Morphology

 

Balantidium coli has 2 developmental stages: a trophozoite stage and a cyst stage. (See also Life Cycle.)

 

Trophozoites live in the large intestines of the host animals.  They are covered in cilia and have boring or rotary motility.  Balantidium coli is known for being the largest protozoan parasite of humans, and it is for the trophozoite stage that it earns this distinction.  Trophozoites can measure between 50-130 mm long by 20-70 mm wide.

 

In trophozoites, the two nuclei are clearly visible.  The macronucleus is long and kidney-shaped, and the spherical micronucleus is nestled next to it.

 

Other distinguishing traits of the trophozoite include the opening, known as the peristome, at the pointed anterior end, which leads to the cytostome, or cell mouth.  Balantidium coli reproduces during the trophozoite stage either by asexual transverse binary fission or sexual conjugation.

 

 

The cyst is the infective stage of the Balantidium coli life cycle.

 

Encystation is the process of forming the cyst; this event takes place in the rectum of the host as feces are dehydrated or soon after the feces have been excreted.

 

Excystation produces a trophozoite from the cyst stage, and it takes place in the large intestine of the host after the cyst has been ingested.

 

Cysts are smaller than trophozoites, measuring 40-60 mm across.  Cysts are round and have a tough, heavy cyst wall made of one or two layers.  Usually only the macronucleus and perhaps cilia and contractile vacuoles are visible in the cyst.

 

 

See the table below for a summary of the differences between trophozoites and cysts.

 

 

 

Trophozoite

Cyst

Shape

Oval, pointed at anterior end

Spherical

Size

50-130 mm long by 20-70 mm wide

40-60 mm across

Surface

Covered in cilia

Covered with thick, hard cyst wall with cilia sometimes visible underneath

Motility

Rotary or boring motility, “like a thrown football”

Non-motile

Infectious

Not infective

Infective

Reproduction

By binary fission or conjugation

Non-reproductive

Nuclei

Macronucleus (kidney-shaped) and micronucleus (spherical, next to macronucleus) visible

Only macronucleus (kidney-shaped) visible; contractile vacuole visible in young cysts; in older cysts, organelle structures look granular

Important cell structures

Funnel-shaped cytostome (cell mouth) near anterior end; 2 contractile vacuoles

Cyst wall made of one or two layers

Diagnosis

Occasionally found in feces, often found in tissue biopsies of infected individuals

Diagnostically found in feces of infected individuals

Drawing

 

 

Introduction

 

The Parasite

Morphology

Life Cycle

Transmission

Animal Reservoirs

Clinical Presentation

 

Diagnosis

Treatment

Epidemiology

Public Health Interventions

 

Glossary of terms

References and links