Biology Reference
In-Depth Information
Although the number of liver lesions indicates the recent passing of
migrating
A. suum
larvae, there exists no relationship whatsoever
between the number of lesions and the number of adult parasites that
eventually end up in the small intestine.
40
Presence of hepatic lesions
predict the presence of intestinal stages with low (29%) positive and high
(94%) negative predictive values.
25
In a study by Bernardo et al.,
41
the
presence of white spots on the liver is more than twice as prevalent as
worms in the small intestine at slaughter. The absence of worms in the
intestine when white spots are visible on the liver can mean two things:
(1) eggs have recently been ingested, but not enough time has elapsed to
allow development of macroscopically detectable ascarids or (2) infec-
tion had been cleared by the host immune response but lesions had not
yet resolved. When milk spots are present, it is highly unlikely that
A. suum
infection is absent. The presence of milk spots has a high
sensitivity, very low specificity, and high negative predictive value as
a screening test for ascariasis in individual hogs.
41
Under continuous
exposure, the number of white spots on the liver increases until weeks
6
e
9, after which there is a gradual decline towards lower levels.
23
This
decline could be attributed to the build-up of immunity as continuous
exposure to
A. suum
infections stimulates the development of strong
protective immunity. Both Eriksen et al.
35
and Urban et al.
42
suggested
the existence of a prehepatic barrier in pigs repeatedly exposed to
A. suum
infections. This would impede freshly acquired larvae from
reaching the liver and therefore prevent the formation of white spots.
Hence, the number of liver white spots is a poor indicator of long-term
A. suum
exposure as it only reflects recent larval migration. Livers
might therefore look normal or only mildly affected at slaughter even
though pigs have been exposed to high numbers of infective eggs during
the course of their life.
The visual assessment of livers is also very subjective. Especially
when pigs with low numbers of white spots are slaughtered the deci-
sion on whether or not an abnormality on the liver is considered a true
white spot largely depends on the perception of the person doing the
assessment. In addition, this usually has to be performed within an
extremely short timespan due to the high speed of slaughter. This
supports the variability in the white spot counts and increases doubt on
the reliability and uniformity of the data on liver lesions at slaughter.
The quality of these data should be improved if they are to be of use in
the future.
Serology
The use of serological tests is widespread and generally accepted in the
pig industry. Tests are available for a number of important bacterial
