Fasciola hepatica in rats: Effects of age and infective dose

Groups of 6 male rats, aged either 5, 10, 15 or 25 weeks old, were infected with either 1, 5, 10, 15 or 20 metacercariae of Fasciola hepatica. The prepaient period among all groups ranged from 57 to 60 days, and the size of worms recovered was inconsistent. Within each age group of rats the proportion of metacercariae which developed to maturity was the same irrespective of the dose of metacercariae. These results indicate that neither a crowding effect nor competitive inhibition occurred. Five-week-old rats were generally more susceptible to infection than other age groups. Some variation in the expression of age resistance in 10-week-old animals suggests that it is at about this age that factors involved in age resistance develop.