List of nine important diseases caused by protozoa in humans:- 1. Leishmaniasis 2. Amoebiasis 3. Diarrhoea 4. Trichomoniasis 5. Tripanosomiasis 6. Lambliasis 7. Toxoplasmosis 8. Malaria 9. Balantidiasis.
Disease # 1. Leishmaniasis:
This disease is caused by Leishmania, a flagellate protozoan which is transmitted from man to man by sandflies belonging to genus Phlebotomus.
Two main forms of leishmaniasis can be distinguished:
(i) Cutaneous (skin) leishmaniasis and
(ii) Visceral leishmaniasis or kala-azar.
Cutaneous (skin) leishmaniasis is due to Leishmania tropica, the protozoan discovered by P. Borovsky in 1898. Two forms of cutaneous leishmaniasis are encountered: theory form (late ulcerating leishmaniasis) and the moist form (acute necrotizing leishmaniasis). The dry form of leishmaniasis is caused by L. tropica var. minor.
A copper-coloured spot appears on the skin of the face, neck, hands and leg. The spot gradually develops into a tubercle that grows and forms necrotized tissue in its centre. The hard skin tubercle becomes covered with a brown-red scab.
When the latter is removed, an ulcerated surface containing large number of pathogen is releaved. The moist form of leishmaniasis is caused by L. tropica var. minor. In it, the ulcer edges are loose and with an uneven outline. The source and reservoir of the moist form are wild rodents which live in burrows.
Visceral leishmaniasis or Kala-azar is caused by Leishmania donovani. W. Leishman and C. Donovan described the causal agent of Kala-azar, the black fever in India, in 1903. In this disease the man is caught by lever and long term remissions.
The temperature is moderately elevated, the spleen and lever become enlarged and progressive anaemia develops. The skin turns a dark colour, therefore, the disease is known as black fever (Kala-azar). Kala-azar is characterised by a chronic course and lasts for many years. Death rate ranges from 20- 30%.
Disease # 2. Amoebiasis:
Amoebiasis, also known as amoebic dysentry, is caused by Entamoeba histolytica which was discovered in 1875 by F. Loesch. The disease is widespread in the Middle East, India, China, North and Central Africa, Indonesia, South America etc. It predominates where sanitary conditions are low.
The trophozoites (E. histolytica forma minuta) are the main form of the pathogen that cause amoebiosis and live in the upper part of the large intestine. They secrete histolytic enzymes, penetrate into the gut wall and feed on the cells of mucous membrane causing ulcers.
The ulcers ultimately burst and discharge blood and mucus that pass along-with trophozoites in the lower part of intestine. Here the conditions are unfavourable for trophozoites and they transform into cysts. The cysts are discharged with stool of the patient and may re-enter the human body with foodstuffs or water generally contaminated by houseflies and may initiate the disease.
Disease # 3. Diarrhoea:
Diarrhoea is caused by Giardia intestinalis (a flagellate protozoan) and is characterized by loose bowls. The causative agent inhabits intestine and feeds upon amino acids and vitamins in the food. It causes intestinal disorders resulting in epigastric pain, abdominal discomfort, loss of appetite and headache.
The disease is transmitted by cysts which come out in stool. The houseflies contaminate foodstuffs and water and cysts may enter the human body through faecally contaminated food or water. Children are more susceptible to this disease.
Disease # 4. Trichomoniasis:
Three species of Trichomonas, a tetraflagellate protozoan, are parasitic in the human body. T. intestinalis is found mainly in the mouth of elderly persons with the bad teeth, periodontosis, etc.; and T. vaginalis inhabits the lower parts of the vaginal apparatus mostly of 18 to 45 years of age and causes vaginitis (vaginal trichomoniasis). The vaginitis is characterized by burning sensation, itch and frothy vaginal discharge. Infection is transmitted through sexual intercourse and toilet articles.
Disease # 5. Tripanosomiasis:
There are two types of tripanosomiasis: African and American. African tripanosomiasis, also called “sleeping sickness”, is a dangerous disease especially in tropical Africa and is caused by two closely related species of Tripanosoma, namely, T.gambliense and T.rhodesiense.
The disease is characterized by cachexia, anaemia, fever, aedema of the brain, chronic leptomeningitis, haemorrhages and kidney-lesions. The patient becomes more somnolent, sinks into a coma and quite frequently dies. The disease is transmitted by a tsetse fly (Glossinia palpalis).
American tripanosomiasis, also called “Chiga’s Disease” is a widespread disease in southern and central part of America and is caused by Schizotryanum cruzi. Children are more susceptible to this disease. Swelling of the body parts, severe headache and continuous fever are the features of the disease. The disease is transmitted by various species of Triatoma, a bug.
Disease # 6. Lambliasis:
This disease is cause by Lamblia intestinalis (a flagellate protozoan). Nausea, pyrosis, hypoacidity, and hepatitis develop quite frequently in this disease. The source of infection is people who discharge cysts in stool. The cysts reach in the intestine through contaminated food or water, dissolve their chitinous membrane and multiply in the small intestine and penetrate into the duodenum and gall bladder.
Disease # 7. Toxoplasmosis:
The causative agent of toxoplasmosis is Toxoplasma gondii, a sporeforming protozoan. Toxoplasma is motile and capable of gliding and rotatory movements although no flagella have been observed. Toxoplasmosis is a widespread disease occurring almost in all countries. Man acquires infection from dogs, cats, sheep and other animals.
Manifestations of this disease in adults include eruptions covering all parts of the body except palms, soles, and scalp. Pneumonia, enterocolitis, nephritis, hepatitis etc. are of quite frequent occurrence. Toxoplasmosis may be congenital or acquired: All women with a history of spontaneous abortion, premature labour, and those who had given birth to dead or deformed children must be examined for prevention of congenital toxoplasmosis.
Disease # 8. Malaria:
Malaria is caused by Plasmodium spp., a spore-forming protozoan. It occurs frequently in tropical and subtropical countries and is characterized by periodic attacks of fever.
Four types of malarial disease are recognised:
(i) Tartian malaria caused by P. vivax,
(ii) Ovale malaria by P. ovale,
(iii) Quaetan malaria by P. malariae, and
(iv) Malignant tartian malaria caused by P. falciparum.
In man, pathogen attacks the liver cells and the red blood cells (RBCs). During the course of a malarian infection, life cycles of Plasmodium involve reproduction within human RBCs with the periodic release of large number of protzoan. A toxic substance called ‘haemozoin’ released by the pathogen is the real cause of malarial fever. The disease is transmitted through female anapheles mosquito.
Disease # 9. Balantidiasis:
Balantidium coli, a ciliate, is responsible for this disease. The pathogen inhabits intestine and causes lesions in the colon producing ulcers, abscesses and colitis characterized by the presence of blood and mucus in the stool. One third of all cases terminate in death. The infection is spread by pigs which carry the parasite in their intestines.