PATHOGENICITY 823 



fected with benign tertian malaria acquires a homologous but not a 

 heterologous tolerance to P. vivax. Later, Boyd, Stratman-Thomas, and 

 Muench (1934) discovered that superinfections with heterologous 

 strains appear to result in clinical attacks of milder intensity than the 

 original attacks. Manwell and Goldstein (1939) have discovered a 

 similar situation in P. civcuni^exum infection in birds. Using six strains, 

 they concluded that immunity was strain specific rather than species 

 specific, although all strains conferred at least partial protection against 

 the others. It should be added that certain strains of P. vivax do have the 

 ability to immunize (or premunize) the patient toward certain other 

 strains. 



Variability in Strains and in Host Response 



Morphological differences between strains of P. vivax have been ob- 

 served. Two strains of this species widely used in Europe for malaria 

 therapy are the so-called Dutch and Madagascar strains. Buck (1935) 

 has found that the Dutch strain consistently exhibits between twelve 

 and thirteen merozoites in both mosquito-inoculated and blood-inocu- 

 lated malaria, while the Madagascar strain exhibits between seventeen 

 and eighteen. The incubation period of the former is twenty-one days, 

 while that of the latter is but twelve days. Whether there is a relationship 

 between merozoite number and incubation period in these cases is some- 

 what of a problem, especially since the discovery of extracellular schi- 

 zogony of malaria organisms in the internal organs. 



Strains of P. vivax likewise differ exceedingly in pathogenicity. Some 

 strains are too low in virulence to be useful in malaria therapy of gen- 

 eral paralysis. The Dutch strain referred to above is said by Hackett 

 (1937) to give higher fever, to be less susceptible to treatment with 

 salvarsan, and to be less virulent than the Madagascar strain. Further- 

 more, it often produced no immediate attack, but in 40 percent of the 

 cases went into a long latency of several months, a phenomenon that 

 occurred with the Madagascar strain in only 6 percent of the cases. 



There appear to be likewise multiple strains of the other human 

 malarias, viz., P. jalciparum and P. malariae (see Hackett, 1937; Boyd 

 and Kitchen, 1937). 



Every case of parasitism exhibits three aspects — the parasite, the host, 

 and the effect of the impinging of the one on the other. It has been 



