Studies of infections with Plasmodium ovale

A careful study has been made of the degree of resistance to reinfection shown by patients who had previously suffered from ovale infections induced by sporozoite inoculation, as compared with that of cases whose previous infections had been induced by trophozoite injection. Under the conditions of our investigations, evidence has been found to suggest that:— (i) Infections induced by sporozoite inoculation give rise to a more effective anti-parasitic immunity than do those caused by trophozoite injection. (ii) Any more effective immunity is probably determined by the more intensive antigenic stimulation of the host, caused by the greater aggressivity of parasites derived directly from sporozoites. (iii) The immunity developed after infections, induced either by sporozoites or by trophozoites, probably acts mainly, if not entirely, against trophozoites and not against sporozoites as such. (iv) Further investigations are needed of these points both in infections with P. ovale and with other species of Plasmodium.