Dengue and Phlebotomus Fever.—In these diseases a leucopenia, which begins to show itself by the second day, is very characteristic.
The average leucocyte count is about 3500 and along with this we have a reduction in the percentage of polymorphonuclears to about 50%. Towards the end of the terminal fever we have an increase in the percentage of large mononuclears.
Bacillary Dysentery.—The agglutination tests are of little value in diagnosing the presence of or type of an infection with dysentery bacilli, as the agglutinating power does not appear until during convalescence.
It is now customary to use a polyvalent antidysenteric serum in treatment so that it is not very essential to ascertain the strain involved in an infection. As a practical matter we make our diagnosis of the presence as well as type of dysentery bacillus involved in an infection by isolating the organism from the dysenteric stool.
During the fever we may have a moderate polymorphonuclear leucocytosis.
Enteric Group of Fevers.—In fevers of atypical course in the tropics one must always remember that typhoid and the paratyphoid fevers are anything but uncommon and blood cultures should always be made when such suspicion arises. In some tropical regions paratyphoid A infections seem most common although the usual experience is to encounter the paratyphoid B infection more frequently. In temperate climates the noting of a moderate leucopenia with an absence of eosinophiles is important in the diagnosis of typhoid, but in the tropics there are so many intestinal parasites and skin infections productive of eosinophilia that we cannot attach any importance to such a finding.
Typhus Fever.—Plotz attaches importance to the culturing of B. typhi exanthematici from the blood of typhus cases, but the relationship is now regarded as not causal. Rickettsia bodies, which can be demonstrated in the louse or in capillaries at autopsy, are now considered to be the exciting organism.
A mononuclear leucocytosis has at times been reported.
Spotted Fever of the Rocky Mountains.—Injection of the blood of the patient into guinea pigs produces the disease in the animal. Frick has reported the finding of chromatin-staining bodies in the red cells of such pigs and Wolbach has found chromatin-staining bacteria in the endothelial cells of such animals. These bodies are now classed as Rickettsia.