that his experiments tend to show that, like yellow fever, dengue is communicated by a mosquito, Culex fatigans. Acting on this hypothesis he was able to protect families from dengue by means of mosquito nets. Ross is said to have extinguished the yearly epidemics in Port Said by anti-mosquito measures. Bancroft, who considers that the germ is ultra- microscopic, has advanced the same view in Australia in favour of Stegomyia fasciata. Experiments by Ashburn and Craig, made under favourable conditions, seemed to show that the germ of dengue resides in the liquor sanguinis, that it is ultramicroscopic and therefore unfilterable, that injected into non-immunes it gives rise to typical dengue, that under natural conditions it is transmitted by Culex fatigans through the salivary glands, that the incubation period of the experimental disease is from three to fourteen days, and that it is not contagious in the ordinary sense of that word. In these and in some other respects dengue has a striking parallelism to yellow fever.
By a series of well-conceived and carefully- carried-out experiments Drs. Cleland, Bradley, and McDonald in Australia have now definitely proved that the virus of dengue is conveyed by Stegomyia calopus, and have rendered it highly improbable that Culex fatigans is a vector. They have shown, further, that the incubation period of the disease, dating from the infective bite, is from six to nine days. Important points still undetermined are the duration of the infective stage in the blood, and the ripening process of the germ in the mosquito.
Influences of meteorological conditions.— When dengue spreads beyond its ordinary tropical limits, as, for example, in the epidemics of Philadelphia and Asia Minor, the extension occurs only during the hottest part of the year— in the late summer and early autumn. Hitherto such epidemics have been arrested on the approach of winter. Even when occurring within the tropics, dengue prevails principally, though not exclusively, during the hottest part of the year. High atmospheric temperature, therefore, seems to be one of the conditions it demands.
