Ring cycle expounded

Lovers of opera may remember notes that appeared on this blog previously. We got about as far as it being all over when a gravitationally advantaged lady sang. But we didn’t get in a mention of the ring cycle: that major work of Wagnerian genius. This time around, we’re taking a look at another kind of valkyrie. We’re back to our tiny airborne nemesis; the malaria parasite Plasmodium.

Malaria info source

In ‘All your malarias‘ we brought together a body of teaching materials on malaria. In our parasitology section you could find a number of ring forms, and the Plasmodium life cycle. But we left out the new star of the ring; Plasmodium knowlesi. The initial work was done by a group at UNIMAS in Kuching, Sarawak. Subsequent reports map out a patchy distribution through Southeast Asia that overlaps areas where you can find macaques and the mosquito Anopheles leukophyrus.

Four Pl. knowlesi trophozoites and a schizont (R). The trophozoites show features that can be found in Pl. falciparum and Pl. malaria.

Four Pl. knowlesi trophozoites and a schizont (R). The trophozoites show features that can be found in Pl. falciparum and Pl. malaria.

Clinical outcomes

Pl. knowlesi can cause serious human infection, particularly in people travelling to remote parts of Southeast Asia for work or pleasure.

Clinical lab

Laboratory confirmation is a bit more complicated than for the other four Plasmodium species because young trophozoites appears as ring forms, similar to the ring forms of Pl. falciparum, and older trophozoites can appear in band forms like Pl. malaria. An additional problem is that rapid diagnostic tests are not yet geared up to detect Pl. knowlesi. This makes molecular methods like PCR assays more attractive for clinical labs with only sporadic demand for malaria microscopy.

Microscopic appearance

Several useful features have been described. Taken together, this presents s different picture from Pl. falciparum and Pl. malariae but individual cases can present the microscopist with a significant challenge. You can obtain a useful chart and more detailed information on the CDC Clinical Parasitology site.

  • varied types of trophozoite at early and mature stages including ring forms, multiple rings per erythrocyte, multiple chromatin dots per ring, signet ring appearance and appliqué forms at the red cell periphery, band forms and more amoeboid distribution within the red cell
  • malaria pigment may be present in the erythrocyte
  • schizonts have up to 16 merozoites inside, and  a prominent nucleus
  • macrogametocytes of both genders are round

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