Immunity and Resistance 629 



bryos of different ages. Embryos are highly susceptible to infection with 

 T. evansi at 8-14 days of incubation, but are quite resistant at 15-17 days 

 (23). Susceptibility to invasion by a particular route also may vary with 

 age of the host. Oral inoculation of rats with T. cruzi is usually possible 

 up to 12 days of age but becomes increasingly more difficult in older 

 animals (102). 



Diet, and particularly the vitamin supply, may influence individual 

 resistance to infection. A well balanced diet is a predisposing factor in 

 human resistance to amoebiasis (2). Conversely, birds on a generally 

 deficient diet suffier abnormally severe attacks of malaria, with greater 

 tendency to relapse than in control animals (36). Certain high-protein 

 diets decrease the severity of flagellosis in rats (133), amoebic infections 

 in mice (134) and dogs (73), and balantodiosis in rats (150). Such effects 

 of proteins have been attributed to modification of the intestinal flora, 

 producing an environment unfavorable to Protozoa. In other cases, par- 

 ticular constituents of the diet, such as vitamins, may exert an important 

 influence. 



Vitamin K, as a dietary supplement, protects chicks against Eimeria 

 tenella, reducing mortality from about 70 to 10 per cent (10). Likewise, 

 supplementary riboflavin (13), thiamine, or a combination of thiamine 

 and pyridoxine (14) decreases the intensity of coccidiosis in rats. A pro- 

 tective influence of ascorbic acid has been reported for Trypanosoma 

 brucei infections in guinea pigs (130). A low-biotin diet favors abnor- 

 mally high parasite densities in chickens and ducks infected with Plas- 

 modium lophiirae and in ducks infected with P. cathemerium (189, 190). 

 A biotin deficiency also prolongs and intensifies Trypanosoma lewisi in- 

 fections in rats, and even a moderate deficiency may lead to death (39, 

 40). Lack of folic acid increases the severity of P. lophurae infections in 

 chickens (152). A pantothenic acid deficiency, in rats infected with T. 

 lewisi, results in unusually high parasite densities, continued multiplica- 

 tion of the flagellates beyond the usual period, and death of the host in 

 extreme cases (20). Even an inorganic supplement — e.g., copper added to 

 the diet of rats infected with T. equiperdum (129) — may increase re- 

 sistance of the host. 



In contrast to such instances in which an adequate supply of a vitamin 

 enhances resistance of the host or a deficiency lowers resistance, there are 

 cases in which development of the parasite is stimulated by a particular 

 vitamin in favorable concentrations. Such a relationship of vitamins to 

 coccidiosis of rats is indicated in a series of papers from Becker's labora- 

 tory. Preliminary observations (18) showed that dietary factors are di- 

 rectly related to the intensity of infections with Eirneria nieschulzi. 

 Addition of yeast to the diet stimulated production of oocysts to a maxi- 

 mum, while certain other supplements were somewhat less stimulatory. 

 Subsequent experiments showed that the yield of oocysts is increased by 



