Hymenolepis microstoma: a change in susceptibility to resistance with increasing age of the parasite.

Hymenolepis microstoma from secondary infections in mice were found to grow initially more slowly than H. microstoma from primary infections. Eventually a similar rate of growth was attained by both kinds of worm. After transplantation, young worms (less than or equal to 4 days old) grew more slowly in previously infected than in naive mice. In contrast, 10-day-old worms grew equally well in naive or resistant mice. The administration of cortisone to mice during a secondary infection of H. microstoma inhibited the stunting of growth in young worms which were able to grow as well as those in a primary infection. The effects of the cortisone persisted for less than 4 days. Worms in a secondary infection given 4 days after cortisone treatment were stunted in growth. These experiments suggest that the susceptibility of the worms to the resistance of the mouse applies to the first 4 days within the host. The loss or reduction of this susceptibility might be associated with the worm's entry into the bile duct which occurs 3--4 days after infection.