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Medical Parasitology

Medical Parasitology deals with the study of three major groups of animals: Protozoa, Helminthes and Arthropods, which act as vectors in transmitting the parasites and causing diseases.


Protozoa and diseases

Protozoa is a group of a cellular microscopic animals which have occupied all the ecological habitats on the earth. The protozoans exhibit mainly two modes of life, free living and parasitic living as ecto and endoparasites. Protozoa cause many diseases in man and animals, more prevalent in tropical and subtropical regions. All classes of Protozoa have pathogenic organisms but they are more in flagellates and sporozoa. Human beings alone have over fifteen species of parasitic Protozoa. They are various amoeba, flagellates, ciliates and sporozoans.Some are harmless but others are responsible for fatal diseases.

Diseases caused by protozoan Parasites:

Amoebiasis cycle

Amoebiasis, also known as amoebic dysentery, is caused by Entamoeba histolytica. Infection generally occurs through drinking water contaminated with cyst of E.histolytica.The parasite penetrates the alimentary canal, secrets histolytic enzymes and feeds upon its cells causing ulcers. These ulcers rupture and discharge mucus and blood in to the intestine that pass along with stools and results in amoebic dysentery. If the infection continues the parasite may reach the liver, lungs and brain where it causes abscess which prove fatal.

Diarrhoea picture
Diarrhoea, which is characteristics by loose bowels, is caused by a flagellate parasite Giardia intestinalis. G.intestinalis is a parasite in the small intestine and colon of man.With the help of sucking disc the parasite attaches itself on the epithelial cells of the intestine and cause a disturbance in the intestinal function leading to malabsorption of fat which causes diarrhoea.The parasite is also capable of producing epigastric pain, abdominal discomfort, loss of appetite and toxic effect.

Trypanosoma gambiense
Trypanosomiasis is caused by the protozoa parasite Trypanosoma which are flagellates parasites of blood plasma (in vertebrate host) and gut (in invertebrate hosts). Trypanosoma is generally transmitted by blood sucking insects.It is the most dreadful pathogen of protozoans. The genus trypanosoma include three species Trypanosoma gambiense and T. rhodesiense is transmitted by Tsetse fly which causes sleeping sickness in man.

Trypanosoma cruziis transmitted by bug which causes Chaga's disease or American trypanosomiasis in South and Central America.


Leishmaniasis is caused by the species of Leishmania, the flagellate parasite in the reticuloendothelial cells of vertebrate host (man) and in the gut of invertebrate host (sandfly).The genus Leishmania includes three species.

Leishmania donovani causes kala-azar or visceral Leishmaniasis.The parasite attacks the endothelial cells, bonemarrow, liver, lymphglands and blood vessels of the spleen .These organs enlarge and cause bloodlessness and high fever.
Leishmania donovani

Leishmania brasilensiscauses a disease called Espundia or American leishmaniasis causes ulceration in nasal cavities, mouth and pharynx.

Leishmania braziliensis

Leishmania tropicacauses oriental sore or Delhi boil .The infection is limited to a local lesion of the skin and subcutaneous tissues which results in ulcer.


Leishmania tropicacauses oriental sore or Delhi boil .The infection is limited to a local lesion of the skin and subcutaneous tissues which results in ulcer.

Trichomonas hominis
Trichomoniasis is caused by the flagellate parasite Trichomonas .Three species are found in man. Trichomonas hominis, Trichomonas lenax, Trichomonas vaginilis. The most common pathogenic species is Trichomonas vaginilis it inhabits the vagina of women and causes vaginitis. The disease is characterized by inflammation of vaginal mucosa, burning sensation, itching and abnormal discharges.Transmission of parasite is always during sexual intercourse by male who act as intermediaries.

Plasmodium falciparum
Malaria is caused by the species of a sporozoan parasite, Plasmodium. It is transmitted through the bite of female anopheles mosquito .In man the parasite attacks the liver cells and red blood cells. A toxic substance, the haemozoin, released by the parasite causes malaria. Four species of plasmodium causes human malaria: Plasmodium vivax, Plasmodium falciparum, Plasmodium malariae, Plasmodium ovale. All the four species multiply asexually in the cells of the liver and erythrocytes of man. In malaria the spleen is enlarged, erythrocytes decrease in number, the blood becomes watery and high temperature accompanied by chills and shivering.

Toxoplasma gondii
Toxoplasmosis is caused by a sporozoan parasite, Toxoplasma gondii. Symptoms caused by the parasite is hydrocephalus, chorioretinitis liver and spleen enlarges.

Balantidial dysentery:
Balantidium coli cystBalantidial dysentery is caused by a ciliate parasite, Balantidium coli. It inhibits the large intestine of man. It causes ulcers which results in dysentery and diarrhoea.


Helminths and diseases:

Helminths include the animals belonging to the Phylum Platyhelminthes and Phylum Nemathelminthes.

Phylum Platyhelminthes are triploblastic, bilaterally symmetrical, dorso ventrally flattened, acoelomate flat worms with organ grade of construction .Platyhelminthes includes the class tremetodes and cestodes. Phylum nemathelminthes are pseudo coelomate, bilaterally symmetrical, tryploblastic, unsegmented vermiform, organ grade of construction. Nemathelminthes includes the class nematodes. Many of the parasitic forms of helminthes are popularly known as parasitic worms.These are the endoparasites of the gut and blood in human and cause various diseases collectively called helminthiasis.

The following three types of helminthiasis are most common and wide spread.

  • Trematodiasis
  • Cestodiasis
  • Nematodiasis


The diseases caused by the trematode helminthes constitute trematodiasis.

Schistosoma mansoni
Schistosomiasis is caused by the parasitic infection of flukes of three species namely Schistosoma manosoni, S.japonicum, S. haemotobium. The flukes cause dysentery and liver diseases followed by fever, sweating, diarrhoea, weightloss and lack of appetite. Intestinal Schistosomiasis is caused by S.mansoni , urinogenital Schistosomiasis is caused by S. haemotobium. The larva of the worm penetrates the skin of man.Through the blood circulation it reaches the lungs to become adult flukes.

Opisthorchis felineus
Opisthorchiasis is caused by the parasite Opisthorchis sinensis (Chinese liver fluke). Human infection is acquired by eating raw or undercooked fish which harbour the larva of the parasite.When human host eat the fish the larvae emerge in the duodenum, enter the common bile duct and mature in the distal bile ducts to an adult fluke. Patients with Opisthorchiasis may die due to variety of complications including biliary obstructions and pancreatitis.

Paragonimus westermani
Paragonimiasis is caused by an infection of the lung fluke, Paragonimus westermani. Adult flukes live in the respiratory tract of man. Human host acquire the infection by eating raw or under cooked infected crustaceans. The lung fluke cause chronic cough with recurring attacks of haemoptysis, night sweats severe chest pain and pleural effusions are common.

Fasciolopsis buski
Fasciolopsiasis is caused by Fasciolopsis buski, the giant intestinal fluke. The adult worm lives in the small intestine of man and pig. The intermediate host is snail. Man acquires the infection of Fasciolopsiasis by eating uncooked aquatic vegetables. The flukes attached to the duodenal and jejunal wall. The point of attachment may ulcerate and get infected.

Fasciola hepatica (liver fluke)
Fascioliasis is caused by Fasciola hepatica, the sheep liver fluke. Human may acquire the infection where ever sheep are raised. The adult fluke live in hepatic bile ducts later penetrate in to the liver where they feed on the parenchymal cells of the liver. Severe untreated infection may become fatal.


Cestodiasis is caused by cestodes (tape worm).

Taenia soliumTaeniasis is caused by the species belonging to the genus Taenia which include mainly Taenia solium (pork tapeworm) and T.saginata (beef tape worm). Man acquires the infection by eating raw or undercooked pork or beef that contains the larvae. The larvae develop in to adult tapeworms in the intestine of man. The presence of tapeworms in the intestine causes gastrointestinal disorders. Occasionally it causes abdominal discomfort, chronic indigestion, anemia and intestinal disorders.

Echinococcosis (Hydatid cyst):

Echinococcus granulosus hydatid cyst
Echinococcosis is an infection caused by Echinococcus granulosus. The definitive hosts are dog, wolf and fox. Man acquires the infection on eating food or drinking water contaminated with onchosphere-containing eggs. Larvae released from the eggs penetrate the wall of the gut, enter the blood stream and disseminate to deep organs such as liver, lungs where they grow to form hydatid cysts. It causes hepatomegaly and lead to jaundice. Rupture of the cyst causes hypersensitivity reaction.


Nematodiasis is caused by the nematode worm.

Ascaris lumbricoides
Ascariasis is caused by the largest nematode Ascaris lumbricoides (round worm). It lives in the small intestine of man lying freely in the lumen. Sexual dimorphism is well distinct, only sexual reproduction takes place. Ascaris infection may be due to migrating larvae and by adult worms. The symptoms caused by the migrating larvae are fever, cough and dyspnoea. The symptoms caused due to adult worms are abdominal pain, vomiting, headache, irritability, dizziness and night terrors.

Ancyclostoma duodenale rhabditiform
Ancylostomiasis is caused by two hook worms Ancylostoma duodenale and Nectar americus. The adult worms live in the intestine of man particularly in the jejunum. Infection occurs when man walks bare foot on the faecally contaminated soil. It penetrates through the thin skin between toes, the dorsum of the feet and the inner side of the soles. The characteristic symptoms of Ancylostomiasis are Ancylostome dermatitis or ground itch, gastrointestinal disorders and severe anemia by adult worms.

Enterobius vermicularis
Enterobiasis is caused by Enterobius vermicualris commonly known as pin worm, thread worm, or seat worm. The pinworm infection is more frequent in children than in adults. The symptoms include severe itching around the anus, loss of appetite, sleeplessness and sometimes inflammation in the vermiform appendix.

Trichuris trichura egg
Trichuriasis is caused by Trichuris trichura, commonly known as whip worm. The adult worms live in the large intestine of man, particularly in the caecum, also in vermiform appendix. The symptoms of the disease are acute appendicitis, mucus diarrhoea and abdominal pain.


Trichinosis is caused by Trichenella spiralis, the trichinia worm. It is one of the smallest nematodes infecting man. Human becomes infected by eating undercooked raw meet containing cyst larvae mainly pork. Larvae mature to adult worms and attaches to the wall of small intestine. The early symptom of trichinosis is eosinophilia. The invasion of larva to the muscle is associated with muscle pain, swelling of the eyelids and facial oedema.

Strongyloides stercoralis larva
Strongyloidiasis is caused by the nematode Strongyloides stercoralis, commonly called thread worm. Parasitic pathogenic females live in the human small intestine and lay eggs in the mucosal epithelium releasing rhabditiform larvae. These larvae becomes infective filariform larvae. Larvae migrating through the lungs may provoke cough, haeomoptysis, and dyspnoea .severe infection of the intestine causes vomiting, diarrhoea and constipation.

Filariasis or Elephantiasis:
Wuchereria bancrofti
Filariasis is caused by Wuchereria bancrofti commonly called as filarial worm. The adult worms inhabit the lymphatic vessels, lymphatic nodes, testes and epididymis. The female worm discharges microfilariae that circulate in the blood. Humans are only definitive host. Insect vectors serve as intermediate host includes species of mosquito's. The infection of the filarial worm causes enlargement of the limbs, mammae and scrotum.

filarial nematode loa loa
Loiasis is an infection caused by the filarial nematode Loa loa, the African eye worm or Loa worm. Humans and baboons are definitive host. The adult Loa worm migrates the skin and occasionally crosses the eye beneath the conjunctiva. Ocular infections include swelling of lids, congestion, itching and pain.


Insects act as vectors in transmitting harmful parasitic infections. Important disease spreading insects are mosquitoes, flies, fleas, bugs, ticks, mites and lice.

Laboratory diagnosis

Depending on the nature of the parasitic infections, the following materials should be collected:

Blood: Where the parasite itself or any stages of development circulates in the blood stream. Examination of blood film forms one of the main procedures for diagnosis.
For eg .In malaria, the parasites are found inside the erythrocytes.
Stool: Examination of stool forms an important part in diagnosis of intestinal parasitic infections and also Helminthic parasites. In protozoal infection, either trophozoites or cystic forms may be detected. In helminthic infection, either the adult worm or their eggs are found.
Urine: when the parasite lives in the urinary tract, examination of urine can diagnose the infection.
Sputum: Examination of sputum will be useful and important if the parasite localizes in the respiratory tract.
Biopsy material: spleen puncture, bone marrow puncture, skin biopsy, muscle biopsy, rectal biopsy, liver biopsy, lumbar puncture
Indirect evidences: Changes indicative of internal parasitic infection are
  • Cytological changes in the blood
  • Biochemical alteration of the blood
  • Serological tests
  • Intradermal reaction test
  • Prophylaxis

Therapeutic Prophylaxis: The parasite is attacked with in the host, there by preventing the dissemination of the infective agents. Eradication of the infection in the reservoir hosts and destruction of intermediary hosts. Personal prophylaxis may be further ensured.
Diagnostic Procedures | Helminths | Introduction to parasitology | Medical Parasitology | Protozoology