The Complement-Fixation Test with Human Sera against the Viruses of St. Louis Encephalitis and Equine Encephalomyelitis

Abstract

Summary Of 137 sera tested from cases of encephalitis, 34 or 24.8 per cent, showed both complement-fixing and neutralizing antibodies for the St. Louis virus alone, 23 or 16.7 per cent for the western equine strain alone and 17 or 12.4 per cent for both viruses. Of these 137 sera, therefore, 73 or 54.0 per cent showed complete correlation between the complement-fixation and the neutralization-tests. With the other 64 sera, there was more tendency for the complement-fixation test to be positive for the St. Louis virus than for the equine, particularly if the neutralization-tests were positive for both the strains. Eleven or 8.02 per cent of the sera gave positive complement fixation with the St. Louis antigen when the neutralization-tests were negative to both strains. Of 42 sera obtained during the first 2 weeks after the onset of disease, 100 per cent neutralized one or the other of the viruses while only 73.8 per cent reacted to complement fixation. Neutralizing antibodies for the western equine strain appeared earlier in the disease than those fixing complement, while the reverse was observed for the St. Louis virus. During the later periods there was not much difference between the presence of either antibody for the 2 strains, except that the complement-fixing reaction remained positive longer for the western equine virus than for the St. Louis. Complement-fixing antibodies for the western equine strain as well as neutralizing substances were found in a serum at least 2 years after the onset of the disease, while those for the St. Louis strain were demonstrated after 10 to 14 months. The complement-fixing ability of a serum may be attenuated after prolonged storage but if the specimen is not anticomplementary or bacterially contaminated, significant reactions may be obtained even after 2 years in the refrigerator. A negative one, however, may not be of value. Of a total of 257 specimens examined by both methods, 120 or 46.7 per cent reacted negatively. 47 of the sera came from the non-endemic Bay Region and 73 from the endemic San Joaquin Valley. 66 of the latter were from cases of poliomyelitis. Complement-fixing as well as neutralizing antibodies were found in sera from a small group of contacts to frank cases of encephalitis. Of 70 sera in the Bay Region tested against the lymphocytic choriomeningitis virus 4 or 5.7 per cent gave a positive complement fixation and only 3 (4.2 per cent) of these a correspondingly positive neutralization-test. Of 121 sera examined none showed complement-fixing antibodies against the eastern equine strain of encephalomyelitis, while 4 out of 86 (4.3 per cent) reacted to a normal mouse brain antigen. The latter sera were likewise positive with the other antigens used. It would be of value to use both the complement-fixation and the neutralization-test for the most accurate differentiation of human cases of encephalitis.