The pathogenesis of primary and secondary infection with Fasciola hepatica in mice.

Primary and secondary infections of F. hepatica in mice were compared to determine how prior exposure to the parasite affected host response. Mice with primary parenchymal Fasciola infections initially had hemorrhagic tunnels filled with inflammatory cells and connective tissue. These lesions were progressive and became most severe 30 days after exposure as the parasites entered the bile ducts. At this time there was much hyperplasia and thickening of all layers of the duct system near the parasites and occasionally severe periportal fibrosis. By 2 months after exposure regeneration of the damaged liver cells was complete although hyperplasia persisted in bile ducts containing flukes. In mice with secondary infections (mice exposed 40 to 50 days after first infection), the inflammatory response was faster and shorter. Most lesions were resolved by 30 days after the second exposure. There was little difference in histology of primary and secondary infections during the chronic phase of the disease.