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
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.
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