306 MOSQUITOES OF NORTH AMERICA 
1907 it again passed through Cairo and other parts of Egypt, but again Ismailia 
and Port Said escaped. Formerly the wards of the hospital in this town were 
full of cases of ‘fever’ during the summer months, but now the beds are used 
for other cases, which no longer contract fever although the mosquito nets have 
been removed.” 
Balfour shows that much the same conditions govern the spread of dengue in 
the Sudan. Thus an epidemic made its appearance in 1906 at Halfa, Port 
Sudan and elsewhere. Khartoum, however, did not have a single case of dengue 
and Balfour points to the scarcity of mosquitoes at this place, particularly of 
Culex quinquefasciatus, as the probable reason. 
An hematozoan, which he compared to the Plasmodiwm of malaria, was 
found by Dr. Graham and considered by him to be the causative organism. 
Other observers have failed to find this organism and have on that account re- 
jected his observations and conclusions. However, as we have seen above, his 
observations and deductions on the mosquito-transmission of the disease are 
confirmed by a number of workers. Bancroft considered that the causative 
organism of dengue is similar to that of yellow fever in respect to its ultra- 
microscopic size. 
Craig has recently discussed dengue in connection with yellow fever and with 
the so-called pappataci fever, this last being transmitted by a minute biting fly 
of the genus Flebotomus, family Psychodide. He shows the close relationship 
of the three diseases and points out that they are most probably due to very 
similar organisms, ultra-microscopic in size and in all probability protozoan. 
The substance of Craig’s discussion, based upon the investigation by Ashburn 
and himself, follows herewith. 
“ After a thorough examination of the blood of numerous patients with 
dengue we concluded that it did not contain any visible organism either bacterial 
or protozoal in nature, which could be considered as the cause of the disease. 
Our attempts at securing blood cultures also resulted negatively and in order to 
determine whether the cause of this fever was present in the blood we were 
forced to attempt the production of the disease by the inoculation of the blood 
from dengue patients into healthy individuals. As the disease is one which in 
the young and robust is not dangerous to life, we felt justified in making such 
experiments, and had no difficulty in procuring volunteers for this purpose. 
“We first undertook to study the effect of the intravenous inoculation of 
unfiltered dengue blood. Eleven men were inoculated, and in seven a typical 
attack of the disease developed, while one case was doubtful. . . . 
“These experiments proved beyond question that the cause of dengue is 
present in the blood of infected individuals, as the intravenous inoculation of 
such blood in healthy men is capable of producing a typical attack of the disease. 
“ As our examinations had proved that it was impossible to demonstrate a 
parasite in the blood we concluded that it must belong to the class of so called 
ultramicroscopie organisms and in order to determine this point we inoculated 
two healthy individuals with filtered blood from dengue patients, with the re- 
sult that severe attacks were produced in both men. In our filtration experi- 
ments we employed a Lilliput filter which was tested and controlled each time it 
was used, and the filtration was done under 730 millimetres pressure. After 
filtration a control test was made of the filter by using a bouillon suspension of 
Micrococcus melitensis, the filtrate then being incubated for two weeks, and 
