus). The macronucleus (vegetative nucleus) regulates metabolic and vital processes, while the micronucleus (generative nucleus) plays a crucial role in sexual reproduction. Most ciliates inhabit aquatic environments, though parasitic species also exist. Balantidium coli is of medical significance.

Balantidium coli is the causative agent of balantidiasis.

Geographical distribution - worldwide.

Localization - Large Intestine.

Morphology. This is the only known parasitic Ciliate of humans, existing in trophozoite and cyst forms. The trophozoite is relatively large, measuring 30-200 x 20-70 µm, with an oval or egg-shaped body. Its organelles of locomotion are cilia; it possesses two contractile vacuoles, two nuclei (a micronucleus and a Kidney-shaped macronucleus), and at its anterior end features a peristome (a pre-oral depression) leading into the cytostome (cell Mouth) and cytopharynx (cell Pharynx) (see Fig.). It feeds on undigested food debris and erythrocytes. Undigested residues are eliminated through the anal pore (cytoproct) located at the posterior end of the body. It reproduces both asexually (transverse binary fission) and sexually (conjugation).

The cyst is oval or spherical, 50-60 µm in diameter, and covered with a double-layered membrane. Both the macro- and micronucleus can be identified within the cytoplasm, along with a posterior contractile vacuole.

Fig. 51. Balantidium coli:

a - vegetative form; b - cyst; 1 - cytostome; 2 - cytopharynx; 3 - food vacuole; 4 - excretory (contractile) vacuole; 5 - macronucleus; 6 - micronucleus; 7 - anal pore.

Life cycle. The infective stage for humans is the cyst. Infection occurs through the ingestion of cysts via contaminated Water, food, or unwashed hands. Cysts can be mechanically vectored by flies and cockroaches. In the digestive tract, cysts transform into vegetative forms. Pigs are considered the primary reservoir of infection, as Balantidium readily encysts within them. Humans suffering from balantidiasis or asymptomatic carriers rarely serve as sources of infection because cyst formation in the human body is uncommon. Individuals working on pig farms and in meat-processing plants are at the highest risk of infection.

Pathogenic effect. These parasites colonize the large intestine, particularly the cecum. Sometimes they persist for long periods without causing clinical symptoms. However, in many cases, they invade the intestinal wall, causing bleeding ulcers. The acute form presents with severe abdominal pain, nausea, vomiting, and frequent loose stools containing mucus and blood. If left untreated, the condition can be fatal.

Laboratory diagnosis. The diagnostic specimen consists of fresh feces. Cysts and vegetative forms are identified via microscopic examination.

Prevention. Personal measures include washing hands before meals and after using the restroom, thoroughly washing raw fruits and vegetables, and drinking only boiled water. Public measures involve preventing soil contamination with pig and human feces, ensuring proper occupational hygiene and working conditions on pig farms, detecting and treating infected individuals, and conducting sanitary education.