The exact way in which the parasites cause such swelling is not very definitely known. Manson, who has done more work on these diseases than any one else, believes that the trouble arises from the clogging of the lymphatic glands or trunks, thus cutting them off from the general circulation, in which case the affected parts may become distorted. This clogging of the passages is believed to be due to the presence of great numbers of immature eggs which have been liberated by parasites injured in some way before their eggs were entirely developed.
This interference with the lymphatic circulation brings about the anomalous condition of a patient with a serious filarial disease with fewer of the filarial parasites in his blood than one who is not so seriously affected. This is supposed to be due to the fact that the disease-producing parasites have died and that the lymphatics have become so obstructed that any microfilariæ they may contain cannot make their way into the general circulation. Such a patient then would not be as likely to infect a mosquito as would one less seriously affected.
It has always been thought that little or nothing could be done in the way of successfully treating this disease, but quite recently a French physician, who has been conducting a long series of experiments in the Society Islands, announced that he is able to cure many cases by certain surgical operations on the affected parts.
DENGUE OR "BREAKBONE FEVER"
This is another disease of the tropics often occurring in widespread epidemics. It is probably most frequently met with in the West Indies, but may occur in any of the tropical countries or islands. Occasionally it spreads into subtropical or even temperate regions. Several extensive epidemics have occurred in the United States. Once introduced into a community it spreads very rapidly and nothing seems to confer immunity.
The various names by which it has been called well describe its effect on the patient; breakbone fever, dandy-fever, stiff-necked or giraffe-fever, boquet (or "bucket") fever, scarlatina rheumatica, polka-fever, etc. While the suffering is intense as long as the disease lasts it seldom terminates fatally.
It has always been classed as a very contagious disease and it has not yet been definitely shown that it is not. Recent observations, however, have shown that it is probably caused by a certain Protozoan parasite that is found in the blood of dengue patients and several experiments have been conducted by Dr. Graham which seem to indicate that it is transmitted by mosquitoes. In these experiments, Culex fatigans, a common tropical or subtropical mosquito, was used. The same parasite that is found in the human blood may be found in the stomach and blood of the mosquitoes up to the fifth day after it has fed on a dengue patient.
Sick and healthy individuals were allowed to remain in close contact in a room from which the mosquitoes had been excluded, and the disease was not spread. Mosquitoes that had bitten dengue patients were taken to a higher region where dengue had never occurred and allowed to bite two healthy persons. Both developed the disease and as they were protected from other mosquitoes until they had recovered, the disease did not spread to others of the community. These and other observations seem to make a complete chain of evidence, and most medical men to-day accept the theory as well proved and in their practice take every precaution to prevent the spread of the disease by keeping the infected patient from being bitten by the mosquitoes.
The yellow fever mosquito is also suspected of carrying this same disease, and it is possible that other species are also concerned. If it is true that the parasite can be carried by several different species of mosquitoes this would account very largely for its rapid spread wherever it is introduced into a community. Where it occurs outside the tropics it is only in the warm summer months when mosquitoes are always abundant.
MALTA OR MEDITERRANEAN FEVER