The genus Plasmodium is a member of the order Haemosporidia. It is the largest genus within this order and currently consists of over 250 species. They cause malaria in many different vertebrates. The species in this genus are entirely parasitic with part of their life cycle spent in a vertebrate host and another in an invertebrate host - usually a mosquito. Vertebrates infected by members of this genus include mammals, birds and reptiles. Host range among the mammalian orders is non uniform. At least 29 species infect non human primates; rodents outside the tropical parts of Africa are rarely affected; a few species are known to infect bats, porcupines and squirrels; carnivores, insectivores and marsupials are not known to act as hosts. The listing of host species among the reptiles has rarely been attempted. Ayala in 1978 listed 156 published accounts on 54 valid species and subspecies between 1909 and 1975. The regional breakdown was Africa: 30 reports on 9 species; Australia, Asia and Oceania: 12 reports on 6 species and 2 subspecies; Americas: 116 reports on 37 species.
Diagnostic criteria of the order Haemosporida Currently there are ~550 species recognised in this order organised into 17 genera. The diagnostic criteria of this family are:
macrogametes and microgamonts develop independently syzygy is absent microgametocyte produces 8 flagellated microgametes zygote is motile (known as an ookinete) conoid present in ookinete stage only sporozoites naked in oocyst (that is without a sporocyst) heteroxenous: merogony and gamogony occur in vertebrate host and fertilization and sporogony in definitive host (a blood sucking insect) hemozoin pigment produced in some genera (including Plasmodium)
Diagnostic criteria of the genus Plasmodium Merogony occurs both in erythrocytes and other tissues Merozoites, schizonts or gametocytes can be seen within erythrocytes and may displace the host nucleus Merozoites have a "signet-ring" appearance due to a large vacuole that forces the parasite's nucleus to one pole Schizonts are round to oval inclusions that contain the deeply staining merozoites Forms gamonts in erythrocytes Gametocytes are 'halter-shaped' similar to Haemoproteus but the pigment granules are more confined Hemozoin is present Vectors are either mosquitoes or sandflies (Lutzomyia). Vertebrate hosts include mammals, birds and reptiles
Note Mammalian erythrocytes do not possess a nucleus. Although it has been suggested that the nucleus was lost in the erythrocytes better to enable them to traverse capillaries evidence for this is lacking. It appears that this loss along with the mitochondria that the erythrocytes also lose may protect the erythrocytes against oxidative stress.
Subgenera The full taxonomic name of a species includes the subgenus but this is often omitted in practice. The full name indicates some features of the morphology and type of host species. Sixteen subgenera are currently recognised. The avian species were discovered soon after the description of P. falciparum and a variety of generic names were created. These were subsequently placed into the genus Plasmodium although some workers continued to use the genera Laverinia and Proteosoma for P. falciparum and the avian species respectively. The 5th and 6th Congresses of Malaria held at Istanbul (1953) and Lisbon (1958) respectively recommended the creation and use of subgenera in this genus. Laverinia was applied to the species infecting humans and Haemamoeba to those infecting lizards and birds. This proposal was not universally accepted. Bray in 1955 proposed a definition for the subgenus Plasmodium and a second for the subgenus Laverinia in 1958. Garnham described a third subgenus - Vinckeia - in 1964. In 1963 Corradetti, Garnham and Laird proposed a new classification of the avian malaria parasites. They created four sub-genera - Giovannolaia, Haemamoeba, Huffia and Novyella - based on the size of the schizonts, the gametocyte forms and the type of exo-erythrocytic schizogony. Additional subgenera have been created since. The currently recognised subgenera are listed below.
Asiamoeba Telford 1988 Bennettinia Valkiūnas 1997 Carinamoeba Garnham 1966 Giovannolaia Corradetti, Garnham & Laird 1963 Haemamoeba Grassi & Feletti 1890 Huffia Garnham & Laird 1963 Lacertamoeba Telford 1988 Laverania Bray 1958 Novyella Corradetti, Garnham & Laird 1963 Nyssorhynchus Poinar 2005 Ophidiella Garnham 1966 Papernaia Landau et al 2010 Paraplasmodium Telford 1988 Plasmodium Bray 1963 emend. Garnham 1964 Sauramoeba Garnham 1966 Vinckeia Garnham 1964
Classification criteria for subgenera The current classification scheme was developed prior to the widespread use of DNA sequence based taxonomy and is based on host and morphological criteria. Plasmodium has since been shown to be paraphytic with the genera Haemoproteus and Hepatocystis (vide infra). Revision of this genus will be undertaken once sufficient DNA sequence material is available. This forthcoming reclassification project is not unique to this genus as DNA based taxonomy is revising many traditional groupings of protozoa. The bird infecting taxa can be separated into two groups on the basis of the gametocytes: species with round gametocytes (Bennettinia, Haemamoeba) and species with elongated gametocytes (Giovanniola, Huffia and Novyella). The monophyly of the Bennettinia, Haemamoeba and Huffia subgenera was subsequently confirmed by molecular studies. The other two genera were found to be paraphytic. The genera were then revised and a new subgenus - Papernaia - was created.
Species with mammalian hosts Laverania Species in this subgenus infect higher primates (including man) and have characteristic sickle shaped female gametocytes. The type species is Plasmodium falciparum. Plasmodium Species infecting higher primates other than those in the subgenus Laverania are placed in the subgenus Plasmodium. The type species is Plasmodium malariae. Vinckeia Parasites infecting other mammals including lower primates (lemurs and others) are classified in the subgenus Vinckeia. The type species is Plasmodium bubalis.
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