Studies on the neurotropic virus of horsesickness. VII. Transmitted immunity

Abstract

1. At birth the serum of foals from immune dams contained no antibodies to neurotropic attenuated horsesickness virus demonstrable by the intracerebral protection test in mice. 2. Less than 30 hours after birth, that is after the first suckle, antibodies are present in a concentration not lower than that of the dam. 3. The titre of the antibodies gradually declines, the duration of their presence being proportional to the original titre. Usually they persist for approximately 6 months. 4. All the evidence indicates that the source of the antibodies is the colostral milk. 5. The acquired immunity was found to protect against virulent pantropic virus for 157 days but not for 239 days. Completely blocked out reactions did not produce a durable active immunity. 6. During the period antibodies could be demonstrated and for an unknown period after their apparent disappearance the inoculation of routine horsesickness vaccine had no immunizing effect. 7. The results obtained in foals were confirmed by passive immunization of adult horses. 8. The significance of the results and their bearing upon routine immunization are discussed. 9. Attention is directed to the necessity for investigating: (a) Mutual interference by antigenically different strains of attenuated horsesickness virus. (b) An observed phenomenon whereby active immunity followed transfusion of blood from an animal showing the presence of high titre antibodies in the serum.