Protozoa

• Infective Stage


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

  
• Pathogenic Stage


  Stage of 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 different into trophozoite forms

  
• Kingdom Protoza


  • 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

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

  
• Facultative parasites


  Capable of free-living state; ex. Acanthamoeba Naegleria (normally reside in soil or water but can cause severe illness when they gain entrance in the CNS or eyes)

  
• Reproduction of Protozoa

  • Majority divide by means of binary fission (asexual reproduction); ex. Flagellates, ciliates and amoebae
  • Sporozoans reproduce by both sexual and asexual means
• Trophozoite


  Motile, feeding, dividing stage of parasite

  
• Cysts


  Dormant non-motile form
• Stages of most intestinal protozoan parasites

  • Trophozoites – pathogenic stage
  • Cyst – infective stage

  
• Except for Trichomonas vaginalis where the cyst forms are not found

  
• Entamoeba histolytica

  • Intestinal and tissue amoeba; only known pathogenic intestinal amoeba
  • 2 stages: non-motile cyst (infective stage) and motile trophozoite (pathogenic stage)

  
• Fecal-oral route


  Transmission via ingestion of the cyst from contaminated food and water

  
• The ingested cyst resist gastric acid in the stomach
• Life cycle of Entamoeba histolytica

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

  2. Trophozoite colonizes the cecum and colon

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

  
• Trophozoites of E. histolytica

  Secrete enzymes that cause local necrosis producing the typical "flask-shaped" ulcer associated with the parasite

  
• Invasion of the portal circulation may occur leading to the development of abscess in the liver, lungs, and brain
• Manifestations of Acute Intestinal Amoebiasis

  • Blood; mucus containing diarrhea (dysentery)
  • Lower abdominal discomfort
  • Flatulence (release of gas)
  • Tenesmus (feeling of incomplete defecation)
• Chronic inflammation may occur, with symptoms such as occasional diarrhea, weight loss and fatigue
• A lesion called an amoeboma may form in the cecum or in the rectosigmoid area of the colon, which may be mistaken for a malignant tumor in the colon
• Manifestations of Extraintestinal Amoebiasis

  • Right upper quadrant pain
  • Weight loss
  • Fever
  • Tender enlarged liver
• Abscess found on the right lobe of the liver may penetrate the diaphragm and cause lung disease (amoebic pneumonitis)
• Other organs that may become infected include the pericardium, spleen, skin, and brain (meningoencephalitis)
• Asymptomatic carrier state

  Occurs if parasite involved is a low-virulence strain, if parasite load is low, or if the patient's immune system is intact

  
• Giardia lamblia (Giardia intestinalis)

  • Initially known as Cercomonas intestinalis; aka Giardia duodenale
  • Parasite exists in a cyst form and trophozoite form

  
• Trophozoite form of Giardia

  • 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

  Oval and thick-walled with four nuclei; divides through binary fissions; each cyst gives rise to two trophozoites during excystation in the intestinal tract

  
• 50% of infected individuals do not present with symptoms and serve as carriers
• Giardia transmission and pathogenesis

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

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

  3. The 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. The trophozoites may also infect the common bile duct and gallbladder

  
• Manifestations of Giardiasis

  • Asymptomatic carrier state
  • Giardiasis (Traveler's diarrhea) - non-bloody foul smelling diarrhea, nausea, loss of appetite, flatulence, abdominal cramps, malabsorption of fat leading to steatorrhea, deficient in fat-soluble vitamins, folic acid, and proteins
• String test


  Patient swallows a weighted piece of string until it reaches the duodenum; trophozoites adhere to the string and can be visualized after withdrawal of the string
• Trichomonas vaginalis is not an intestinal pathogen. It causes urogenital infections and transmitted through sexual intercourse

  
• Infection is highest among sexually-active women in their thirties and lowest in postmenopausal women
• Infants may be infected as they pass through the infected birth canal during delivery
• In men, the most common infection site is the prostate gland and urethral epithelium
• Men with symptoms, manifestations are related to the urogenital infection
• T. vaginalis


  Not an intestinal pathogen, causes urogenital infections and transmitted through sexual intercourse

  
• T. vaginalis


  • Isolated from urethra and vagina in infected women
  • Isolated in the urethra and prostate gland in infected men

  
• Infection with T. vaginalis

  Highest among sexually-active women in their thirties and lowest in postmenopausal women