Lizard Malaria Parasites

Plasmodium giganteum

Two species of Plasmodium infect the rainbow lizard, Agama agama, over a large area of mesic Africa: P. agamae and P. giganteum. These are two of the earliest species of Plasmodium described, P. agamae by Wenyon in 1909 and P. giganteum by Theiler in 1903. P. giganteum is shown in these photographs. It is much larger parasite of the two, producing probably over 100 merozoites per schizont. The gametocytes are also much larger and often fill the cell. P. giganteum is also distinctive because it primarily infects immature red blood cells which have a larger nucleus and stain darker with Giemsa (although this is not obvious if the parasite is large and fills the cell).

The interaction between P. giganteum and P. agamae is described in Schall and Bromwich (Oecologia, 1994). There is a positive association between the two species; that is, mixed infections are found far more often than expected by chance. Uninfected lizards have few immature red blood cells in their circulation, not a problem for P. agamae, but not a prime habitat for P. giganteum. Once a lizard is infected with P. agamae, immature red blood cells flood the circulation, providing habitat for P. giganteum. Thus, there may be a facilitation, or kind of succession, at work: P. giganteum needs P. agamae to enter the host first. This would argue that at a geographic location, P. giganteum cannot occur alone. To date, no study (with large sample sizes) has found P. giganteum alone at a site, but P. agamae does occur alone.

The pictures above are from infections found in A. agama in Sierra Leone, west Africa. The geographic distribution of both species is given in Schall and Bromwich (Oecologia, 1994).

The virulence of P. giganteum was difficult to measure because few solitary infections were found, but when mixed with P. agamae, clutch size of females was reduced substantially. Details on the virulence of P. giganteum are described in Schall (1990, Parasitology), Schall (1996, Advances in Parasitology), and Schall (2002 in The Behavioural Ecology of Parasites edited by E. E. Lewis, J. F. Cambell, and M. V. K. Sukhdeo).