time because of the economic consequences. Some countries insist that all persons entering their borders have documentation proving they have received certain vaccinations, such as for yellow fever. However, there are few anti‐parasite vaccines and even where effective prophylactic medicines are available to treat parasites, such as anti‐malarial drugs, it is notoriously difficult to persuade people to take them as prescribed.
Another of the major reasons why parasites remain a problem is the lack of suitable drugs and vaccines to treat them. The development of drugs for use in human medicine takes many years and is extremely expensive. Consequently, the drug companies need to be sure that they will obtain a good rate of return for their investments. See Chapter 14 for more information on the treatment of parasitic diseases. Unfortunately, those who suffer most severely from parasitic diseases are usually poor and cannot afford expensive drugs. Similarly, the development of anti‐parasite vaccines is hampered by a combination of cost and the difficulty of generating protective immunity against parasitic infections. These issues are dealt with in detail in Chapter 15.
The control of parasites by targeting their vectors/intermediate hosts is also becoming more problematic. For many years, this approach proved highly effective, and in the 1950s, it was even believed possible that malaria might be eradicated by killing the anopheline mosquito vectors. However, some vectors are exhibiting increasing resistance against a wide range of insecticides and new chemicals are not being developed to replace those in current use. Furthermore, there are mounting concerns for the environmental damage that can result from inappropriate use of insecticides and fears over risks they pose to our health.
2 Taxonomy
CONTENTS
2.2 Viruses: A Special (Unresolved) Case
2.1 Introduction
In this chapter, we will provide a very brief introduction to the study of taxonomy. Correct diagnosis is essential for treatment and control of any disease and that requires consensus on the names and terms used in the identification process. Without it, there cannot be effective communication between workers both within and between countries. For example, even within a country, a disease or organism may be known by various common names, and language differences further complicate communication. Therefore, before we begin to consider specific parasites, it is necessary to understand of how the taxonomic system works and its relevance to parasitology.
Those who study the identification of organisms are called taxonomists, and they arrange organisms into a hierarchy of categories to demonstrate their relationship to one another. Phylogeny is the study of the evolutionary relationships between organisms. This is increasingly informed by comparisons of gene sequences in a process called molecular phylogeny in which phylogenetic trees are generated to represent the closeness of relationships.
The Ancient Greek philosopher Heraclitus of Ephesus is accredited with the well‐known saying that ‘All is flux. Nothing stays still’. This is certainly true of taxonomy, and frequent name changes and taxonomic re‐arrangements will be a constant refrain throughout this book. One needs to be aware of these changes in order to compare past reports with those published more recently. For example, an organism might now be known under a different name or what was once described as a single species is now considered to consist of two or more distinct species with different biological characteristics.
Over the years, taxonomists have identified numerous organisms and grouped them together in many different arrangements. Primarily, this has been on the basis of their morphology, and this remains a major feature of taxonomy. Increasingly, morphological studies are complemented by molecular phylogeny, and this is having have a major impact on our understanding of animal relationships, confirming some groupings whilst questioning the validity of many others. However, molecular phylogenetics does not always provide clear evidence of the relationships between species. Consequently, there is often a lack of consistency between texts, and there are frequent rearrangements.
There is some debate about how many kingdoms exist although most modern textbooks refer to six: Archaea, Bacteria, Protista, Fungi, Plantae, Animalia. Parasitic species are common in all the kingdoms but traditionally, parasitologists deal almost exclusively with organisms belonging to the kingdoms Protista and Animalia. Although many prokaryotes (archaeans and bacteria) are parasitic, their study falls within a remit of microbiology. Similarly, parasitic fungi fall within the realm of mycology; parasitic plants are reserve of botany (although, these days, many practitioners prefer the title of plant scientist).
2.2 Viruses: A Special (Unresolved) Case
Viruses are not usually considered to be living entities and therefore do not have a kingdom of their own. This, however, is a hotly debated topic. For example, although Moreira and Lopez‐Garcia (2009) argue strongly against viruses being living entities, Koonin and Starokadomskyy (2016) consider the very question of whether viruses are alive to be unscientific because the definition of what one means by life is arbitrary. Didier Raoult and his co‐workers argue that the giant viruses called nucleocytoplasmic large DNA viruses (NCLDVs) should be considered as an additional distinct domain of living organisms (Boyer et al. 2010). NCLDVs are so large that they can be mistaken for bacteria, and their genomes are typically twice the size of other viruses. The suggestion that a specific group of viruses might be living organisms has generated a great deal of controversy for which no resolution is in sight. Nevertheless, phylogenetic analysis suggests that NCLDVs evolved before modern eukaryotes, that is, before the organisms that are their current hosts. Furthermore, they may have been the source of two DNA‐dependent RNA polymerases and a DNA topoisomerase that are found in modern eukaryotes (Guglielmini et al. 2019).
2.3 Taxonomic Hierarchy
Kingdoms are subdivided into units or taxa (singular taxon) such as class, family, genus (Table 2.1). There are no rules about how many species constitute a genus, how many orders constitute a class, or whether families are divided into subfamilies. However, it is essential that a ‘taxon’ forms a natural grouping. Consequently, research, especially molecular phylogeny, causes taxonomists to re‐arrange the hierarchy of individual species and groups of organisms on a regular basis. A class, family or any other category within one group of organisms is therefore
