What happens during the trophozoite stage?

Early development: The starting point is the ring form

The earliest stage following red cell invasion has a typical ring form:

  • a = chromatin dot
  • b = digestive vacuole
  • c = parasite cytoplasm

Later development: Growth and development

The later stages include red cell modification:

  • A = parasites increase size and may change shape
  • B = the red cell may alter size and shape or acquire dots
  • C = haemoglobin is metabolised - cells become pale and pigment is formed

Differences between the trophozoites of the different species

At the earliest stage of red cell infection parasites have a ring form and distinction between species may not be easy (or even possible); however, as the parasites mature and develop differences become more apparent. In some species the ring form is maintained until late stages of maturation, in others there is loss of a typical ring appearance

  • P.falciparum: Typically small and delicate, these ring forms tend to persist from early trophozoite until late stages, during this time they may thicken
  • P.ovale: Typically large and robust rings, but maintain a ring shape until late stages of development (contrast with P.vivax)
  • P.malariae: Small, but more substantial than P.falciparum, these parasites become more solid or band like, losing the ring form as they develop
  • P.knowlesi: Early forms resemble P.falciparum, but as they mature they often extend and become solid or band-like resembling P.malariae
  • P.vivax: Early stages appear as rings, but then become irregular and eventually the more typical amoeboid appearances develop

These changes are illustrated for early and late trophozoites below:

The sequence of development: P.falciparum

Small and often delicate rings at early developmental stages, these thicken at later stages, although they remain as "rings"

The sequence of development: P.ovale

Robust rings that enlarge during parasite development, although the ring form can often still be distinguished

The sequence of development: P.malariae

Small but more robust than P.falciparum at early stages, the parasites become more solid as they develop and may extend as a band

The sequence of development: P.knowlesi

Beginning their development as fine rings (like P.falciparum), at later stages they elongate (resembling P.malariae)

The sequence of development: P.vivax

Begin as large rings, then become irregular and often amoeboid in shape at later stages of development