Twenty-five grade cattle were infected subcutaneously with 1040to 104·6TCD 50 of a highly-attenuated strain of rinderpest virus which is used as a vaccine. No clinical reaction was observed but the proliferation of virus was studied in twenty-two tissues harvested at daily intervals from the first to the 10th days after inoculation. Serum samples collected at the same times were examined for rinderpest-neutralizing antibody.There was an ‘eclipse’ phase of 3 days during which no infectivity could be demonstrated in any tissue. On the 4th day virus had generalized, as shown by its detection in lymphoid tissues which were not associated with the site of inoculation; occasional animals showed evidence of viral proliferation in the local muscle and subcutaneous tissue. A considerable growth of virus, with peak titres between 104·0and 105·0TDC50/g., was demonstrated in the prescapular, pharyngeal and mesenteric lymph nodes, also in the palatal tonsils and Peyer's patches of the ileum. Highest titres (105·4TCD50/g.) were recorded in the prescapular haemo-lymph nodes, but less virus (up to 103·4TCD50/g.) appeared in the spleen.A low-level viraemia was detected in eight of the thirteen cattle killed on the 5th to 8th days inclusive. Minimal quantities of virus were found on two occasions each in the bone marrow and lung. No virus was recovered from the mucosae of the base of the tongue, abomasum, ileum, caecum and colon; liver, heart, kidney and brain tissue also failed to support its multiplication.Neutralizing antibody was present in all cattle by the 10th day after inoculation, its appearance being associated with the abrupt decline in virus titres, which was usually demonstrable on the 8th day.The behaviour of the attenuated virus was compared with that of virulent strains, and it was concluded that its lack of pathogenicity was due primarily to its failure to proliferate in the mucosae of the gastro-intestinal and respiratory tracts. Vaccine virus was, in fact, exclusively ‘lymphotropic’, a characteristic which may account for the solid, lasting immunity it confers and for the considerable antibody response it provokes in inoculated cattle. Inability to spread by contact amongst susceptible cattle may be a result of the absence of virus in mucosae or parenchymatous organs and hence in excretions.
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