Malaria 613 



are able to penetrate the tissues of man after inoculation by a mosquito. 

 Within a short time, perhaps half an hour (42), they have entered 

 tissue cells to initiate the pre-erythrocytic phase. 



TRANSFER OF MALARIAL 

 PARASITES 



The relation of mosquitoes to malaria apparently was first suspected 

 by Lancisi, who stated in 1717 that marshes cause malaria through the 

 transformation of minute worms into mosquitoes which infuse a poisoned 

 liquid into the wounds they inflict. In 1883, Krieg and King again sug- 

 gested that malaria might be spread by mosquitoes, and this same opinion 

 was held by Laveran, Manson, Pfeiffer, and others. The theory was con- 

 firmed for bird malaria by Ross in 1898, and later in the same year, by 

 Grassi and his colleagues for P. falciparum. By the end of 1899, Grassi and 

 his associates had demonstrated similar cycles in P. malariae and P. vivax 

 and had transferred malaria to man from infected mosquitoes. 



Aside from the dubious possibility that apes may serve as reservoirs, the 

 source of mosquito infection is a human reservoir with both types of 

 gametocytes. Young children are often the major source in tropical coun- 

 tries. For example, in Central African areas where malaria is endemic, 

 adults usually show only trophozoites of P. falciparum, or rarely a few 

 gametocytes of this species, whereas the gametocytes of all three major 

 species may be expected in children (92). 



The ability to transmit human malaria is limited to anophelines. Of 

 these mosquitoes, only the "domesticated" types are usually important 

 because they are most likely to become infected. Once they have acquired 

 P. falciparum, mosquitoes may remain highly infective for about 10 days 

 after sporozoites appear but are no longer infective to man after 40 days 

 (27). The period of infectivity is somewhat longer for P. vivax (24). If 

 malaria is to be maintained in a human population, suitable mosquitoes 

 must be present in at least a minimal density. Hence, climatic conditions, 

 which affect both mosquito breeding and development of the parasites 

 in mosquitoes, exert an important influence on transfer of malaria. 



Seasonal variations in incidence are more or less noticeable in malarial 

 regions. In general, vivax malaria is most common in the early spring 

 and through mid-summer in temperate regions. Initial attacks of malig- 

 nant tertian rarely occur before early summer and are usually to be ex- 

 pected in late summer and early autumn. Quartan malaria is more likely 

 to reach its peak in late autumn and early winter. The rainy and dry 

 seasons in the tropics are obviously major influences on seasonal inci- 

 dence. In temperate climates, however, changes in temperature may be 

 more important. For example, the biothermal range for P. vivax in 

 Anopheles quadrimaculatus is about 15-30°, with an optimum near 28°. 

 At temperatures above 30° development is inhibited, and the parasites 



