THE MALARIAL FEVERS 



The presence of Anophelinae, however, does not indicate that 

 there must be malaria in the locality, an important point to which 

 Celli was the first to call attention. 



England, for example, though it possesses three species of the 

 Anophelinae — viz., Anopheles maculipennis Meigen, 1818; A. hifur- 

 catus Linnaeus, 1758; and A. nigripes St., of which the two former 

 certainly can carry malaria — is practically free from the disease. 

 Nuttall, Cobbett, and Strangeways-Pigg believe that this condition 

 has been arrived at by the reduction of the numbers of the mos- 

 quitoes by drainage. 



Since the war, however, a certain number of indigenous cases of benign 

 tertian malaria have been reported. 



On the other hand, there is no evidence of the existence of malaria 

 without some of the Anophelinae. On the contrary, there is evi- 

 dence that where there are no Anophelinae there is no malaria 

 (Barbados, for example, as first noted by Low). Further, it is 

 highly probable that the endemic malaria of Mauritius and Reunion 

 is due to the introduction of Pyretophorus costalis Loew, 1866, 

 as has been pointed out by Ross. There is also the evidence that 

 in places where successful anti-anopheline measures have been 

 carried out, as in Ismailia, malarial fever has ceased to exist. 



In order that there may be plenty of these insects, there must be 

 a certain degree of warmth; for as a rule they hibernate in the 

 winter of the temperate zone, coming out in the spring, and in- 

 creasing in numbers, to reach a maximum in the warm days of 

 autumn. In the tropics, of course, the heat is present all the year 

 round. Heat alone, however, will not suffice for the mosquito, for 

 there must be water for the development of the larvse and pupae. 

 Hence, in the dry season in the tropics, there may be few mosquitoes 

 visible, while their numbers will increase remarkably after the rains. 

 In the dry season the mosquitoes and their larvae aestivate, and 

 wait for more suitable conditions. 



There is, however, a third factor to be considered, and that is the 

 effect of the temperature upon the parasite in the mosquito. For 

 ages it has been noted that the fevers due to P. malaricB and P. vivax 

 occur in the temperate zone in the earlier period of the year — i.e., 

 in the spring — while the worst fevers, due to L. malaricB, do not 

 occur until the summer or autumn. This point has been carefully 

 studied by Grassi, Jancs6, Hollander, and others, and the result of 

 their experiments tends to show that temperature has most effect 

 upon the odkinete before it pierces the wall of the stomach of the 

 mosquito and becomes encysted. It would appear that if the 

 temperature is below 15° to 16° C, no further development of the 

 oocyst will take place in any form of parasite. Further, P. malancB 

 will develop at a lower temperature than the other two, while 

 P. vivax will also develop at a low temperature, but L. malavice 

 requires a distinctly higher one. 



This may be the reason of the scarcity of L. malaricR in the tem- 

 perate zone, except in the summer and autumn, and its common 



