Showing papers in "Advances in Parasitology in 1984"

The piroplasms: life cycle and sexual stages.

Abstract: Publisher Summary The piroplasms are protozoa that are highly pathogenic to cattle, sheep, goats, and occasionally even to man. They comprise two genera—namely, Theileria and Babesia . The diseases they induce, known collectively as “theilerioses” and “babesioses,” cause fevers and lead to important economic losses in the tropics, subtropics, and southern Europe. This chapter highlights the differences between the life cycles of Babesia and Theileria species with respect to their morphology, studied by means of light and electron microscopy. The chapter describes the life cycle of piroplasms. They have a typical sporozoan life cycle comprising three phases: (1) Schizogony, an asexual reproduction phase in the vertebrate host. (2) Gumogony, the formation and fusion of gametes inside the intestinal cells of ixodid ticks. (3) Sporogony, an asexual reproduction in the salivary gland of the tick leading to the infectious, saliva-transmitted sporozoites. Comparative biological and morphological studies show that the economically important piroplasms comprise three groups: (1) Babesia species sensu strictu ; (2) Bubesia equi , B. microti ; and (3) Theileria species.

Metabolism of Entamoeba histolytica Schaudinn, 1903

Abstract: Publisher Summary This chapter focuses on the metabolism of typical Entamoeba histolytica originally isolated from human infections. It presents evidence that E. histolytica, grown axenically, have a complete but circumscribed, metabolic capability; and that when living in contact with other cells, they have a faculty for expanding their metabolic competence. The chapter addresses the possible modes of transfer of activities from associated cells to the amoeba and evidence that pathogenicity is thereby influenced. It also considers growth factors and nutrients, metabolic regulation, and speculative views on the ways the metabolism of amoebae may shed light on primitive metabolic pathways. The chapter illustrates allosteric modulation or some system for the regulation of amoebic glycolytic enzymes. However, the results of probing for regulatory features have been disappointing. They have yielded only thermodynamic explanations of enzyme regulation—that the rate of a reversible reaction slows as equilibrium is approached. An understanding of the pathogenicity of the organism is likely to come from the investigation of the interactions among amoebae, the host, and the associate organisms of the gut.

Serotonin receptors in parasitic worms

Abstract: It is evident from the above review that during the last two decades a great deal of interest in investigating the action of serotonin in parasitic worms has been shown by parasitologists as well as by scientists from several other disciplines What we have initially reported concerning the effect of serotonin on motility and carbohydrate metabolism of F hepatica has been pursued on several other parasitic worms The studies so far indicate that serotonin stimulates motility of every species tested among the phylum Platyhelminthes The indoleamine also stimulates glycogenolysis in the few flatworm parasites that have been investigated The information in nematodes is scanty and the role of serotonin in these parasites is still open for experimentation Recent biochemical investigations on F hepatica and S mansoni demonstrated that serotonin and related compounds utilize a common class of receptors in plasma membrane particles which I designate as 'serotonin receptors' These receptors are linked to an adenylate cyclase that catalyses the synthesis of the second messenger, cyclic 3',5'-AMP Serotonin and its congeners increase the concentration of cyclic AMP in intact parasites whereas antagonists inhibit such an effect Cyclic AMP stimulates glycogenolysis, glycolysis and some rate-limiting glycolytic enzymes It activates a protein kinase that may be involved in activation of glycogen phosphorylase and phosphofructokinase Serotonin-activated adenylate cyclase in S mansoni is activated early in the life of the schistosomule The possibility is discussed that the availability of cyclic AMP through serotonin activation in these parasites may be a prelude to the development processes that take place in the parasite The different components of the serotonin-activated adenylate cyclase in the parasite are the same as those that have been previously described for the host Binding characteristics of the receptors indicate that the receptors in F hepatica appear to be different from those that have been described in the host The discovery of these receptors and their differences from those in the host offer a new site which is amenable to pharmacological manipulation The search for new agents that influence serotonin receptors in these parasites could be included in a strategy for the development of new chemotherapeutic agents against these parasites

Giardia and Giardiasis

Abstract: Publisher Summary Organisms in the genus Giardia are widely distributed, flagellated protozoan parasites of all classes of vertebrates. Giardia is an intestinal protozoan parasite of humans.. Under appropriate conditions, they can cause disease. This chapter summarizes the progress in the study of Giardia and its relation with its host. Studies in this regard have resulted in a number of developments, most occurring within the past decade. These include (1) information regarding the nature of Giardia infection and disease, the host–parasite interaction, and the development of a mouse model of disease; (2) their axenic cultivation and the determination of conditions for excystment; (3) information regarding their spread via water, via sexual contact, and from one animal species to another; (4) data regarding the role of the immune system in affecting host susceptibility to these organisms and the possibility of severe disease in immunodeficient individuals; (5) recognition of the frequent difficulty in detecting this parasite, the introduction of a new method of obtaining trophozoites from the patient, and efforts to diagnose the infection immunologically; and (6) the demonstration that some Giardia infections are not eradicated with presently recommended drugs.