The size and fecundity of parasitic nematodes are constrained by the host immune response. For the parasitic nematode of rats, Strongyloides ratti, parasitic females infecting immunized rats are smaller and less fecund than those infecting naïve rats. Here, we investigated whether these constraints on size and fecundity are life-long. This was done by comparison of worms from different immunization and immunosuppression regimes. It was found that the per capita fecundity of parasitic females of S. ratti is fully reversed, but that their size is only partially reversed, if previously immunized hosts are subsequently immunosuppressed, suggesting that fecundity is not subject to life-long constraints. The host immune response also resulted in allometric changes in the parasitic females. The significance of these results with respect to the growth and control of nematode fecundity are discussed.
