Plasmodium

Associate Prof. Dr. Yulin WangEmail:yulinwang@dlmedu.edu.cn

Tel: +86 411 86110306

Department of parasitologyDalian Medical University

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Malaria

• The causal agents of malaria are blood parasites of the genus Plasmodium, family Plasmodiidae in the suborder Haemosporina. There are approximately 156 named species of Plasmodium, which infect various species of vertebrates. Four are known to infect humans: P. falciparum (恶性疟原虫 ), P. vivax (间日疟原虫 ), P. ovale (卵形疟原虫 ) and P. malariae (三日疟原虫 ). In China P. falciparum and P. vivax are common; P. ovale and P. malariae infection are rarely seen.

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Life cycle

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Sexual cycleAsexual cycle

LIFE CYCLE

There are two phases in the life cycle: the sexual cycle, which occurs primarily in mosquitoes, and the asexual cycle, which occurs in humans, the intermediate hosts. The vector and definitive host is the female Anopheles mosquito (only the female take a blood meal).

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Life cycle in mosquitoThe sexual cycle begins in the human red blood cells when some merozoites develop into male and others into female gametocytes. The gametocyte-containing red blood cells are ingested by the female Anopheles mosquito and, within her gut, produce a female macrogamete and eight spermlike male microgametes. After fertization, the diploid zygote differentiates into a motile ookinet that burroes into gut well, where it grows into an oocyst within which many haploid sporozoites are produced. The sporozoites are released and migrate to the salivary glads, ready to complete the cycle when the mosquito takes her next blood meal.

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Life cycle in RBC The merozoites invade red blood cells; Then transformed into ring stage and trophozoite stage Finally, develop into erythrocytic schizonts. Infected RBC ruptured and release merozoites

P. vivax and P. ovale prefer invading young cells; P. malariae invade usually mature older cells, rarely reticulocytes; P.falciparum EE merozoites invade both the reticulocytes ( 网织红细胞 ) and

erythrocytes (young and old). Erythrocytic stages such as trophozoites , schizonts and gametocytes are present.

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Life cycle in hepatocytesSporozoites move away with the blood stream from the site of injection to the liver, and then infect liver cells. In hepatocytes, each sporozoite undergoes extensive replication within a parasite-derived vacuole, essentially walling off the parasite from the liver cell cytoplasm. When sporozoites mature into schizonts, the infected liver cells rupture and release merozoites. Of note, in P. vivax and P. ovale a dormant stage, hynozoites, can persist in the liver and cause relapses by invading the bloodstream weeks, or even years later

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Summary on life cycle

• Hosts:– Intermediate host: Human– Definitive host: female Anopheles mosquito

• Infective stage: – Sporozoites

• Infected cells: hepatocyte, RBC

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Morphology_ P. falciparum The ring forms of P. falciparum are very small (1μm in diameter), which a very thin circle of cytoplasm; some appear to have two nuclei, and some are closely pressed to the periphery of the cell, the infected host red cells are not enlarged;

In P. falciparum, the schizonts are small, and rarely seen in peripheral blood, because infected cells adhere to the endothelium of capillaries in the internal organs. The erythrocytic is completed within 48 hours and always takes place inside the capillaries and vascular beds of internal organ.

P. falciparum gametocytes are crescent-shaped but those of other species are spherical.

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GametocyteSchizoite

TrophozoiteRing

Normal; rarely, Maurer's clefts (under certain staining conditions)

As the trophozoite grows , its food vacuoles become less noticeable bylight microscopy, but pigment granules of hemozoin ( 疟色素 ) in the vacuoles may become apparent. Hemozoin is the end product of the parasite's digestion of the host's hemoglobin but is not a partially degraded form of hemoglobin.

Morphology_ P. vivax

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Ring Trophozoite

Schizoite Gametocyte

Schüffner's dots refers to a hematological finding that is associated with malaria, exclusively found in Plasmodium ovale and Plasmodium vivax. Schüffner's dots are visible by light microscopy in Romanovsky-stained blood smears as multiple brick-red dots.

The ring forms of P. vivax are larger (2μm in diameter), and as the parasite grows the infected cell becomes enlarge and develop red-staining Schüffner‘s dots ( 薛氏点 ) on its surface;

As the trophozoite grows , its food vacuoles become less noticeable bylight microscopy, but pigment granules of hemozoin ( 疟色素 ) in the vacuoles may become apparent. Hemozoin is the end product of the parasite's digestion of the host's hemoglobin but is not a partially degraded form of hemoglobin.

In P. vivax, erythrocytic schizonts are large, round and irregular in form and occupy the entire red cell, which are enlarged. All the developing stages of schizonts can be seen which contain pigment granules. A mature schizont contains usually 16 merozoites but may contain more even up to 24.

Round to oval; compact; may almost fill RBC; chromatin compact, eccentric (macrogametocyte) or diffuse (microgametocyte); scattered brown pigment

Parasites comparisons between P. falciparum and P. vivax at ring stage

• The ring forms of P. falciparum The ring forms of P. falciparum are very small (1μm in diameter),

which a very thin circle of cytoplasm; Some appear to have two nuclei Some are closely pressed to the periphery of the cell the infected host red cells are not enlarged Multiple infection of RBC is more common Maurer's clefts may be present rarely

• The ring forms of P. vivax The ring forms of P. vivax are larger (2μm in diameter) The parasite grows the infected cell becomes enlarge Develop red-staining Schüffner’s dots ( 薛氏点 ) on its surface

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PATHOGENESIS AND CLINICAL MANIFESTATIONS

Anemia in malaria:Most of pathologic findings of malaria result from the destruction of red blood cells.

Red cells are destoryed by the release of the merozoites Red cells are destoryed by the action of spleen to first sequester the infected red cells and then to lyse them. Bone marrow hematopoietic function inhibition (Inadequate erythropoietic response of the bone marrow to anemia). Autoimmune ( 自身免疫 ) hemolysis

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PATHOGENESIS AND CLINICAL MANIFESTATIONS

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Serious Anemia in P. Falciparum:Short life cycle duration (36-48 h)P.falciparum causes a high level of parasitemia, because it can infect red cells of all ages. In contrast, P.vivax infects only reticulocytes and P.malariae infects only mature red cells; therefore, they produce much lower levels of parasites in the blood. Significant bone marrow hematopoietic function inhibition.

PATHOGENESIS AND CLINICAL MANIFESTATIONS

Incubation period (潜伏期 ) is the time interval between the infective bite of Anopheline mosquito and the onset of the clinical symptoms. It includes the period of the time for the sporozoite to reach the liver, the duration of the development in the liver, and the time of development in the RBC to produce sufficient erythrocytic merozoites to cause clinical symptoms.

P. vivax: 8 - 31 days; P. falciparum: 7 - 27 days. P. malariae: 18 - 35 days; P. ovale: 16 - 18 days.

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PATHOGENESIS AND CLINICAL MANIFESTATIONS

Malarial paroxysm (疟疾发作 ) Malarial paroxysm is preceded by a prodromal period. A few days before the first paroxysm, the patient may feel malaise, muscle pain, headache, loss of appetite, and slight fever; or the first paroxysm may occur abruptly, without any prior symptoms.

The classic malarial paroxysm comprises three successive stages: cold stage, hot stage and sweating stage.Cold-stage: A typical attack of benign tertian or quartan malaria begins with a feeling of intense coldness as the hypothalamus, the body's thermostat, is activated, and the temperature then rises rapidly to 41 . The teeth ℃chatter (牙打战 ), and the bed may rattle from the victim's shivering. The skin is warm and dry. Nausea, vomiting, severe headache, backache, and hypotension are usual.Hot stage: It begins after 1/2 to 1 hour, with intense headache and feeling of intense heat.Sweating stage: It is the final stage, often a mild delirium stage lasts for several hours. As copious perspiration signals the end of the hot stage, the temperature drops back to normal within 2 to 3 hours, and the entire paroxysm is over within 8 to 12 hours.

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PATHOGENESIS AND CLINICAL MANIFESTATIONS

The timing of the fever cycle:

P.vivax and P.ovale : 48 hours P.malariae: 72 hours P.falciparum:36-48 hours

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PATHOGENESIS AND CLINICAL MANIFESTATIONS

Relapse (复发 ) In the cases of P. vivax and P. ovale infections, two populations of exoerythrocytic forms have now been shown. One develops rapidly into schizonts, as previously described, but the other remains dormant as hypnozoites (“sleeping animalcules”). Relapse is that symptoms reappear 8 to 24 weeks later when malaria parasites have been eliminated from blood , which is caused by hypnozoites.

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PATHOGENESIS AND CLINICAL MANIFESTATIONS

Recrudescence (再燃 ) is a feature of P. malariae and P. falciparum, which is caused by small numbers of viable erythrocytic forms of the parasite in the internal organs around 2 months after the initial infection. This is believed to be due to change of the surface antigens of the parasites, thus evading themselves from the immune responses of the host. Attacks even up to 40 years after the initial infection, notably after splenectomy has also been reported.

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PATHOGENESIS AND CLINICAL MANIFESTATIONS

Transfusion malaria Transfusion of infected blood and the use of contaminated needle of the intravenous drug addicts can cause transfusion malaria. Pre-erythrocytic development is absent, and incubation period is short. Clinically, it behaves like a naturally acquired infection. Relapse does not occur.

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IMMUNITYNatural (innate) immunity (先天性免疫 ) Natural immunity in malaria refers t

o the inherent but non-immune mechanisms of the host defence against malaria. It is mainly based on the nature of the red cells. The nature of the red blood cells that determine the susceptibility of the cells to invasion by malaria parasites and development in the cells include: Age of red blood cell Nature of haemoglobi: Individuals with sickle cell trait(heterozygotes) are pro

tected against malaria because their red cells have too little ATPase activity and cannot produc sufficient energy to support the growth of the parasite.

Enzyme content of erythrocyte: People with glucose-6-phosphate dehyfrogenase(G6PD) deficiency re also protected against the severe effects of falciparum malaria.

Presence or absence of certain factors

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Microscopy detection Microscopic identification is the method most frequently used to demonstrate an active infection. A blood specimen collected from the patient is spread as a thick and thin blood smear, stained with a Romanovsky stain (most often Giemsa).Thin smear is used for detecting parasites and species of the infecting parasite. This technique remains the gold standard for laboratory confirmation of malaria. For P. vivax, microscopy detection should be performed 10 hours after paroxysm. The parasites at ring trophozoite, schizonts and gametocyte could be seen. For P. falciparum, microscopy detection should be performed during paroxysm. Only ring stage parasites could be seen. However, the parasites at gametocyte stage could be seen 10 days after paroxysm.

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PREVENTION AND CONTROLTreatment of infected individuals Acute attacks by any of malaria parasites should be treated wi

th chloroquine(氯喹 ). P. vivax and P. ovale infections acquired through mosquito bit

es may have persistant hepatic forms(hypnozoites). These must be eliminated with primaquine( 伯氨喹 ) to prevent relapse.

For P.falciparum: As a response to increasing levels of antimalarial resistance, WHO recommends the following therapeutic options: artemether/lumefantrine, artesunate plus amodiaquine, artesunate plus ulfadoxine/pyrimethamine (in areas where SP efficacy remains high), artesunate plus mefloquine (in areas with low to moderate transmission), and amodiaquine plus sulfadoxine pyrimethamine.

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