List of seven parasitic forms of flagellates:- 1. Giardia Lamblia 2. Trichomonas Hominis 3. Trichomonas Vaginalis 4. Haemoflagellates 5. Trypanosoma Cruzi 6. Trypanosoma Gambiense 7. Leishmanias.
Parasitic Form # 1. Giardia Lamblia:
It is also known as Giardia intestinalis and it lives as parasite in the intestine of man and causes a disease called giardiasis. The distribution is cosmopolitan. Trophozoites measure 9-20 by 6-15 micra. Cytostome is absent. Protoplasm is clear. Ventral side of the body is flat and dorsal side is convex.
The posterior end is pointed but anterior end is round. A bean- shaped sucking disc is present on the ventral surface. Two elongated nuclei and two parabasal bodies are present.
There are eight rhizoplasts and flagella in following arrangements—right-1, left—1, anterolateral-1, postero-lateral-1, ventral-2 and caudal-2. Cysts measure 8-14 by 6-10 micra and contain 2 to 16 nuclei. Infection occurs through as a trailing or posterior flagellum beyond contaminated drink or food (Fig. 10.7).
Parasitic Form # 2. Trichomonas Hominis:
These cosmopolitan parasites have trophozoites of 5-20 micra and inhabit the intestine of cattle. It also lives as commensal in the colon of man. The cytostome is distinct and parabasal bodies are absent. The protoplasm contains single nucleus and food vacuoles. Number of free flagella varies from 3-5.
Two blepharoplasts are situated in front of and anterior to the nucleus. Three to four flagella arise from the blepharoplast close to the nucleus and are directed anteriorly.
From the other blepharoplast arises the fixed flagellum, costa and axostyle. The fixed flagellum is accompanied by the undulating membrane throughout the whole length of the body and then continues as a posterior flagellum beyond the body length (Fig. 10.8).
Parasitic Form # 3. Trichomonas Vaginalis:
Cosmopolitan; inhabits as a parasite in the vagina of women and also found in the urethra of man. Approximately 10-30 micra in length and more or less oval in shape (Fig. 10.9). Nucleus is elongated; cytostome is less distinct; parabasal body is large; undulating membrane is short. No cyst formation; does not survive more than 24 hours outside the body of the host. Transmission is direct through males.
Parasitic Form # 4. Haemoflagellates:
The Haemoflagellates are a group of flagellates which habitually live in the blood or tissues of man and other vertebrates. The haemoflagellates of man belong to the family Trypanosomatidae.
The family includes two genera namely, Trypanosoma and Leishmania. These parasites are structurally complex and are of considerable pathogenic importance to man. Trypanosomes are uniflagellated blood parasites of man and other vertebrates.
They occur in variety of forms and all these forms (Fig. 10.10) are represented in the life cycle of Herpetomonas muscarum (a member of the family Trypanosomatidae) which is a parasite in house fly. The life cycle of Trypanosoma revolves round two hosts—one vertebrate and the other invertebrate. The trypanosomes show polymorphism, presenting different morphological forms under different conditions.
The polymorphic forms are (Fig. 10.10):
I. Leishmanial form or Amastigote Form:
The body is round or oval with a nucleus and a kinetoplast but no flagellum is present.
II. Leptomonad form or Promastigote Form:
The whole structure is thread-like, nucleus centrally located, blepharoplast is anterior to the nucleus, the rhizoplast arises from the blepharoplast and runs straight up to the anterior extremity and then emerges as a free flagellum twice as long as the body.
III. Crithidial form or Epimastigote Form:
The flagellum is not completely free and runs along the surface and up the anterior end. It is in association with the undulating membrane which is short. Beyond the anterior end the flagellum is free.
IV. Trypanosoma form or Trypomastigote Form:
The blepharoplast is situated behind the nucleus. Flagellum skirts the whole length of the body and remains attached to the undulating membrane.
Leishmanial forms leave the body along with the excreta of the fly. Ingestd Leishmania reaches the oesophagus of vertebrate host and is transformed into leptomonad form.
The genus Trypanosoma includes the typical blood parasites of man and other vertebrates. They are transmitted by the bloodsucking invertebrates from vertebrate to vertebrate. Trypanosomas occur in all vertebrates, but are pathogenic to man and some domestic mammals.
The major pathogenic trypanosomes of man are: Trypanosoma gambiense and T. rhodesiense—the causative agents of African sleeping sickness. The pathogenic trypanosomes have a similar life history. The biological account of some trypanosomes will give an account of the group in general.
Parasitic Form # 5. Trypanosoma Cruzi:
Causative agent of Schizotrypanosomiasis or Chagas’ disease. Distribution has recorded in Central and South America. Trypanosoma forms occur in the blood stream of man but do not multiply there. They are 20 micra in length and 3-7 micra in breadth. Kinetoplast is big and situated posterior to nucleus. Nucleus is elongated. Undulating membrane is narrow, free end of flagellum is not more than half the body length.
Trypanosoma forms change to leishmanial forms and the change is reversible. Leishmanial forms are ovoid and 4 micra in diameter. Presence of distinct nucleus and rod-shaped kinetoplast, short rhizoplast perpendicular to kinetoplast may be seen. Leishmania forms reproduce by binary fission and take refuge in muscle fibres, neurons, testis, thyroid gland and skin (Fig. 10.11).
The blood-sucking hemipterous bugs of the family Triatomatidae are the intermediate hosts. Parasites in trypanosoma forms enter the gut of the bugs and transform to crithidial forms; several weeks after, the crithidial forms switch over to trypanosoma forms and are then called Metacyclic trypanosoma.
Man becomes infected by the deposition of excreta of bugs on the bruised skin, conjunctiva of eye and even lips.
Parasitic Form # 6. Trypanosoma Gambiense:
Causative agent of west African trypanosomiasis or sleeping sickness. They occur in the lymph glands, in reticular tissue of spleen, blood and at a later stage in the cerebro-spinal fluid in trypanosoma forms only and divide by binary fission. The trypanosoma forms are 15-32 micra in length.
The undulating membrane is much convoluted, nucleus is posteriorly placed and kinetoplast is round. Cytoplasm bears volutine granules (Fig. 10.12).
Three types of trypanosoma forms are known:
(ii) Stumpy, and
The intermediate host is the blood-sucking tsetse fly, Glossina palpalis, which infects man in two ways:
(a) Direct Transmission:
When a fly bites a man infected with trypanosoma, some trypanosomas stick to the proboscis and when this fly bites another man the trypasnosoma are introduced into him provided the time between the successive bites do not exceed few hours.
(b) Cyclical Transmission:
When the fly takes the infecting meal the parasites enter the midgut, remain there for two days and start multiplying. To avoid washing out by the movement of gut, the parasites take refuge in the extraperitrophic space—the space between gut wall and peritrophic membrane (a thin membrane which envelops the blood imbibed by the fly) and multiply.
Then they come out in huge numbers to the proventriculus after ten days and reach the salivary gland on the 12th day. They become ready for infection after 20 day.
The fly introduces the trypanosomas in the blood stream of man along its bite (Fig. 10.13).
Among the other Trypanosomas, the Trypanosoma rhodesiense causes east African sleeping sickness; Trypanosoma brucei causes nagana fever of African domestic animals and transmitted by Glossina; Trypanosoma evansi causing surra disease of Indian horses, cattle, camels and transmitted by tabanid flies; Trypanosoma equiperdum causing dourine disease of horses and mules are transmitted directly during coition.
The non-pathogenic trypanosomes also occur in man. Trypanosoma primatum of anthropoid apes, Trypanosoma rangeli of man in Venezuela and Columbia and Trypanosoma rotatorium of frogs are some of the typical non-pathogenic trypanosomes.
Parasitic Form # 7. Leishmanias:
Members of the genus Leishmania which are parasitic to man and other vertebrates occur in Leishmanial forms (flagellaless forms) and in the intermediate hosts they are seen in leptomonad forms (flagellated forms). Three members of the genus are parasites in man and they offer close resemblance to one another.
In man the leishmanias (Fig. 10.14) are intracellular parasites of the reticuloendothelial system namely, the endothelial cells, large mononuclear leucocytes and Kupffer cells of liver. In case of heavy infection they have been found to invade ectodermal cells and polynuclear leucocytes.
Leishmanias are oval in shape and measure 2-4 micra by 1.5-2 micra. The nucleus is elongated with a rod-shaped kinetoplast which is perpendicular to the nucleus. Flagellum is absent. Binary fission is the mode of multiplication. By their successive divisions the parasites become overcrowded in the host cell, which is ultimately destroyed.
The intermediate host is the sand fly belonging to the genus Phlebotomus. The fly ingests leishmanias along with the blood of the vertebrate host. In the midgut of the fly the parasites increase in size, develop flagella and are metamorphosed into long, slender Leptomonad forms in four days. The leptomonas multiply vigorously by binary fission and reach the proventriculus of the fly.
Repeated multiplication inside the proventriculus causes complete obstruction of the organ. As a result, when the sand fly tries to ingest blood, the meal goes no further than the oesophagus. This causes a regurgitation of the sucked blood and the leptomonads are introduced in the blood stream along with the regurgitation.
It resides in the viscera and is the causative agent of visceral leishmaniasis or fatal kala-azar. It is prevalent in eastern India, China, central Asia, east Africa, South America and Russia.
Man becomes infected by becoming associated with a sylvatic or non-domestic reservoir jackal and domestic reservoir dog. Spreading of the disease among human being is caused by the intermediate host Phlebotomus (Indian vector, Phlebotomus argentipes).
(i) In kala-azar, the parasite attacks the endothelial cells, bone marrow, Kupffer cells of liver, blood vessels of the spleen and lymph glands (lymph nodes).
(ii) These organs are enlarged and there is a symptom of bloodlessness and high fever.
The control of sand fly (Phlebotomus sp.) is to control of mosquito of malaria.
(i) If antimony compounds are treated to the patients, proves successful.
(ii) Urea stibamine, Amino stiburea, Solistibosan, Pentamidine isothionate may be the effective drugs.
Resides in human skin and is the causative agent of the cutaneous leishmaniasis or oriental boil and oriental sore. It is most predominant in the old world. Sylvatic reservoir is wild rodents and the domestic reservoir is dog. Transmission is through Phlebotomus (Indian vector, Phlebotomus sergenti).
It resides in the cutaneous and mucocutaneous parts of human body and is the causative agent of Espundia—a serious disease of buccal and nasal cavities. It is prevalent in the New- world. Sylvatic reservoir is a rodent and opossum and the domestic reservoir is dog. Transmission occurs, through Phlebotomus.