Plasmodium vivax

Plasmodium vivax is a protozoal parasite and a human pathogen. This parasite is the most frequent and widely distributed cause of recurring malaria. Although it is less virulent than Plasmodium falciparum, the deadliest of the five human malaria parasites, P. vivax malaria infections can lead to severe disease and death, often due to splenomegaly (a pathologically enlarged spleen). P. vivax is carried by the female Anopheles mosquito; the males do not bite.

Biology

Life cycle Like all malaria parasites, P. vivax has a complex life cycle. It infects a definitive insect host, where sexual reproduction occurs, and an intermediate vertebrate host, where asexual amplification occurs. In P. vivax, the definitive hosts are Anopheles mosquitoes (also known as the vector), while humans are the intermediate asexual hosts. During its life cycle, P. vivax assumes various different physical forms (see below). Asexual forms:

Sporozoite: Transfers infection from mosquito to human Immature trophozoites (ring or signet-ring shaped), about one-third of the diameter of a red blood cell. Mature trophozoites: Very irregular and delicate (described as amoeboid); many pseudopodial processes seen. The presence of fine grains of brown pigment (malarial pigment) or hematin is probably derived from the haemoglobin of the infected red blood cell. Schizonts (also called meronts): As large as a normal red cell; thus the parasitized corpuscle becomes distended and larger than normal. There are about sixteen merozoites. Sexual forms:

Gametocytes: Round. P. vivax gametocytes are commonly found in human peripheral blood at about the end of the first week of parasitemia. Gametes: Formed from gametocytes in mosquitoes. Zygote: Formed from combination of gametes Oocyst: Contains zygote, develops into sporozoites

Human infection P. vivax human infection occurs when an infected mosquito feeds on a human. During feeding, the mosquito injects saliva, along with sporozoites, through the skin. A proportion of these sporozoites reach the liver. There they enter hepatic cells, on which they feed, and reproduce asexually, as described in the next section. This process gives rise to thousands of merozoites (plasmodial daughter cells) in the body. The incubation period of human infection usually ranges from ten to seventeen days and sometimes up to a year. Persistent liver stages allow relapse up to five years after the elimination of red blood cell stages and clinical cure.

Liver stage The P. vivax sporozoite enters a hepatocyte and begins its exoerythrocytic schizogony stage. This is characterized by multiple rounds of nuclear division without cellular segmentation. After several nuclear divisions, the parasite cell will segment, and merozoites are formed. There are situations where some of the sporozoites do not immediately start to grow and divide after entering the hepatocyte, but remain in a dormant, hypnozoite stage for weeks or months. The duration of latency is thought to be variable from one hypnozoite to another and the factors that will eventually trigger growth are not known; this might explain how a single infection can be responsible for a series of waves of parasitaemia or "relapses". It has been assumed that different strains of P. vivax have their own characteristic relapse pattern and timing. However, such recurrent parasitemia is probably being over-attributed to hypnozoite activation. Two newly recognized, non-hypnozoite, probable contributing sources to recurrent peripheral P. vivax parasitemia are erythrocytic forms in bone marrow and the spleen. Between 2018 and 2021, it was reported that vast numbers of non-circulating, non-hypnozoite parasites occur unobtrusively in tissues of P. vivax-infected people, with only a small proportion of the total parasite biomass present in the peripheral bloodstream. This finding supports an intellectually insightful, paradigm-shifting viewpoint, which had prevailed since 2011 (albeit not believed between 2011 and 2018 by most malariologists and therefore ignored), that an unknown percentage of P. vivax recurrences are recrudescences (having a non-circulating or sequestered merozoite origin), and not relapses (which have a hypnozoite source). The recent discoveries concerning bodily parasite biomass distribution did not give rise to this new theory; it was pre-existing, as explained above. The recent bone marrow and spleen, etc., findings merely confirm the likely validity of the theory.

Erythrocytic cycle

P. vivax preferentially penetrates young red blood cells (reticulocytes), unlike Plasmodium falciparum which can invade erythrocytes. In order to achieve this, merozoites have two proteins at their apical pole (PvRBP-1 and PvRBP-2). The parasite uses the Duffy blood group antigens (Fy6) to penetrate red blood cells. This antigen does not occur in the majority of humans in West Africa [phenotype Fy (a-b-)]. As a result, P. vivax occurs less frequently in West Africa. The parasitised red blood cell is up to twice as large as a normal red cell and Schüffner's dots (also known as Schüffner's stippling or Schüffner's granules) are seen on the infected cell's surface. Schüffner's dots have a spotted appearance, varying in color from light pink to red, to red-yellow, as coloured with Romanovsky stains. The parasite within it is often wildly irregular in shape (described as "amoeboid"). Schizonts of P. vivax have up to twenty merozoites within them. It is rare to see cells with more than one parasite within them. Merozoites will only attach to immature blood cells (reticulocytes) and therefore it is unusual to see more than 3% of all circulating erythrocytes parasitised. Unusual erythrocytic forms were detected in a few cases of an outbreak in Brazil.

Mosquito stage Parasite life cycle in mosquitoes includes all stages of sexual reproduction:

Infection and Gametogenesis Microgametes Macrogametes Fertilization Ookinite Oocyst Sporogony

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