614 Malaria 



are usually eliminated after 24 hours at 37.5°. P. vivax is more resistant 

 to low temperatures and arrested oocysts may pass the winter in mos- 

 quitoes and complete their development in the following spring (111). 



In any season, a combination of circumstances may lead to a severe 

 outbreak of malaria, as opposed to the more common endemic condition. 

 Favorable climatic changes, permitting a marked increase in the anoph- 

 eline density, may produce such a result in a susceptible population 

 containing enough gametocyte carriers. An unusually wet season may 

 serve the purpose in an area normally too dry for dangerously heavy 

 mosquito densities. An unusually dry period might exert the same effect 

 by converting rapidly flowing streams into isolated pools suitable for 

 mosquito breeding. Importation of a prolific vector into new malarial 

 territory offering little hindrance to breeding may be followed by a severe 

 outbreak of malaria. Human activities, such as the migration of gameto- 

 cyte carriers into anopheline territory, also may start an outbreak in a 

 population relatively free from malaria. 



Congenital transfer of malaria has been reported occasionally under 

 conditions which eliminate other possibilities, but there are no adequate 

 data for estimating the frequency of such transfer. Mechanical transfer 

 by inoculation of blood is a routine measine in malarial therapy of 

 syphilis and has occurred occasionally in blood transfusions. Storage of 

 blood in a blood bank for a week is not a complete safeguard against 

 the transfer of parasites in transfusions (94). Erythrocytic stages of P. 

 vivax may be stored, at —70°, in citrated or defibrinated blood for at 

 least five months without eliminating infectivity upon inoculation (91). 

 Mechanical transfer also may be accomplished by drug addicts through 

 common use of a hypodermic needle (75). 



THE HUMAN MALARIAS 



The incubation period 



The number of sporozoites introduced is probably the most im- 

 portant influence on length of the incubation period (9, 16). The min- 

 imum for establishment of human infections is unknown, although 

 inoculation of single trophozoites has produced infections with P. know- 

 lesi in Macaca mulatta (30) and with P. cathemeriiim in canaries (104). 

 Relative susceptibility of the individual ranks next in importance. In 

 addition, the incubation period may vary with the strain of malarial 

 parasites. Climatic conditions also may have some significance, since 

 incubation periods in falciparum malaria may be relatively short from 

 October through December and relatively long during the winter months 

 (14). 



The usual incubation periods are 14-18 days, with a common range of 

 ^-35 days, for P, pivax; 18-21 days for P. ynaJariae; and 9-12 days for P. 



