Advances in Parasitology (eBook)

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2015 | 1. Auflage
182 Seiten
Elsevier Science (Verlag)
978-0-12-803334-0 (ISBN)

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First published in 1963, Advances in Parasitology contains comprehensive and up-to-date reviews in all areas of interest in contemporary parasitology. Advances in Parasitology includes medical studies of parasites of major influence, such as Plasmodium falciparum and trypanosomes. The series also contains reviews of more traditional areas, such as zoology, taxonomy, and life history, which shape current thinking and applications. The 2013 impact factor is 4.36 - Informs and updates on all the latest developments in the field - Contributions from leading authorities and industry experts

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2. Transport Hosts and Metacercariae: Some Basic Ecological Concepts

From a general assessment of the relevant scientific literature it is apparent that there is a degree of underestimation and confusion regarding the exact nature of the relationship between transport host and free-living metacercariae. It was therefore considered worthwhile to set out a few basic concepts in order to place these associations in the context of more traditional host–parasite interactions. Although the definition of transport hosts as a theoretical model has been much debated (Sprent, 1963; Odening, 1976; Macko, 1980), many of the fundamental practical aspects of the ecological relationship between free-living metacercariae and chosen transport host remain sadly neglected, simply because they interact on a much more limited basis compared to the intricate ecophysiological relationships present in more standard host–parasite associations. Nevertheless, the vast amount of literature on epibiosis provides a relevant basic conceptual framework (e.g. Wahl (1989)), for evaluating likely key aspects of their interactions. However, this can only represent potential scenarios and should not be interpreted as indicating any degree of definitive evidence for these kinds of interactions between metacercariae and transport hosts.

To begin with it will be useful to address weaknesses of a couple of common basic terms used in relation to free-living metacercarial ecology. Firstly, transport hosts are often in the scientific literature simply referred to as ‘substrate’. Although this is technically correct, it does give the false impression that the host has the same level of interactions with the metacercariae as an inert surface would. Evidence from epibiosis studies would suggest that this is unlikely to be the case and therefore ‘transport host’ may be a better general term, with ‘substrate’ confined to defining the exact location on the host where encystment takes place. Secondly, many studies describe species as encysting ‘on any hard surface’ (e.g. Yamaguti (1975)). Such statements are unhelpful as observations usually took place under artificial laboratory conditions in glass containers or on glass slides, presenting the cercariae with an overload of cues from hard surfaces, potentially masking other settlement prompts. It seems unlikely that under natural conditions cercariae would adopt such indiscriminate behaviour, particularly where many habitats, such as coastal rocky shores, have bed rock which represents an abundance of hard surfaces that cannot possibly aid the transport of the parasite to the target host. Use of this term should therefore be avoided.

Both metacercariae and transport host will potentially benefit or be hindered by their relationship. For metacercariae, determining the most optimum form of transport host must be a key objective. In theory, because the transmission of metacercariae is dependent on trophic interactions any organism that regularly forms the diet of the target definitive host may have the potential to act as a transport host. Laboratory studies have generally demonstrated that many species of free-living metacercariae will encyst on a wide range of organisms. However, field studies are rare and it therefore remains largely unknown if such experimental host choices would be replicated under natural conditions, although the available evidence would suggest that some differences occur (see Section 4.5.1). Host choice is likely to be determined by a number of factors. Settlement by metacercariae will probably occur in hydrodynamically favourable sites, either within habitats or upon individual transport hosts, presumably where turbulence is reduced and potential transport hosts remain visible to predators/grazers.

A significant problem for free-living metacercariae is the unstable character of the living substratum of the transport host, with both natural mortality and physical disturbance of the transport host potentially interfering with the parasites transmission success. Further instability is caused by morphological changes of the transport host during its life cycle. In a similar manner to those changes encountered by epibionts (Wahl, 1989), these variations can include host growth and shrinking, fission and fusion, shedding of fruiting bodies and leaves, and moulting. The longer a metacercariae remains attached to a transport host the greater the risk morphological changes may pose to the parasites viability.

Similarly, fluctuations in the physiological activity of the transport host may also induce instability. Production and exudation of metabolites such as waste, nutrients and toxins may change both temporally with season, predation pressure, developmental stage and biological cycles and spatially according to habitat and different locations on the same host (Wahl, 1989). Although metacercarial cysts are considered to show a great tolerance to a range of abiotic environmental conditions (see Section 6.1), their responses to biotic conditions associated with the transport host remain unknown and their viability may be dependent on phases when, or on host sites, where the composition and quantities of exudate are not considered harmful.

A final risk factor for metacercariae is the dangers of physiological stress due to potential drastic environmental changes associated with the transport host migrations or shifts in habitat stability. For example, transport hosts in marine intertidal zones will experience daily extremes in temperature, salinity and humidity, while the growth of plant transport hosts may shift metacercariae encysted upon them from favourable to unfavourable climatic conditions over time.

For the transport hosts themselves colonization by free-living metacercariae may indirectly affect their own ecological and physiological homeostasis, although such changes are likely to depend on the hosts relative size compared to the parasite, smaller hosts being more vulnerable, and the intensity of the metacercarial population. A covering of metacercariae may potentially have a protective role, possibly slowing down the rate of desiccation in air-exposed hosts or by providing a ‘camouflage’ through their own surface properties that may interfere with the cues used by nontarget host predators to identify prey.

More often, free-living metacercariae will likely have detrimental effects on the transport host. Colonization by parasites may cause an increase in weight and consequently reduce buoyancy, while rigid cysts may reduce the elasticity of the settlement site on the host hindering motion and flexibility, thereby increasing the likelihood of breakage in highly turbulent environments. Metacercariae may cause shading of plant surfaces, reducing photosynthesis, which under particularly heavy parasite burdens may induce a negative energy budget resulting in the ultimate mortality of the host. Similarly, transcutaneous uptake or excretion of nutrient salts, dissolved organic matter, ions and/or gases by transport hosts may be affected, although any impact will be dependent on the specific organism's biology and the settlement location of the parasite.

It is clear that free-living metacercariae have the potential to be unfavourable for the transport hosts ecology and functional biology. Many organisms, in order to prevent colonization by epibionts have developed a number of strategies to protect themselves (Wahl, 1989; Wahl et al., 1998; Krug, 2006), which may also protect against free-living metacercariae. Three classes of adaptations are considered to have evolved: tolerance, avoidance and defence (Wahl, 1989). Many species, particularly sedentary ones, which secrete mineral outer shells are capable of tolerating extensive colonization of these surfaces. Nevertheless, an important prerequisite for this tolerance is a prevailing indifference to increased friction and weight and a physiological inactive outer surface. However, even in heavily colonized individuals the body's orifices (shell borders, tubes and siphons) and external sense organs are maintained clean (Wahl, 1989).

Avoidance of colonization may include movement in space, time or dissimulation. Accelerated growth or reproduction may produce tissue or offspring at a rate higher than the colonization rate; this is particularly relevant for plant growth which may allow for the continued maintenance of a photosythetically active zone (e.g. Sand-Jensen (1977) and Bultlauistand Woelkerling (1983)). Similarly, the migrations of species into biologically less vulnerable habitats or optical and chemical camouflage may also be considered avoidance.

Defensive mechanisms can include mechanical, physical and chemical elements. Mechanical aspects that may impede colonization include special surface structures such as spicules, intense surface production of mucus, periodic shedding of the cuticula or epidermis, scale-casting, friction between the body surface of burrowing or fast-swimming species and the surrounding sediment or water, and the active removal of colonizers by the scraping of the surface with specialized appendages (Dyryndd, 1986; Wahl, 1989; Wahl et al., 1998). Nevertheless, the efficiency of such mechanical mechanisms depends on many factors such as the proportion of the surface cleaned at any given time, the size and intensity of colonies, and the regularity of surface disturbance by the hosts' mechanical defences. The properties of the physical surface of transport hosts, in particular low-energy surfaces where there is a minimum of free ions in the...

Produkt-Details

• Erscheint lt. Verlag: 2.7.2015
• Mitarbeit: Herausgeber (Serie): David Rollinson, Russell Stothard
• Sprache: englisch
• Themenwelt: Medizin / Pharmazie ► Gesundheitsfachberufe
• Themenwelt: Medizin / Pharmazie ► Medizinische Fachgebiete ► Mikrobiologie / Infektologie / Reisemedizin
• Themenwelt: Naturwissenschaften ► Biologie ► Mikrobiologie / Immunologie
• Themenwelt: Technik
• ISBN-10: 0-12-803334-7 / 0128033347
• ISBN-13: 978-0-12-803334-0 / 9780128033340