Parasites, in general, may be single-celled or multicellular.

Single celled parasites are known as Protozoa.

Multi cellular parasites are known as Metazoa or Helminths or more commonly as worms.

Protozoans can be further sub divided into

Sarcodina, includes the amebas like Entamoeba histolytica (amebiasis), Entamoeba coli, Entamoeba dispar; Acanthameba and Naeglaria spp.(meningoencephalitis).

Sporozoa, includes sporozoans like Cryptosporidium spp.(Cryptosporidiosis), Plasmodium spp. (malaria), Toxoplasma spp.(Toxoplasmosis), Cyclospora spp., Isospora spp. Microsporidia(diarrhoea), Babesia spp.(influenza like symptoms.

Mastigophora, includes flagellates such as Giardia lamblia (Giardiasis, watery foul-smelling diarrhea), Trichomonas vaginalis (Trichomoniasis), Trypanosoma spp. Such as cruzi (Chagas disease), gambiense (Sleeping sickness) , Leishmania spp. donovani ( Kala-azar), tropica, mexicana and brasiliensis ( Leishmaniasis.

Ciliata, includes ciliates like Balantidium (diarrhea).

Metazoans are sub-divided into Platyhelminthes, commonly known as Flat-worms and Nemathelminthes commonly known as roundworms or nematodes.

Platyhelminthes have two important classes that are clinically important that are:

Cestoda or tapeworms, such as Taenis spp. solium(cysticerosis), saginata(taeniasis); Diphyllobothrium spp.(diphyllobothriasis); Echinococcus spp.(Echinococcosis)

Trematoda or flukes, such as Schistosoma spp.(schistosomiasis); Clonorchis spp.(clonorchiasis); Paragonimus(paragonimiasis).

Diseases caused by Intestinal protozoa:

Entamoeba histolytica causes amebic dysentry and liver abscesses.

The life cycle includes two significant stages of Trophozoites (motile, feeding, reproducing form surrounded by a cell membrane) and

Cysts ( nonmotile, nonreproducing surrounded by a thick wall).

Pathogenesis and Clinical findings:

The parasite is acquired by the ingestion of the cysts through the fecal-oral route (contaminated food and water).

The ingested cysts differentiate into trophozoites in the ileum.

These trophozoites then colonize the cecum and the colon.

The trophozoites have the capability to secrete necrotic enzymes and cause localized necrosis in the colonic epithelium.

Further invasion into the muscularis layer results in flask shaped ulceration that damages the intestinal epithelium significantly.

They can also form a granulomatous lesion called an ameboma in the cecum or recto-sigmoid areas of the colon, that may resemble as and must be distinguished from a adenocarcinoma of the colon.

Any further invasion results in the entry of the trophozoites into the portal circulation, now having the potential to cause a systemic disease involving the liver causing abscess that can penetrate the diaphragm and cause lung disease.

Acute intestinal amebiasis presents as a bloody, mucus containing diarrhea accompanied by lower abdominal discomfort, and flatulence.

Chronic amebiasis presents with milder symptoms such as occasional diarrhea, weight loss and fatigue.

Amebic abscess of the liver is characterized by more severe symptoms with a right upper-quadrant pain, weight loss, fever and a tender enlarged liver.

Aspiration of the liver abscess yields a brownish-yellow pus.

Laboratory diagnosis:

Examination of diarrheal stools reveal trophozoites characteristically containing ingested red blood cells.

Non-diarrheal stools often reveal the presence of cysts. Cysts however are passed intermittently and hence at least three specimens should be examined.

The cysts typically contain four nuclei and are an important diagnostic criteria.

Serological testing can also be useful as antibodies although non-protective are produced against the trophozoite antigens especially in invasive amebiasis.

The indirect hemagglutination test is usually positive and diagnostic in patients with an invasive disease.

It is especially important to distinguish E.histolytica from other spp. of Entamoeba.

Their trophozoite nucleus, typically possess a central nucleolusand fine chromatin granules along the nuclear membrane, unlike other Entamoebas.

The E.histolytica cysts are smaller and contain four nuclei unlike others that may contain eight nuclei.

PCR based assays to detect E.histolytica antigens and nucleic acids are however highly specific.

A wet mount in saline or iodine may also be useful to distinguish between amebic and bacillary dysentry which may contain many inflammatory cells such as polymorphonuclear leucocytes.

Treatment of choice for symptomatic intestinal amebiasis and liver abscesses is Metonidazole.

Asymtomatic carriers treated with iodoquinol or paromomycin.

Prevention to avoid fecal contamination of cultivated and consumed food.

Boiling water is effective as cysts not very resistant and get killed.

Chlorination, however may not be effective.

Giardia lamblia causes Giardiasis.

Giardiasis is characterized by watery but non-bloody, foul smelling diarrhea accompanied by persistent nausea, flatulence and abdominal cramps but no fever.

Their life cycle also include the trophozoite and a cyst stage.

The trophozoite is pear-shaped with two nuclei, four pairs of flagella and a suction disk that helps attach to the intestinal walls.

The cysts are oval, thick walled and several characteristic internal fibres. On encystation in the intestinal tract, each cyst gives rise to two trophozoites

Pathogenesis:

Ingested cysts through contaminated food and water (fecal-oral route) results in encystation in the duodenum.

The trophozoite thus formed now attaches to the intestinal walls. They do not invade the mucosa and do not enter the blood stream.

This however leads to inflammation of the duodenal mucosa affecting the absorption of proteins and fats.

Laboratory diagnosis:

Examination of diarrheal stools may show the presence of both trophozoites and cysts.

Non-diarrheal stools may contain cysts alone.

ELISA test to detect the presence of cyst wall antigen is a important diagnostic criteria.

Treatment and prevention:

Treatment of choice is Metronidazole or quinacrine hydrochloride.

Prevention includes boiled, filtered or iodine treated water.

Cryptosporidium parvum or Cryptosporidium hominis causes Cryptosporidiosis.

Cryptosporidiosis manifests itself as a watery, non-bloody diarrhea causing large fluid loss, especially in the more severe forms in immunocompromised individuals.

The life cycle of Cryptosporidium parvum, belonging to sporozoa sub-group, involves both the sexual and asexual cycles involving oocytes, that result in schizonts, then merozoites , followed by micro(male) and macro(female) gametes , that ultimately unites to form a zygote and differentiates finally into an oocyst.

Pathogenesis:

Involves the transmission or ingestion of oocytes through the fecal-oral route.

The oocysts excyst in the jejunum of the small intestines, where the trophozoites attach to the gut wall.

Invasion does not occur. The pathogenesis of diarrhea is as yet

uncertain.

Laboratory diagnosis involves the finding of oocysts in fecal smears using a modified acid-fast staining technique.

Treatment may not be required in immunocompetent patients as the disease is self-limiting. Nitazoxanide is the drug of choice, if necessary.

For immuno-compromised patients however, paromomycin may just help in reducing diarrhea.

Prevention using purified water, either boiled or filtered . Chlorination is not effective.

Among the clinically significant urino-genital protozoa is Trichomonas vaginalis.

They cause Trichomoniasis, one of the most common infections world-wide, that presents a watery, foul-smelling, greenish vaginal discharge in women.

Infected males are generally asymptomatic.

Trichomonas vaginalis is a pear shaped flagellate, possessing four anterior flagella and a central nucleus.

It has an undulating membrane. It exists only as a trophozoite.

Pathogenesis:

Transmission is through sexual contact, hence does not need a cyst form.

It locates itself in the vagina (female) and the prostate(male).

Laboratory diagnosis generally involves a wet mount of vaginal/prostate secretions to find the pear shaped trophozoites with a typical, jerky motion, along with neutrophils.

The drug of choice is Metronidazole for both partners to prevent reinfection.

Maintaining a low pH of the vagina is helpful.