Protection Against R. equi in Foals

Foal sleep in an old barn and is light with sunshine.
A PNAG vaccine might protect foals from R. equi in their first week of life.
Presently, no vaccine is available to protect foals from R. equi, but researchers are exploring potential means of improving a foal’s success at challenging infection. iStock

Rhodococcus equi is an intracellular bacterium that causes devastating respiratory disease in foals and is difficult to control. Presently, no vaccine is available. Plasma transfusions might be preventive against R. equi, but they are costly and time-consuming. Use of macrolide antibiotics is discouraged due to the potential for development of antimicrobial resistance and possible poor efficacy.

Studies at Texas A&M College of Veterinary Medicine and Biomedical Sciences explored a potential means
of improving a foal’s success at challenging infection [Cohen, N.D.; Kahn, S.K.; Cywes-Bentley, C.; et al. Serum antibody activity against poly-N-acetyl glucosamine (PNAG), but not PNAG vaccination status, is associated with protecting newborn foals against intrabronchial infection with Rhodococcus equi. Microbiology Spectrum 2021, 9:e00638-21;].

PNAG Vaccination for Mares

In a previous study, mares were immunized at six and three weeks prior to parturition. They received a conjugate vaccine (not yet licensed) “targeting the highly conserved microbial surface polysaccharide, poly-N-acetyl glucosamine (PNAG).” PNAG is a conserved surface antigen found especially on intracellular microbes, like R. equi. [Cywes-Bentley, C.; Rocha, J.N.; Bordin, A.I.; et al. Antibody to Poly-N-acetyl glucosamine provides protection against intracellular pathogens: Mechanism of action and validation in horse foals challenged with Rhodococcus equiPLoS Pathog 2018, 14(7): e1007160.].

Foals obtain the antibodies through colostrum and showed good protective antibody response to challenge at 28 days. In the study, 91% born to vaccinated mares were protected against R. equi pneumonia. Alternately, 86% of non-vaccinated controls developed pneumonia.

Protecting Younger Foals from R. equi

The more recent study sought to evaluate protection with this protocol in foals younger than 6 days old. These younger foals are often exposed at birth due to the presence of this bacteria in the environment. The results indicated that “vaccination of mares did not significantly reduce the incidence of pneumonia in foals. However, activities of antibody against PNAG or for deposition of complement component onto PNAG was significantly higher among foals that did not develop pneumonia than among foals that developed pneumonia.”

The research also showed that “Vaccination of mares against PNAG protected their 28-day-old foals against intrabronchial infection with virulent R. equi, but vaccination of mares failed to protect foals against intrabronchial infection with R. equi at age 6 days.” This is possibly due to reduced complement activity and functional neutrophil responses in foals less than a week of age compared to slightly older foals. Complement increases killing effects of neutrophils and lymphocytes on R. equi.

More investigation is needed for a potential PNAG vaccine under field conditions to determine if its efficacy is useful for foals against R. equi in their first week of life.

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