The inhibitory activity of secretions in cattle against foot and mouth disease virus.

Natural and induced factors inhibiting foot and mouth disease virus were investigated in bovine secretions, especially in those from the upper respiratory and oro pharyngeal areas. Techniques were devised to collect lachrymal, nasal, buccal and pharyngeal fluids from normal, convalescent and passively or actively immunised steers. The pH and total protein content of secretions were established in normal cattle. Immunoglobulin types IgA and IgGl predominated. Interferon was not detected. Normal tears exhibited no antiviral activity but nasal secretion, oral saliva and p~al fluid were inhibitory due to their alkaline pH and, in the case of salivary fluids, to the presence of an additional anti viral factor which was partially characterised. Virus lost infectivity in vitro due to natural, non specific factors at rates which varied with the strain of virus to a maximum of 1.25 log units per hour. Clinical disease, viral excretion, interferon and antibody were studied following infection with virus of types 0, A and C. Interferon was detected for up to 4 days at the period of maximum viral excretion. Secretory neutralising antibody developed , - 5 days after exposure, reached a peak within 21 - 28 days and was associated principally with IgA and IgGl. Serum levels were consistently greater than those in secretions. Passive immunisation studies showed that some secretory antibody was derived from serum. years after infection. Antibody persisted for at least ~ Secretory antibody levels increased with successive subcutaneous doses of inactivated vaccine. A single dose elicited good humoral but

poor secretory responses. Levels in serum and secretions rose after a second dose and approached convalescent levels after a third. Secretory antibody was principally IgGl but after a third vaccination IgA was also detected. The relation between secretory antibody and the outcome of exposure to infection was studied. In steers vaccinated once or twice and exposed 14 days after the last vaccination, no lesions were observed and little virus was recovered from their secretions excepting pharyngeal fluid. After three vaccinations much less pharyngeal virus was recovered following exposure. Convalescent animals reexposed 5 and a half years after primary infection were immune and did not become carriers, pharyngeal samples remaining negative. These observations correlated well with the presence of neutralising IgA in the secretions.