Introduction to Parasitology

The Importance of Life Cycles

Life cycles of parasites may provide two important pieces of information.

  1. Information that has predictive value with respect to the pathogenic importance of each particular parasite.
  2. Information of epidemiological significance that is indispensable to developing  effective control programs.


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.




Parasites and Parasitic Diseases of Domestic Animals
Dr. Colin Johnstone (principal author)
Copyright 1998 University of Pennsylvania
This page was last modified on January 24, 2000