It has been shown that whereas blood obtained during the breakdown
reaction in immunized horses may be non-pathogenic for mice it is still capable
of infecting horses, dogs and ferrets. Due to this fact it is often. impossible to
isolate virus directly in mice from cases of horsesickness in immunized horses.
This difficulty may be overcome by first isolating the virus in the horse
or ferret from which animal mice may readily be infected. It is probable that
the dog may be used in a similar manner.
Attempts to reactivate for the mouse virus present in the blood from
immunized horses by various methods were unsuccessful.
Just how this virus is inactive for the mouse while active for the horse, dog
and ferret is not understood, although the serum antibody produced as a result
of immunization is believed to be involved.
By means of neutralization tests in mice with the neurotropic form of a
virus which produced a fatal breakdown in an immunized horse it was shown
that such immune horses may react severely to infection with virus which is
related antigenically to strains of virus incorporated in the vaccine.
Ferrets have been shown to be susceptible to viscerotropic horsesickness
virus. The disease in ferrets is mild and results in a febrile reaction with viraemia.
I should like to thank Dr. D. A. Haig for his helpful advice and criticism
and Mr. P. J. van Rooy for carrying out the high speed centrifugation