Expulsion of Nippostrongylus brasiliensis from rats protected with serum. I. The efficacy of sera from singly and multiply infected donors related to time of administration and volume of serum injected.

Several of the parameters related to the efficacy of passive protection against Nippostrongylus brasiliensis were studied in female hybrid (PVG/c x DA)F1 and outbred Wistar rats. The time after infection at which immune serum was given did, to some extent, alter the degree of protection conferred. There was substantial protection 8 days after challenge in rats given hyperimmune serum (HIS) either as daily injections 4-7 days post-infection or as a single dose on day 4. Eight separate experiments in which HIS was injected 4 days after challenge with 1000 l3 resulted in expulsion of 96 +/- 2% of the worm burdens by day 8. In a further six experiments, following infection with 2000 l3 and using the same protocol, 85 +/- 3% of the worm burden was expelled by day 8. A lag of 2 days between serum transfer and commencement of worm expulsion was consistently observed and, within the space of a further 2-4 days, more than 95% of the parasites were expelled. Transplanted 'normal' and 'damaged' adult worms were also susceptible to the passive transfer of HIS. Sera recovered from rats immunized with two or three challenges (hyperimmune sera) were more protective than sera from rats given one challenge. Serum from rats challenged for the first time 6 days previously conferred significant protection against a 1000 l3 infection and sera recovered 8 and 10-12 days post-infection with 4000 l3 protected recipients with increasing effectiveness. Thoracic duct lymph collected on the tenth day of infection with 4000 l3 was also protective. The response to both primary infection and hyperimmune serum was dose-dependent. The consistently good protection achieved in the present study when compared with the variable success of previous experiments (reviewed by Ogilvie & Jones, 1971) may be a function of the strain and sex of the rats used, together with modifications of the immunization protocols. The relevance of these findings to mucosal defences against N. brasiliensis is discussed.