Antibody to equi factor(s) in the diagnosis of Corynebacterium equi pneumonia of foals.

Antibody to equi factor(s) in cases of Corynebacterium equi pneumonia in foals was detected using C. pseudotuberculosis exotoxin sensitized calf red blood cells. The test was standardized using antitoxin produced in rabbits by injection of equi factor(s). All sera from ten foals with culture-diagnosed C. equi pneumonia had antibodies to equi factor(s) (titre range 8-256, mean 74.0) and nine sera from 11 foals with suspected C. equi pneumonia also showed antibodies (titre range 4-512, mean 136.4). Two of five pneumonia foals with transtracheal aspirate cultures not yielding C. equi had such antibodies. Fifty-eight of 59 control horse sera had no antibodies; the one positive serum came from a foal on a farm where C. equi pneumonia was endemic. By contrast only five of 15 foals with experimentally-induced C. equi pneumonia had antibodies to equi factor(s), probably because the acute nature of the disease produced did not mimic the chronic course of the natural disease. Antibody to equi factor(s) can be used in the diagnosis of naturally-occurring corynebacterial pneumonia in foals.