Serum antibody response to Listeria monocytogenes, listerial excretion, and clinical characteristics in experimentally infected goats.

We induced an experimental listeriosis in five goats by oral inoculation of Listeria monocytogenes serovar 1/2a. The resulting generalized bacteremia was reflected by systemic illness in four of the five animals inoculated and by excretion of L. monocytogenes in milk by all five animals. By using an enzyme immunoassay, we recorded a markedly enhanced immunoglobulin G (IgG) antibody response in the two youngest and most seriously ill test animals. In these, the elimination of L. monocytogenes from the gastrointestinal tract occurred simultaneously with the development of the highest antibody levels at 14 to 15 days postinoculation. In the case of the oldest test animal, a preexisting, persistent IgG antibody response was recorded which was associated with the total absence of clinical symptoms and the shortest observed fecal carriage of L. monocytogenes, lasting for only 3 days. Two animals remained practically seronegative, and an IgM antibody response was not recorded for any of the animals. The findings suggest that an association exists between the humoral immune defense against Listeria infections, the clinical course of the infection, and the elimination of the Listeria organisms from the gastrointestinal tract.