INFECTION 



1133 



sites have been given in Chapter XXI. (p. 504), and need not be 

 referred to here. 



Infection. — The parasites are introduced into the human body 

 as sporozoites, which pass from the salivary glands down the hypo- 

 pharyngeal canal of the mosquito's proboscis into the blood when 

 a human being is bitten by an infected anopheline, 



Incubation. — Once in the blood, three possibilities confront the 

 parasites: they may either be killed and no infection result, or the}^ 

 may remain dormant in some form in the spleen, and not develop 

 until predisposing causes, by lowering the vitality, give them oppor- 

 tunity, or they may at once proceed to develop and give rise to fever. 



In this latter event, there is usually an interval between the period 

 of the bite and the attack of fever, for it is evident that in order to 

 influence the metabolism of the body so profoundly as to produce 

 a rise of temperature a certain amount of toxin must be generated. 

 This requires a definite number of parasites, and hence the incuba- 

 tion period may be short if many parasites are simultaneously 

 inoculated by the mosquitoes, or long if but few. As a rule it may 

 be said that some nine to twelve days are required for the develop- 

 ment of a sufficient number of parasites to produce fever. Acton 

 gives the following table of the length of the incubation period: — 



p ., Maximum in Minimum in Average 



arasi e. Days. Days. Days. 



Quartan . . . . . . 18 11 14 



Tertian .. .. .. 21 | 6 11 



Subtertian ...... 14 j 2 6 



During the incubation period, however, there may be slight 

 symptoms of lassitude, or pains in various parts of the body coming 

 on at regular intervals, and often not much regarded, as they may 

 pass off quickly. We are inclined to consider these as being due to 

 parasites as yet not numerous enough to produce fever, and we 

 think these symptoms of importance, as an attack of fever may be 

 prevented by the prompt administration of quinine. This subject 

 will, however, be further discussed under the Pathology of Latent 

 Malaria. 



The Fever. — The life-history of the parasite has a definite re- 

 lationship to the disease, as can be noted by studying the diagrams. 

 The rise of body temperature is always associated with the sporu- 

 lation of the parasite and the liberation of pyrogenetic toxins, the 

 apyretic interval with the growth and maturation of the parasites 

 in the red corpuscle. This point has been recently carefully studied 

 by Ross and Thomson b}^ their enumerative methods, and they 

 find that there is a distinct correlation between the number of the 

 asexual forms found in the peripheral blood and the fever, and that 



