Cases first appear in the late Spring and the disease becomes epidemic during the Summer.
An attack produces quite an immunity.
The disease has chiefly been studied in the Balkan States but undoubtedly it is widespread.
The disease is almost never fatal so that we know nothing of its pathology.
Symptomatology.—The symptoms will answer perfectly for cases of dengue one sees in a dengue epidemic in which, instead of the saddle-back course of fever, we have a three-day primary rise and then a fall to normal without any secondary fever rise. Cases of phlebotomus fever are occasionally reported where the fever continues seven or eight days.
The symptoms as usually given are as follows: After a period of incubation of from three to six days there is an abrupt onset with congested face and injected conjunctivae. There is pain in head, eyes and back. There is marked malaise with great depression of spirits. There is anorexia with coated tongue and rarely vomiting and diarrhoea. There may be some congestion of the pharynx and even a slight bronchitis. So much in common with influenza has it clinically that a synonym is summer influenza. The liver and spleen are normal. Mental depression is frequently noted. Epistaxis is rather common.
There is a leucopenia and polymorphonuclear percentage decrease. The two points which are chiefly advanced in its clinical differentiation from dengue are (1) slow pulse, a bradycardia, and (2) only three days of fever and absence of eruption.
Seven-day Fever
Rogers first described a dengue-like fever which occurred in India during the summer months as seven-day fever. The fever course was at times typically saddle-back and again would be of continuous type. A pulse relatively slow for the temperature is generally recorded, together with sudden onset and general malaise. The spleen is at times enlarged and there is a definite leucopenia and polynuclear percentage reduction. The eruption is only occasionally present (about 10%) and is an erythema which makes its appearance on the extensor surfaces of the forearm about the fourth day after the onset.