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.