all about parasitology

1. CHAPTER 11
WORMS / OVA CALCULATION TECHNIQUE

2. Parasitology lab techniques can be divided
into two techniques:
• Qualitative
• Quantitative
Qualitative Quantitative
Simple sedimentation technique Stoll technique
Formalin ethyl acetate technique Beaver technique
Zinc sulphate flotation technique Kato-Katz technique
Sheather’s sugar flotation
technique
Nucleopore technique

3. Methods for estimation of worm burden
• Quantitative techniques: mathematical models provide
quantitative results, numbers. Used to predict the parasite
burden of individual host.
• Estimates of daily egg output have been made for a number of
hepatic and intestinal worms.
• Estimates total number of eggs in 24 hour stool specimen, it is
possible to calculate the approximate number of adult worms
present.
• To follow the results of therapy, affording a basis for
comparison of the efficacy of various medications.
• Egg laying capacity seems to vary inversely with total numbers
of worm present. e.g:
• Chinese liver fluke: 2400 in 24hrs
• Ascaris: 200 000 in 24 hrs(1000-2000 eggs per gram in a
patient infected with one pair of Ascaris.

4. Kato katz technique
• Principles
– It is a quantitative technique.
– This process facilitates the recovery of eggs for the
intestinal parasites (e.g: Ascaris lumbricoides, Trichuris
trichiura, hookworm, Schistosoma mansoni, S.
japonicum, etc)
– It used for estimating the amount of worms and
determine the severity of the infection (light, moderate
or heavy infection).
• Materials
– Medium size of cellophane that be soaked – cut into
strips, Glycerine –malachite green solution, wire net
(40 mesh), spatula, template, applicator stick,
absorbent paper, rubber stopper.

5. • Method
– Place 60-70mg of feces on absorbent paper.
– Press the upper part of the sample with the wire net.
– Scrape the feces that pass through the net and place
at the central hole of the template lying over the glass
slide.
– Carefully withdraw the card, leaving feces on the glass
slide.
– Put cellophane soaked in glycerin-malachite solution
over the sample.
– Incubate for one hour at room temperature. Purpose:
to allow clearing of fecal material but not for the eggs.
– Examine the entire film under low magnification.

6. Expected results
Estimate total worms:
A = total amount of eggs of the species
inside feces.
B = amount of feces used (60-70mg)
Total eggs / feces (g) = A / B x 1000

7. Stoll techniques
Introduction
– It is used for estimating the amount of worms and to
determine the severity of the infection (light, moderate
or heavy infection).
– Can also be used to estimates the effectiveness of any
antihelminthic treatments toward patients.
– This technique is applicable to the study of soil-
transmitted helminthiasis.
– Advantage – the counting takes a long time because
the amount of extra (non-egg) material on the slides.

8. Materials
• 0.4% NaOH, Erlenmeyer-like flask which has been
marked at the level 60ml and 56ml, glass beads, slide and
coverslip, stoll pipette and rubber stopper.
Method
• Add NaOH in the flask until the level of 56ml.
• Using applicator stick, add feces in the solution until the
level of 60ml.
• Add glass beads.
• Close the flask with rubber stopper and mixed well.
• Leave for 12-24 hrs (shake from time to time)
• Shake vertically for 20-30 seconds and transfer 0.075ml
immediately using stoll pipette onto a slide.

9. • Cover with coverslip and count the ova.
• Correct the result with stool consistency:
– X1 – formed stool
– X2 – soft stool
– X3 – half soft stool/ loose stool
– X4 – liquid stool
• Report ova calculation as ova / ml f.s.b (formed stool
basis)

10. • Example:
– In 60ml stool suspension containing 4ml stool.
– In 0.075ml stool suspension containing: 0.075 x 4 = Xml stool
60
– In Xml stool containing Y ova
– Therefore, in 1ml stool = 1 x Y ova
X
– Multiply answer with correction factor.
Beaver technique
Materials
– Light meter, wood block with round window (16mm in diameter,
18mm deep), adjustable source of light, slide and coverslip.