exo-erythrocytic schizogony. Gametocytes are the infective stages 
for the mosquito and, in the arthropod stomach, develop into gametes 
which initiate the sexual cycle. They do not multiply in the human 
host, and unless ingested by the appropriate mosquito, will degener- 
ate and die within a few days. The sexual development, ending in 
production of sporozoites, the infective stage for man, is influenced 
by such extrinsic factors as temperature and humidity. The average 
length of both sexual and asexual cycles for each species is in- 
cluded in the table below. These are based on reports in the liter- 
ature (by various investigators) and represent the more common 
average development periods as recorded. 
The Anopheles species involved as hosts for human malaria vary 
with the geographical area. There are over sixty species which are 
considered vectors of malaria in different areas of the world. In the 
United States, there are only two that are considered to be important 
vectors: A . quadrimaculatus in the east and A . freebomi in the west. 
Not all species of Anopheles serve as malaria vectors, either be- 
cause they are not good biological hosts, or because they normally 
do not feed on human blood. 
Malaria infections can be spread from man to man via blood 
inoculations, for example, transfusions or common hypodermic needles 
used by drug addicts, but an exo-erythrocytic phase does not occur. 
Exo-erythrocytic development takes place only after sporozoite 
inoculation. 
Man is considered to be the only natural vertebrate host for the 
four species of human malaria. However, recently it has been found 
that P. cynomolgi , a parasite of monkeys resembling P. vivax, can 
be transmitted by mosquitoes to humans (Eyles, et al., 1960). While 
there is at present no clear-cut evidence that transmission occurs 
naturally, the possibility of monkey to man infection exists. Since 
this early report of Eyles, man to man transmission of P. cynomolgi 
by mosquitoes has been demonstrated by Contacos et al. (1962). 
The susceptibility of humans to simian malaria species may influence 
control and eradication programs in areas in which they are endemic. 
References 
Eyles, D. E., Coatney, G. R., and Getz, M. 1960. Vivax-type malaria 
parasites of Macaques transmissible to man. Science 131 : 18 1 2-1 813. 
Contacos, P. G., Elder, H. A., Coatney, G. R., and Genther, C. 
1962. Man to man transfer of P lasmodium cynomolgi by mosquito 
bite. Am. J. Trop. Med. Hyg. 11 ( 2) : 1 86 -1 93. 
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