W. E. Stehbens

Bio: W. E. Stehbens is an academic researcher. The author has an hindex of 1, co-authored 1 publications receiving 27 citations.

The biology of some intraerythrocytic parasites of fishes, amphibia and reptiles.

Abstract: Fishes, amphibia and reptiles, the ectothermic vertebrates, are hosts for a variety of intraerythrocytic parasites including protists, prokaryotes, viruses and structures of uncertain status. These parasites may experience host temperature fluctuations, host reproductive strategies, population genetics, host habitat and migratory behaviour quite unlike those of endothermic hosts. Few blood infections of fishes, amphibia and reptiles have proven pathogenicity, in contrast to the many intraerythrocytic parasites of mammals and some birds which harm their hosts. Although not given the attention afforded to intraerythrocytic parasites of endotherms, those of ectotherms have been studied for more than a century. This review reports on the diversity, general biology and phylogeny of intraerythrocytic parasites of ectotherms. The existence of taxonomic confusion is emphasized and the main taxonomic features of most of the 23 better characterized genera, particularly the kinetoplastid and apicomplexan protists, are summarized. Transmission of protistan infections of aquatic ectotherms is also discussed. Leeches can transfer sporozoties or merozoites to the vertebrate host during feeding. Dormant sporozoites of Lankesterella may permit transmission of species of this genus between vertebrates by predation. The fish haemogregarine, Haemogregarina bigemina, probably has gnathiid isopods, rather than leeches, as its definitive hosts. Hepatozoon spp. in aquatic hosts, and Progarnia of caiman, may also use invertebrate hosts other than leeches. Protistan infections of terrestrial or semi-terrestrial hosts are transmitted by a variety of arthropods, or, in some cases, leeches, contaminated paratenic hosts, or sporocysts free in water. Transfer of protists between vertebrates by predation and congenitally may also occur. The biology of the host cells of these infections, the red blood cells of ectotherm vertebrates, is summarized and compared with that of mammalian erythrocytes. Erythropoiesis, the nature of the surface molecules (especially the possible existence of a major histocompatibility complex), the haemoglobins, and the shape and size of erythrocytes are discussed. The exoerythrocytic sites in which protists, prokaryotes, viruses and structures of uncertain status exist before erythrocyte entry are described. Tissue merogony, tissue cysts and invasion of the white cell series occur in a variety of protistan infections. Intraerythrocytic stages of protistan infections are also discussed, including modes of entry to erythrocytes, survival mechanisms, and multiplication. The impact of infection on host populations is difficult to assess, in part because there is no agreement in the literature on the criteria used to evaluate parasite-induced cost to the host. Almost all studies have been on haemogregarine and Plasmodium infections in, mainly, lizards, but also fishes and snakes. Some infections may be responsible for mortality in their hosts, but hosts themselves may be short-lived, or have a limited ability to recover from infection.

Ultrastructural study of characteristic organelles (paired organelles, micronemes, micropores) of sporozoa and related organisms.

Abstract: By means of electron microscopy a study has been made of different developmental stages of Eimeria callospermophili, E. falciformis, Toxoplasma gondii, Frenkelia spec. (= M-organism), Babesia bigemina, and B. ovis. Major emphasis was given to the analysis of some characteristic organelles of the motile stages of the sporozoans. These organelles were the paired organelle, the micronemes and the micropores.

Ultrastructure of Cryptosporidium wrairi from the guinea pig.

Abstract: SYNOPSIS. The ultrastructure of the known tissue stages of Cryptosporidium wrairi Vetterling, Jervis, Merrill, and Sprinz, 1971 parasitizing the ileum of guinea pigs is described. Young trophozoites are surrounded by 4 unit membranes, the outer 2 of host origin, the inner 2 the pellicle of the parasite. Each trophozoite contains a vesicular nucleus with a large nucleolus. Its cytoplasm contains ribosomes, but eventually fills with cisternae of the rough endoplasmic reticulum. As the trophozoite matures the area of attachment of the parasite to the host cell becomes vacuolated, with vertical membranous folds. It is apparent that the parasite acquires nourishment from the host cell thru this area of attachment. As schizonts develop, (a) multiple nuclei appear, (b) the endoplasmic reticulum enlarges, (c) the attachment zone increases in area, (d) large vacuoles, which develop as endocytotic vesicles in the attachment area, are found in the cytoplasm and (e) the inner unit membrane of the parasite pellicle is resorbed around the sides of the developing schizont. Following nuclear division, merozoites develop from the schizont by budding. Merozoites have an ultrastructure similar to that described for other coccidia except that no mitochondria, micropores, or subpellicular tubules were observed. Merozoites penetrate the epithelial cell causing invagination of the microvillar membrane and lysing it. No unit membrane is formed between the parasite and the host cell. However, the cell produces one or 2 dense bands adjacent to the parasite attachment area. The macrogamete contains a nucleus, endoplasmic reticulum, attachment zone, and large vacuoles. It also contains a variety of granules, some of which are polysaccharide. The immature microgametocyte contains multiple compact nuclei. No mature microgametocytes or zygotes were found.

Electron microscope studies on motile stages of malaria parasites VI. The oöknete of Plasmodium berghei yoelii and its transformation into the early oocyst

Abstract: The ookinete and early oocysts of Plasmodium berghei yoelii in Anopheles stephensi have been studied by electron microscopy. The ookinete resembles that previously described for other species of the genus. It contains a single nucleus, one or more crystalloids, mitochondria, a number of anterior elongated, convoluted rods or tubules, and it is bound by a pellicle of 2 unit membranes and a layer of subpellicular microtubules. The structure of the anterior end of the ookinete was revealed in more detail than hitherto. There is an annular thickening of the pellicle, the polar ring, from which the subpellicular microtubules appear to arise. In some specimens, the polar ring was on a narrow anterior protuberance; in others, it was retracted and, behind it, there was a cavity between the tubules and the pellicle apparently formed by a split in the plasmalemma.

The fine structure of the conoid of sporozoa and related organisms

Abstract: The merozoites ofEimeria callospermophili andE. stiedae as well as the zoites ofToxoplasma gondii and the M-organism and the erythrocytic merozoites ofBabesia bigemina were studied by means of electron microscopy. The fine structure of the apical pole was analysed, and compared with the results of other studies of sporozoa and related organisms. The following conclusions were made: