LEC1. Parasitology;Amoeba PDF

LEC.1 Parasitology Dr. Maysoon A.Merdaw Parasitology is the area of biology concerned with the phenomenon of dependence of one living organism on another. Medical parasitology deals with the parasites which infect man, the diseases they produce, the response generated by him against them, and variou...

LEC.1 Parasitology Dr. Maysoon A.Merdaw Parasitology is the area of biology concerned with the phenomenon of dependence of one living organism on another. Medical parasitology deals with the parasites which infect man, the diseases they produce, the response generated by him against them, and various methods of diagnosis, prevention and treatment. Parasite: is an organism that is entirely dependent on another organism, referred to as its host, for all or part of its life cycle and metabolic requirements. Strictly speaking, the term parasite can be applied to any infectious agent but, by convention, it is generally restricted to infections caused by protozoa and helminths and excludes the viruses, bacteria and fungi. Parasite is of two types: 1-Microparasite: small, unicellular and multiplies within its vertebrate host, often inside cells. Protozoa are microparasites. 2-Macroparasite: large, multicellular and has not direct reproduction within its vertebrate host. This category include helminths. On the basis of their location, parasites may be divided into: -Ectoparasites: which live on the surface of the body, e.g., the human louse, Pediculus humanus. The infection by these parasites is known as infestation. They are important as vectors transmitting pathogenic microorganisms. -Endoparasites: which live within the body of the host. All protozoan and helminthic parasites of man are endoparasites. The invasion by endoparasites is known as infection. These can be further subdivided into following types: -Obligate parasites: organisms cannot exist without a host e.g. Toxoplasma gondii. -Facultative parasites: organisms that under unfavorable circumstances may live either a parasitic or free- living existence e.g. Naegleria fowleri. -Accidental parasites: organisms that attack an unusual host e.g. Echinococcus granulosus in man. 1 -Aberrant parasites: organisms that attack a host where they cannot live or develop further e.g. Toxocara canis in man. -Free- living: the term free-living describes the nonparasitic stages of existence which are live independently of a host e.g. hookworms have active free-living stages in the soil. Host: organism which harbors the parasite and provides the nourishment and shelter. It is of following types: -Definitive host: harbors the adult parasite, where the parasite replicates sexually. -Intermediate host: the host which alternates with the definitive host and harbors the larval or asexual stages of a parasite. Some parasites require 2 intermediate hosts for completion of their life cycle. -Paratenic host: a host in which larval stage of a parasite survives but does not develop further. It is often not a necessary part of life cycle. -Reservoir host: can harbor a pathogen indefinitely with no ill effects. Once discovered, natural reservoirs elucidate the complete life cycle of infectious diseases, providing effective prevention and control. -Compromised host: one in whom normal defense mechanisms are impaired e.g. AIDS. Such hosts are extremely susceptible to a variety of pathogens. Host- parasite relationships: -Symbiosis: an association in which both host and parasite are so dependent upon each other that one cannot live without the help of the other. Neither of the partners suffers from any harm from this association. -Commensalism: an association in which only parasite may benefit without detectable damage to the host as in case of Entamoeba coli in the large intestine of man, (One partner benefits but the other is not hurt). -Parasitism: One partner (the parasite) harms or lives on the expense of the other (host). The degree of dependence of a parasite on its host varies. Classification of animal parasite and vectors: Subphylum Class Species 2 Order Family Genus All of these names must be of Greek or Latin origin or have a classical termination. Species: it designates a population, the members of which have essentially the same genetic characters and are capable of continued reproduction of their kind, but usually cannot interbreed with individuals of other species. Genus: is a group of closely related species. The scientific designation of a species is a combination of the genus and species name. This is referred to as binomial nomenclature. Ex. Entamoeba histolytica. Vector: an agent, usually an insect that transmits infection from one human host to another. Vector is only transport, helping the pathogen to complete its life cycle, while intermediate host existence is essential in completion of some parts of life cycle (asexual only). Zoonosis: term describe a disease communicable from animals to humans (enzootic infection acquired by man) under natural conditions. Examples include; leishmaniasis, hydatid cyst and fascioliasis. Section I. Protozoa Protozoa are single-celled animals; each cell performs all of the necessary functions of life, majority of which are free-living. Classification of the protozoa: Human parasites in the kingdom Protista, subkingdom Protozoa are classified under four phyla: 1- Sarcomastigophora (containing amoeba and flagellates) 2- Apicomplexa (containing Sporozoa) 3- Ciliophora (containing Ciliates) 4- Microspora 1- Sarcomastigophora This phylum is subdivided into two subphyla: 1- Sarcodina 2- Mastigophora Sarcodina (Amoebae): Amoeboid organisms using pseudopodia for both locomotion and feeding. Only Entamoeba histolytica is of medical importance. 3 Entamoeba histolytica Morphology: the parasite exists in three forms; trophozoite, precyst and cyst. Trophozoites; 10-60 µm size, unidirectional motility, single pseudopodium, ingest erythrocytes, granular cytoplasm small, central karyosome, and beaded chromatin. Habitat: trophozoites reside in mucosa and submucosa of large intestine of man. Life cycle: It passes its life cycle in only one host. Cysts are passed in faeces. Man acquires the infection by ingestion of mature quadrinucleate cysts in faecally contaminated food or water. Trophozoites can also be passed in diarrhoeal stools, but are rapidly destroyed once outside the body, and if ingested would not survive exposure to the gastric environment. In contrast, cysts may remain viable in a humid environment and stay infective for several days. Flies and cockroaches can also serve as vectors for the transmission of E. histolytica cysts. In the small intestine the cyst wall is lysed by trypsin and a single tetranucleate amoeba (metacyst) is liberated. Each nucleus divides by binary fission giving rise to eight nuclei, thus from each mature cyst eight small metacystic trophozoites are produced. This process is known as excystation. Metacystic trophozoites are carried in the faecal stream into the caecum. They invade the mucosa and ultimately lodge in the submucous tissue of large intestine. Here they grow and multiply by binary fission. During growth, E. histolytica secretes a proteolytic enzyme which brings about destruction and necrosis of tissue and produces flask- shaped ulcers. Encystation: occurs in the intestinal lumen in which chromatin materials are concentrated into bars (chromatoidal bodies) in the cytoplasm of the cyst. The nucleus of cyst divides into two, then each of the two daughter nuclei divided once again so mature cyst 4 has four nuclei. The amoebae are mostly present at the periphery of the lesion. A large number of trophozoites are excreted along with blood and mucus in the stool leading to amoebic dysentery. In a few cases, erosion of the large intestine may be so extensive that trophozoites gain entrance into the radicles of portal vein and are carried away to the liver where they multiply leading to amoebic hepatitis and amoebic liver abscess. It has been established that the invasive and noninvasive forms represent two separate species, respectively E. histolytica and E. dispar. These two species are morphologically indistinguishable unless E. histolytica is observed with ingested red blood cells (erythrophagocystosis). Pathogenesis: Intestinal amoebiasis: After an incubation period of 1-4 weeks, the amoeba invade colonic mucosa. During growth, E. histolytica secretes a proteolytic enzymes, producing flask- shaped ulcers and profuse bloody diarrhea (amoebic dysentery). Ulcers may be deep or superficial. 5 E. histolytica may also cause amoebic appendicitis and amoebomas (pseudotumoral lesions associated with necrosis, inflammation and oedema). Extraintestinal amoebiasis: About 5-10% individuals with intestinal amoebiasis, 1-3 months after disappearance of dysentery, develop hepatic amoebiasis. Tophozoites are carried from the ulcer in the large intestine and multiply in the liver, lead to cytolytic action then small abscesses merge to form big liver abscesses. The abscesses may grow in various directions; it may enter into general circulation involving lungs, brain, spleen, skin, etc. Pathogenicity depends on: 1. Virulence of strain 2. Resistance of the host (depends on the innate immunity). 3. State of nutrition of the host. 4. Infection with other agents (free of other infections mean less susceptible to infection). 5. Some drugs may irritate the intestinal wall so irritated intestine is more susceptible to infection. 6. Bacterial flora (metabolic processes can enhance the invasiveness). Symptoms: Fever, chills, and diarrhea, sometimes bloody or white mucus and often with cramps. Some people may have only mild abdominal discomfort or no symptoms at all. Symptoms can start 2 or more weeks after infection. 6 Laboratory Diagnosis: Entamoeba histolytica must be differentiated from other intestinal nonpathogenic amebae. The nonpathogenic Entamoeba dispar is morphologically identical to E. histolytica, and differentiation must be based on isoenzymatic or immunologic analysis. Molecular methods are also useful in distinguishing between E. histolytica and E. dispar. -Microscopic identification of cysts and trophozoites in the stool is the common method for diagnosing E. histolytica. This can be accomplished using wet mount and permanently stained preparations as Iodine or trichrome or by Flotation or Sedimentation method for stool samples. -Blood examination: shows moderate leukocytosis. -Serological tests: in later stages of invasive amoebiasis, antibodies appear. Tests include ELISA, IHA and IFA. -Histology: trophozoites can be identified in aspirates or biopsy samples obtained during colonoscopy or surgery. - Molecular methods: DNA probe and PCR. Treatment: For asymptomatic infections, paromomycin or iodoquinol are the drugs of choice. For symptomatic intestinal or extraintestinal infections, the drugs of choice are metronidazole or tinidazole, immediately followed by treatment with paromomycin or iodoquinol. Failure of metronidazole therapy may be an indication for surgical intervention. 7 Nonpathogenic amoebae: Entamoeba Morphologically identical to E. histolytica. It must be separated by dispar isoenzymatic, immunologic or molecular analyses. Entamoeba Some consider this a separate species. It differs from E. histolytica by hartmanni being smaller in size. Distinguished from E. histolytica by having an eccentric endosome, and Entamoeba mature cysts with 8 nuclei. If chromatoidal bodies are present, they have coli splintered ends, rather than rounded as in E. histolytica. Endolimax This is a very small amoeba (6-15um) with a large, eccentric endosome nana and thin nuclear envelope. Mature cysts contain 4 nuclei. Both the trophozoite and cyst have one nucleus with a large Iodamoeba endosome. The cyst contains a large glycogen vacuole that stains darkly butschlii with iodine. Opportunistic amoebae Naegleria fowleri: It is a free-living amoeba that can be pathogenic, causing a fulminant brain infection called naegleriasis. Found in warm freshwater and in the soil near warm-water discharges of industrial plants, and in unchlorinated or minimally-chlorinated swimming pools. It can be seen in either an amoeboid or temporary flagellate stage. N. fowleri is inhaled through the nose, where it then enters the nasal and olfactory nerve tissue, travelling to the brain. N. fowleri normally eat bacteria, but when it enters humans, it uses the brain as a food source. It does not form a cyst in human tissue, where only the amoeboid trophozoite stage exists. Cysts and flagellate forms of N. fowleri have never been found in tissues or CSF. 8 Life cycle and pathogenicity The amoeboid form of N. fowleri is the invasive stage of the parasite. Man acquires infection by nasal contamination during swimming in freshwater lakes, ponds or swimming pools. Infection may also be acquired by inhalation of dust containing infective forms. It is likely that flagellate forms or cysts of N. fowleri could enter the nose. However, since the amoeboid form is the invasive stage of the parasite, therefore, it appears that flagellate forms revert to amoeboid forms and the amoeboid forms escape from the cysts in the nose. The amoeboid forms invade the nasal mucosa, and travel along the olfactory nerves to brain leading to a rapidly fatal infection known as primary amoebic meningoencephalitis (PAM). Patient develops severe frontal headache, fever (39°–40°C), anorexia, nausea, vomiting and signs of meningeal irritation. Involvement of the olfactory lobes may lead to disturbances in smell or taste. Patient may also develop visual disturbances, confusion, irritability, seizures and 9 coma. The disease usually results in death within 72 hours of the onset of symptoms. The period between contact with the organism and onset of clinical symptoms vary from 2–3 days to as long as 7–15 days. PAM may resemble acute purulent bacterial meningitis, and these conditions may be difficult to differentiate particularly in the early stage. Laboratory diagnosis Microscopy of CSF: Motile amoebae with characteristic morphology can be readily demonstrated in simple wet-mount preparation of fresh CSF specimen. Culture: N. fowleri may be cultivated by placing some of the CSF on nonnutrient agar (1.5%) spread with a lawn of washed Escherichia coli or Enterobacter aerogenes and incubated at 37°C. The amoebae will grow on the moist agar surface and will use the bacteria as food, producing plaques as they clear the bacteria. As colonies grow and expand, cysts that survive moderate desiccation are formed; thus, strains can be maintained by transfer of either trophic or cystic forms. -Almost all cases are diagnosed during autopsies due to the rapid progression of the disease. Treatment At present there is no satisfactory treatment for PAM. Antibacterial antibiotics and antiamoebic drugs are ineffective. Amphotericin B, a drug of considerable toxicity, is the antinaeglerial agent for which there is evidence of clinical effectiveness. 10 Notes: Vector:- an agent, usually an insect that transmits infection from one human host to another. Vector is only transport, helping the pathogen to complete its life cycle, while intermediate host existence is essential in completion of some parts of life cycle (asexual only). A vector is any organism acting as an intermediary host for a parasite.... Good examples of vectors are the mosquito in transmitting malaria and ticks in transferring Lyme disease, Vector: Any living creature that can pass an infection to another living creature. Humans are technically vectors, but the term is more commonly applied to nonhuman organisms. There are two types of vectors -: 1-Mechanical vector term used to describe a vector which assists in transfer parasitic forms between hosts but is not essential in the life cycle of the parasite, e.g. a housefly and Cockroaches in the case of Entamoeba, that transfers amoebic cysts from infected faeces to food that is eaten by humans. 2-Biological vector in whose body the pathogenic organism develops and multiplies before being transmitted to the next host, then it is essential in the While life cycle of the the (reservoir parasite.. host):- can harbor a pathogen indefinitely with no ill effects. Once discovered, natural reservoirs elucidate the complete life cycle of infectious diseases, providing effective prevention and control. Or The reservoir is a host which allows the pathogen to live, and possibly grow, and multiply. Humans, animals and the environment can all be reservoirs for microorganisms. Sometimes a person may have a disease but is not symptomatic or ill. 11