The Natural Resistance of the White Rat to Nematospiroides dubius and the Effect of Cortisone on This Resistance

Abstract

Nematospiroides dubius develops normally in the white mouse, but the white rat is naturally resistant to this parasite. Larvae penetrated the intestinal mucosa and started development similarly in both hosts. In the rat an intense inflammatory reaction around the larvae resulted in a connective tissue cyst from which the worms were unable to emerge and re-enter the lumen. At the time when worms in the mouse were in the lumen, some worms were still alive in the rat, but they eventually died without leaving the tissues. Cortisone was administered to rats in daily doses of 5 mg/100 g of body weight in an attempt to inhibit the inflammatory reaction. The worms in these rats developed essentially as those in the mouse, re-entering the intestinal lumen and ovipositing only a day or 2 later than in mouse infections. Other phases of resistance, such as longer prepatent period, smaller percentage of worm development, and shorter period of patency, were not influenced by cortisone. Treatment with cortisone started 4 days prior to infection did not affect these additional phases nor further influence the development of the parasite in the rat. Treatment with cortisone beginning 4 to 7 days after infection was less effective than treatment started on the day of infection. Cortisone administered to the mouse had no influence on the life cycle of the parasite.