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HLT-VET42-04 · Lesson 6 · 24 clock hours · lab

Parasitology: Fecal Diagnostics, External Parasites & Zoonotic Risk

Objective. Students will perform a fecal flotation and direct smear, identify common gastrointestinal parasites and their eggs to genus level, and perform a skin scrape and tape preparation to identify common external parasites, noting zoonotic implications for each.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed clinician in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it to make decisions about any patient or medication. Always follow your instructor, your supervising professional, and current official references and regulations.

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Parasitology in the Veterinary Clinic

Parasitology is one of the most routinely performed diagnostic areas in veterinary practice. As a veterinary assistant or technician, you will prepare and read fecal samples, scrape skin for external parasites, and communicate zoonotic risks clearly to pet owners and clinic staff. Accuracy protects both animals and people.

Fecal Flotation

Fecal flotation is the standard method for recovering gastrointestinal parasite eggs (ova), oocysts, and larvae from a stool sample. The technique relies on a flotation solution that has a higher specific gravity than most parasite eggs, causing eggs to rise to the surface while fecal debris sinks.

Zinc sulfate centrifugal flotation is widely preferred because it preserves protozoal cyst morphology better than Sheather's sugar solution and is the recommended method for detecting Giardia cysts. Sheather's sugar solution works well for most nematode eggs but can distort protozoa.

Centrifugal flotation procedure (summary):

1. Weigh or measure approximately 2 grams of fresh feces.

2. Mix thoroughly with flotation solution in a centrifuge tube, filling to within a few millimeters of the rim.

3. Place a coverslip on top of the tube so it contacts the meniscus.

4. Centrifuge at the speed and time specified by your laboratory protocol.

5. Carefully lift the coverslip straight up and place it on a labeled glass slide.

6. Examine the entire coverslip area under 10× (low power) first, then confirm morphology under 40× (high power).

Sketch and label all ova observed, comparing them to a reference atlas such as Georgis' Parasitology for Veterinarians (Bowman, 11th ed.).

Common Gastrointestinal Parasites and Their Ova

Direct Fecal Smear

A direct smear complements flotation by detecting motile organisms that may not float. Prepare a thin smear by mixing a small amount of fresh feces with a drop of saline on a glass slide, apply a coverslip, and examine immediately under 10× and 40×.

Safety note: wear gloves and handle all fresh fecal material as potentially zoonotic.

External Parasite Identification

Tape preparation: Press clear tape firmly against the skin surface or coat, then adhere tape to a glass slide. Examine under low power. This method is ideal for Cheyletiella ("walking dandruff") and lice.

Skin scrape: Apply mineral oil to skin, scrape with a #10 scalpel blade until capillary bleeding (for Sarcoptes, which burrows deeply) or a more superficial scrape (for Demodex, which lives in follicles). Transfer material to a slide, add a drop of mineral oil, apply a coverslip, and examine under 4× and 10×.

Zoonotic Risk and Safety Protocols

Consult CDC and CAPC (Companion Animal Parasite Council) guidelines for current zoonosis information. Key points:

Key terms8

Flotation solutionA liquid with a higher specific gravity than parasite eggs, used in fecal flotation to cause eggs to rise to the surface for collection on a coverslip.
Centrifugal flotationA fecal diagnostic technique in which centrifugation is used to concentrate parasite eggs at the surface of a flotation solution, improving sensitivity over passive flotation.
OvaParasite eggs; the plural of ovum. Identification of ova by morphology allows genus-level diagnosis of gastrointestinal parasites.
TrophozoiteThe actively motile, feeding stage of a protozoan parasite such as Giardia, detectable in fresh fecal smears.
Skin scrapeA diagnostic procedure in which a scalpel blade is used to collect skin material mixed with mineral oil for microscopic examination of mites such as Demodex and Sarcoptes.
ZoonosisAn infectious disease or parasitic infestation that can be transmitted from animals to humans under natural conditions.
CheyletiellaA surface-dwelling mite of dogs, cats, and rabbits that appears as moving scales ('walking dandruff') and can cause a pruritic rash in human contacts.
Tape preparationA diagnostic technique in which clear tape is pressed to the skin or haircoat to collect surface parasites or eggs, then examined microscopically.

Worked examples3

1. You weigh a fecal sample and need 2 grams. Your scale reads in 0.1 g increments. You place the weigh boat first and tare the scale, then add feces until the display reads 2.0 g. How many grams of feces have you added, and why is this correct?
  1. Taring sets the scale to zero with the empty weigh boat, so the weight of the weigh boat is excluded from the reading.
  2. When the display reads 2.0 g after taring, the scale is measuring only the feces added.
  3. 2.0 g − 0 g (tared) = 2.0 g of feces.
Answer: You have added exactly 2.0 g of feces. Taring ensures the container weight does not affect the measurement.
2. You are performing a McMaster egg count. Each McMaster grid has two chambers; you count 14 eggs in chamber 1 and 11 eggs in chamber 2. The McMaster slide factor provided by the manufacturer is 50. What is the eggs-per-gram (EPG) count?
  1. Add the counts from both chambers: 14 + 11 = 25 eggs total.
  2. Multiply the total count by the McMaster factor: 25 × 50 = 1250.
  3. Report the result as 1250 eggs per gram (EPG).
Answer: 1250 EPG. (Note: always verify the McMaster factor printed on your specific slide; 50 is a common factor but slides may vary.)
3. Zinc sulfate flotation solution should have a specific gravity of approximately 1.18–1.20 for optimal parasite egg recovery. You use a hydrometer and read 1.15. What does this mean and what should you do?
  1. A specific gravity of 1.15 is below the recommended range of 1.18–1.20.
  2. A solution that is too dilute may not float heavier eggs (e.g., tapeworm eggs) reliably, reducing test sensitivity.
  3. Dissolve additional zinc sulfate into the solution, stir well, and recheck with the hydrometer until the reading falls within 1.18–1.20 before using the solution for patient samples.
Answer: The solution is too dilute and must be corrected by adding more zinc sulfate until the specific gravity reaches 1.18–1.20.
In class: Fecal flotation: each student weighs a fecal sample, prepares a centrifugal flotation using zinc sulfate or sugar solution, places a coverslip, and examines under low and high power; they sketch and label any ova observed using a parasite egg reference atlas. · Direct fecal smear: students prepare a thin direct smear from the same sample, add a coverslip, and scan for motile protozoa (Giardia trophozoites) and Tritrichomonas, recording presence or absence with a description of morphology. · External parasite lab: students examine instructor-prepared tape preps and deep skin-scrape slides for Demodex, Sarcoptes, Cheyletiella, and flea dirt; they complete an identification worksheet and note which organisms pose a zoonotic risk to the owner and clinic staff using CDC and CAPC zoonosis guidelines.
Materials: Fecal samples (preserved or simulated), centrifuge, flotation solution (zinc sulfate), centrifuge tubes, McMaster counting slides, coverslips, compound microscopes, parasite egg identification atlas (Bowman: Georgis' Parasitology for Veterinarians 11th ed.), tape, mineral oil, skin-scrape slides, CDC/CAPC zoonosis reference cards
Check: Parasite identification worksheet scored against answer key: student must correctly identify genus for at least six of eight parasite examples shown and correctly flag all three zoonotic species. 80% or higher required.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Laboratory Procedures
1. Which fecal flotation solution is most preferred for preserving Giardia cyst morphology?
Laboratory Procedures
2. A technician prepares a direct fecal smear from a cat with chronic large-bowel diarrhea and observes elongated motile organisms with an undulating membrane and a single posterior flagellum. The most likely organism is:
Parasitology / Zoonosis
3. During a skin-scrape examination, you observe oval mites with long unjointed pedicel-like leg stalks and dorsal spines. The patient has severe pruritus. Which mite is most consistent with these findings, and what is its zoonotic status?
Laboratory Procedures
4. A veterinary assistant examines a tape preparation from a rabbit and notices large mites moving among the scales — so-called 'walking dandruff.' This finding is most consistent with:
Parasitology
5. You observe barrel-shaped parasite eggs with bipolar plugs on a canine fecal flotation. These eggs are most consistent with:
Parasitology / Zoonosis
6. Which of the following gastrointestinal parasites is associated with cutaneous larva migrans (creeping eruption) in humans?
Laboratory Procedures
7. When performing a deep skin scrape for Demodex, the correct endpoint for scraping is:
Occupational Safety / Zoonosis
8. According to standard safety practice, which action is most important immediately after completing any fecal parasitology procedure?
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