Abstract
The most important recent laboratory contributions to the control of yellow fever will be briefly summarized under three headings: (1) Methods of diagnosis, (2) Transmission, and (3) Protection.
(1) Methods of diagnosis.—The development of improved methods of identification, in particular by immunity tests, has made it possible to diagnose yellow fever with much greater certainty. Moreover, since the immunity following an attack of the disease is usually of life-long duration, it is possible to determine what proportion of any particular population has been infected and also how long any district has been free from infection. The application of immunity tests to the delimitation of endemic zones, especially in West Africa, has led to a great increase in our knowledge, and yellow fever has been found to have a much wider distribution than was previously suspected. Among other methods of recognizing the disease may be mentioned complement-fixation tests and also in post-mortem material the histopathology of the liver.
(2) Methods of transmission. Indirect.—The main factors in the transmision of the disease by mosquitoes have been elucidated, and the relations between the course of the infection in monkeys (and also presumably in man) and the infectivity of these animals to mosquitoes. It is found that the blood becomes infective at a very early stage, before febrile symptoms develop, and that infectivity usually disappears three to four days after the onset of fever, owing to the presence of immune bodies in the blood. It is evident that any yellow fever patient must be considered to have been capable of infecting mosquitoes before showing any signs of the disease. Many other species of mosquitoes in addition to the Aëdes ægypti have now been shown capable of transmitting yellow fever.
Direct.—It is now known that it is possible to acquire yellow fever in the absence of mosquitoes, through handling infected material. Many cases of laboratory infection have now been recorded in which other sources of infection can be definitely excluded.
(3) Protection.—The most important advance in this direction has been the development of practicable methods of vaccination. The use of attenuated virus was followed by the use of virus and immune serum. The development of the latter has depended mainly on the discovery that when yellow fever virus is inoculated intracerebrally into mice, after a few passages it acquires neurotropic affinities and loses to a great extent its capacity for producing a general infection. The use of such virus, combined with human or animal immune serum, has been found to result in the development of a high degree of immunity comparable in intensity with that following an attack of the disease.