Thieme 2003, 348 pp, numerous figures and tables. Tranlation.
Parasites can be found in all phyla and classes of organisms. Parasitism has developed polyphyletically. Arranging the various aspects of human parasitology according to the natural system of the parasites will impede the formulation of questions concerning its biology . Parasitology is a discipline of ecology, whereas parasitism is a special case of symbiosis. In heteroxenous, cyclically transmitted parasitoses warm-blooded and cold-blooded animals alternatively provide the specific habitat for a protozoon. In comparison to a monoxenic parasitosis with a single host what kind of advantages would such a lifestyle bring ? The chance of survival must be much better and the advantage of evolutionary selection considerable, otherwise such a mode of host alternation by a parasite would not have developed so often in proto- as well as in metazoan parasites.
Therefore we focus on the symbiotic lifeform the "clically transmitted parasite" and start with malaria This is not because the pathogen is a protozoan, but because of all heteroxenous parasitoses malaria is the most thoroughly investigated and generally best known. Onchocerciasis, which follows in the book, is in this respect the total opposite : the parasite is metazoan and its larvae do not circulate in the blood but live in the lymph of the skin tissue. In this case the vectors are also nematocerous Diptera; however, they breed in rapidly flowing water, are active during the day and orient themselves visually to find a mate and to some extent to find a host. We continue in this manner, avoiding any scheme in the selection of the topics and their succession.
Every biologist should ask questions such as: Why do the larval stages, the microfilariae, in some but not all filariae, accumulate in the lung capillaries where they will never reach a bloodsucking vector? Why do schistosomes waste a lot of their eggs by drift into the liver instead of being excreted with the faeces? Or, why does the amoeba which causes dysentery sometimes penetrate the host tissue, even though its transmission will thereby not be improved, yet the chance of survival is reduced?
For the biologist the relation of human parasitology to medicine is important but can be ambivalent. The hypothesis of attack and defence limits the perspective, being governed by wishful thinking concerning new therapeutic possibilities. On the other hand, parasites cannot be understood without basic knowledge of the diseases they cause. Also disease - in biological terms the reduction of fitness and reproduction - has an ecological function: without parasites any biocoenosis would maintain its balance. This has certain consequences for the relation of a parasite to its human host . In principle the biologist has to be aware of the possibilities and technical costs of diagnosis and therapy. The validity of the data received is crucial for obtaininginsights in epidemiology and determination of the effectiveness of pest control.
Ecology is a discipline of biology. A biologist has to regard parasites without prejudice, even on an individual level. Even though parasitology can be regarded as a supporting branch of medicine, one can obtain life-saving insights, but might miss its intrinsic nature.
Epidemic diseases which are transmitted by arthropods such as louse borne typhus, rocky mountain spotted fever, drug resistant scrub typhus, relapsing fever, plague and yellow fever have been included, because their epidemiology and control are essentially determined by the biology of their vectors. The occurrence of Chlamydia indicates the extreme importance of hygiene. Clean water not only prevents the omnipresent intestinal diseases, it is also indispensable for personal hygiene. There is a clear difference between these microbial or viral infectious diseases and the invasive diseases caused by proto- and metazoan parasites.
The chapters on entomology/arachnology and malacology repeate general zoological knowledge in relation to vectors and intermediate hosts. Flies as parasites of warm-blooded animals, the parasite - host interactions of intermediate snail hosts and their control are described in detail. We concentrate on the organs that are important for the development as well as for the epidemiology of the parasite. General zoological knowledge is not common today - especially among microbiologists. Furthermore we deal with the organ systems used for defence against pathogens. Descriptions of the dissection of a reduviid bug and an intermediate snail host are available on the internet.
The defence systems against cyclically transmitted parasites within either a warm-blooded or invertebrate host are analogous, not homologous: the immune system of vertebrates and the internal defence system (IDS) of invertebrates. In the case of the immune reaction, the ability to increase resistance is based on memory and not genetics in comparison to the internal defence system where the ability is inherited and not memorized. Descriptions of cyclically transmitted parasitoses should differentiate these two kinds of resistances by using different terminologies . A terminology without consistency is like a graph without coordinates; it is as useless as a ladder without rungs. Therefore the principles of the respective defence systems and their effects on the epidemiology had to be outlined.
In the chapter on The general biology of parasitism we try to generalize the findings discussed in detail earlier. The survival strategies of parasites are unique in the living world. We distinguish between alternative and balancing strategists, depending on whether the persistence of the parasite as well as that of its host is based on an equilibrium which will be achieved either after many generations or by a physiological balance achieved in their individual life-time.
The economic importance of parasitoses caused by eukaryotic pathogens involves the impact of the Disability Adjusted Life Year (DALY). Instead of the often cited death rate, prevalence and lethality of a parasitosis, it is the socioeconomical impact, which justify the costs of its control, the social burden caused and eventually the general reduction in fitness. A chapter on statistics was included because parasites have a negative binomial distribution within their hosts. Indeed data resulting from experiments with parasites can be evaluated statistically by using parameter-free methods without the knowledge of the distribution pattern of the characteristics. However, the respective parameters are subject to numerous feed-back mechanisms and the resulting functions are not linear. The conclusions of such experiments ,especially in respect to their future therapeutic potential, are usually overestimated; this actually occurred recently in the assessment of a vaccine against malaria. Generally, in textbooks of statistics, the negative binomial distribution is mentioned at best but not explained in further detail. Not every biologist is aware that the well-known binomial distribution is in fact an artefact of the laboratory: Nowhere in nature do two organisms of the same species differ in one characteristic genetic trait only, the prerequisite for a binomial distribution pattern. Such a situation cannot be created for parasites as long they live within their hosts, even under strict experimental conditions. The conditions are too complex and, as mentioned above, the interactions are not linear. Consequently the criteria for effectiveness during, for example, the development of an epidemiologically effective vaccine, must be defined in a new way.
Today all problems with medical relevance are attempted to be solved by molecular biological methods, e.g. the docking mechanisms of protozoan parasites on their host cells. In addition, parasites are being used as models for basic research. The antigenic variation of Trypanosoma brucei, for example, is particularly well suited for the analysis of gene expression. For the parasitologist the variation of surface antigens is seen as one of the numerous physiological survival strategies of a parasite. The molecular-biological differentiation of parasite strains is as important in the study of the epidemiology of a parasitosis as are the shifting interactions between immune cells via lymphokines in the study of the physiology of each individually surviving parasite. Particularly in diagnosis, molecular test kits replace time consuming microscopical approaches, which are, however, still absolutely necessary in endemic regions.
The layout of the book reflects that of our integrated lectures and microscopy courses given to students of biology for more than three decades, which are still well liked and attended. The complex pathogen-vector-definitive host is not split into protozoology, helminthology and entomology but considered as a ecological unit.
The following colleagues have helped us with advice: Prof. Dr. Wilhelm Becker (Hamburg), Dr. Dorrit Grobe (Basel), PD Dr. Jörg Grunewald (Tübingen), Prof. Dr. Winfried Haas (Erlangen), Prof. Dr. K. Lingelbach (Marburg), PD Dr. Rolf Lorenz (Tübingen), Prof. Dr. Peter Kimmig (Stuttgart), Dr. Jürgen Priemer (Berlin), Prof. Dr. Hartwig Schulz-Key (Tübingen) and Dr. Brigitte Walderich (Tübingen). The competent corrections by the microbiologist Dr. Willi Kuhn (Tübingen) are particularly appreciated .
With such a big project errors cannot be avoided. In view of the breadth of the subject it is hardly possible to avoid any mistakes. Letters with corrections will be gratefully received.
The figures were designed and produced by the first author, if not mentioned otherwise. Figures from other sources have been redrawn to a uniform standard and often modified to our requirements. All figures were transformed digitally by “epline Comp” , Kirchheim/Teck. We express our special thanks to all involved.
We also thank the publishing house Georg Thieme, especially Dr. Margitt Hauff-Tischendorf, head of the program planning biology, for their understanding and support which enabled us to finalize the project.
Peter Wenk and Alfons Renz, Tübingen 2003