620 Malaria 



malarias. Recrudescences, which occur shortly after recovery from a 

 primary attack, are sometimes distinguished from relapses following a 

 fairly long period of latency. 



The greatest tendency to relapse is noted in tertian malaria. In many 

 experimental infections (18), relapses have occurred after most primary 

 attacks interrupted by small doses of quinine, and after half of the spon- 

 taneously terminated primary attacks. The tendency to relapse varies 

 with the strain of P. vivax (18). Seasonal factors also may be significant, 

 since July-September cases have shown a greater tendency to relapse than 

 the January-March group (22). The pattern of relapse also varies with 

 the strain (29). The course of tertian malaria often involves a series of 

 "recrudescences" and then a period of latency, which may last 6-12 months 

 before the next relapse occurs. In stubborn cases, this sequence may be 

 repeated for several years after the primary attack. The St. Elizabeth 

 strain (United States) usually does not show marked recrudescence, but a 

 prolonged latent period and eventually a relapse are characteristic. The 

 Chesson strain (New Guinea) usually shows fairly regular renewals of 

 activity without prolonged latency. Relapses of the St. Elizabeth strain 

 seem to coincide approximately Avith the mosquito season in the southern 

 states. The Chesson strain is native to a region in which mosquitoes are 

 reasonably available throughout the year. 



In jalciparum malaria, renewed activity shortly after the primary at- 

 tack is generally to be expected, but relapses after long latency are much 

 less common than in benign tertian. The tendency to produce "short- 

 term" relapses may vary with the strain, and the incidence of relapses 

 has ranged from 8.3 per cent (11) to 80.6 per cent (56) in experimental 

 infections. 



Although infections with P. malariae sometimes last a long time and 

 relapses occur after apparently long periods of latency, little is known 

 about the pattern and incidence of relapses. 



Malariologists now believe that relapses involve two different phases 

 in the life-cycle. A reactivation (recrudescence) occurring shortly after 

 the primary attack depends upon renewed multiplication of erythrocytic 

 stages not yet eliminated from the blood. The relapse following a long 

 period of latency involves a persistent exo-erythrocytic infection which 

 eventually supplies the merozoites for repopulation of the blood. 



Blackwater fever 



Blackwater fever occurs most frequently in individuals coming 

 from malaria-free areas into a region where malaria is highly endemic. 

 The specific cause is unknown. When it occurs, blackwater fever follows 

 prolonged cases of malaria which have been imperfectly treated, and 

 there is good evidence that P. jalcipariwi is always, or nearly always, the 

 only species involved. Inadequate dosage with quinine is not essential to 



