Plasmodium azurophilum

Species Infecting White Blood Cells

Plasmodium azurophilum is one of the more interesting species in the genus. The parasite infects Anolis lizards throughout the eastern Caribbean, including Puerto Rico and many Lesser Antilles islands (biogeography is covered by Staats and Schall, 1996, Biotropica).

The species was described by S. R. Telford Jr. (Telford, 1975, International Journal for Parasitology) who noted that the parasite cells seldom store pigment and infect both erythorcytes and several classes of white blood cells. Thus, one of the diagnostic features of the genus Plasmodium, infecting of erythrocytes only, seems to be violated by this species. Later, S. Ayala (Ayala and Hertz, 1981, Revista do Instituto de Medicine Tropicale Sao Paulo) noted from lizards studied in Martinique that infections tended to be entirely infecting red or white blood cells and they wondered if there might be two species involved. Schall noted the same phenomenon on St. Martin (Schall, 1992, Oecologia). This suggested that the parasite might enter the host, first infecting red blood cells, then switching to the white cells. This was suggested because infected lizards tended to express more white blood cells in their blood than noninfected lizards. Thus, the parasite could be manipulating its host to produce its second class of host cell.

S. Perkins then demonstrated that P. azurophilum indeed is two species with identical morphlogy under the light microscope (Perkins, 2001, Journal of Evolutionary Biology; Perkins, 2000, Proceedings of the Royal Society of London). Sequencing the cytochrome b gene, Perkins found that on each island there were different sequences for infections only in white vs. red cells. This was repeated on each island throughout the eastern Caribbean. Phylogenetic study showed that parasites in red cells are more closely related to those in red cells on other islands, even far away, than parasites in white cells on the same island. The ancestor of both species appears to have been a parasite in erythrocytes from either St. Kitts or nearby islands. Thus, the ancestor species (infecting red blood cells) produced a new species that infects only white cells -- a rare resource!

We thus consider P. azurophilum to be two species, P. azurophilum (red) and P. azurophilum (white). These are cryptic, sister species. Perkins has not named a new species because it is possible that P. azurophilum (red) is already known under another name from Central America.

A number of studies on the ecology (including virulence) of lizard malaria parasites in the Caribbean Anolis have not distingished between the two "P. azurophilum" species (Schall and Pearson, 2000, Journal of Herpetology; Schall and Vogt, 1993, Biotropica; Schall and Staats, 1997, Oecologia; Staats and Schall, 1996, Journal of Parasitology; Schall, 1992, Oecologia), whereas more recent studies divide data for the two species (Schall, Pearson, and Perkins, 2000, Journal of Parasitology; Schall, 2002, in The Behavioural Ecology of Parasities; Schall and Staats, 2002, Copeia).

In the pictures below are shown P. azurophilum (white), both asexuals and gametocytes. Note the lack of malaria pigment. Often the parasite is not easy to detect, especially the male gametocytes, because they stain very pale in an already pale-staining cell.