Biology Reference
In-Depth Information
if the concentration of eggs in feces is very low. The sensitivity of diag-
nosis depends on three main factors, namely, the presence of mature
female worms, as males do not produce eggs; the number of eggs
produced by mature female worms; and the dilution of eggs in the feces.
The number of eggs released by female worms into the gut depends on
several factors. First, whether the worms have been fertilized by a male,
although infertile “decorticated” eggs can be released in small numbers
by mature, unfertilized female worms. Infertile eggs tend to be longer and
thinner than normal.
42
Second, the presence of other A. lumbricoides in the
gut, which act to inhibit the production of eggs in a process called density-
dependent fecundity so that the number of worms per female decreases in
proportion to the worm burden.
44,45
This is a non-linear effect as the
presence of a few worms appears substantially to affect the concentration
of eggs in feces. Third, there is evidence of a wide range variation in the
per capita fecundity of worms in different parts of the world.
46
This means
that the relationship between the concentration of eggs in feces and the
number of worms in the gut is specific to the locality, but may also affect
the diagnosis of infections. However, a single female A. lumbricoides can
produce up to 200,000 eggs per day,
47
which is 1 egg per milligram in
200 g of stool, if thoroughly mixed.
The probability of seeing an egg in a sample of feces depends on their
dilution and dispersion in the fecal mass, which is smaller in children
than adults. There is evidence from examining multiple samples from the
same stool that eggs may be clumped,
48
a characteristic that may result
from the aggregation of worms within hosts.
49
Ideally every fecal sample
should be thoroughly mixed before a subsample is taken for diagnosis,
perhaps with a known volume of 0.9% saline containing a surfactant to
separate and disperse the eggs. The probability of seeing an egg will
depend on the amount of feces examined and on whether or not any fecal
detritus is removed or clarified.
There are several alternative methods of microscopical diagnosis. The
simplest is to examine a direct smear of feces (typically 5
10 mg) on
a glass slide. A study in Brazil found this method to be more sensitive
than the Kato-Katz method, which samples 3
e
6 times as much feces.
50
The Kato-Katz method applies glycerol to a standard volume of filtered
feces deposited in a template on a slide; the glycerol serves to clarify the
fecal detritus so that after 30 minutes the eggs of worms become easier to
see.
51
The amount of filtered feces placed on the slide depends on the
diameter of the hole in the template. The number of eggs on the slide are
counted and then multiplied by a standard factor depending on the
template used. However, in an experiment a template 6 mm in diameter
and 1.0 mm thick delivered between 12 and 34 mg of feces in 95% of
samples,
52
which should have led to multiplication factors of between
29 and 83
e
to calculate the concentration of eggs per gram (epg) of feces.
