A Serological Difference Between Eastern and Western Equine Encephalomyelitis Virus

Abstract

From cases of equine encephalomyelitis in New Jersey and Virginia a virus has been secured which closely resembles that found in a similar disease in the western United States by Meyer, Haring, and Howitt. 1

Like the western virus the strains isolated from eastern cases are highly pathogenic for guinea pigs, less so for mice, and mildly virulent for rabbits. Tests on other animals and detailed studies on the routes of infection have not been made. Infection has been produced in guinea pigs by subcutaneous, intraperitoneal, and intracerebral injection; mice have been infected by these routes and also by intranasal injection. Rabbits given virus intracerebrally usually die, but if it is given intra- or subcutaneously they recover and are then immune to intracerebral injection. Their sera will neutralize the virus injected. The virulence of the eastern virus for laboratory animals appears to be greater than that of 2 western strains we have obtained from horse brains sent from Utah. It differs little in virulence from a Nevada and a South Dakota strain that have both been adapted to guinea pigs.

Strains of the western and the eastern virus have been compared serologically and a distinct difference has been found. In order to detect this difference it was necessary to test decreasing amounts of the virus (brain suspension from infected guinea pig) against undiluted serum. An example of a test is given in Table I. It will be seen that had a 10% brain suspension alone been used, neutralizing antibodies could not have been detected.

After a series of tests, a dilution of 10^-3 of a 10% brain suspension was selected as suitable for neutralization experiments. Each mixture of serum and virus was tested by intracerebral injection into 2 guinea pigs. The animals were etherized before the injection. A summary of the tests made is given in Table II.

It is apparent that the western strains are serologically identical and that the eastern strains differ from them but not from each other. The fact that the difference between them is not merely one of virulence is shown by the failure to neutralize in both directions. Since the Nevada and South Dakota strains do not differ from the 2 freshly obtained Utah strains there is no reason to believe that their serological character has been modified by adaptation to guinea pigs.