Biology Reference
In-Depth Information
DIAGNOSIS OF INFECTION
Current infections with A. lumbricoides can be diagnosed in five main
ways: (1) by seeing the worms in the gut either during endoscopy, in
sonographic images,
35
in X-ray photographs or in tomographic images;
36
(2) by seeing worms after they are expelled, either naturally or after
treatment; (3) by detecting the worms' metabolites in urine;
37
(4) by
detecting the worms' DNA in feces;
38,39
and (5) by seeing the worms' eggs
in feces under a microscope. Although the last method is the most simple,
direct and most widely used in developing countries, methods using
polymerase chain reactions followed by electrophoresis or fluorescence of
the DNA
38,39
may well become more commonplace in developed coun-
tries as microscopy skills are lost, or not trusted, and as “multiplex”
methods to detect several intestinal parasites at once are developed.
Two diagnostic methods are not useful: clinical signs or symptoms and
antibody tests. There are no specific clinical signs or symptoms of infec-
tion, and if the worm load is light the effects may be negligible. Antibodies
to parasite-specific antigens can be detected in blood
40
or saliva
41
but are
not useful for diagnosis as they cannot distinguish between past and
current infections. However, they may be useful to estimate the propor-
tion of the population that is or has been infected with A. lumbricoides.
The metabolites of A. lumbricoides detected in the urine of infected
people are 2-methyl-butyramide and 2-methyl-valeramide, and their
concentration is proportional to the worm burden.
37
However, gas
liquid
chromatography is required to detect these substances after they have been
extracted fromurine in a solvent, so it is not a simple or practicable method.
The recent development of methods involving amplifying worm DNA
by polymerase chain reaction has included primers for A. lumbricoides,
and several species of worms can be detected in the same assay using
fluorescent probes.
39
However, it is not clear how sensitive this method is
in comparison with microscopy.
Compared with these highly technical methods, the most simple, reli-
able and accurate diagnosis of infection is still achieved by seeing the
worms' eggs under a microscope at a magnification of 100
e
, as they are
m) and are orange, yellow or brown,
42
so are
easy to identify. A slide can be prepared and examined in a few minutes
and the only equipment needed is a microscope. However, the eggs of
A. lumbricoides cannot be distinguished fromthose of A. suum, a species that
can infect humans in addition to its normal host, the pig. A. lumbricoides
and A. suum are identical in terms of their method of transmission, infec-
tion, pathophysiology and treatment.
43
Although the specificity of diagnosing A. lumbricoides/A. suum should
usually be 100% as the eggs are pathognomonic, infections can be missed
large (55
e
75
m
mby35
e
55
m
