The first objective of this study was to develop an immunohistochemical procedure for rapid detection of Rhodococcus equi in impression smears from affected organs of foals on postmortem examination. The second aim was to demonstrate whether R. equi can be detected in smears of tracheal exudates collected from the same foals using an immunohistochemical method. Impression smears and cryostat and paraffin-embedded sections were made from the lungs and mediastinal lymph nodes of three foals (A, B and C) that had died of respiratory disease caused by R. equi, and also from the caudal mesenteric lymph node of foal A. Impression smears were made from the tracheal exudates of all foals. An affinity purified rabbit IgG was used for the immunohistochemical demonstration of R. equi. This antibody reacted with serotype 1 of R. equi in Ouchterlony's immunodiffusion and in the passive haemagglutination test, but not with other serotypes or with Streptococcus equi ssp. equi or Staphylococcus aureus, and failed to give an immunohistochemical reaction with Mycobacterium boris or M. paratuberculosis. The immunohistochemical method proved to be of identical sensitivity to bacterial culture; moreover, from the lungs and mediastinal lymph nodes of one foal, R. equi could only be detected by this method. R. equi was demonstrated in smears of the tracheal exudates of all three foals. The results of this study indicate that the immunohistochemical method may be used for the rapid detection of R. equi in impression smears from the affected organs, especially abscesses, obtained postmortem, and possibly as a tool for diagnosing R. equi pneumonia in live foals by examining smears of tracheal aspirates.
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