Acquired immunity to Fasciola hepatica in splenectomised rats

Effect of inoculum on the size and location of Fasciola hepatica subsequently recovered from the livers of rats

Two outbred strains of Sprague-Dawley rats were given, by intraperitoneal injection, 5, 10, 20, 30 or 50 newly excysted juvenile (NEJ) Fasciola hepatica. Ninety days post-infection all rats were killed and their livers teased apart under 10 X magnification for quantitation of the flukes present. There was no significant difference in worm numbers between the rat strains. However, several significant differences (P less than 0.05) between the various infection groups were observed. As the size of the infective dose was increased from 5 to 50 NEJ, the percentage of the infective dose recovered from the livers of the infected rats decreased from 36.7 to 12.1%. With the 5 and 10 NEJ infective doses, the worms recovered were large mature flukes (1.52 and 1.68 cm, respectively) and were found in the common bile ducts. In the rats receiving infections of 30 and 50 NEJ, the flukes were smaller (0.88 and 0.55 cm, respectively), immature, and were primarily located in the liver parenchyma. These findings are important in light of previous studies on the development of resistance in the rat to challenge infections in which immunity was based on both the size and the location of the flukes recovered. The results from our study indicate that in a primary infection of 20 or greater NEJ, many small immature flukes remain in the liver parenchyma, even after several months. When testing for resistance to F. hepatica in rats, these flukes may erroneously be thought to comprise a portion of the challenge infection.

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.