Abstract
Six strains of circumflexum were used. These were secured from 5 different species of hosts, 3 of which were from widely separated localities (Syracuse, N. Y., Cape Cod, Mass., and Germany). The technic consisted in inoculating chronic infections (infections which had been made 2 or more months previously) of a given strain with parasites of another strain. The results indicated that a chronic infection with one strain protected against subsequent infection with another in most cases, although in general the degree of immunity to infection with another strain was somewhat less than when super-infection with the same strain was done. It was found, however, that the 4 strains originally isolated from birds caught in Syracuse (although they were isolated from 3 different host species) conferred much higher immunity against reciprocal or cross-infection with some other of the 4 than toward the strains derived from Cape Cod or German hosts. Of all 6 strains, that derived from a Cape Cod blackbird proved most able to break down the resistance conferred by chronic infection with some other strain, but previous infection with it conferred strong protection against the other five strains.
These results show that at least in this species of avian malaria immunity (“premunition”) is more a matter of strain than previous work with other species had indicated (Gingrich, 1 Manwell 2 ), and it was thus more like the monkey and human malaria parasites in this respect. Indeed the degree of immunity exhibited by a chronic infection of one strain towards another may be less than that occasionally observed between species. As an example, Plasmodium rouxi may be mentioned. This species has been found to confer a strong immunity towards subsequent infection with circumflexum, but the relationship is not reciprocal.
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