Parasitology: Introduction, Specimens, & Examination

facultative parasites

parasites that can exist as a parasite AND on its own

accidental/incidental host

a host that the parasite usually does not use as a reservoir

definitive host

the host where the adult and sexual stages of the parasite develop and reproduce

intermediate host

larval or asexual reproductive phases occur in these hosts (parasite can then transfer to definitive host)

vector

arthropods or other carriers of a parasite that serves as a mode of transmission to the host

ectoparasites vs. endoparasites

• ectoparasites

  • lives on or outside of the body (ticks, lice, or fleas)

• endoparasites

  • lives within the host (intestinal worms or Plasmodium)

What is looked for in stool specimens for parasites?

eggs or larva

What should be avoided when collecting stool for testing?

• collection after radiologic procedures with barium

• contact with urine

• collection if the patient takes certain medications (mineral oils, bismuth, antidiarrheals, antimalarials, antibiotics)

A minimum of ___ specimens should be collected over a ___ day period for parasitic testing.

3, 10

What happens to a portion of the stool specimen when received in the lab?

• it is placed in a preservative such as polyvinyl alcohol (PVA)

• or refrigerated for up to 4 hours before examination

When should formed stool and liquid stool be examined?

• formed

  • within 2-3 hours of collection

  • best for observing amoeba cysts and flagellates

• liquid

  • within 30 minutes of collection

  • tends to contain trophozoites of amoeba and flagellates

What is used to prepare direct wet mounts for stool specimens?

• saline

  • detects motile amoebas and flagellates

  • can also detect helminth eggs, larvae, and protozoan cysts

• iodine

  • highlights internal structures

  • detects protozoan cysts (shows yellow cytoplasm)

  • glycogen inclusions dye brown

What are the two types of concentration methods of stool specimens for concentrating parasitic elements?

• floatation

  • zinc sulfate separates cysts and eggs, which floats to the top and are skimmed off

  • provides a clean concentrate, but certain eggs and protozoa do not float

• sedimentation

  • formalin ether is used

  • parasites are freed from the stool and are sedimented by centrifugation (eggs, protozoa, and larvae)

Why are permanently stained mounts used in the lab?

• to keep a permanent record of the organisms identified

• slides are fixed with Schaudinn’s fixative or PVA

merthiolate-iodine-formalin (MIF) stain

• combination preservative, fixative, and stain for parasitic specimens

trichrome stain (Wheatley adaptation)

• used for fresh and PVA-preserved stool

• shows blue-green cytoplasm and red-purple nuclei

• eggs and larvae appear red

iron hematoxylin stain

• used for fresh and PVA material

• parasites and background material stain gray-blue and nuclei stain black

modified acid fast stain

• used for identifying coccidia-like organisms like cyclospora

• enhances the detection of oocysts (they appear magenta)

Blood specimens are collected in ___ consecutive days at ___ hour intervals.

3, 6-18

Blood specimens are stained with

Giemsa or Wright stain

When are thick smears and thin smears made with blood specimens?

• thick smears

  • screening for parasites at 100x (for detecting microfilaria)

  • 1000x used for screening for Plasmodium sp. and Babesia

  • not fixed - cells lyse on the slide

• thin smears

  • fixed and stained

  • prepared when thick smear screens are positive

Which identification methods are used to diagnose parasitic infections?

• direct antigen detection

  • EIA, DFA, and immunochromatogenic assays

• molecular testing

  • PCR (good for speciating Plasmodium)

  • Multiplex PCR