A comparison of the acquired resistance to Dipetalonema viteae stimulated in hamsters by trickle versus tertiary infections.

Two groups of hamsters were hyperinfected with Dipetalonema viteae. Each of the 15 hamsters in the first group received a total of 900 larvae given in three equal doses on days 0, 150 and 250 from the start of the experiment (tertiary-infection group). Each of the 20 hamsters in the second group received a total of 900 larvae given in 18 equal doses (50 larvae per dose) at 14 day intervals. Thus the final dose was given on day 238 from the start of the experiment (trickle-infection group). About half of the hamsters in each group were killed 100 days after the last sensitizing infection and their adult worm burdens and subcutaneous nodules were counted. The tertiary-infection group had a higher average number of adult worms per hamster, but fewer subcutaneous nodules than the trickle infection group. However, when the number of adult worms and subcutaneous nodules were added together, the sums from both groups were similar. The remaining hamsters of the above two groups, along with a group of nine previously uninfected hamsters, were given a challenge infection of 500 larvae per animal. Necropsy data taken 70-80 days after the challenge infection indicated inhibition and/or destruction of developing larval stages in the trickle + challenge-infection group, but no such acquired resistance phenomenon in the tertiary + challenge-infection or challenge-infection groups. While the mechanism remains unknown, it is clear that prior exposure to repeated small infections over an extended period stimulated a protective response to D. viteae in hamsters. This response was not seen in animals given three large infections over a similar time period.