|If we know the migration patterns of a parasite's life
cycle in its host, this information can be used to predict the
parasite's involvement in disease. For example by knowing the migratory
pathways of Ascaris suum in pigs we can predict where the
significant pathological changes will occur. Ascaris suum larvae migrate
from the small intestine (F) to the
liver (G) via the hepatic portal
vein and then to the lungs (H) via
the heart. Knowing this, we can predict that important pathological
changes will be found in the liver and lungs.
|Migrating larvae in the liver cause an inflammatory
reaction, intralobular tissue destruction and hemorrhage. This is
followed by an intense infiltration of eosinophils and
collagen production. These lesions are visible at necropsy on liver
surfaces as whitish areas and are commonly called "milk spots"
since they resemble splashes of milk.
|Migration of larvae in the lungs also produces hemorrhagic
lesions and intense infiltrations of eosinophils around alveoli into
which larvae are migrating on their way up the bronchial tree. Repeated
infections will produce more widespread hemorrhages, edema and
emphysema. The image to the right shows significant hemorrhagic lesions
in the apical and cardiac lobes of Ascaris- infected lungs.