The biology of Echinococcus, the causative agent of echinococcosis (hydatid disease) is reviewed with emphasis on the developmental biology of the adult and metacestode stages of the parasite. Major advances include determining the origin, structure and functional activities of the laminated layer and its relationship with the germinal layer; and the isolation, in vitro establishment and characterization of the multipotential germinal cells. Future challenges are to identify the mechanisms that provide Echinococcus with its unique developmental plasticity and the nature of activities at the parasite-host interface, particularly in the definitive host. The revised taxonomy of Echinococcus is presented and the solid nomenclature it provides will be essential in understanding the epidemiology of echinococcosis.

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Biology and Systematics of Echinococcus.

We examine patterns of community richness among intestinal parasitic helminth communities in fishes, herptiles, birds and mammals with respect to the comparative number of component species in a host population. We show that terrestrial hosts have, on average, fewer component species than aquatic hosts. We also show that the mean number of component species in aquatic hosts increases from fishes through herptiles to birds before declining slightly in mammals. For terrestrial hosts, the mean number of component species increases from herptiles, through birds, reaching a maximum in mammals. We conclude that: (i) habitat of the host is more important in determining community richness than is host phylogeny; (ii) the phenomenon of ‘host capture’ may be largely responsible for increased species richness in some host groups; (iii) aquatic birds harbour the richest intestinal helminth communities; and (iv) as we interpret them, our data refute the time hypothesis, which would predict that fishes as the oldest lifestyle should have the richest helminth communities.

Ecological versus phylogenetic determinants of helminth parasite community richness

Recently, some authors (Kennedy, 1981; Price & Clancy, 1983) have argued that there are fundamental differences between the communities of helminths in fish and bird hosts. Such differences are foreshadowed by the work of Dogiel (1964) and are apparent from survey data (e.g. Threlfall, 1967; Bakke, 1972; Hair & Holmes, 1975 on birds, and compare Chubb, 1963; Mishra & Chubb, 1969; Wootten, 1973; Ingham & Dronen, 1980 on fish). Questions still remain, however, as to whether the distinctions are truly justified and whether the differences are really fundamental. In this paper, we address these questions by examining helminth diversity in a series of hosts. We then discuss and provide explanations for the observed differences.

Patterns in helminth communities: why are birds and fish different?

The Biology and Evolution of Trematodes

Fasciolosis is an economically important disease of domestic livestock, in particular cattle and sheep, and occasionally man. The disease is caused by digenean trematodes of the genus Fasciola, commonly referred to as liver flukes. The two species most commonly implicated as the aetiological agents of fasciolosis are F. hepatica and F. gigantica (family Fasciolidae). F. hepatica has a worldwide distribution but predominates in temperate zones while F. gigantica is found on most continents, primarily in tropical regions. The class Trematoda belongs to the phylum Platyhelminthes, and can be divided into two subclasses, the Digenea and Aspidogastrea. The Digenea are characterized by a complex life cycle in which one or more intermediate hosts are involved. Many variations on the life cycle exist, but each typically includes a molluscan primary or intermediate host in which larval multiplication occurs, and a vertebrate final or definitive host in which sexual reproduction occurs. Members of the family Fasciolidae are hermaphroditic and self-fertilization can occur, although sexual reproduction is normally by cross-fertilization in the final host. The story of the discovery of the life cycle of F. hepatica has been recounted before; therefore only a short account will be given here. A list of the principal discoveries is shown in Table 1.1. Integral to the successful completion of the life cycle, the biological factors involved will be discussed in the second part of the chapter. Additional details on many of the aspects of the life cycle will be covered in more depth in subsequent chapters.

The Life Cycle of Fasciola hepatica

Publisher Summary This chapter discusses interaction of helminths and their hosts within the alimentary canal and, especially the stomach and intestines. Many such parasites live in the gastro-intestinal canal while others penetrate into the wall of the canal and a few invade organs, such as the liver and pancreas. The dynamic relationship between parasite and host is clearly shown and a good example set for future researchers. First, there is a study of the parasite–host interface, including the various kinds of organs of attachment, the varied nature of the lumen and the mucosa of the gastrointestinal canal, the nature of adhesion, and absorptive surfaces of the helminth and the host. The chapter describes the ecology of helminth site selection by trematodes, cestodes, and nematodes and matters concerning concurrent infections, transplantation, and migration. The chemical and physical characteristics of the intestinal lumen ionic and osmotic characters, microbial ecology, enzymes, bile acids and dietary fats, nutritive gradients, and matters of luminal homeostasis are discussed. The functional gradients in the gastrointestinal tract, bearing on the absorption of electrolytes and nonelectrolytes, water absorption, and malabsorption are also discussed.

Ecological and Physiological Aspects of Helminth-Host Interactions in the Mammalian Gastrointestinal Canal

Publisher Summary This chapter discusses parasite behavior in understanding platyhelminth responses. Parasitic organisms are similar to free-living organisms in that they respond to changes in their environment. These responses can take the form of measurable behavioral changes, underlying which there obviously has to be mechanisms involving the biochemistry and physiology of the parasitic organism. This chapter focuses on the importance of behavioral analyses to elucidate the mechanisms of orientation. In the free-living stages of parasitic platyhelminths, the ability to orient to specific cues is crucial. The host-finding behavior of the free-living miracidia involves at least three sequential steps where, at a minimum, the parasites must be able to respond directionally to cues that characterize the host habitat and the specific host. The long-term goal is to explain the reciprocal inhibition between the behavior of the pharynx (used in feeding) and the behavior of the ventral sucker (used in attachment and locomotion). The study of platyhelminth neurobiology can only advance through an approach that integrates behavioral analyses with neurobiology.

Parasite Behaviour: Understanding Platyhelminth Responses

Consistent in vitro behavioral patterns were found in the scolex and strobila of adult Hymenolepis diminuta. These patterns were measured with a force transducer and the behavior analyzed with a slow motion closed circuit T.V. Varying concentrations of serotonin (5-HT), acetylcholine (Ach), histamine and somatostatin, in the range of 10(-3) to 10(-9) M, were tested for their influence on the rhythmic patterns of behavior. High concentrations of 5-HT and of Ach decreased scolex motility. While 5-HT significantly increased motility in the anterior-, mid- and posterior regions of the strobila at 10(-3) M, Ach inhibited motility in all 3 regions of the strobila at the same concentrations. At high concentrations, somatostatin had a smaller stimulatory effect on worm motility in the anterior and mid-regions; histamine only significantly affected worm motility in the posterior region of the strobila. Depending on concentration, the action of 5-HT, Ach and histamine can be reversed, particularly in the anterior and posterior regions of the strobila. The in vivo assay for worm migrational responses suggests that the action of the neuromuscular stimulators and inhibitors on worm migration is indirect.

Hymenolepis diminuta: Behavioral effects of 5-hydroxytryptamine, acetylcholine, histamine and somatostatin

The behavioral repertoire of the infective stage of Fasciola hepatica was qualitatively characterized. During activation, a primary activity was the emptying of the ceca by peristaltic-like contractions. Emergence behavior comprised coordinated patterns of body movement and sucker activity specifically directed at disruption of the ventral plug. The stimulus specificity of the emergence response for glycine-conjugated cholic acid and the log dose-effect relationship of this response with glycocholic acid suggested a receptor-mediated sensory recognition. Extracts from the duodenum (departure organ) and the liver (arrival site) significantly affected the rate of locomotion and the orientation of the migratory stage. The evidence for orientation in the migrating stages is unequivocal, but the mechanisms by which they orient are unclear.

https://www.jstor.org/stable/pdfplus/3281496.pdf

The behavior of juvenile Fasciola hepatica.

ABsTRAcr: Acetylcholine, cholinergic agonists and acetylcholinesterase inhibitors significantly decrease the amplitude and frequency of spontaneous longitudinal muscle contractions in Fasciola hepatica. In order of their effects on the inhibition of muscle contractions, the cholinergic agonists can be ranked as nicotine > carbachol > acetylcholine. High calcium ion concentration also causes a significant inhibition of contractions. Atropine, a cholinergic antagonist that acts on muscarinic receptors, significantly increases the amplitude and frequency of spontaneous contractions and completely reverses the effects of cholinomimetic drugs, including nicotine. In adult F. hepatica, the levels of acetylcholine and its precursor choline are 3.14 ? 0.55 and 13.75 ? 3.72 pmol/ mg wet weight, respectively. The activities of choline acetyltransferase, specific acetylcholinesterase and the nonspecific cholinesterase are 1.25 + 0.19, 238.0 ? 13.0, and 83.0 ? 33.0 nmol/hr/mg protein, respectively. Early studies of motor activity in Fasciola hepatica showed that acetylcholine and cholinergic drugs decreased the rate and amplitude of muscle contractions and muscle tension (Chance and Mansour, 1949, 1953). These authors were first to suggest that an acetylcholine-like compound was a putative inhibitory neurotransmitter in this parasite (Chance and Mansour, 1953). Biochemical demonstration of the presence and formation of acetylcholine (Chance and Mansour, 1953) and histochemical demonstration of acetylcholinesterase activity in flukes (Halton, 1967; Probert and Durrani, 1977) supported this suggestion. Holmes and Fairweather (1984) also demonstrated that cholinomimetic drugs inhibited con

D. F. Mettrick

Papers

Ecological and Physiological Aspects of Helminth-Host Interactions in the Mammalian Gastrointestinal Canal

Parasite Behaviour: Understanding Platyhelminth Responses

Hymenolepis diminuta: Behavioral effects of 5-hydroxytryptamine, acetylcholine, histamine and somatostatin

The behavior of juvenile Fasciola hepatica.

Effects of cholinergic drugs on longitudinal muscle contractions of fasciola hepatica