Failure of passive immunisation by colostrum from immunised mares to protect foals against Rhodococcus equi pneumonia

Abstract

Summary The immunoprophylactic capacity of colostrum‐derived Rhodococcus equi antibodies was evaluated in pony foals experimentally infected with R. equi. Six pony mares (Group 1) were injected serially with live R. equi during pregnancy; a further six pony mares were not vaccinated and used as controls All (Group 2) foals were permitted to suck from their dams and the adequacy of IgG, passively transferred via colostrum, was determined by single radial immunodiffusion. The presence and relative value of R. equi specific antibodies were determined by ELISA. Mares vaccinated with R. equi developed significantly (P<0.05) increased R. equi antibody levels, and the principal foals from these mares had specific antibody values that were significantly (P<0.01) higher than control foals. Rhodococcus equi were aerosolised into a caudal lung lobe of all foals at seven days of age. Clinical signs, haematological alterations, immune responses and thoracic radiographs were monitored. Foals were subjected to euthanasia and complete post mortem examinations were performed. All foals developed pneumonia as evidenced by clinical and radiographic signs. The survival rate for Group 1 foals did not differ significantly from that of control foals. Two of the six foals in each group survived the bacterial challenge. Also, there were no significant differences between groups for duration of survival, peak febrile responses, duration of fever, time from infection to onset of fever, or peak concentrations of leucocytes and neutrophils. Post mortem examinations of four foals from each group, destroyed approximately three weeks post infection because of terminal disease, revealed severe pyogranulomatous pneumonia involving primarily the caudal portion of one lung. One control foal had miliary pyogranulomatous lesions in both lungs. Rhodococcus equi was cultured from the pulmonary lesions in these foals. Two principal and two control foals that recovered from the infections had mild pulmonary fibrosis at three months post infection. Rhodococcus equi was not cultured from any of these foals at necropsy. The results of this investigation suggest that passive immunisation of newborn foals, by ingestion of colostrum derived R. equi specific antibodies, does not provide adequate protection against experimental infection with R. equi.

Rhodococcus equi: Clinical Manifestations, Virulence, and Immunity

“…The role of antibody in partial protection against R. equi infection would suggest that vaccination of mares could confer at least some degree of protection. However, in both a field study and an experimental challenge, vaccination of mares did not provide protection against R. equi pneumonia despite a significant increase in colostral R. equi ‐specific antibody and transfer of these antibodies to foals . More recently, vaccination of a small number of mares with VapA associated with a water‐based nanoparticle adjuvant led to high anti‐VapA IgG concentrations in mares and foals and might have conferred protection against natural challenge compared to nonvaccinated controls .…”

Prophylaxis of Rhodococcus equi pneumonia in foals – a review of literature

“…Versuche, Stuten vor der Geburt mit Lebend-oder Totimpfstoff gegen R. equi zu immunisieren, haben dazu geführt, dass die R. equi-AK im Kolostrum signifikant anstiegen (Madigan et al 1991, Martens et al 1992. Zusätzlich wurden in postkolostralen Serumproben von Fohlen geimpfter Stuten hohe AKKonzentrationen ermittelt.…”

“…Wie in der ersten Impfstudie wurden auch in dieser keinerlei Angaben darüber gemacht, ob parallel zur Impfung der Stuten auch Änderungen in Haltung und Management durchgeführt worden sind. Die positiven Ergebnisse der Stutenimpfung dieser Studien lassen sich nicht in Einklang mit anderen Untersuchungen bringen, nach denen es keine Korrelation zwischen den AK im Kolostrum und dem Schutz vor der R. equi-Pneumonie gibt (Martens et al 1992, Triskatis 2005.…”

“…Der Zeitpunkt zu dem HIP verabreicht werden soll, um den bestmöglichen Schutz zu gewähren, ist unbekannt. Empfohlen wird eine Transfusion vor der ersten R. equi-Exposition (Martens et al 1992) und eine zweite im Alter von etwa einem Monat (Giguère und Prescott 1997, Giguère et al 2002, Perkins et al 2002.…”

Transfer of the Virulence‐Associated Protein A‐Bearing Plasmid between Field Strains of Virulent and AvirulentRhodococcus equi

“…Exposure occurs early in life and infection is believed to result primarily through inhalation of aerosolized bacteria . Control and prevention of foal pneumonia secondary to R. equi has proven to be challenging and development of an effective vaccine remains elusive to date …”