Protective Antibodies against Equine Encephalomyelitis Virus in the Serum of Laboratory Workers

It has been shown recently 1 that a large proportion of mice and guinea pigs develop, with increasing age, physiological or structural barriers that prevent certain viruses from invading the central nervous system. This resistance is demonstrable when virus is given peripherally, as, for example, intraäbdominally or intramuscularly, but not when it is injected directly into the brain. It is not a result of prior infection nor is it associated with the presence of protective substance in the serum. Furthermore, in the recent epidemic of equine encephalomyelitis (E. E.) in man in southeastern Massachusetts, 2 children were predominantly affected. The older animals which resist the E. E. viruses develop systemic infection, as is evidenced by the finding of virus in the circulation and later the presence of protective antibodies in the serum. In view of this suggestive relationship of age of both the experimental animal and man to clinically apparent infection with this virus, it was thought desirable to undertake a study of the protective capacity of the serum in certain individuals in our laboratory, who had been in contact with the E. E. virus for a period extending from 1 to over 6 years. The results would indicate whether a clinically inapparent infection, as determined by the presence of protective antibody, could possibly have occurred during that time.

Serum-protection tests were carried out in mice by the intraabdominal method of Olitsky and Harford; 3 that is, by injecting by the intraäbdominal route, 0.03 cc of a mixture of equal parts of test serum and virus-dilutions (tenfold dilutions were used) into 15-day-old mice. Each mixture was given to groups of 3 or 4 mice. The Eastern strain of virus (E. E. E.) was derived from a stock which was frequently passaged in mouse brain and was again passaged through mouse brains immediately before use.