REVIEW — TEOPICAL MEDICINE, ETC. 37 



9. Dengue can be transmitted by the mosquito, Culex fatigans, and this is probably the most common Dengue — 

 method of its transmission. continual 



10. The period of incubation in experimental dengue averages three days and fourteen hours. 



11. Certain individuals are absolutely immune to dengue, as proved by our experiments. 



12. Dengue is not a contagious disease, but is infectious in the same manner as is yellow fever and the 

 malarial fevers. 



In another paper^ they deal with diagnosis. In differentiating from yellow fever the 

 slower pulse, jaundice and hgematemesis occurring in that disease are helpful. The same 

 would hold good in the Egyptian disease most resembling yellow fever, namely, infectious 

 jaundice. As regards influenza, they point out the association of dengue with the presence 

 of mosquitoes, while influenza occurs where they are absent and often in cold weather. 

 They mention the catarrhal symptoms in the latter and lay stress on the leucopaenia and 

 lymphocytosis found co-existing in dengue. Early small-pox has to be difierentiated, and 

 sometimes an acute follicular tonsillitis simulates dengue. The prophylaxis resolves itself 

 into protection against mosquitoes. In this paper are recorded the differential blood counts 

 by Vedder who assisted in the work. The variation in the relative proportion of the large 

 and small lymphocytes found by Stitt was not confirmed, but, as already stated, his other 

 results were substantiated. 



Eoss" has recently advanced strong confirmatory evidence to show that the immunity 

 of Port Said from dengue fever since 1906, while epidemics raged elsewhere in Egypt, was 

 due to the abolition of Gulex fatigans in that town. The same is true of Ismailia, which 

 escaped during the epidemic of 1907. 



One^ has been able to make some personal observations regarding dengue in the Sudan, 

 but only on a small scale. As the disease was very prevalent in Egypt and parts of the 

 Sudan during the summer and autumn of 1906, one was in hopes of being able to carry out 

 a study of the blood in dengue. Fortunately in one sense, unfortunately in another, though 

 Port Sudan and Haifa were visited by epidemics, Khartoum, so far as can be told, wholly 

 escaped. Not a single case of dengue fever was notified, and this, although it is more than 

 likely that several persons suffering from dengue must have reached Khartoum by railway 

 from the north, while I saw one case which arrived in Khartoum before convalescence was 

 fully established, and while he was probably still in an infectious state. 



Is it not possible, then, that the immunity which Khartoum has enjoyed is due to the 

 comparative freedom of the town from mosquitoes, and especially from Cidex fatigans ? No 

 species of mosquito was at all common in Khartoum during the months when dengue was 

 prevalent in other parts of the Sudan and in Egypt. Thus, during June, 1906, there were in 

 Khartoum 689 water collections which might have served as mosquito breeding places. Of 

 these 17 were infected with larvae or pupae, being 2-47 per cent. The corresponding figures 

 for Khartoum North were 125 ; 4 ; 3-2 per cent. During July the percentage infected in 

 Khartoum was 4-35, in Khartoum North 3-2 ; August, Khartoum 7-22, Khartoum North 3-20 ; 

 September, Khartoum 9-94, Khartoum North, 3-20 ; October, Khartoum 8-32, Khartoum 

 North 4-76. The slight rise in August, September and October was due to heavy rainfall, 

 but adult mosquitoes were not numerous. 



The figures are only approximately correct, but they give a good idea of the state of the 

 town. One does not wish to press the point too much, but the observation is interesting and 

 suggestive so far as it goes. 



I append a table of differential leucocyte counts made on blood films from cases of dengue 

 and supposed dengue sent me by Dr. Crispin from Port Sudan. It is necessary to note 

 that one has classed as lymphocytes, both true lymphocytes and lymphocytes with irregular 

 nuclei, while under the term mononuclears, are included both large lymphocytes and large 

 mononuclears in accordance with the very useful classification of Dutton and Todd.* 

 Transitionals, however, have been placed separately. 



My cases were few in number, and most of the bloods were taken only in the early stages 

 of the fever. Moreover, in one or two cases, I do not know what the eventual diagnosis was. 



' Ashburn, P. M., and Craig, C. P. (May, 1907), " Experimental Investigations regarding the Etiology of 

 Dengue Fever." Philippine Journal of Science, p. 71, Vol. II. 



" Ross, E. H. (July 1st, 1908), "The Prevention of Dengue Fever." Annals of Tropical Medicine and 

 Parasitology, Series T. M., Vol. II., No. 3. 



' Balfour, A. (April 1st, 1907), " Notes on the Differential Leucocyte Count, with Special Reference to Dengue 

 Fever." Journal of Tropical Medicine awl Hygiene, p. 113, Vol. X. 



■• Dutton, J. E., and Todd, J. L. (1903). The Thompson, Yates ami Johnson Laboratories' Report, Vol. V., New 

 Series, Part 2, Liverpool. 



