Phylum APICOMPLEXA Levine, 1970

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Introduction

Apicomplexan cells possess a distinctive apical complex of organelles, comprising a conoid, polar ring, rhoptries, micronemes and subpellicular microtubules, which facilitate entry into host cells as they are obligate intracellular parasites for most of their life-cycles. These parasites undergo cyclic development involving three divisional processes: merogony (schizogony), gamogony and sporogony. Cell division may occur by fission (splitting of the maternal cell) or endogeny (internal formation of daughter cells). Over 8,000 species have been described as monoxenous or heteroxenous parasites of vertebrate and invertebrate hosts. There are four main apicomplexan groups: gregarines, haemogregarines, coccidia and haematozoa.

Gregarines form large extracellular gamonts which may be septate (cephaline) or aseptate (acephaline); the former being divided by a septum into an anterior protomerite and a posterior deutomerite. The conoid is modified, forming an anterior holdfast organelle (epimerite in septate species or mucron in aseptate species). Equal numbers of gametes are produced by male and female gamonts. Most species have monoxenous life cycles in the digestive tracts or body cavities of invertebrates and lower chordates although some have heteroxenous life-cycles cycling between molluscan and crustacean hosts involved in predator–prey relationships.

Haemogregarines are adeleorin coccidia which form small intracellular gamonts; microgamonts producing from 1 to 4 non-flagellated microgametes which associate pairwise with macrogametes (syzygy). Over 400 species have been recorded as heteroxenous parasites in vertebrate leucocytes and erythrocytes with haematophagous invertebrates acting as vectors.

Coccidia form non-motile resistant oocysts that contain infective sporozoites usually confined within secondary spores (sporocysts). The gamonts of eimerian coccidia develop separately and many flagellated microgametes are produced. Numerous species have been described predominantly on the basis of oocyst morphology and host occurrence. Infections may be confined to the gut or undergo extra-intestinal development leading to marked histopathological changes.

Haematozoa are small blood-borne parasites which undergo merogony and gamogony in vertebrate blood cells. They are transmitted by blood-sucking invertebrates where fertilisation occurs forming a motile zygote (ookinete). Gamonts do not exhibit syzygy and sporozoites are not enclosed within sporocysts. Two main groups are recognised in terrestrial vertebrates: pigment-forming haemosporidia with insect vectors; and non-pigment forming piroplasms with arachnid vectors.

Diagnosis

Unicellular protistan parasites, cells with apical complex organelles.

General References

Lee, J.J., Leedale, G.F. & Bradbury, P. (eds) 2000. An Illustrated Guide to the Protozoa. Lawrence, Kansas : Society of Protozoologists, Allen Press Inc. Vol. I.

Levine, N.D. 1988. The Protozoan Phylum Apicomplexa. Boca Raton, Florida : CRC Press Vol. I & II.

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