Parasitology: Teaching notes and resources main page
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Below is a parasitology lecture course covering the major parasitic groups, parasitic protozoa, monogeneans, digeneans, cestodes, nematodes, acanthocephalans, and parasitic arthropods, including life cycles, parasite biochemistry, host specificity, molecular parasitology and parasites and behaviour. Full details can be found in the contents sections below.
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Please note that although these pages can be accessed by non-students of Aberystwyth University this parasitology lecture course and its contents remain the copyright of Professor J. Barrett, Professor P.M Brophy, Dr J. Hamilton, Dr R. Morphew and Professor C. Thomas. No part of the lectures may be used for anything other than the personal and private use of an individual without prior permission of Professor J. Barrett (email@example.com).
Contents: Lecture Numbers and Titles.
Contents: Additional Information.
Parasitism is a very successful way of life, all of the major groups of animals have parasitic members and 50% of all known animal species are parasitic at some stage of their life cycle. No one definition of parasitism is totally satisfactory, but that of Crofton is a good starting point.
This definition of parasitism emphasises the ecological nature of the association. Parasites are usually smaller and more numerous than their hosts and there may be some degree of genetic complementation.
The parasite niche is defined by the host resources (metabolites, space, homeostatic mechanisms) exploited by the parasite, moreover by affecting the host parasites can modify their environment. Most ecologists are agreed that the majority of parasite niches are still unfilled.
The host influences the evolution of its parasites, conversely parasites influence the evolution of their hosts in terms of both macro and micro-evolution.
There are over 50,000 species of protozoa, of which a fifth are parasitic. They infect vertebrates and invertebrates and in some cases plants. Parasitic protozoa are in general small, have short generation times, high rates of reproduction and a tendency to induce immunity to reinfection in those hosts that survive. Structurally a protozoan is equivalent to a single eukaryotic cell, although many species contain more than one nucleus for all or part of their life cycle. The success of protozoa is the result of their remarkable development of organelles, which perform the same function as organs in metazoa.
The protozoa that infect man range from forms which are never pathogenic to those that cause malaria, sleeping sickness, Chaga's disease and leishmaniasis, all now regarded as being among the major diseases of tropical countries. Other less serious protozoal diseases in man are amoebiasis, giardiasis and toxoplasmosis. In domestic animals, nagana, babesiosis and theileriosis take a major toll of cattle in Africa, whilst coccidiosis, in its various forms, presents a continuing threat to poultry and cattle, particularly under conditions of intensive rearing. Even fish and invertebrates suffer from a variety of protozoan infections which create major problems in fish farming and oyster culture.
Adult cestodes are all endoparasitic in the intestines of vertebrates. Physiologically they are an interesting group since they have no gut at any stage of their life cycle. The outer syncitial tegument of the adult tapeworm is a naked cytoplasmic layer covered with microvilli.. Cestodes produce no digestive enzymes of their own and instead rely on their hosts' enzymes. Tapeworms have evolved a whole repertoire of uptake mechanisms in their teguments which are able to compete successfully with those of their hosts' mucosa for the low molecular weight nutrients released during digestion.
Tapeworms have a typical platyhelminth excretory system based on flame cells and collecting ducts and a simple nervous system that, pharmacologically, is very different from that of vertebrates. The majority of tapeworms are protandrous hermaphrodites and, in many cases, self-fertilization is the norm. Asexual reproduction in the life cycle is rare.
Digenetic trematodes are among the most common and abundant of the helminth parasites. The adults are parasitic in all classes of vertebrates, especially marine fish, and they inhabit almost every organ of the vertebrate body. Their development requires at least two hosts. The first a mollusc (or rarely an annelid), the second a vertebrate. Many species include a second or even a third intermediate host between the mollusc and the vertebrate. Second or third intermediate hosts can be vertebrates or invertebrates. In a typical life cycle, the egg (passed out in urine, faeces or sputum) hatches to give a ciliated miracidium which invades the snail to give a sporocyst. Within the sporocyst a number of embryos develop asexually to become rediae. The redia is more differentiated than the sporocyst and has a pharynx and gut. Additional embryos develop within the rediae and these become cercariae. The cercariae, which usually have tails, emerge from the snail to infect the final host. Variations occur in the number of generations of daughter sporocysts or daughter rediae produced before the cercariae are formed, whether there is a redial generation at all (it is absent in several groups) and whether the cercaria needs to develop further as a metacercaria before it can become infective.
Of the 6000 or so known species of digeneans only about a dozen are important parasites of man. Of these probably the most important are the Schistosomes, of which there are four major species infecting some 200 million people in 75 countries.
The Monogeneans are Platyhelminths and they live as ecto-or mesoparasites on marine or freshwater fish, with a few species occurring on amphibians and reptiles. Their main feature is a complex opisthohaptor. Like Cestodes and Digeneans, Monogeneans have a syncitial tegument with scattered microvilli.
Monogeneans are hermaphrodite with a typical platyhelminth reproductive system. They all have direct life cycles with a free-living, ciliated larva called an onchomiracidium and tend to show a high degree of host specificity. Unlike most parasites, Monogeneans produce a relatively small number of large eggs.
The Acanthocephala are a small, but unusual group of parasites. The Acanthocephala have no gut at any stage of their life cycle, they have a complex, syncitial body wall, a retractible proboscis for attachment and a pair of glandular lemnisci. Sexes are separate and the structure of the reproductive organs is unique. Acanthocephalans are all parasitic in the guts of vertebrates. Their life cycles all require an invertebrate intermediate host and paratenic hosts are a frequent feature.
Monogeneans and Acanthocephalans can cause outbreaks of disease in fish farms.
Nematodes are major parasites of plants and animals, although the vast majority of them are free-living. The nematodes are characterized by a stylized body plan and a stylized life cycle. The animal parasitic nematodes show little morphological adaptation to parasitism, other than a general increase in size. Plant parasitic nematodes frequently have an eversible stylet that they use to puncture cells during feeding and are major vectors of plant viruses. Sexes are generally separate in the nematodes, although hermaphrodites and parthenogenic females do occur. In most nematodes sex is determined genetically, but in many plant and insect parasitic nematodes sex is determined environmentally. The life cycle of nematodes always includes four moults and four larval stages and this is retained, even in the most complex, multi-host life cycles. In contrast to their rigid morphological and developmental organization, nematodes are biochemically and physiologically highly adaptable.
The Phylum Arthropoda is enormous. At least three quarters of a million species have been described - more than three times the number of all other animal species combined. It is not surprising, therefore, that many species are parasites. Arthropods cause injury or discomfort through annoyance, blood loss, dermatitis, envenomization and allergy. They are also important vectors of parasitic disease. Insects are the intermediate hosts for some of the most important tropical diseases infecting man and his domestic livestock (e.g. malaria, trypanosomiasis, leishmaniasis and filariasis). Huge tracts of land are rendered unsuitable for agriculture because of the diseases insects carry. Disease transmission can occur via the arthropods mouthparts or through food or skin contamination by the arthropod or its discharges.
Many parasites appear not to elicit or not to be affected by their hosts' immune response. Any parasite which can survive in its mammalian host for appreciably more than nine days must be assumed to have some mechanism for avoiding or mitigating their hosts' immune response. The evasion strategies can include: surface absorption of host antigen, molecular mimicry, loss or masking of surface antigens, antigenic variation, the occupation of immunologically incompetent sites and immunosuppression. There is no evidence for specific immunological tolerance.
Invertebrates have innate, but not acquired immune responses, invertebrates have no 'immunological memory'. However, the evasion strategies used by parasites of invertebrates show many parallels with those used by the parasites of mammals.
There is a complex interaction between parasites and their hosts' immune system and parasites may provide unique systems in which to study immune responses.
Studies on parasite ecology are concerned with patterns in the distribution of parasites and the mechanisms responsible for generating those patterns. Regulatory mechanisms may be density independent, e.g. temperature or density dependent, e.g. host immune response. Autoecology deals with the individual species and so is concerned with populations. Synecology deals with groups of species and so is concerned with communities.
Mathematical models of parasite populations can be used to predict population behaviour over time. Such models can be used to give warnings of epidemic situations, to evaluate control methods and predict the likely results of man made environmental changes. Models are also increasingly employed to carry out cost/effectiveness and cost/benefit analyses.
Parasites show a wide range of life cycles. Life cycle studies are important in designing control programmes, they can also give clues to evolutionary relationships. Parasite transmission can be horizontal or vertical and may or may not involve multiplication in the final host. Life cycles may also be direct (no intermediate host) or indirect (one or more intermediate hosts).
Natural selection will tend to favour maximum fecundity and maximum survival. However, there must be a trade off between these two strategies, since resources invested in one are not available for the other. The solution to the trade off will depend on other environmental factors. K-strategists put maximum investment into survival, whilst r-strategists put maximum investment into fecundity. On balance, parasites are r-strategists exploiting a variable environment in which there is only limited inter-and intraspecific competition. In general parasites are short lived with type III survivorship curves and have a high reproductive rate and variable population size, characteristics all typical of r-strategists.
During the life cycle, intermediate hosts act as temporal residual components, extending the range of the parasite in both space and time. Complex life cycles are built up by the process of host capture and in many cases the intermediate host appears to have been the original one.
Why should we study the behaviour of the organisms that are infected, or are at risk of infection, from parasites? There are many reasons. For example, changes associated with infection may profoundly alter predator-prey interactions or skew mating success in natural ecosystems, whilst a knowledge of how host behaviours affect parasite transmission could lead to the development of novel husbandry strategies that maximise animal welfare.
We shall examine how the behaviour of animals may have significant consequences for their susceptibility to, and for their avoidance of, parasite infections. In particular we will consider how parasites have evolved to exploit natural patterns of host behaviour � such as sexual contact, feeding behaviour and habitat use � for transmission, and conversely how behavioural strategies exhibited by hosts may offer some protection against infectious parasites. In addition we shall cover the role of complex, highly evolved social behaviours such as the use of cleaning stations by reef fish in regulating parasite loads in natural ecosystems.
We shall turn our attention to the effects parasites have on the behaviour of infected hosts, and try to understand �why� such changes occur by examining causal, developmental, functional and evolutionary aspects of behaviour change. Detailed examples will be used to illustrate the many types of changes in the behaviour of parasitised animals that have been reported. Such behaviour changes are frequently attributed to �adaptive� (i.e. evolutionarily beneficial) manipulation of the host by parasites, which in turn may potentially benefit from the behaviour change, for example by maximising the chance of being transmitted to other hosts. However, there are alternative explanations, and carefully designed experiments need to be undertaken to demonstrate adaptive manipulation unambiguously. We will end by examining the current state of knowledge regarding the evolutionary basis and ecological consequences of infection-associated changes in the behaviour of host organisms.
Precise identification of the cause of a parasite infection is essential for successful treatment, for understanding the epidemiology of the parasite and in the implementation of control measures. To date, none of the morphological based laboratory techniques, which rely on finding the causative agent are entirely satisfactory, and new molecule based approaches are gradually being developed as alternatives. However, there are advantages and disadvantages in using biochemical (enzyme activity assays and isoenzyme patterns ), molecular biological (DNA probes and PCR) and serological approaches (ELISA and agglutination tests) to identify protozoan and helminth infections.
Vaccination is viewed as an essential part of integrated infection control since it has many advantages over chemotherapy. There are successful vaccination programmes against viral and bacterial diseases, but unfortunately, progress towards parasite vaccines has been slow. This limited progression is due in part to the multifactorial immune response to parasite infection, parasite immune avoidance strategies and the complex nature of parasite antigens. However, in recent years molecular biology research has identified and produced new protective recombinant antigens for vaccine trials. DNA based vaccination has also been successful in trials.
Chemotherapy is the most cost-effective way of treating parasitic diseases. However, the discovery of new anthelmintics still relies heavily on a trial and error approach and with the increasing problems of drug resistance there is a need to develop new compounds whose mode of action is different from the drugs already in use. The need for new drugs is particularly acute with malaria.
Central to the whole concept of chemotherapy is selectivity; the parasite is affected, but the host is not. Selectivity can result from a variety of mechanisms, involving both host and parasite, including differential distribution, detoxification and activation of the drug, differential binding to receptors, the presence of unique receptors and differences in the relative importance of different biochemical pathways between host and parasite.
Drugs rarely remove 100% of the parasites from the host. The development of drug resistance is the outcome of the tremendous selection pressure exerted on parasite populations by drug treatment and can be due either to a single resistance gene or it may be polygenetic.
In 1947 Norman Stoll published estimates of the number of human infections with different parasites. Since then the world population has increased from 2.2 x 109 to 5.7 x 109, most of this increase being in poor countries where the risk of infection is greatest. Even with those parasites where prevalence has decreased, the population increase has meant that the number of individual cases has continued to increase. The spread of AIDS has also led to an increase in opportunistic infections.
The range of methods used to control parasites can be grouped under i) prevention of environmental contamination ii) destruction of free-living stages iii) destruction of intermediate hosts iv) destruction of reservoir hosts v) prevention of infection and vi) prevention of parasite maturation. It as been found that an integrated approach to parasite control, where a number of different tactics are used, is normally more effective than using a single method on its own. Control methods must be based on a thorough knowledge of the ecology and life cycle of the parasite concerned, protozoa for example which normally multiply in their definitive hosts require different tactics from helminths which, in general, do not.