Plasmodium vivax is the most widely distributed malarial organism. Infections occur worldwide in both the tropics and subtropics. In addition, unlike the other Plasmodium species, Plasmodium vivax is also seen in temperate regions.
Morphology of Plasmodium vivax
The cytoplasmic ring of the typical Plasmodium vivax ring form measures approximately one third the diameter of the red blood cell in which it resides. A single chromatin dot serves as the connecting point of this delicate ring.A vacuole can be seen inside the ring. At the outer edge of the red blood cell, a place known as accolé or appliqué, the parasite may first be visible as a crescent- shaped mass.
Although remains of the cytoplasmic ring can be seen, the parasite takes on a more irregularly shaped ameboid look. A single large point of chromatin is found in the cytoplasmic material. The vacuole remains visible and essentially intact until the development stage is finished.The presence of hemozoin( a remnant of the parasite feeding on RBC hemoglobin visible as a brown pigment) becomes apparent in the cytoplasm of the parasite in this stage and increases in amount and visibility as the parasites mature
The continuing chromatin division results in 12 to 24( average 16) merozoites. Surrounded by cytoplasmic material, these merozoites occupy most of the RBCs. In some cases, it is hard to detect RBCs. Brown pigment may also be present.
The average Plasmodium vivax macrogametocyte is characterized by a round to oval homogeneous cytoplasm and an eccentric mass of chromatin, often located at the edge of the parasite. Diffuse, delicate, light brown pigment throughout the parasite may be visible.
The typical Plasmodium vivax microgametocyte is a large pink to purple chromatin mass surrounded by a colorless, pale halo in Giemsa. Cytoplasmic hemozoin is generally visible evenly distributed.
Other Morphologic Characteristics
Red blood cells infected with Plasmodium vivax tend to enlarge and distort as the parasites grow.Plasmodium vivax ‘s morphological forms, with the exception of early ring forms less than 8 to 10 hours after infection, may contain small granules in the cytoplasm known as Schüffner ‘s dots( also known as eosinophilic stippling).This characteristic is also typically seen in Plasmodium ovale infected RBCs. Although their presence may not be helpful in speciating these two Plasmodium species, Schüffner ‘s dots may prove helpful in the preliminary exclusion of the species that do not contain them, Plasmodium malariae and Plasmodium falciparum.
Life Cycle of Plasmodium vivax
Plasmodium vivax typically tends to invade young RBCs. These immature cells are the primary target of invasion, because they are typically pliable. This feature allows RBCs to respond to the presence of the replicating parasite by increasing its size. Thus, RBC distortion occurs.
Clinical symptoms of Malaria
Benign Tertian Malaria
Patients infected with Plasmodium vivax typically begin to develop symptoms of benign tertian malaria following a 10- to 17-day post- exposure incubation period. These vague symptoms are similar to those usually seen in cases of flu, including nausea, vomiting, headache, muscle pain and photophobia.
As infected RBCs start to rupture, the resulting merozoites, hemoglobin and toxic cellular waste products initiate the first in a series of paroxysms. Typically, these paroxysms occur every 48 hours( thus, tertian malaria).Untreated patients may experience and resist numerous attacks over several years. However, chronic infections can cause serious damage to the brain, liver and kidney.Blockage of these organs occurs when toxic cellular waste products and hemoglobin, as well as RBC clumps, accumulate in the corresponding capillary veins, resulting in ischemia or tissue hypoxia. Dormant hypnozoites may cause relapses months to years after the initial infection.
Laboratory Diagnosis of Malaria
All morphological stages of Plasmodium vivax can be seen on thick and thin, peripheral blood films. However, thin blood films are most beneficial in the diagnosis of species.Although the best time to observe numerous infected RBCs is halfway between paroxysms, blood samples can be taken at any time during the disease. The morphological forms present at a given time reflect the developmental stage that occurs at that time.
Treatment of Malaria
The choice of appropriate treatment for malaria is relatively more complex than the selection of chemotherapy for other parasite infections. For this reason, malaria treatment for the Plasmodium species will be discussed as a group in this section.
Many antimalarial drugs are available on the market( not all available in the United States), including quinine, quinidine, chloroquine, amodiaquine, pyrimethamine, sulfadoxine, dapsone, mefloquine, tetracycline, doxycycline, halofantrine, atovaquone, proguanil, qinghaosu, artemisinin, artemether, artesunate, pyronaridine, Fenozan B07, dioxanes, nonane endoperoxides, azithromycin. It is important to note that these available malarial medications affect the parasite in a variety of ways, depending on the specific phases of the morphological life cycle. Furthermore, specific plasmodium species differ in their response to these treatments.Drug- resistant malaria has emerged in recent years, and the threat of a continued increase in these strains remains a concern in the medical community.Physicians must take into account the known medication information, including the possibility of potential drug toxicity, when selecting the course of treatment for people suffering from malaria, as well as the patient’s G6PD status(glucose 6 phosphatase).Drug- resistant malaria has emerged in recent years, and the threat of a continued increase in these strains remains a concern in the medical community.
Prevention and Control of Malaria
Prevention and control measures designed to stop the spread of Plasmodium vivax( as well as other Plasmodium species) include personal protection such as netting, screening, protective clothing and repellents for persons entering known endemic areas.In some cases, prophylactic treatment may be used based on the geographical location and length of exposure as well as other factors.Ideally, although it is difficult to achieve mosquito control or better yet total eradication, the life cycle of the organism would definitely break in addition to the prompt treatment of infected persons.In some cases, prophylactic treatment may be used based on the geographical location and length of exposure as well as other factors.Avoiding the sharing of intravenous needles and thorough screening of donor blood are additional measures aimed at eliminating the risk of transmission of non- mosquito Plasmodium species.
A number of studies have focused on developing potential malaria vaccines for Plasmodium vivax and other Plasmodium species.Just as consideration of the specific malaria species and multiple morphological forms in each organism’s life cycle is necessary for the selection of proper treatment, this information is also important when designing vaccines.Unfortunately, questions have arisen as to the effectiveness of such a control measure. Additional research is essential to answer these questions. Although not currently available, the prospects are hopeful that viable vaccines long awaited and much needed will be developed in the future.