Protozoan Diseases and Humans

The following points highlight the eight significant protozoan diseases caused in humans. The diseases are: 1. Amoebiasis 2. Diarrhoea 3. Trypanosomiasis 4. Leishmaniasis 5. Trich moniasis 6. Malaria 7. Toxoplasmosis 8. Balantidial Dysentery.

Protozoan Disease # 1. Amoebiasis:

Amoebiasis, also known as amoebic dysentery, is caused by Entamoeba histolytica. Infection generally occurs through drinking water. The trophozoite of E. histolytica penetrates the wall of the colon, secretes histolytic enzymes and feeds upon its cells causing ulcers.

These ulcers rupture and discharge mucus and blood into the intestine that pass along with stools and results in amoebic dysentery. If the infection is allowed to continue the parasite may reach the liver, lungs and brain where it causes abscesses which prove fatal.

There is no intermediate host in the life cycle of E. histolytica. Transmission of the parasite from man to man takes place through the tetra nucleate cysts. Before the cyst-formation the trophozoite changes into a smaller minuta form, which then encysts to form a tetra nucleate cyst.

These tetra nucleate cysts are voided with the faecal and contaminated water and food and are then transmitted into new hosts. 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 has often led to the occurrence of disease.

Occasionally drinking water supply contaminated with infected faeces gives rise to epidemics. Houseflies may transmit cysts while passing from faeces to unprotected foodstuffs. The cysts of E. histolytica have been found in the droppings of cockroaches which also serve as a source of infection.

Amoebiasis is endemic in tropical countries. It can be treated with Emetine, Fumagillin, Metronidazole, Tinidazole, Terramycin, Erythromycin, Aureomycin and Chloroquine, etc.

Protozoan Disease # 2. Diarrhoea:

Diarrhoea, which is characterised by loose bowels, is caused by a flagellete parasite Giardia (=Lamblia) intestinalis and a sporozoan Isopora hominis. Giardia intestinalis is a parasite in the small intestine and colon of man.

It has an elliptical or pear-shaped body which is bilateraly symmetrical with dorsal side convex and ventral side flattened and deepened anteriorly to form a concave sucking disc. It bears two nuclei and four pairs of long flagella arranged symmetrically.

With the help of sucking disc the parasite attaches itself on to the convex surface of the epithelial cells in the intestine and may cause a disturbance of intestinal function leading to malabsorption of fat which causes diarrhoea. Consequently the patient may complain of persistent looseness of bowels. The parasite is also capable of producing epigastric pain, abdominal discomfort, loss of appetite, headache and toxic effect (allergy).

Infection in man is brought about by ingestion of cysts. Transmission of the parasite takes place through cysts which are voided with faeces and are transmitted to new hosts with contaminated water and food. The infection of Giardia is more common in children than in adults.

Atebrin, Chloroquine and Acranil are effective drugs in the treatment of Giardia. Metronidazole has also been reported to be quite effective in its treatment.

Isopora hominis is a rare infectious parasite amongst human beings inhabiting the small intestine of man and may cause intestinal symptoms like colic and diarrhoea. Dogs are supposed to be reservoir hosts of I. hominis.

Protozoan Disease # 3. Trypanosomiasis:

Trypanosomiasis is caused by the species of Trypanosoma which are flagellate parasites of blood plasma (in vertebrate hosts) and gut (in invertebrate hosts). Trypanosoma is generally transmitted by blood-sucking insects. It is the most dreadful of all pathogenic protozoans. Sleeping sickness is a dangerous disease of man in Africa.

Three species cause sleeping sickness in man which are as follows:

(i) Trypanosoma gambiense is transmitted by tsetse flies, Glossina palpalis and G. tachinoides. It causes Gambian or Central African sleeping sickness.

(ii) Trypanosoma rhodesiense is transmitted by tsetse fly Glossina morsitans. It causes Rhodesian or East African sleeping sickness.

Both the above species of Trypanosoma are confined to those parts of Africa where tsetse flies, their vectors, are found.

On infection by the parasite trypanosome fever is caused during which the parasite lives freely in the blood, then the parasites collect in the lymph glands, spleen and liver causing their enlargement, finally they enter the cerebrospinal fluid causing sleeping sickness which results in coma and eventually in death.

Suramin and Pentamidine are considered to be the drugs of choice for early and acute infection. As they cannot pass the blood-brain barrier, they are not of any value when the central nervous system is involved in which case an arsenical is needed. The arsenicals include Tryparsamide, Melarsen, Melarsoprol (Mel B) and Trimelarsen. Nitrofurazone, an oral trypanoside may be used in cases resistant to arsenic.

(iii) Trypanosoma cruzi is transmitted by a bug called Triatoma megista. Transmission to man is not due to bug’s bite but through its faeces. It causes Chagas’ disease or American trypanosomiasis in South and Central America. Chagas’ disease is similar to sleeping sickness. It causes continuous fever, lymph glands, spleen and liver are swollen with degeneration of infected cells, and disorders of the nervous system.

Anaemia and injury to heart muscles lead to death. No permanent cure was suggested for this disease. Recently Melzer and Kollert (1963) suggested successful treatment of a case of T. cruzi with Nitrofurazone tablet, giving a total dosage of 18.375 gm in 27 days.

Protozoan Disease # 4. Leishmaniasis:

Leishmaniasis is caused by the species of Leishmania, the flagellate parasite in the reticulendothelial cells of vertebrate host, man and in the gut of an invertebrate host, the blood sucking fly, Phlebotomus.

The genus Leishmania includes three species which are common parasites of man, viz.:

(i) Leishmania donovani;

(ii) Leishmania tropica and

(iii) Leishmania brasiliensis.

(i) Leishmania donovani causes kala-azar or visceral leishmaniasis which is widespread and endemic in many places in India, China, Africa, Southern Europe, South America and Russia. Its vector is a sand fly, Phlebotomus. In kala-azar the parasite attacks the endothelial cells, bone marrow, liver, lymph glands and blood vessels of the spleen.

These organs are enlarged and there is a bloodlessness and high fever. If left untreated, 75 to 95 per cent of the patients die within a period of two years. Treatment with antimony compounds proves successful. Urea stibamine, Aminostiburea, Neostibosan, Solistibosan, Sodium-antimony-gluconate and Pentamidine isoethionate are most effective drugs.

(ii) Leishmania tropica causes Oriental sore (Tropical sore) or Delhi boil. The infection is limited to a local lesion of the skin and subcutaneous tissues which turn into ulcerating wounds.

Its vector is Phlebotomus, a sandfly. This parasite is found along the shores of Mediterranean through Syria, Arabia, Mesopotamia, Iran to Central Asia, the drier parts of Central and Western India and also in many places of Central Africa. Treatment includes regular cleaning and dressing of the boils, Pentavalent preparation of antimony.

Dehydroemetine orally in doses of 100 mg daily for 10-21 days has given satisfactory result.

(iii) Leishmania brasiliensis causes a disease called Espundia or American leishmaniasis producing multiple sores over large areas of the skin and oro-nasal mucosa. Ulceration in nasal cavities, mouth and pharynx is quite frequent. The vectors are anthropophilic sandflies.

The parasite is confined to Central and South America. Treatment includes Pentavalent preparation of antimony. In resistant cases Pyrimethamine or Amphotericin B may be useful.

Protozoan Disease # 5. Trich moniasis:

Trichomoniasis is caused by the species of flagellate parasite, Trichomonas. Its body is pear-shaped provided with one nucleus, an axostyle, a parabasal body, 3- 5 anterior free flagella, and one backwardly directed flagellum along the side of the body. Trichomonas are parasites in vertebrates and many invertebrates.

Three species are found in man which are:

(i) Trichomonas hominis,

(ii) Trichomonas lenax and

(iii) Trichomonas vaginalis.

The most common pathogenic species is Trichomonas vaginalis. It inhabits the vagina of women and causes vaginitis. The disease is characterised by inflammation of vaginal mucosa, burning sensation, annoying itch and abnormal discharges.

Transmission of parasite is always during sexual intercourse by male members who act as intermediaries. T. vaginalis is also found in urinary tract of men infecting the urethra and prostate. Arsenic and iodine drugs and antibiotics such as Terramycin and Aureomycin have proved useful in the treatment of the disease.

Protozoan Disease # 6. Malaria:

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.

There are following four species of Plasmodium which cause human malaria:

(i) Plasmodium vivax causes benign malaria in which fever comes on every 48 hours;

(ii) Plasmodium malaria causes quartan malaria in which fever comes every 72 hours;

(iii) Plasmodium falciparum causes malignant sub-tertian malaria in which the fever is more or less continuous;

(iv) Plasmodium ovale causes mild tertian malaria in which fever comes on every 48 hours.

All the four species multiply asexually in cells of the liver and erythrocytes of man. In malaria the spleen is enlarged, erythrocytes decrease in number, the blood becomes watery, pigment granules collect in the spleen and there is high temperature accompanied by chills and shivering.

Malaria not only causes millions of deaths annually in the tropics but it also prevents the cultivation of the most fertile regions of the earth. Various drugs which are now used for the treatment of malaria include Quinine, Camoquine, Chloroquine, Plasmoquine, Resochin, Pentaquine, Pamaquine, Paludrine, etc.

Protozoan Disease # 7. Toxoplasmosis:

Toxoplasmosis is caused by a sporozoan parasite, Toxoplasma gondii. Human infection of Toxoplama gondii has been reported from European countries, Middle East, Sri Lanka, U.S.A, Australia, Hawaii and many other places. The infection appears to be cosmopolitan.

The dissemination of the parasite occurs through the blood stream ultimately localising in various organs such as brain, spinal cord, eyes, lungs, liver, spleen, bone marrow, lymph nodes, heart muscles and skeletal muscles.

The parasites multiply by endodyogeny but under certain conditions large cysts are also formed. Symptoms of the disease are hydrocephalus and chorioretinitis. Infection occurring in early months of pregnancy results either in abortion or still birth of the foetus. Pyrimethamine (Daraprim) combined with Sulphadiazine have been found to be an effective remedy.

Protozoan Disease # 8. Balantidial Dysentery:

Balantidial dysentery is caused by a ciliate parasite, Balantidium coli. It inhabits the large intestine of man. It may bore into the tissues of the intestine causing ulcers which results in dysentery and diarrhoea. This may prove fatal. The transmission of the parasite to a new host takes place through cysts in contaminated water and food.

Drugs used for the treatment are Carbarsone, Diodoquin and Oxytetracycline have been found to be effective.