Parasitology

General introduction:

Parasite:

A living organism which depends on a living host for its survival and derives nutrient from the host without giving any benefit to hosting.

A living organism which depends on a living host for its survival and derives nutrient from the host without giving any benefit to hosting.

1. Ectoparasite:A parasite which lives on the external surface of the host is an ectoparasite.Eg leech,mites,ticks etc.
2. Endoparasite:A parasite which lives within the body of the host is called endoparasite.All protozoan and helminth parasite of human beings are endoparasite.
3. Facultative parasite:Organism which under favour condition may line a parasite life but free-living existence may also occur.These are named as the facultative parasite.eg naegleria.
4. Obligate:Organism which cannot exist without their host are known as the obligate parasite.Malarial parasite is an example of the obligate parasite.
5. Accidental parasite:Parasite which infects an unusual host are known as the accidental parasite egEchinococcus granululosusinfect human being accidentally and lead to hydatid disease.
6. Aberrant/Wandering parasite:Organism which infects a host where it can not develop further.These parasites are known as aberrant or wondering parasite egToxoplasma Gandhi,Echinococcus granulosa.

Host:

An organism which harbour the parse is called host.It also provides shelter and nourishment to the parasite.

1. Defensive host/Primary host:The host which the parasite undergoes sexual reproduction is called defensive host.In mammal parasite, sexual reproduction takes place in mosquitoes.So mosquitoes are the defensive host.
2. Intermediate host:The host in which larval or asexual stage of the parasite are found is called intermediate host.Human is the intermediate host of the malarial parasite.Asexual multiplication takes place in human.
3. Paratenic host:The host in which larval stage of parasite but does not develop further is called paratenic host.
4. Reservoir host:It is a host that harbours the parasite and acts as an important source of infection to another susceptible host is called reservoir host.

Entamoeba histolytic

The parasite causes dysentery and liver abscess human.It is worldwide in distribution most common in the typical and subtropical than temperate zone.Trophozoites ofE.histolytica live in the mucous and submucous layer of the large intestine of the human.

Trophozoite:

It is a growing or feeding stage of the parasite.The morphology of host-parasite shows following characteristics.

• Shape:Not fixed because of constantly changing the pattern.
• Size:Ranging from 18-40nm
• Cytoplasm:Divided into two [portion namely, a clear translucent ectoplasm and a granular endoplasm.Ingested RCB particles and tissue debris are often found inside plasma membrane.
• Nucleus:Spherical shared and varying in size from 4 to 6 nm.It stained preparation,the nucleus structure shows

1. Karyosomes-Small dots like,central in position
2. Nucleus membrane-Contain a single layer of uniformly distributed fine chromatin granules.

Cyst:

It is a round or oval,12 to 15 nm in diameter and is surrounded by the highly refractive membrane called the cyst wall which makes it highly resistant to digestion by gastric juice and adverse environmental conditions.A mature cyst is quadrinucleate.The cytoplasm of cyst shows chromatoid based glycogen mass.

Lifecycle

Pathogenesis:

1. Reservoin:Hu,mans are commenest source of infection.
2. Mode of infection:TransmissionE.histolyticaoccurs through ingestion of guard nucleated cyst.Faecal contamination of drinking water,vegetables and food are the primary causes.Eating of uncooked vegetables and fruits which have been fertilised with infected human faeces can also transmit it.Houseflies may transmit cyst from faeces to unprotected food.
3. Incubation period:It varies widely but generally 4 to 5 days
4. Clinical feature:infection caused byE.histolyticais commonly called amoebiasis.It can be divided into Intestinal amoebiasis and Extraintestinal amebiasis.

• Intestinal amoebiasis:The infection is limited entirely to the large intestine.After the liberation of trophozoite,they directly penetrate epithelial cell by their amoeboid activity and finally reaches to the submucosa of the large intestine.Here the trophozoite rapidly multiplies and colonised.trophozoide feed on a cytolytic material.
• Extraintestinal amoebiosis:Small individual with intestinal amoebiasis develops extraintestinal amoebiasis.It includes

1. Hepatic amoebiasis:The trophozoite ofE.histilyticaare carried out by blood to the liver via hepatic vein.Once establish,in liver,trophozoites multiply in large number by binary fission,proceed to carry on their cytolytic action resulting in necrosis of the liver cell.
2. Pulmonary amoebiosis:Trophozoites gain entrance to pulmonary circulation and enter into lungs causing abscesses.It is called pulmonary amoebiosis.

• Cerebral amoebiosis,Cutaneous amoebiasis:These are rare varieties of amoebiasis which may have arisen from either hepatic or pulmonary source.

Lab diagnosis:

Lab diagnosis ofE.histolyticadepends on demonstration of trophozoites and cyst in specific specimens such as stool,pus sputum etc.

• Examination of stool:
• Collection of stool
• Stool should not be mixed with urine
• fresh stool
• clean,dry,no disinfectant use in container
• Macroscopic examination:Faeces should be examined for its consistency,colour,odour and presence of blood and mucus.An offensive,dark,brown,acidic stool mixed with blood and mucus is suggestive of amoebic dysentery.
• Microscopic examination:The microscopic examination of the stool can be carried out unstained in a slain preparation and also stained with iodine.
• Examination of push for hepatic amoebiasis:Trophozoites ofE.histolyticamay be demonstrated by microscopy or pus aspirated from hepatic abscesses and liver biopsy.In about 15% of case of hepatic abscess cyst ofE.histolyticamay demonstrate in the stool

Prevention and control:

• Primary prevention:Preventing contamination of water food vegetables and fruits with human faeces.
• Sanitation:Safe disposal of human faeces.Washing hands after defecation and before eating
• Water supply:Protection of water supply from faecal contamination.Filtration and boiling of drinking water
• Food hygiene:Protect the food from contamination.

References:

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.