158 MALARIA 



eles have been described, less than one-third have been proved 

 to be carriers of malaria. Some species will carry certain types 

 of malaria and not others (see p. 439). A knowledge of the 

 malaria-transmitting ability of various species of mosquitoes 

 and their habits is of the utmost importance in any attempt to 

 exterminate malaria by exterminating mosquitoes. The knowl- 

 edge that A. malefactor of Panama, breeding in cavities of stumps 

 and trees, was not a malaria carrier saved several hundred thou- 

 sand dollars in the anti-malarial campaigns in the Canal Zone. 

 The distinguishing characteristics of Anopheles and a brief 

 account of a few of the more important malaria-carrjdng species 

 will be found on pp. 439-441. 



Reports of malarial outbreaks have occurred which were said 

 to be due to some other cause than mosquito transmission, l)ut 

 when completely investigated there has always been found to be 

 a " leak " somewhere. Sometimes the presence of mosquitoes 

 was unsuspected, sometimes other fevers have been mistaken 

 for malaria, and sometimes the malarial parasites have been 

 harbored for weeks or months in " latent " form. This is a 

 phase of malaria which is little understood, but it is a well-known 

 fact that long after symptoms of the disease have disappeared, 

 and the parasites can no longer be found in the blood, a fresh 

 outbreak may occur, coincident with some loss of vitality, or 

 some physiological shock on the part of the host from some 

 other cause. Often a mere change of climate and environment is 

 sufficient to precipitate " latent " malaria. It is highly probable 

 that the ordinary blood parasites are carried in the meantime 

 in such small numbers as to be practically impossible to find. 

 Ross has pointed out that if 1000 parasites in the body were 

 able to withstand the unfavorable conditions and existed there 

 during the "latent" stages, a man working 12 hours a da}' 

 searching blood smears would have a chance of finding one 

 only once in five years. Some authors have advanced the theory 

 that the gametocytes, suddenly stimulated by some unknown 

 cause, develop by parthenogenesis, i.e., without the ordinary sexual 

 mosquito cycle, and thus cause the relapse. This idea has been 

 widely accepted but there seems to be little ground for it and some 

 positive evidence against it. The parasites naturally thrive 

 best when their host is weakened by some other influence which 

 then acts as an accomplice for them. Such influences are ex- 



