Chapter 9

• Ineffective Stage


  Stage of the parasite that enters the host or the stage that is present in the parasite's source of infection
• Pathogenic Stage


  Stage of the parasite that is responsible for producing the organ damage in the host leading to the clinical manifestations
• Encystation


  Process by which trophozoites differentiate into cyst forms
• Excystation


  Process by which cysts differentiate into trophozoite forms
• Kingdom Protozoa


  • Single-celled eukaryotic organisms
  • Spherical or elongated in shape
  • Classification is based on the organ of locomotion
  • Not all protozoan are parasitic

  
• Types of protozoa based on locomotion

  • Amoeba – pseudopods (false feet)
  • Flagellates – flagella
  • Ciliates – cilia
  • Sporozoans – non-motile (obligate parasites)

  
• Facultative parasites


  Capable of free-living state
• Facultative parasites


  • Acanthamoeba and Naegleria
• Majority of protozoa divide by means of binary fission (asexual reproduction)
• Sporozoans reproduce by both sexual and asexual means
• Stages of parasitic protozoa

  • Trophozoite – motile, feeding, dividing stage of parasite
  • Cysts – dormant non-motile form

  
• For most intestinal protozoan parasites, trophozoite is the pathogenic stage and cysts are the infective stage

  
• For Trichomonas vaginalis, cyst forms are not found
• Intestinal protozoa


  • Entamoeba histolytica
  • Giardia lamblia (Giardia intestinalis)
  • Trichomonas vaginalis
  • Balantidium coli
• Entamoeba histolytica


  • Intestinal and tissue amoeba
  • Only known pathogenic intestinal amoeba
• Stages of Entamoeba histolytica

  • Motile Trophozoite (Pathogenic Stage)
  • Non-motile Cyst (Infective Stage)
• Epidemiology of Entamoeba histolytica

  • More common in tropical countries, in areas with poor sanitation
  • Transmitted via fecal-oral route through ingestion of the cyst from contaminated food and water
  • Water serves as a major source of infection
  • Sexual transmission may occur
• Life cycle of Entamoeba histolytica

  1. Ingested cyst resists gastric acid in the stomach

  2. Ingested cyst undergoes excystation in the ileum where it differentiates into a trophozoite (pathogenic stage)

  3. Trophozoite colonizes the cecum and colon

  4. Trophozoite may then undergo encystation and become converted into cysts, which then passed out with the feces

  5. Trophozoites with active infection are found in diarrheic stools while cysts are found in formed, non-diarrheic stools
• Manifestations of Amoebiasis


  • Acute Intestinal Amoebiasis
  • Extraintestinal amoebiasis
  • Asymptomatic carrier state
• Acute Intestinal Amoebiasis


  • Presents as blood : mucus-containing diarrhea (dysentery) accompanied by lower abdominal discomfort, flatulence, and tenesmus
  • Chronic inflammation may occur, with symptoms such as occasional diarrhea, weight loss and fatigue
  • Amoeboma may form in the cecum or in the rectosigmoid area of the colon
• Extraintestinal amoebiasis


  • Occurs when the parasite enters the circulatory system
  • Amoebic liver abscess is the most common extraintestinal form
  • Characterized by right upper quadrant pain, weight loss, fever, and a tender, enlarged liver
  • Abscess found on the right lobe of the liver may penetrate the diaphragm and cause lung disease
  • Other organs that may become infected include the pericardium, spleen, skin and brain
• Asymptomatic carrier state


  • Occurs if the parasite involved is a low-virulence strain, if the parasite load is low, or if the patient's immune system is intact
  • Patient presents no symptoms but the parasite reproduces and is passed out with the patient's feces
• Laboratory diagnosis of Entamoeba histolytica

  • Trophozoites in diarrheic stools or cysts in formed stools
  • Trophozoites characteristically contain ingested red blood cells
  • Stool specimen should be examined within an hour of collection to see the motility of the trophozoites
  • Serologic testing may be useful for the diagnosis of invasive amoebiasis
• Treatment of Entamoeba histolytica

  • Metronidazole is the treatment of choice for symptomatic intestinal amoebiasis or hepatic abscess
  • Tinidazole is an alternative drug for both intestinal and extraintestinal amoebiasis
  • Asymptomatic carriers should be treated with diloxanide furoate, metronidazole, or paromomycin
• Prevention and control of Entamoeba histolytica

  • Good personal hygiene; by washing of hands, especially for food handlers
  • Proper waste disposal to avoid fecal contamination of water sources
  • Avoid the use of "night soil" (human feces) for fertilization of crops
  • Adequate washing and cooking of vegetables should be observed
• Giardia lamblia (Giardia intestinalis)

  • Initially known as Cercomonas intestinalis
  • Another name used is Giardia duodenale
• Stages of Giardia lamblia

  • Trophozoite form
  • Cyst form
• Trophozoite form of Giardia lamblia


  • Pear-shaped or teardrop-shaped with four pairs of flagella and has a motility likened to a falling leaf
  • Resembling to an old man with whiskers ("old man facies")
  • Possess a sucking disc which the parasite uses to attach itself to the intestinal villi of the infected human
• Cyst form of Giardia lamblia

  • Oval and thick-walled with four nuclei
  • Divides through binary fission
  • Each cyst gives rise to two trophozoites during excystation in the intestinal tract
• Epidemiology and pathogenesis of Giardia lamblia


  • Worldwide distribution through contaminated water sources
  • 50% of infected individuals do not present with symptoms and serve as carriers
  • Many species of mammals may act as reservoirs
  • Infection is also common among individuals engaging in oral-anal contact
  • High incidence has been seen in daycare centers and among patients in mental hospitals
• Life cycle of Giardia lamblia

  1. Parasite is transmitted through ingestion of the cyst from fecally-contaminated water and food

  2. Cyst enters the stomach and is stimulated by the gastric acid to undergo excystation in the duodenum

  3. Trophozoites then attach themselves to the duodenal mucosa through the sucking disks

  4. Damage to the intestines is not due to invasion of the parasite but because of inflammation of the duodenal mucosa, leading to diarrhea with malabsorption of fat and proteins

  5. Trophozoites may also infect the common bile duct and gallbladder
• Manifestations of Giardiasis


  • Asymptomatic carrier state
  • Giardiasis (Traveler's diarrhea)
• Asymptomatic carrier state of Giardiasis

  • Infection with the parasite is usually completely asymptomatic
  • Infected individual unknowingly passes out the parasite with the feces which can contaminate water
• Giardiasis (Traveler's diarrhea)


  • Infection is characterized by a non-bloody, foul smelling diarrhea accompanied by nausea, loss of appetite, flatulence, and abdominal cramps
  • Symptoms may persist for week or months
  • Malabsorption of fat may lead to the presence of fat in the stool (steatorrhea)
  • Patients are usually afebrile
  • Manifestations resulting to malabsorption may include deficient in fat-soluble vitamins, folic acid and proteins
  • Self-limiting infection, lasting one to two weeks. Relapse may occur, esp in patients with IgA deficiency
• Laboratory diagnosis of Giardiasis

  • Diagnosis is made by demonstration of the cyst or trophozoite (or both) in diarrheic stools
  • Only cysts are isolated from the stools of asymptomatic carriers
  • If microscopic examination of stool is negative, String test may be performed
• Treatment of Giardiasis


  • Metronidazole
  • Tinidazole
  • Nitazoxanide
• Prevention and control of Giardiasis

  • Avoidance of fecal contamination of water supplies through proper waste disposal
  • Drinking water should be boiled, filtered or iodine-treated especially in endemic areas
  • Proper hand washing is recommended
• Trichomonas vaginalis


  • Parasite is pear-shaped organism; central nucleus, four anterior flagella, and an undulating membrane
  • Exists only in the trophozoite form (infective and pathogenic)
• Epidemiology and pathogenesis of Trichomonas vaginalis

  • T. vaginalis is not an intestinal pathogen
  • Causes urogenital infections and transmitted through sexual intercourse
  • Isolated form the urethra and vagina in infected women
  • Isolated in the urethra and prostate gland in infected men
  • Infection is highest among sexually-active women in their thirties and lowest in postmenopausal women
  • Occasionally, the parasite may be transmitted through toilet articles and clothing of infected individuals
  • Infants may be infected as they pass through the infected birth canal during delivery
  • Parasite invades the vaginal mucosa of infected women, where they multiply through binary fission
  • Trophozoites feed on local bacteria and leukocytes
  • In men, most common infection site is the prostate gland and the urethral epithelium
• Manifestations of Trichomoniasis


  • Infection in Men
  • Infection in Women
  • Infection in Infants