638 



Immunity and Resistance 



lates at appropriate intervals. This procedure is based upon the premise 

 that in a rapidly dividing population, there is greater variation in length 

 and a larger coefficient of variation than in an adult population of fully 

 grown flagellates. The rate of reproduction may be estimated also by 

 determining the percentage of dividing flagellates (175). Any destruction 

 of parasites may be estimated on the basis of significant decrease in 

 parasite density. Such methods have been applied to the analysis of 

 acute lethal infections, relapsing lethal infections, and infections with 

 non-pathogenic species. 



Acute lethal infections (Taliaferro's "continuous fatal" type) are the 

 simplest type, showing merely an incubation period and then a sharp 



thousands/mm^ 



200 



days 



Fig. 14. 2. A relapsing lethal infection: Trypanosoma rhodesiense in a 

 guinea pig (after Taliaferro and Taliaferro). 



increase in the parasite population. In mice infected with T. rhodesiense 

 (178), the flagellates are detectable about four days after inoculation, 

 and they continue to multiply until the host dies on the seventh or 

 eighth day (Fig. 14. 1.). The coefficient of variation remains fairly constant, 

 indicating a uniform fission-rate, and there is no significant break in the 

 growth-curve. The obvious conclusion is that the mouse develops no 

 appreciable resistance to T. rhodesiense. 



Relapsing lethal infections (Taliaferro's "intermittent fatal" type) are 

 produced in a number of host-parasite combinations — T. brucei, T. 

 gambiense, and T. rhodesiense in guinea pigs, rabbits, and rats; T. eqiii- 

 num and T. evansi in rats; T. rhodesiense in man and the cat. With 

 T. rhodesiense in the guinea pig (178), there are irregular increases and 

 decreases in parasite density (Fig. 14. 2.). The decreases are referred to as 

 crises; the subsequent increases in parasite population, as relapses. Very 



