Trophozoite

3 Key excerpts on "Trophozoite"

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  eBook - ePub

  Principles of Veterinary Parasitology

  • Dennis Jacobs, Mark Fox, Lynda Gibbons, Carlos Hermosilla(Authors)
  • 2015(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  Figure 4.5 Schizont: SEM (with artificial colouring) showing merozoites (pink) within host cells (blue). Reproduced with permission of D.J. Ferguson.

  4.2.3 Nutrition

  Protozoa feed mainly on particulate material. The cell membrane indents and folds slowly over, thereby entrapping a small quantity of food and drawing it into the cell. This process is known as pinocytosis or phagocytosis, depending on the size of the particle (in ascending order). In ciliates, food particles are directed by the action of cilia towards the base of a funnel-like structure (the ‘cytostome’). When sufficient food has accumulated in this, a vacuole forms which is engulfed into the cytoplasm. Many parasitic protozoa are also able to absorb liquid nutrients and in some cases this may be their main source of nourishment.

  The feeding stage in a protozoan’s life-cycle is called a ‘Trophozoite’. For many species this is the only form that exists, but others have a succession of stages with different functions, appearances and, of course, names.

  4.2.4 Transmission

  Trophozoites adapted to a parasitic existence are often vulnerable to unfavourable conditions in the external environment. Those of many species, therefore, secrete a protective wall around themselves to form a resistant ‘cyst’ prior to leaving their host.

  Passage from host to host can take place in many ways, ranging from passive transfer of Trophozoites or cysts (by faecal-oral transfer, for example) to transmission of various life-cycle stages by arthropod vectors or the ingestion by carnivores of organisms within intermediate hosts.

  Extra information box 4.1

  Explanation of life-cycle jargon

  Protozoan life-cycles can be direct (i.e. completed without an intermediate host) or indirect (with an essential intermediate host) and, for simplicity, this is the terminology used in this book. Other terms may be found elsewhere:

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  eBook - ePub

  The Biology of Parasites

  • Richard Lucius, Brigitte Loos-Frank, Richard P. Lane, Robert Poulin, Craig Roberts, Richard K. Grencis, Ron Shankland, Renate FitzRoy(Authors)
  • 2017(Publication Date)
  • Wiley-VCH

    (Publisher)

  Toxoplasma gondii. (a) Sporozoite. (b, c) Tachyzoites in macrophage. (d) Bradyzoites in tissue cysts. (e) Bradyzoite. (f) Infection of intermediate hosts with bradyzoites from tissue cysts. (g) Trophozoite in intestinal epithelial cell. (h) Schizont. (i, j) Merozoites. (k) Macrogametocyte. (l) Macrogamete. (m) Microgametocyte. (n) Microgamete. (o) Zygote. (p) Sporont inside oocyst. (q) Sporulated oocyst with two sporocysts, each containing four sporozoites (infectious for cat or intermediate host). (r) Sporozoite, infectious for cat.

  Intermediate hosts can be infected by either sporozoites or bradyzoites. The parasites invade subepithelial cells, where an asexual multiplication by endodyogeny follows. This is a special type of schizogony (see Figure 2.41 ), which proceeds rapidly in diverse types of cells. As the divisions during endodyogeny can occur as rapidly as every 5–9 h, the stages are known as “tachyzoites” (Greek: tachys = quick). The number of tachyzoite generations is not genetically fixed. In this phase of rapid multiplication, the parasites are disseminated via the blood and are able to actively penetrate the barriers of tissues and permeate, among others, the placenta, pass into the offspring and multiply in them. This allows vertical transmission, which may be a relevant mode of sustained transmission in rodents and is certainly of great clinical importance in humans.

  After some days, the multiplication of tachyzoites in the infected host slows down as a result of a strong host imune response. Now bradyzoites develop (Greek: bradys = slow), which divide only slowly within their host cells. At the same time, the wall of the parasitophorous vacuole is remodeled to form a 2-µm thick cyst wall. The new tissue cysts can occur in all organs, but are concentrated in the brain and skeletal or heart muscle. Recently, it has been shown that T. gondii tachyzoites can infect endothelial cells, which enables them to cross the blood brain barriere and establish chronic infections. These stages can persist for years, probably in neurons, and are infectious for the cat as definitive host. They are also infective for intermediate hosts, if ingested during carnivorism. Tissue cysts can interfere with neurological functions of the host and can alter the cat-avoidance behavior of rodents: cat urine is atractive for T. gondii

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  eBook - ePub

  Practical Medical Microbiology for Clinicians

  • Frank E. Berkowitz, Robert C. Jerris(Authors)
  • 2015(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  SECTION V Parasitology Passage contains an image

  CHAPTER 22 Parasitology

  General description of parasites

  The microorganisms that are, by tradition, referred to as “parasites” are eukaryotes. They are classified, for convenience, into protozoa (single-celled organisms), worms, and ectoparasites. As a general principle, protozoa multiply within the host (as do viruses and bacteria). In contrast, with a few exceptions, e.g. Strongyloides stercoralis, worms do not multiply within the host. The “worm burden” is the number of worms entering the host. The protozoa are classified into four groups: flagellates, ciliates (of which only one infects humans), amebae, and apicomplexa (sporozoa). The worms are classified into three groups: roundworms (nematodes), flukes (trematodes), and tapeworms (cestodes). From a clinical standpoint, it is also convenient to classify the internal parasites into those infecting primarily the gut (even if they pass through the blood and tissues to reach there, in the case of some worms) and those infecting primarily the blood or tissues other than the gut.

  Life cycle

  This describes the pathway that the parasite takes from the time of entering the host, its pathway through the host, its exit and spread to another host, until its progeny reach the same point in another host. An understanding of the life cycle is essential for understanding the pathogenesis of the infection, its epidemiology, and its clinical manifestations. It thus provides the information on which methods for preventing transmission can be based. Many parasites that affect humans, in particular worms, have an animal host as part of their life cycle in the following ways.

  • The parasite is an animal parasite and humans are incidental hosts, e.g. Babesia, Toxocara, Echinococcus.
  • An animal is one of the hosts in a cycle that normally involves humans, e.g. Schistosoma spp., in which a snail is the intermediate host.

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