Trophozoites

Active feeding and growing stage of the protozoa is called the trophozoites. It derives  nutrition from  the environment by diffusion, pinocytosis, and  phagocytosis. 

1.0Giardia lamblia

• These organisms, also referred to as G. duodenalis or G.intestinalis, were initially described by van Leeuwenhoek.
• They are the most common pathogenic parasitic infection in humans and are spread by fecally contaminated water or food.
• Infection may occur after ingestion of as few as 10 cysts.. 
• Giardia trophozoites can be identified in duodenal biopsies by their characteristic pear shape Infectious Enterocolitis with two nuclei of equal size, each of which contains a complete copy of the genome. 
• Despite large numbers of trophozoites, which are sickle-shaped in profile and tightly bound to the brush border of villous enterocytes, there is no invasion and small intestinal morphology may be normal by light microscopy .
• In the gut, cysts convert to motile trophozoites and attach to sugars on the intestinal epithelia through surface lectins.

Habitat:

• G. lamblia lives in the duodenum and upper jejunum and is the only protozoan parasite found in the lumen of the human small intestine.

Morphology:

• It exists in 2 forms: 
• Trophozoite (or vegetative form) 
• Cyst (or cystic form). 

Trophozoite

• The trophozoite is in the shape of a tennis racket(heart shaped or pyriform shaped) and is rounded anteriorly and  pointed posteriorly . 
• It measures 15 µm x 9 µm wide and 4 µm thick.  Dorsally, it is convex and ventrally, it has a concave  sucking disc, which helps in its attachment to the  intestinal mucosa.
• It is bilaterally symmetrical and possesses. 1 pair of nuclei  4 pairs of flagella Blepharoplast, from which the flagella arise (4 pairs) 1 pair of axostyles, running along the midline 
• Two sausage shaped parabasal or median bodies,  lying transversely posterior to the sucking disc.
• The trophozoite is motile, with a slow oscillation about  its long axis, often    resembling falling leaf.

Life Cycle:

• Giardia passes its life cycle in 1 host.
• Infective form: Mature cyst.

Mode of transmission:

• Man acquires infection by ingestion of cysts in  contaminated water and food. 
• Direct person to person transmission may also  occur in children, male homosexuals, and mentally ill persons.  
• Enhanced susceptibility to giardiasis is associated  with blood group A, achlorhydria, use of cannabis,  chronic pancreatitis, malnutrition, and immune  defects such as 19A deficiency and hypogammaglobulinemia. 
• Within half an hour of ingestion, the cyst hatches out  into two trophozoites, which multiply successively by  binary fission and colonize in the duodenum.
• The trophozoites live in the duodenum and upper part  of jejunum, feeding by pinocytosis. 
• During unfavorable conditions, encystment occurs  usually in the colon.
• Cysts are passed in stool and remain viable in soil and  water for several weeks. 

Pathogenicity and Clinical Features:

• G. lamblia is typically seen within the crypts of duodenal  and jejunal mucosa.
• It does not invade the tissue, but  remains tightly adhered to intestinal epithelium by means  of the sucking disc. 
• They may cause abnormalities of villous architecture  by cell apoptosis and increased lymphatic infiltration of  lamina propria. 
• Variant specific surface proteins (VSSP) of giardia play an  important role in virulence and infectivity of the parasite. 
• Often they are asymptomatic, but in some cases,  Giardia may lead to mucus diarrhea, fat malabsorption  (steatorrhea), dull epigastric pain, and flatulence. The  stool contains excess mucus and fat but no blood. 
• Children may develop chronic diarrhea, malabsorption  of fat, vitamin A, protein, sugars like xylose disaccharides,  weight loss, and sprue-like syndrome. 
• Occasionally, Giardia may colonize the gallbladder,  causing biliary colic and jaundice.
• Incubation period is variable, but is usually about 2 weeks. 

Diagnosis:

• Giardiasis can be diagnosed by identification of cysts of  Giardia lamblia in the formed stools and the trophozoites  and cysts of the parasite in diarrheal stools.

2.0Balantidium coli 

• Balantidium coli belongs to the Phylum Ciliophora and Family Balantididae.
• It is the only ciliate protozoan parasite of humans. 
• It is the largest protozoan parasite of humans.

1. History and Distribution

• It was first described by Malmsten in 1857, in the feces of dysenteric patients.
• It is present worldwide, but the prevalence of the infection is very low.
• The most endemic area is New Guinea, where there is a close association between man and pigs.

2. Habitat

• B. coli resides in the large intestine of man, pigs, and monkeys.

3. Morphology

• B. coli occurs in 2 stages - trophozoite and cyst.

Trophozoite

• The trophozoite lives in the large intestine, feeding on cell debris, bacteria, scratch grains and other particles.
• The trophozoite is actively motile and is an invasive stage of the parasite found in dysenteric stool.
• It is a large ovoid cell, about 60-70 µm in length and 40-50 µm in breadth. Very large cells, measuring upto 200 µm are sometimes seen.
• The cell is enclosed within a delicate pellicle showing longitudinal striations.
• The motility of trophozoite is due to the presence of short delicate cilia over the entire surface of the body.
• Its anterior end is narrow and the posterior end is broad.
• At the anterior end, there is a groove (peristome) leading to the mouth (cytostome), and a short funnel- shaped gullet (cytopharynx).
• Posteriorly, there is a small anal pore (cytopyge).
• The cilia around the mouth are larger (adoral cilia).

Key points of Balantidium coli:

• It is the only ciliate parasite of humans.
• Largest protozoan parasite residing in the large intestine.
•  It occurs in 2 stages: trophozoite and cyst.
• Trophozoite is oval shaped with a slightly pointed anterior end with a groove, peristome leading to the mouth, cytostome. Rounded posterior end has a small anal pore, cytopyge and has a large kidney- shaped macronucleus and small micronucleus.
• Cyst: It is the infective stage of the parasite.
• Mode of infection: Infection is acquired from pigs and other animals by ingestion of cysts in contaminated food and drink.
• Infection leads to mucosal ulcers and submucosal abscess in the intestine.
• Clinical features: Most infections are asymptomatic. In mild infections, it causes diarrhea, abdominal colic, tenesmus, nausea, and vomiting.
• Diagnosis: Based on demonstration of trophozoites and cysts in feces and examination of biopsy specimens and scrapings from intestinal ulcers.
• Treatment: Tetracycline is the drug of choice.
• Prophylaxis: Avoiding contamination of food and water and treatment of infected pigs and persons.

3.0Malaria Parasite 

• Phylum: Apicomplexa  
• Class: Sporozoa  
• Order: Haemosporida  
• Genus: Plasmodium.  
• The genus Plasmodium is divided into 2 subgenera, P.  vivax, P. malariae and P. ovale belong to the subgenus  Plasmodiumwhile P. falciparum is allocated to subgenus  Laverania because it differs in a number of aspects from  the other 3 species. 
• P. vivax, P. malariae, and P. ovale are closely related to  other primate malaria parasites. 
• P. falciparum on the  other hand, is more related to bird malaria parasites  and appears to be a recent parasite of humans, in  evolutionary terms. 
• Perhaps for this reason, falciparum  infection causes the severest form of malaria and is  responsible for nearly all fatal cases.

Life Cycle:

• Malaria parasites pass their life cycle in 2 hosts.  Definitive host: Female Anopheles mosquito. Intermediate host: Man. 
• The life cycle of malarial parasite comprises of 2 stages— an asexual phase occurring in humans, which act as  the intermediate host and a sexual phase occurring  in mosquito, which serves as a definitive host for the  parasite 

Asexual Phase:

• In this stage, the malaria parasite multiplies by division  or splitting a process designated to as Schizogony (from schizo: to split, and gone: generation).  
• Because this asexual phase occurs in man, it is also called the vertebrate, intrinsic, or endogenous phase.  
• In humans, schizogony occurs in 2 locations—in the  red blood cell (erythrocytic schizogony) and in  the liver cells (exoerythrocytic schizogony or the  tissue phase).  
• Because schizogony in the liver is an essential step  before the parasites can invade erythrocytes, it is  called pre-erythrocytic schizogony.  
• The products of schizogony, whether erythrocytic  or exoerythrocytic, are called merozoites (meros: a  part, zoon: animal). 
• In the erythrocyte, the merozoite loses its internal  organelles and appears as a rounded body having a  vacuole in the center with the cytoplasm pushed to the  periphery and the nucleus at one pole. These young  parasites are, therefore, called the ring forms or young  trophozoites.
• The parasite feeds on the hemoglobin of the erythrocyte. It does not metabolize hemoglobin completely and therefore, leaves behind a hematin-globin pigment called the malaria pigment or hemozoin pigment, as residue.
• The malaria pigment released when the parasitized cells rupture is taken up by reticuloendothelial cells. Such pigment-laden cells in the internal organs provide histological evidence of previous malaria infection.
• As the ring form develops, it enlarges in size becoming irregular in shape and shows amoeboid motility. This is called the amoeboid form or late trophozoite form.
• When the amoeboid form reaches a certain stage of development, its nucleus starts dividing by mitosis followed by a division of cytoplasm to become mature schizonts or meronts.

Plasmodium Vivax

• P. vivax has the widest geographical distribution, extending through the tropics, subtropics and temperate regions. It is believed to account for 80% of all malaria infections.
• Trophozoite is actively motile, as indicated by its name vivax. The ring form is well-defined, with a prominent central vacuole. One side of the ring is thicker and the other side thin. Nucleus is situated on the thin side of the ring. The ring is about 2.5-3 µm in diameter, about a third of the size of an erythrocyte. The cytoplasm is blue and the nucleus red in stained films. The ring develops rapidly to the amoeboid form and accumulates malarial pigment.
• The infected erythrocytes are enlarged and show red granules known as Schuffner's dots on the surface. They become irregular in shape, lose their red color, and present a washed out appearance. A few of the parasitized erythrocytes retreat into the blood spaces of the internal organs.

Plasmodium Falciparum

• The name falciparum comes from the characteristic sickle shape of the gametocytes of this species (falx: sickle, parere: to bring forth). This is the highly pathogenic of all the plasmodia and hence, the name malignant tertian or pernicious malaria for its infection.
• Ring form: The early ring form in the erythrocyte is very delicate and tiny, measuring only a sixth (1/6) of the red cell diameter. Rings are often seen attached along the margin of the red cell, the so-called form applique or accole. Binucleate rings (double chromatin) are commonly resembling stereo headphones in appearance. Several rings may be seen within a single erythrocyte. In course of time, the rings become larger, about a third of the size of the red cell and may have 1 or 2 grains of pigment in its cytoplasm.
• The subsequent stages of the asexual cycle–late trophozoite, early and mature schizonts–are not ordinarily seen in peripheral blood, except in very severe or pernicious malaria. The presence of P. falciparum schizonts in peripheral smears indicates a grave prognosis