1246 



DENGUE AND ALLIED FEVERS 



42° N. and 28° S. in exceptional cases. It is, therefore, a disease 

 of tropical climates, and of warm weather in other climates. It 

 appears to be favoured by low-lying lands near the sea, well 

 supplied with water, and not to be found at high altitudes ; but 

 there are exceptions to this. It occurs in Australia, where it has 

 appeared since 1885, and is common in the Anglo-Egyptian Sudan 

 and along the Red Sea. 



When the distribution of dengue is compared with that known 

 for Cul&x fatigans, it will be seen that the two coincide most re- 

 markably. In fact , at the present time it appears as though the geo- 

 graphical distribution depends upon the distribution of that insect, 

 together with the importation or presence of infected persons. 



etiology. — ^W^e are indebted to Graham and Ashburn and Craig, 

 and to Cleland, Bradley, and McDonald, for our knowledge of the 

 aetiology of this disease. 



There appears to be no doubt that dengue fever is caused by some 

 unknown living organism which requires over two days to increase 

 so as to produce the symptoms of the disease when inoculated into 

 human beings, and generally five to nine and a half days when 

 produced by bites of an infected stegomyia. 



Both Graham and Ardate have observed small bodies in the red blood- 

 corpuscles, which are described as small, usually round, but sometimes 

 elongated, bodies about one-fifth to one-third of the size of a red corpuscle. 

 They divide up into minute granules, which become extra-corpuscular. 



This organism exists in the blood, as can be shown by the intra- 

 venous injection of 20 c.c. of dengue fever blood producing typical 

 attacks of the disease, the incubation usually being two to three 

 days; but it is so small that it will pass through the pores of a filter 

 which will retain Micrococcus melitensis, which is only 0-4 /x in 

 diameter, as is demonstrated by the production of the disease by 

 intravenous injections of filtered dengue fever blood, the incubation 

 being two or three days, as before. It may also be present in the 

 lymphatic glands, the gland juice being at times infective, as proved 

 by experiments. 



Blood is infective from two to eight days, but not after the four- 

 teenth day, and the virus is said to be transmitted by the bites of 

 Culex fatigans, which is thought to be an agent in the spread of the 

 disease, but Cleland, Bradley, and McDonald's experiments have 

 thrown considerable doubt on this. There may, however, be two 

 distinct diseases, one caused by a stegomyia and one by a culex, 

 the incubation period being about three days sixteen hours. It can 

 also be transmitted by the bites of Stegomyia calopus, when, further, 

 there is a natural immunity against the disease in some people, and 

 also some degree of acquired immunity of unknown duration obtained 

 by an attack. 



The points in the aetiology which require further research are the 

 nature of the organism and its chemical products; the changes, if 

 any, which the parasite undergoes in the mosquito; the length of 

 time during which the mosquito can carry the infection ; whether any 



