Malaria test (Malaria plasmodia in blood)

Plasmodia are absent in the blood smear of healthy people. Malarial plasmodia alternately parasitize in 2 hosts: in the body of the female mosquito of the genus Anopheles, where sexual reproduction, sporogony, occurs, and in the human body, where asexual reproduction, schizogony, takes place. The initial phase of schizogony occurs in hepatocytes (extraerythrocytic schizogony), the subsequent one - in erythrocytes (erythrocytic schizogony). Developing in erythrocytes, plasmodia feed on hemoglobin and destroy the affected erythrocytes. All pathological manifestations of malaria [fever attacks, anemia, splenomegaly, damage to the central nervous system (CNS) in the tropical form of malaria] are associated with erythrocytic schizogony.

There are 4 types of plasmodia:

• P. falciparum is the causative agent of tropical fever, the most dangerous form of malaria, which requires urgent treatment. In P. falciparum, erythrocytic schizogony begins in the peripheral bloodstream and ends in the central bloodstream, due to the retention of affected erythrocytes in the capillaries of internal organs. As a result, at the beginning of the infection, only young trophozoites ("rings") are present in blood products. Gametocytes, after maturation in the capillaries of internal organs, are detected in the peripheral blood on the 10th-12th day of the disease. Detection of adult trophozoites or schizonts of any age in the peripheral blood indicates the onset of a malignant course of tropical malaria and an imminent fatal outcome if emergency measures are not taken. In other types of malaria, erythrocytic schizogony occurs entirely in the peripheral bloodstream. Unlike other types of plasmodia, P. falciparum gametocytes are not round but oblong in shape and have a long lifespan. They die within 2-6 weeks (other types - within 1-3 days), so the detection of P. falciparum gametocytes for many days after the patient has been cured (termination of erythrocytic schizogony) due to the action of schizonticidal drugs is a common occurrence that is not considered an indicator of the ineffectiveness of therapy.
• P. vivax is the causative agent of three-day malaria.
• P. malariae - the causative agent of quartan malaria
• P. ovale is the causative agent of ovale malaria (three-day type).

The cycle of erythrocytic schizogony is repeated in P. falciparum, P. vivax and P. ovale every 48 hours, and in P. malariae - 72 hours. Malarial attacks develop at that phase of the cycle of erythrocytic schizogony when the bulk of the affected erythrocytes is destroyed and the daughter individuals of plasmodia (merozoites) released from them invade intact erythrocytes.

The following is important for establishing the species affiliation of malaria parasites: the presence of polymorphism of age stages or one leading stage, their combination with gametocytes; morphology of different age stages, their sizes in relation to the affected erythrocyte; character, size of the nucleus and cytoplasm; pigment intensity, its shape, size of grains/granules; the number of merozoites in mature schizonts, their size and location in relation to the pigment accumulation; the tendency of the parasite to affect erythrocytes of a certain age (tropism); the tendency to multiple lesions of individual erythrocytes by several parasites and its intensity; the size of affected erythrocytes in relation to unaffected ones, the shape of affected erythrocytes, the presence of azurophilic granularity in affected erythrocytes; the shape of gametocytes.

During acute attacks of malaria, a certain pattern of changes in the blood is observed. During chills, neutrophilic leukocytosis with a shift to the left appears. During fever, the number of leukocytes decreases slightly. With the appearance of sweat and apyrexia, monocytosis increases. Later, after 2-4 attacks, anemia appears, which develops especially early and quickly with tropical fever. Anemia is mainly hemolytic in nature and is accompanied by an increase in the content of reticulocytes. Poikilocytosis, anisocytosis, and polychromatophilia of erythrocytes are found in blood smears. With the addition of bone marrow suppression, the number of reticulocytes decreases. Sometimes a picture of pernicious-like anemia is noted. ESR increases significantly with malaria.

During the interictal (afebrile) period, adult trophozoites predominate in the blood in all forms of malaria except tropical malaria. During this period of the disease, certain stages of plasmodia are constantly present in the blood, until the complete cessation of erythrocyte schizogony. In this regard, there is no need to take blood for testing only at the height of a malarial attack, but it can be tested at any time. The absence of malaria plasmodia in blood smears and a thick drop of a patient with malaria reflects only the thoroughness of the study and the professional competence of the laboratory specialist.

When assessing the intensity of parasitemia, the total number of asexual and sexual forms is taken into account, with the exception of P. falciparum. The intensity of parasitemia is assessed using a "thick drop" per 1 μl of blood. The number of parasites is counted in relation to a certain number of leukocytes. When 10 or more parasites are detected per 200 leukocytes, the counting is stopped. When 9 or fewer parasites are detected per 200 leukocytes, the counting is continued to determine the number of parasites per 500 leukocytes. When single parasites are detected in a "thick drop" of blood, their number is counted per 1000 leukocytes. The number of parasites in 1 μl of blood is determined using the following formula: X = A × (B / C), where: X is the number of parasites in 1 μl of blood; A is the counted number of parasites; B is the number of leukocytes in 1 μl of blood; C - the counted number of leukocytes.

In cases where it is not possible to determine the number of leukocytes in a given patient, their number in 1 µl, according to WHO recommendations, is conventionally taken as equal to 8000.

The effectiveness of treatment is monitored by examining a thick blood drop with a parasite count in 1 μl of blood. The test should be performed daily from the 1st to the 7th day from the start of chemotherapy. If the parasites disappear during this period, further blood testing is performed on the 14th, 21st and 28th day from the start of treatment. If resistance is detected (assessed by the level of parasitemia) and, accordingly, treatment ineffectiveness, the antimalarial drug is replaced with a specific drug of another group and the blood test is performed according to the same scheme.

Patients who have had tropical malaria are placed under dispensary observation for 1-2 months, with parasitological blood tests performed at intervals of 1-2 weeks. Patients who have had malaria caused by P. vivax, P. ovale and P. malariae should be monitored for 2 years. Any increase in body temperature in these individuals requires laboratory blood tests to detect malarial plasmodia.

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