By the usual methods, a diagnosis of yellow fever in man early during the course of the illness is impossible, since these are based on the isolation and identification of the specific virus or on the demonstration that specific antibodies appear as a result of the infection. Davis (2) as early as 1931, however, showed that the complement-fixing antigen appears with regularity in the blood of monkeys succumbing to infection with yellow fever virus, and Hughes (11) in 1933 made a similar observation with regard to the precipitinogen. Both authors suggested that the occurrence of the antigens they were dealing with might afford a means for arriving at a positive diagnosis of yellow fever in the acute phase of the disease. No further work appears to have been done on this subject, however, and because of the practical considerations involved a reinvestigation of the matter was considered worthwhile.
The complement-fixation test was chosen in preference to the precipitin test since in our experience it proved to be the more reliable, and the presence of the complement-fixing antigen could be readily demonstrated in sera apparently devoid of the precipitinogen.
Eighteen monkeys (Macaca mulatta) and 31 marmosets (Callithrix jacchus) were employed in the present investigation. These animals were inoculated with the classic Asibi strain of yellow fever virus or with the O.C., A.C.-Bol., or Volcanes jungle strains of the virus, and their sera were tested for the presence of complement-fixing antigen on one or more occasions after inoculation of the virus. The complement-fixing antigen was present in the blood of all those animals which died or were sacrificed when moribund, but was not detected in the blood of those which survived infection and subsequently developed complement-fixing as well as neutralizing antibodies.
The lack of correlation between the virus content and the antigen content of a serum indicates that complement-fixing antigenicity must be referred not to the virus per se but to some substance resulting from the reaction provoked in the host's tissues by the virus. A specific febrile response following inoculation of virus indicates only that the reaction to the parasite is of sufficient magnitude to evoke the appearance of the antigen. The absence of a postinfection temperature rise does not necessarily imply the converse, however, since the antigen has been detected both in monkeys and in marmosets which succumbed to infection without showing a febrile reaction.
At least two obvious explanations exist for the failure to demonstrate antigen in the blood of those few animals which survived infection; either (a) the serum specimens tested were not taken at the appropriate intervals, or (b) the antigen was present in concentrations too small to be detected by the technique used. Experiments now in progress indicate that the latter explanation is probably the correct one, and it would seem, therefore, that the antigen appears in demonstrable amounts chiefly (and perhaps solely) in the blood of those animals in which the disease may be expected to terminate fatally.
In 10 of 37 animals a post-mortem diagnosis of yellow fever virus infection could not be made by histologic examination of the liver, although it was made during life by examination of the serum for complement-fixing antigen. This suggests that the serologic test may therefore constitute a more accurate diagnostic method than does microscopic examination of the liver, although a broader experience is necessary before any unqualified statement is possible as to the greater accuracy of one method over the other.
Opportunity to examine human yellow fever sera has not been available, but if the antigen occurs in the blood of man also it should be possible to use the complement-fixation test for the early diagnosis of the disease in this host, provided that the method is applicable to all degrees of infection, and is not limited to severe or fatal cases.
From the Serviço de Estudos e Pesquisas sôbre a Febre Amarela, Rio de Janeiro, Brazil.