The immunization of horses against horsesickness by the use of formalised virus. Part II
1933
(1) The antigenic value of individual spleens varies greatly. (2) The product of several pooled spleens is more potent than that obtained from a single spleen. (3) The potency of different batches of pooled spleens varies. (4) The keeping quality of formalized vaccines for four months is good. After that time there is evidence of decrease in value (not tested after seven months). (5) The addition of phenol as a bactericide has no detrimental effect. (6) Filtration through a Seitz filter is contra-indicated. (7) The lowest concentration of formaldehyde required to produce a safe vaccine is 1 : 1000. (8) The lowest concentration of formaldehyde able to produce a solid immunity to 0-virus is 1 : 4000. (9) It is not safe to proceed from a 1 : 1000 vaccine to a 1 : 4000 vaccine in less than two intermediate steps. (10) The best results have been obtained with four injections of 1: 1000, 1 : 2000, 1 : 3000 and 1 : 4000 vaccine. (11) The interval between injections should not be less than 14 days, or greater than 21 days. (12) The dose of vaccine should not be smaller than 20 c.c. A dose of 30 c.c. has given the most constant results. (13) The immunity produced by the vaccine may last for as long as six months. Usually it is transient and on occasion has markedly decreased in 28 days. (14) It is necessary to complete immunization by a dose of fully virulent virus. (15) The margin of safety of the method is narrow. (16) The application of the method to the field cannot be recommended in the present state of our knowledge.
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