Immunity to Litomosoides carinii in Mastomys natalensis. I. Effect of immunization with microfilariae and existing primary infections on the parasitaemia after microfilariae injection and challenge infection.

Subcutaneous injections of intrauterine stages of Litomosoides carinii into Mastomys natalensis induced strong immunity to i.v. injected blood microfilariae. Immunity, developed after boostering with an i.p. and an i.v. injection of microfilariae, did not totally suppress the parasitaemia of a challenge infection but reduced significantly the microfilaraemia level. No effect was found on number and size of the worms of the challenge infection, the number of microfilariae or the number of leucocytes in the pleural cavity. Delayed type hypersensitivity reactions in challenged animals were similar to those in non-immunized, infected controls. Sera of immunized animals agglutinated microfilariae and mediated cell attachment to microfilariae. Challenge infections did not change this until the end of the fourth week post infection but sera taken 32 days after challenge and later failed to induce such reactions. Challenge infections performed 120 or 240 days after a primary infection did not increase the parasitaemia of recipients. Dissections carried out 130 days after the challenge showed that (a) the developmental rate of the challenge infection was reduced by about 50%; (b) the size of the challenge parasites was reduced; and (c) that these worms produced significantly less embryonic stages in comparison to worms of primary infections, of which about 90% were abnormal.