Subcutaneous injection of nursing mothers from day 16 to day 20 post partum with infective larvae of Strongyloides ratti or Nippostrongylus brasiliensis does not result in the development of worms in the litters if 1 h is allowed between injection and resumed nursing, and suckling is terminated 24 h later. Thus the low numbers of N. brasiliensis (1% of the dose) which develop in litters after 24 h, 4 day or 5 days suckling when mothers are injected and returned to their young immediately, represent skin invasion and not milk-borne infection.

Taking precautions consistent with the foregoing, S. ratti equivalent to 28%, 45%, 45% and 48% of the dose were shown to be transmitted in the milk to suckling rats of 4 mothers injected with 4000 L 3 on day 18 post partum and 72 h before weaning. One mother of the same batch failed to transmit worms (1% of the dose in the litter) and the take in all 5 mothers was insignificant (max. = 3% of the dose versus an average of 21% in controls).

Large numbers of S. ratti were subsequently found in the intestines of mothers whose litters were weaned immediately after (21%) or 6 h after (16%) injection, whereas very few (less than 1%) developed in mothers deprived of their offspring 24 h after injection. Dynamic, rather than static, determinants of larval routes inside the host present the only logical basis for an explanation of these facts.