Plasmodium ovale

Plasmodium ovale is a protozoan parasite belonging to the Apicomplexa phylum, one of the several species known to cause malaria in humans. It was first described in detail by the British parasitologist Clifford Dobell in 1922, following earlier observations. This species is responsible for a form of malaria known as ovale malaria, which is typically less severe than that caused by Plasmodium falciparum but shares clinical similarities with Plasmodium vivax, including the ability to form dormant liver stages. Its geographical distribution is primarily restricted to tropical regions, particularly in West Africa and islands of the Western Pacific Ocean.

Taxonomy and classification

The species was formally named by Clifford Dobell after examining blood films from a patient in British India. Within the genus Plasmodium, it belongs to the subgenus Plasmodium (Plasmodium), which includes other human-infecting species. Molecular phylogenetic studies, often analyzing genes like the cytochrome b gene, place it closely related to Plasmodium vivax, suggesting a common evolutionary ancestor. Historically, two subspecies or variants were proposed, curtisi and wallikeri, based on genetic distinctions observed through techniques like polymerase chain reaction; these are now often considered distinct cryptic species. The classification is continually refined by organizations like the World Health Organization and research from institutions such as the Centers for Disease Control and Prevention.

Morphology and life cycle

The life cycle involves both a human host and an Anopheles mosquito vector, specifically species like Anopheles gambiae. In humans, sporozoites injected by a mosquito infect hepatocytes, where they develop into schizonts; unlike Plasmodium falciparum, some forms become dormant hypnozoites, capable of causing relapses months later. In the bloodstream, the parasite invades erythrocytes, displaying distinctive morphological features: enlarged, oval-shaped infected red blood cells with fimbriated edges, and Schüffner's dots visible with Giemsa stain. The asexual replication cycle produces merozoites that perpetuate the blood-stage infection, while some parasites differentiate into sexual forms, gametocytes, which are ingested by a feeding Anopheles mosquito, completing the cycle in the midgut and salivary glands.

Epidemiology and distribution

The parasite has a patchy, focal distribution, being most prevalent in Sub-Saharan Africa, particularly West Africa and countries like Liberia, Ghana, and Nigeria. It is also found, though less commonly, in parts of Asia and the Western Pacific, including Papua New Guinea and the Philippines. Due to its lower prevalence and often mild symptoms, it has historically been overshadowed in epidemiological studies by Plasmodium falciparum. However, improved diagnostic methods, including polymerase chain reaction, have revealed its presence in regions like South America and Madagascar. Travel-associated cases are reported in non-endemic areas, documented by health agencies like the European Centre for Disease Prevention and Control.

Clinical features and diagnosis

Ovale malaria typically presents with classic malarial symptoms: fever, chills, headache, and fatigue, following an incubation period that can be extended due to dormant liver stages. The fever paroxysms often occur every 48 hours, similar to Plasmodium vivax infection. A key clinical characteristic is its tendency to cause relapses from reactivated hypnozoites, sometimes months after the initial infection. Diagnosis relies on microscopic examination of blood films using Giemsa stain to identify the distinctive oval erythrocytes and Schüffner's dots. However, it can be misidentified as Plasmodium vivax; molecular techniques like polymerase chain reaction at reference laboratories, including those of the World Health Organization, provide definitive speciation.

Treatment and prevention

The standard treatment for the blood-stage infection involves chloroquine, to which the parasite usually remains sensitive, unlike many strains of Plasmodium falciparum. To prevent relapses from hypnozoites, the 8-aminoquinoline drug primaquine is required, following screening for glucose-6-phosphate dehydrogenase deficiency to avoid hemolytic risk. In cases of chloroquine resistance, alternatives like atovaquone-proguanil or artemisinin-based combination therapies may be used, as per guidelines from the World Health Organization. Prevention strategies mirror those for all malaria: use of insecticide-treated bed nets, indoor residual spraying with compounds like DDT, and chemoprophylaxis for travelers to endemic areas such as West Africa, advised by the Centers for Disease Control and Prevention.

Category:Apicomplexa Category:Parasites of humans