Parasitology: Protozoa

What types of protozoa are covered in this unit?

• intestinal amoeba

• free living amoeba

• flagellates

• intestinal ciliates

• intestinal coccidia

Most intestinal amoeba are ___

non-pathogenic

What do intestinal amoeba use for motility?

pseudopods (cytoplasmic extensions, “false feet”)

How are amoeba identified?

• by their:

  • size

  • shape

  • number of nuclei

  • granularity of cytoplasm

  • vacuoles

  • ingested materials

Protozoa cyst form

• found in formed stool

• rigid, non-motile, infectious

• resistant to environmental changes

• found in asymptomatic hosts or carriers

Protozoa trophozoite form

• found in liquid stool

• pleomorphic (appears in various forms)

• seen in active disease

• sensitive to environmental changes

What is it called when a protozoan in cyst form converts to its trophozoite form?

excystation

• occurs when the host ingests cysts

Entamoeba histolytica

• causes amoebic dysentery

• symptoms

  • cramping, fever, bloody/mucusy diarrhea, characteristic lesions in the intestines (flask-shaped ulcers)

• can cause hepatic abscess

  • organism can erode intestinal mucosa, enter circulation, and colonize the liver (causes increased WBCs and liver enzymes)

• picture: trophozoite form

Entamoeba histolytica cyst and trophozoite forms

• cyst form (picture)

  • contains 1 to 4 nuclei with a central karyosome, cytoplasm may have bars

• trophozoite form

  • “bullseye” nucleus

  • seen in wet mounts and trichrome stains

Blastocystis hominis

• most common intestinal protozoa, but not considered a common cause of diarrheal disease

• associated with traveling abroad

Toxoplasma gondii accidental host and definitive host

• accidental: humans

• definitive: cats (oocysts are shed from cat feces)

Where do the sexual and asexual life cycles of Toxoplasma gondii occur?

• sexual (enteric cycle)

  • cats’ intestinal tract, caused by ingesting reservoir meat

• asexual (extraintestinal cycle)

  • humans, ingested oocysts or trophozoites

  • can multiply in the brain and other host cells

What infections does Toxoplasma gondii cause? What happens to pregnant women who are infected?

• encephalitis, myocarditis, pneumonitis, and dissemination

• in pregnant women, the infection can be spread congenitally, causing CNS involvement in the fetus, severity depends on when the exposure occurs during the pregnancy

Toxoplasma gondii diagnosis

• extremely difficult

• oocysts are pear or crescent-shaped and are called tachyzoites (visualized by hematoxylin stain)

• IgM specific tests can diagnose acute infections in neonates

• IFA and EIA tests are used routinely

What does Naegleria fowleri cause, and what are the stages?

• Primary Amoebic Meningoencephalitis (PAM)

  • early stage: vague upper resp distress, headache, lethargy

  • acute stage: sore throat, nasal congestion, severe headache, fever, vomiting, stiff neck, coma, and death

How does Naegleria fowleri cause infection?

• by penetrating nasal mucosa and traveling by the olfactory nerve to the brain

• causes infection of the brain and meninges

• rapidly fatal

What is the 3 stage life cycle of Naegleria fowleri?

• cyst

• trophozoite

• flagellated

Infections caused by Acanthamoeba spp. (free living amoeba)

• granulomatous amoebic encephalitis (GAE)

  • brain lesions, acute fever, headache, and neck pain

• amoebic keratitis

  • occurs in those wearing soft or extended-wear contacts

  • trichrome stains and giemsa stains visualize trophozoites in specimens

flagellates

• more rigid than amoeba

• causes infections of the GI tract, blood, and urogenital tract

• Giardia duodenalis

Giardia duodenalis trophozoite and cyst forms

• Giardiasis

• low infectious dose

• trophozoites

  • pear or teardrop shaped, face-like, bilateral symmetry

  • falling leaf motility and a large ventral sucking disk

• cysts

  • oval, up to 4 nuclei, retracted flagella, “cluttered” internal appearance

How does Giardia duodenalis cause infection and what are some common symptoms?

• organism adheres to intestinal mucosa after ingestion and interferes with nutrient absorption

• symptoms

  • acute diarrhea, cramps, nausea, malaise

  • 12 to 14 days after: severe diarrhea with no blood

Giardia duodenalis treatment

• metronidazole or albendazole

Dientamoeba fragilis

• only binucleate flagellate that infects humans

• sluggish motility

• diarrhea

• lacks a cyst stage

• trophozoite (picture)

Which intestinal flagellate is this?

Chilomastix mesnili

Trichomonas vaginalis

• STD

• associated with adverse pregnancy outcomes

  • transmission to newborn, cervical neoplasia, pelvic inflammatory disease

• increased risk of HIV due to compromised mucosal barriers

• infections: vaginalis and urethritis

Trichomonas vaginalis in the lab

• lacks a cyst stage!

• trophozoite

  • pear-shaped with an undulating membrane, 4 flagella, jerky motility

• wet mounts used for observation

• giemsa stain shows a prominent axostyle (midline) and a single nucleus

Balantidium coli

• only pathogenic intestinal ciliate

• lives in the large intestine and does not migrate

• cyst (infective form)

• trophozoite

  • oval shaped, slightly pointed end

Where can Cystoisospora belli (intestinal coccidia) be found? What infection does it cause? What stains are used to identify?

• location

  • the cytoplasm of the epithelial cells of the small intestine

• infection

  • cystoisosporiasis

• identification

  • modified acid fast stain or iodine prep

• oocysts are not infective when passed in the stool

  • but once ingested, they excyst, releasing sporozoites that invade the epithelial cells

How are the Cryptosporidium infections transmissed and how are they diagnosed?

• transmission: contact with contaminated water (including chlorinated pools)

• diagnosis: modified acid fast stain with carbolfuschin

Which species of Cryptosporidium causes apicomplexa?

C. parvum

What is the vector of Plasmodium? How is it transmissed? What type of host are humans?

• vector: mosquitoes

• transmission: bites from female mosquitoes, blood transfusions, infected needles, or transplacental transmission

• humans are intermediate hosts

What types of patients are immune from Plasmodium infections?

patients with sickle cell anemia, hemoglobin C, or G6PD deficiency

schizogony

The asexual reproduction process of Plasmodium

exoerythrocytic schizogony of Plasmodium

• occurs after a mosquito bite

• sporozoites from the mosquito travel from the blood to liver cells where the first asexual cycle occurs

• merozoites (daughter cells) are released and enter the host’s RBCs

erythrocytic cycle of Plasmodium

• in the RBCs, the merozoites (daughter cells) develop into ring forms and mature into trophozoites

  • hemozoin pigment can be seen as the merozoites use Hgb for metabolism

• followed by the formation of schizonts (asexual forms), which then rupture and release new merozoites

symptoms of Malaria

• prodromal period: headache, anorexia, and low grade fever

• paroxysms (shaking chills) occur as RBCs rupture

  • includes fever, limb and back pain, disorientation, night sweats, and fatigue

• anemia, splenomegaly, and black water fever (hemozoin pigment released into urine)

Plasmodium vivax

• usually invades reticulocytes

• enlarged, infected RBCs

• Schuffner’s stippling may be present (fine pink dots)

Plasmodium malariae

• invades older RBCs

• may have a band appearance with dark, course, brown pigment

• mature schizont has 8 merozoites

Plasmodium ovale

• least common

• resembles vivax

• enlarged RBCs, may have an oval shape with fringed edges

• mature schizont has 6-12 merozoites

Plasmodium falciparum

• infects any and all RBCs

• cresent-shaped gametocyte (sexual stage) (upper left of picture)

• frequently shows multiple rings in one RBC

• 20-24 merozoites in schizonts

Babesia (malaria-like parasite)

• tickborne parasite (humans are accidental hosts)

• infection resembles malaria, involves hemolytic anemia, splenomegaly, and hepatomegaly

Babesia microti

• vector: ixodes species of tick (same tick that spread lyme disease)

• symptoms generally only appear in the young, elderly, and immunocompromised

• diagnosis

  • Wright or giemsa stains show pleomorphic ring forms

  • trophozoites form tetrad arrangments of 4 merozoites, “Maltese cross”

What does the Trypanosomes species of hemoflagellates cause?

African trypanosomiasis (sleeping sickness) and Chagas disease

African trypanosomiasis

• caused by T. brucei subsp. gambiense (West African sleeping sickness) and T. brucei subsp. rhodesiense (East African sleeping sickness)

• infects the blood and lymph nodes

• vector: tsetse flies

T. cruzi (Chagas Disease)

• vector: kissing bugs

• transmission can also occur through blood transfusion, transplant, or congenitally

• diagnosis: C-shaped trypomastigotes found in thick and thin blood smears and body fluids

What infections does Leishmania spp. cause?

• cutaneous Leishmaniasis

  • skin ulcers

• visceral Leishmaniasis

  • organism spread through the lymphatics and invades the liver (increased liver enzymes), spleen, BM, and lymph nodes