Biology Reference
In-Depth Information
sedimentation/flotation technique is a qualitative technique and is
applied to investigate whether parasites are present in the host or not. To
investigate and compare the quantity of eggs that are excreted, the
McMaster test is used.
Ascaris
eggs are round or oval shaped with a thick
brown irregular wall and are easily recognized. Although coprological
examinations are easy to perform and require relatively cheap materials,
these tests are time consuming and therefore not cost effective or optimal
for the screening of large numbers of samples.
The amount of eggs excreted by a pig can give an impression of its
intestinal worms burden because, in continuously exposed pigs, the
quantity of eggs that are shed seems to be correlated with the number of
adult worms in the intestine.
29,30
However, the aggregated distribution of
adult worms within the infected population will be reflected in the
number of pigs excreting eggs. When using coprological examinations
for the detection of
A. suum
infections in pigs, there is a significant
possibility of false positive and false negative egg counts. False positives
are the result of coprophagia and/or geophagia and their prevalence and
magnitude depend on different management and housing factors.
31
For
diagnosis at farm level, however, these false positive samples are not as
important as the false negatives. Boes et al.
31
has shown that as much as
23% of the investigated pigs that harbored worms in their intestine did
not excrete any eggs.
31
These false negative results are possible when
only immature worms are present or when only worms of a single sex are
present. Evidently, these factors have an impact on the sensitivity of fecal
examination for the diagnosis of
A. suum
infection. Moreover, as
addressed above, the number of adult worms and therefore the number
of excreted eggs are not representative of the level of parasite exposure.
Liver White Spots
Hepatic white spots are the most characteristic lesions caused by
migrating
A. suum
larvae in pigs. After larvae migrated through the liver,
the destroyed liver cells are replaced by interlobular depositions of
fibrous tissue and cellular infiltrates, producing the typical white
spots.
32,33
Milk spots have the property to heal, causing the livers to
appear normal about 35 days after primary inoculation with
Ascaris
eggs.
34
In pigs that have primed immune responses due to vaccination or
previous exposure to the parasite, white spots are larger, more distinct,
and more persistent.
34
e
37
This would suggest that white spot formation is
the result of secondary immunological reactions to antigenic material
released by migratory or dying larvae, which seems to be in accordance
with previous studies showing a significant relationship between the
number of white spots and antibodies against
Ascaris
antigens.
12,36,38,39
