630 Immunity and Resistance 



supplementary pyridoxine (14) or pantothenate (19). Comparable find- 

 ings have been reported for malarial parasites. A riboflavin deficiency, in 

 chickens infected with Plasmodium lophurae, reduces the parasitemia to 

 less than one-fifth that in birds on a high-riboflavin diet (151). Panto- 

 thenic acid shows a similar influence on P. gallinaceum. Chickens with 

 a pantothenate deficiency develop much less severe trophozoite-induced 

 infections than controls on a normal diet. Oral dosage with analogues of 

 pantothenic acid produces much the same eflect as pantothenate defi- 

 ciency, the most active analogue (pantoyltauramido-4-chlorobenzene) 

 being at least four times as active as quinine (34). 



The relative resistance of a particular host is influenced also by viru- 

 lence of the parasite, which may vary within a species. Such differences 

 in virulence are apparent in strains of avian malarial parasites (135), in 

 Plasmodium cynomolgi, P. inui, and P. knoxvlesi of monkeys (160), and 

 in malarial parasites of man (25, 82). Some strains of P. vivax have such 

 low virulence that they are of no value in malarial therapy. Differences 

 in virulence may be correlated with rates of reproduction. The relatively 

 virulent Madagascar strain of P. vivax averages 17-18 merozoites at merog- 

 ony; the less virulent Dutch strain, only 12-13. Strains of Entamoeba 

 histolytica also may vary in virulence, as indicated by their effects on 

 kittens (112), and strains retain their general characteristics in cultures 

 (114). Experimental reduction of virulence may be possible. For example, 

 a human passage strain of P. knoxvlesi, a species normally lethal to cer- 

 tain monkeys, has produced a mild chronic infection in these animals 

 (93). Likewise, virulence may be increased experimentally. A typical 

 strain of Trypanosoma gambiense, after seven passages through young 

 rats, caused death of adult rats in 4-7 days. Survival of adults infected with 

 the original strain ranged from 25 to 95 days (145). In similar fashion, 

 two strains of E. histolytica showed increased virulence for kittens after 

 seven passages through dogs. Upon return of the strains to cultures, how- 

 ever, virulence dropped to approximately the original levels after several 

 months (113). 



ACQUIRED RESISTANCE 



Resistance may be acquired actively, as a result of infection or 

 vaccination, or passively by transfer of antibodies from an actively im- 

 munized animal. The resistance which is acquired actively in certain 

 protozoan infections is a resistance to superinfection — "premunition" of 

 Ed. Sergent — and is dependent to a considerable extent upon persistence 

 of a latent infection, as in malaria. However, this resistance may last for 

 some time after apparent elimination of the parasites. In other cases, such 

 as Trypanosoma leioisi in the rat and coccidiosis in mammals, the para- 

 sites are finally eliminated so that a so-called "sterile" immunity is 

 developed. 



