IMMUNITY IN CALVES AGAINST RINDERPEST              147

      In Table IV we have attempted to place the calves in groups, as numerically
equal as possible, according to their age at the time of immunisation and the
interval of time between immunisation and test, and we consider that the figures in
the " total " columns, vertical and horizontal, of this Table indicate the relation-
ship of these two factors to the duration of immunity, and also support our belief
stated above.

      In general the results obtained in these experiments are in conformity with
those obtained by other workers, and we conclude that in India calves under 4
months should not be submitted to the serum-simultaneous method of inoculation
as the immunity conferred, if any, is too short to be of any practical value.

      In calves over this age, however, the method is applicable as the economy
effected in serum, and the early reduction of the number of unprotected animals in
a herd, will more than counterbalance the small percentage of cases of incomplete
immunity of short duration.

      As a further precaution to eliminate these doubtful cases, it appears to us
advisable to re-inoculate all animals immunised in calf-hood with a dose of virus—
the cost of which is negligible in comparison with serum—about 12-18 months after
the first inoculation.

      With regard to the relationship of the degree of reaction to the subsequent dura-
tion of immunity we have observed that in those cases where a definite rinderpest
reaction has occurred the duration of complete immunity has always been longer
than the test period—the maximum test period in this experiment being 4 years.
A durable immunity can, however, be conferred without the appearance of any
appreciable symptoms, provided that the virus is capable of diffusing throughout
the body in spite of the inhibiting influence exerted either by the antibodies of the
simultaneously inoculated serum, or by a natural resistance on the part of the
tissues of the inoculated animal. In cases where either or both of these factors
prevent or limit the contact between virus and reacting tissue, little or no immunity
is developed. From this it appears that the severity of the reaction, as judged by
the presence or absence of symptoms, does not give a useful indication of the
probable duration of immunity.

      In this experiment the greater resistance to rinderpest infection of indigenous
breeds, except the Sahiwal, as compared with European × indigenous cattle is
evident, but we do not consider that we have sufficient data to enable us to formu-
late an opinion upon the relative duration of immunity in Indian breeds as
compared with European × indigenous breeds, or with other exogenous breeds.

                                                                                                                    C