Nematospiroides dubius: arrested development of larvae in immune mice.

When immune NIH mice were killed 10 days after a challenge infection with Nematospiroides dubius, approximately 10% of the inoculated larvae were recovered from the intestinal lumen, irrespective of the dose administered. When such mice were treated with cortisone from Day 10 for a period of 8 to 14 days and were subsequently killed for worm counts, it was found that they had significantly more worms than the immune control mice killed on Day 10. During the week following the beginning of treatment with cortisone there was little change in the low worm burdens in immune mice. However, 9 to 11 days after this treatment worm counts indicated that worms were accumulating in the intestinal lumen, and concurrently eggs were recorded in the feces of the mice. These observations indicated that a period of 9 to 11 days was required after the initiation of cortisone treatment on Day 10 for the worms in immune mice to complete their development to the adult lumen-dwelling stage. It is suggested that the larvae in the challenge infection became arrested early in their development in the intestinal wall and that growth resumed only after cortisone treatment. When treatment with cortisone was initiated later after challenge, it was still effective in reactivating arrested worms, but the lower worm recoveries in these mice indicated that the arrested larvae were being slowly rejected by the host. In subsequent experiments it was established that the arrested larvae of N. dubius were insusceptible to the activity of pyrantel embonate, an anthelmintic which is 99% effective against adult worms in the intestinal lumen. The mechanism whereby the larvae of N. dubius became arrested in immune mice and subsequently resumed their development after cortisone treatment is discussed.