Topic: introduction to medical parasitology

Дата канвертавання26.04.2016
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Source and mode of parasitic infection or infestation

1. Food and drink:

Ingestion of raw or undercooked food or drinking water or other beverages containing the infective stage of the parasite.

2. Soil, dust and water (canals and streams)

a) Ingestion of food or drink contaminated with soil or dust containing the infective stage of the parasite.

b) Inhalation of dust.

c) Direct contact with soil (handling, walking barefooted); the infective stage penetrates the skin.

d) Using water streams (washing, swimming, wading, irrigation, etc.); the infective stage penetrates the skin or mucous membrane.

3. Vector

a) Bite of vector inoculating the infective stage.

b) Feces of vector containing the infective stage contaminate the skin (intact or wounded).

c) Ingestion of vectors containing the infective stage.

d) Direct penetration of an arthropod into the skin.

4. Direct contact:

  1. Skin contact.

  2. Sexual contact.

  3. Autoinfection or direct infection.

5. Congenital infection.
Effect of the parasite on the host (Pathogenicity)
The effect depends on the number, size and shape of the parasite; its activity (movement and migration); site (habitat); specific toxin and host reaction.

The effect may be due to:

1. The parasite abstracting nourishment from the host.

2. Mechanical effect leading to tissue destruction as a result of trauma, pressure, compression or obstruction; feeding on tissues or irritation of tissues leading to inflammatory or neoplastic reactions.

3. Toxic effects from toxins secreted or waste products excreted by the parasite, leading to poisoning or allergic reactions.

4. Secondary infection with other organisms as bacteria. The host reaction to the invading parasite may be:

a) Generalized in the form of fever, anaemia, leucocytosis, leucopaenia, eosinophilia, allergic reactions, weakness, etc.

b) Localized according to the tissue or organ affected, e.g. gastro­intestinal disturbances (colic, dyspepsia, diarrhea, and dysentery), itching, ulceration, hepatomegaly, splenomegaly, etc.

Protozoa are single celled organisms. There are four classes of Protozoa commonly found in concentrated faecal samples. These are differentiated by the method of motility. Protozoa include Entamoeba, Giardia, Trichomonas, Cryptosporidium, Isospora, Pneumocystis and Balantidium. There are two diagnostic life-cycle stages commonly seen in parasites – the cyst and the adult trophozoite stage. The trophozoite stage is analyzed directly on a slide without concentration. Cysts require concentration. The key diagnostic factor is that Protozoan cysts are typically 5-30 microns in diameter, and as such that are smaller than most Helminths eggs. Due to the size they are particularly difficult to see under the microscope if the sample clarity is bad.

The medically important Helminths are nematodes (roundworms), cestodes (tapeworms) and trematodes (flukes). Genera include: Fasciola, Schistosoma, Ascaris, Hookworm, Trichuris, Taenia and Enterobius. The normal stage for examination is the egg stage, although larvae may develop in some organisms (Strongyloides). The diameter of the eggs ranges from 30-150 microns.

The other major groups of parasites are known as blood-borne parasites where they are transmitted by an arthropod vector. Far more important arthropods for transmitting parasitic infections are the mosquitoes. Mosquitoes are known to carry malaria and filarial nematodes. Different types of biting flies transmit African trypanosomiasis, leishmaniasis and several kinds of filariasis.

Most protozoan and helminthic infections that are transmitted by arthropods can readily be diagnosed, on clinical basis alone, but are usually identified by fairly simple techniques designed to reveal the presence of the causative parasite by microscopy. Sophisticated techniques are also being employed including highly sensitive and specific simple monoclonal antibody tests, DNA probes and PCR primers.

Protozoa exhibit a wide variety of morphologies. There is no one shape or morphology, which would include a majority of protozoa. Shapes range from the amorphous and ever-changing forms of amoeba to relatively rigid forms dictated partially by highly ordered cytoskeletons or secreted walls or shells. Many protozoan species exhibit complex life cycles with multiple stages. Sometimes the different life cycle stages are so dissimilar that they have been mistaken for completely different species.

The vast majority of protozoa are microscopic. However, they do exhibit an incredibly large range of sizes. Extant species range in size from < 1 μm (10-6 meter) to several mm. Fossilized Forminiferida of several cm have been identified. Most of the organisms discussed in Parazitology will be 3 – 50 μm. This small size necessitates the use of a microscope to detect protozoa. An electron microscope is needed for detailed morphological studies.

Protozoa are found virtually everywhere. As a group, the protozoa are extremely adaptable. Individual species, though, may have very specific-niches.

Like all other organisms, protozoa must be able to acquire and metabolize nutrients from their environment. Many protozoa simply absorb fluids (i.e., osmotrophy) from their media, while some are scavengers that ingest solid material (i.e., phagotrophy). Predatory protozoa either actively hunt down or passively ambush other organisms (typically bacteria or other protozoa). Some protozoa are photosynthetic and can capture the energy of the sun and convert it to usable chemical energy (i.e., autotrophic or phototrophic). Many protozoa are not restricted to a single feeding mechanism and can utilize combinations mentioned above (i.e., heterotrophic, mixotrophic).

Protozoa are conveniently divided into free-living and symbiotic with a few that are facultative symbionts. Generally free-living organisms are found in the soil or aqueous environments, whereas symbionts live in close association with another organism. Symbiosis implies a physiological dependency on another organism for its nutrition. Different forms of symbiosis can be distinguished on the nature of the association between the dissimilar organisms:

commensalism: denotes an interaction that is beneficial to one organism but has no affect on the other organism. For example, many protozoa live in the alimentary canal of another organism without doing any harm. These commensals are often scavengers or predators that exploit the abundance of nutrients or bacterial fauna provided by the host organism;

mutualism: is a special form of commensalism in which both organisms derive some benefit and become dependent on each other. The classic example of mutualism is the protozoan Trichonympha found in the gut of termites. Trichonympha, with the assistance of symbiotic bacteria, digests the wood particles (i.e., cellulose) ingested by the termite;

parasitism: denotes a relationship in which one organism (the parasite) benefits at the expense of the other organism (the host). Generally this host expense implies that the parasite takes in macromolecules from the host and releases others into the host. In some instances the parasitism will be overtly harmful to the host and referred to as being pathogenic. These pathogenic protozoa will be the primary focus of this course.

The earliest observations of protozoa noted their motility. However, motility is not a universal feature of protozoa and different protozoa utilize different mechanisms for their movement. In fact, protozoa were initially classified based in part on their motility. Cilia and flagella are subcellular structures, which propel protozoa through a fluid medium. Flagella are long whip-like structures, which propel the organism as a result of wave-like beat, which is propagated through their length. Flagellated protozoa typically have one or a few flagella per organism. In contrast, ciliated protozoa are usually covered with rows of numerous cilia. Cilia and flagella can also assist in the procurement of food, reproduction and other functions.

In contrast to the swimming exhibited by flagellates and ciliates, amoeba are protozoa that crawl along a solid substratum in a fashion known as “amoeboid movement”. The amoeba projects out a pseudopod, or false foot, and then pulls the rest of the body forward.

The most common form of reproduction in protozoa is asexual binary fission. In other words, a single organism will divide into two equal organisms. A slight modification of this binary fission, called budding, is when one of the newly formed cells is smaller than the other. Typically the larger cell is called the mother and the smaller is the daughter. Some protozoa will form an intracellular bud and essentially give birth. Another variation of binary fission is a multiple fission or segmentation. In this situation, several rounds of nuclear replication occur without cytokinesis. This multinucleated cell will then form multiple progeny simultaneously.

In summary, protozoa are unicellular eukaryotic microorganisms. However, the amount of diversity concerning morphology, size and life styles exhibited by protozoa makes it difficult to develop a more precise definition. However, protozoa do exhibit many of the features found in other eukaryotes.
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