Microbiology Case 5-9 Extra Credit

5 Microbiology Case 5-9
A 21-year-old female student, Maria P., reported to the University Health Office with a
complaint of diarrhea and severe flatulence of 9 days’ duration. She also reported anorexia,
malaise, abdominal bloating, and cramping with nausea. A complete physical and history were
performed. They physical examination revealed an otherwise healthy young woman with a
slight heart murmur. The murmur was first discovered when she was a young child and is
considered a minor congenital defect. The murmur did not interfere with her living an active
life, and no therapy was required. Her physical also revealed that she had lost 13 lbs. since her
last visit. Maria indicated that this was not due to dieting, but rather to the fat that she had
been feeling very ill lately and that she “hadn’t eaten much” in the past week and a half. The
history revealed that the student was a biology major who returned 3 weeks ago from an
externship at a beaver sanctuary in Colorado. She was a research assistant involved in a study
that required fieldwork at the beaver dams, with regular handling of the animals to monitor
weight and other parameters. She had also traveled to Portugal with her parents 9 months ago
to visit her grandparents and extended family. She had a 6-year-old Great Dane and two cats
but no “exotic” pets. Maria had no allergies, and she did not regularly take any medications,
although she did take a daily multivitamin with iron. She also admitted that when she first
started feeling ill, she “took some antibiotic that was left over from when I had Strep throat 2
years ago.”
Stools were collected for routine culture, ova and parasite analysis, and C. diff toxin assay.
Laboratory results were as follows:
 Stool culture: “No Salmonella spp, Shigella spp. or Campylobacter spp. isolated,
please consult laboratory if other enteric bacterial pathogens are suspected.”
 C. diff toxin assay was negative.
 The ova and parasite examination was positive for characteristic cysts in the
concentrate and for trophozoites in the permanent stained smear. They cysts
were thin-walled, smooth, oval-shaped, and approximately 8 to 10 μm wide. The
trophozoites also showed centrally located dark bands and two eccentrically
located nuclei, each with a prominent central karyosome. The arrangement of
the nuclei and dark bands created an “old-man face” like image.
QUESTIONS
1. Based on Maria’s history, presentation, and the description of the organism, what
parasite is causing the problem?
Giardia lamblia
2. What was significant from Maria’s history that would make the physician suspect this
pathogen?
Her recent class trip to a beaver sanctuary in Colorado where she worked with beavers
in their natural habitat was one indicator for suspecting this pathogen. Beavers are well
known to carry parasites so, handling them in their natural habitat more than likely
exposed her to contaminated water that contained many types of bacteria and
parasites. Another indicator was that she was losing weight even though she was not
dieting. Because parasites feed off the host, malabsorption is a common symptom of
parasites. Symptoms of Giardia lamblia occur 1-3 weeks after exposure
3. What term is used for the centrally located bands often seen in this organism?
Axostyle
4. What other intestinal parasite(s) might the physician have suspected?
Cryptosporidium parvum because the parasite is the biggest reason for water-related
diseased in the U.S.
5. What are the recommended timing and number of specimens for diagnosis of
intestinal parasites?
Parasites shed ova intermittently, so samples MUST be collected intermittently for

proper diagnosis. This is typically done by collecting three samples on three different
days at three different times but, depending on the kit being used the times can vary.
6. Traditional methods for O & P examination include a concentrated and a permanent

stained slide. Why are both methods suggested?
Wet mounts show the parasite’s motility also, it is a quick and simple thing to do but, it
doesn’t reveal the parasites identifying features not the cysts. Permanent stained slides
reveal the parasite’s ova and cysts upon examination, as well as, preserves the
specimen.
7. What disease process did the physician suspect when the C. diff toxin assay was
ordered?
That the antibiotics the patient took affected the balance of their normal flora because
antibiotics kill normal flora, allowing C. diff to grow.
8. What was significant from Maria’s history that would make the physician suspect this
disease?
Maria taking antibiotics that were prescribed long ago and for a different reason which
put her at risk for contracting C. diff because the antibiotics were killing off her normal
flora. A C. diff infection could cause stomach aches and diarrhea as well so when the C.
diff toxin assay came back negative, the next parasite to look at was one that can be
found in contaminated waters.
9. Describe the C. diff toxin assay.

The C. diff toxin assay test amplifies 2 genes that are specific to toxigenic strains of C.
diff using PCR. What occurs is that a DNA sample is denatured, DNA strands are
separated, using heat. Then, a primer is added to the sample and the temperature is
lowered to allow the primer to bind specific genes that are being targeted for the test.
Then, the temperature is raised again so that the DNA enzymes copy the DNA sequence
on each individual DNA strand so that two new strands are formed. This process is then
repeated until billions of DNA copies are formed.
10. Why is C. diff toxin assay favored over culture as a method to diagnose this disease?
Because detecting toxins using PCR is a faster and more sensitive method than culturing.
Culturing requires days to grow and it also does not distinguish between colonization
and infection.

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5 Microbiology Case 5-9

4. What other intestinal parasite(s) might the physician have suspected?

Parasites shed ova intermittently, so samples MUST be collected intermittently for

6. Traditional methods for O & P examination include a concentrated and a permanent

9. Describe the C. diff toxin assay.

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