Parasitology

An Integrated Approach

Alan Gunn, Sarah J. Pitt

• Publisher: Wiley
• Year: 2012
• ISBN:
  9781119945086
• Edition: 1
• Topic: Medicine
• Subtopic: Medical Microbiology & Parasitology

Parasitology: An Integrated Approach, provides a concise, student-friendly account of parasites and parasite relationships that is supported by case studies and suggestions for student projects. The book focuses strongly on parasite interactions with other pathogens and in particular parasite-HIV interactions, as well as looking at how host behaviour contributes to the spread of infections. There is a consideration of the positive aspects of parasite infections, how humans have used parasites for their own advantage and also how parasite infections affect the welfare of captive and domestic animals. The emphasis of Parasitology is on recent research throughout and each chapter ends with a brief discussion of future developments. This text is not simply an updated version of typical parastitology books but takes an integrated approach and explains how the study of parasites requires an understanding of a wide range of other topics from molecular biology and immunology to the interactions of parasites with both their hosts and other pathogens.

1

Animal associations

1.1 Introduction

In this introductory chapter we will introduce the concept of parasitism as a lifestyle and explain why it is such a difficult term to define. We shall also introduce some of the terms that are commonly used by parasitologists. Like all branches of science, parasitology has a number of associated specialist terms such as ‘intermediate host’, ‘definitive host’ and ‘zoonosis’ that need to be understood before it is possible to make sense of the literature. We will explain why the study of parasites is so important and why parasites are likely to remain a problem for many decades to come. We will end by introducing the study of taxonomy because this will help inform the chapters on specific groups of organisms as well as the chapters on diagnosis, treatment, and control. Taxonomy is nowadays something of a Cinderella subject among biologists but it cannot be ignored because scientists must agree on the names things are to be called if they are to communicate with one another.

1.2 Animal associations

All animals are in constant interaction with other organisms. These interactions can be divided into two basic types: intra-specific interactions and inter-specific interactions.

Intra-specific interactions are those that occur between organisms of the same species. They range between relatively loose associations such as those between members of a flock of sheep, to highly complex interactions such as those seen in colonial invertebrates (e.g. Bryozoans and some of the Cnidaria (jellyfish and sea anemones)). For example, the adult (medusa) stage of certain jellyfish may appear to be a single organism but it is actually composed of colonies of genetically identical but polymorphic individuals. These colonies divide labour between themselves in a similar manner to that of organ systems within a non-colonial organism, for example, some colonies are specialised for reproduction while others are specialised for feeding.

Inter-specific interactions are those that take place between different species of organism (Figure 1.1). As with intra-specific interactions, the degree of association can vary between being extremely loose to highly complex. Odum (1959) classified these interactions on the basis of their effect on population growth using the codes ‘+’ = positive effect, ‘−’ = negative effect, and ‘0’ = no effect. This leads to six possible combinations (00, 0−, 0+, etc.) and these too can be broken down into further subdivisions (Toft et al., 1993). Some authors also include a consideration of the direction and extent of any physiological and biochemical interactions between the two organisms. A wide range of terms have been suggested in an attempt to compartmentalise these interactions (e.g. phoresis, mutualism, predation) but these are merely convenient tags and they cannot be defined absolutely. This is because the variety of organism interactions is extremely broad and even within a single interaction there are a host of variables such as the relative health of the two organisms that determine the consequences of the interaction for them both. It is therefore not surprising that there is a multiplicity of definitions in the scientific literature and it is not unusual for two authors to arrive at two different terms for the same type of interaction between species. In this section, we will discuss symbiosis, commensalism, phoresis, mutualism and finally parasitism, with some examples of each.

Figure 1.1 Different species will occasionally co-operate for mutual benefit

1.2.1 Symbiosis

The term symbiosis is usually translated as ‘living together’ and is derived from the Greek syn meaning ‘with’ and biosis meaning ‘life’. It was originally used in 1879 by Heinrich Anton de Barry to define a relationship of ‘any two organisms living in close association, commonly one living in or on the body of the other’. According to this original definition, symbiosis covers an extremely wide range of relationships. Some authors state that both organisms in a symbiotic relationship benefit from the association (i.e. it is [++]) although this is clearly a much more restrictive definition and it is more appropriately referred to as mutualism. However, some authors state that symbiosis and mutualism are synonymous – this only adds to the confusion. For the purposes of this book we will keep to de Barry's original definition.

Symbionts

Strictly speaking, a ‘symbiont’ is any organism involved in a symbiotic relationship. However, the vast majority of scientists tend to restrict the term to an organism that lives within or upon another organism and provides it with some form of benefit – usually nutritional. The association is therefore referred to as a host: symbiont relationship and the majority of symbionts are microorganisms such as bacteria, algae or protozoa. Where the symbiont occurs within the body of its host, it is referred to as an endosymbiont, while those attached to the outside are referred to as ectosymbionts. Two types of endosymbiont are recognised: primary endosymbionts (or ‘p-endosymbionts’) and secondary endosymbionts. Primary endosymbionts form obligate relationships with their host and are the product of many millions of years of co-evolution. They are usually contained within specialised cells and are transferred vertically from mother to offspring. As a consequence, they undergo co-speciation with their host and form very close host-specific relationships. By contrast, secondary endosymbionts are thought to be the product of more recent host: symbiont associations and, in the case of insects, the symbionts are contained within the haemolymph (blood) rather than specialised cells or organs. Secondary endosymbionts tend to be transmitted horizontally and therefore do not show the same close host: symbiont relationship. It is not known how endosymbionts begin their association with their hosts but some authors suggest that they arise from pathogens that attenuated over time. The suggestion that a parasite–host relationship tends to start off acrimoniously and then mellow with time was once widespread in the literature, but while this may sometimes occur, it is not a foregone conclusion.

The importance of symbionts to blood-feeding organisms

Although blood contains proteins, sugars and lipids as well as a variety of micronutrients and minerals, it lacks the complete range of substances most organisms require to sustain life and to reproduce. Consequently, many of the animals which derive most or all of their nutrition from feeding on blood (haematophagy) have evolved symbiotic relationships with a variety of bacteria that provide the missing substances, such as the B group of vitamins. The need for supplementary nutrients is particularly acute in blood-sucking lice (sub-order Anoplura) because they have lost the ability to lyse (break up) red blood cells and therefore many nutrients will remain locked within these cells. In many cases, the bacteria are held within special cells called mycetocytes that are grouped together to form an organ called a mycetome. Although these terms appear to indicate the involvement of fungi, they originate from a time when scientists did not distinguish between the presence of yeasts and bacteria within cells. Many scientists continue to use the term ‘mycetocyte’ regardless of the nature of the symbiont but others use the term ‘bacteriocyte’ where it is known that the cells harbour only bacteria. In blood-feeding leeches belonging to the order Rhynchobdellida (there is a popular misconception that all leeches feed on blood; many of them are actually predatory), mycetomes are found surrounding or connected to the oesophagus. Mycetomes are not found in all blood-feeding leeches and in the medicinal leech, Hirudo medicinalis, the symbiotic bacteria are found within the lumen of the gut (Graf et al., 2006). The bacteria present in Hirudo medicinalis have been identified as Aeromonas veronii (earlier work on leeches often refers to it as Aeromonas hydrophila), a species of bacteria that has been associated with a number of other blood-feeding organisms. Aeromonas veronii has also been reported as causing wound infections in humans and inducing septicaemia and gastroenteritis. (Graf, 1999). Leeches are extremely useful in modern medicine, particularly to aid wound drainage following plastic surgery, but one of the risks associated with their application is that the patient acquires an Aeromonas infection. These infections are often trivial but they can become serious and lead to the formation of an abscess or cellulitis (e.g. Snower et al., 1989). This is a difficult problem to solve because the symbiotic bacteria are essential for the leeches.

Box 1.1 The role of symbionts in the life of tsetse flies and their transmission of trypanosome parasites

Tsetse flies, like most other blood-feeding organisms, harbour bacterial symbionts that facilitate the breakdown of the blood meal and provide essential nutrients to the fly. In the case of tsetse flies, these are principally B group vitamins, vitamin H (Biotin), folic acid and pantothenic acid and in the absence of the symbionts, the adult female fly is unable to reproduce. Tsetse flies have at least three different symbionts that are found within certain gut epithelial cells and these are passed on from the female fly to her larvae as they develop in her uterus. Of these symbionts, Sodalis glossinidius is thought to be the most important in influencing the establishment of trypanosomes in the tsetse fly. Tsetse flies have an effective immune system that protects them from invading micro-organisms. This includes the production of lectins that attach to and kill the invading organisms and toxic reactive oxygen species such as superoxide and hydrogen radicals (Macleod et al., 2007). However, Sodalis glossinidius releases N-acetylglucosamine which interferes with the activity of the lectins and scavenges reactive oxygen species, thereby allowing the trypanosomes to establish. It is possible that there are differences between strains of Sodalis glossinidius in the production of N-acetylglucosamine and this may be (to a greater or lesser extent) the reason why there are differences in the susceptibility of tsetse flies to infection with trypanosomes.

In nymphs and adult males of the human body louse, (Pediculus humanus; sub-order Anoplura) intracellular symbionts are found within a mycetome that is sometimes referred to as the ‘stomach disc’. This mycetome is located on the ventral side of the mid-gut but unlike the leeches mentioned above, there is no actual connection between the mycetome and the lumen of the gut (Sasaki-Fukatsu et al., 2006; Perotti et al., 2008). In adult female lice, the bacteria re-locate to the oviducts and the developing eggs. This is in keeping with the observation that primary endosymbionts are transmitted within the eggs (i.e. transovarially) to the offspring. The bacteria associated with Pediculus humanus have been identified as belonging to the gamma (γ) proteobacteria and have been given the name Riesia pediculicola. Interestingly, molecular phylogenetic analysis is unable to distinguish between the symbiotic bacteria isolated from human head lice (Pediculus humanus capitis) and human body lice (Pediculus humanus humanus). This adds support to phylogenetic analysis of the lice themselves (Light et al., 2008) that indicates that although head lice and body lice occupy different ecological niches and body lice tend to lay their eggs on clothing while head lice attach their eggs to hair shafts, they are two morphotypes of the same species rather than two separate species. One suggestion is that the body lice evolved from head lice relatively recently in human evolution following the common practice of wearing clothing. The association between Riesia and Pediculus is estimated to be between 12.95 and 25 million years old, which makes it one of the youngest host: primary endosymbiont relationships so far recorded (Allen et al., 2009). In common with other primary endosymbionts, Riesia has undergone a reduction in genome complexity and lost genes: this is because it has come to rely on its host for the provision of many nutrients, protection from the environment and protection from predators. In addition, because its transmission is via the eggs of its host, each louse symbiont population is in reproductive isolation and unable to undergo recombination with other strains of Riesia in other lice. This has led to the suggestion that Riesia will lack the capacity to develop rapid resistance mechanisms to antibiotics, and because the Riesia is essential for the lice, killing the symbiont would result in host mortality (Perotti et al., 2008).

1.2.2 Commensalism

The term ‘commensalism’ is derived from the Latin commensalis and means ‘at the same table together’. Most definitions indicate that one species benefits from the association and the other is unharmed (0+). The concept of ‘harm’ within any definition leads to complications because it may be difficult to measure and depends upon the circumstances. Similarly, a ‘benefit’ may not be immediately apparent and it is possible that some of t...