Protective immune response against Rhodococcus equi: An innate immunity-focused review.
Authors: da Silveira Bibiana Petri, Cohen Noah D, Lawhon Sara D, Watson Robert O, Bordin Angela I
Journal: Equine veterinary journal
Summary
Rhodococcus equi remains a leading cause of pneumonia and death in foals despite decades of research, with no effective vaccine currently available to prevent infection. This narrative review synthesises current knowledge of how R. equi evades and manipulates the foal's innate immune system, focusing particularly on the virulence-associated protein A that allows the bacterium to survive intracellularly by blocking macrophage acidification—a critical survival mechanism that distinguishes pathogenic from non-pathogenic strains. The authors highlight that foal susceptibility stems from developmental immaturity of both innate and adaptive immunity, whereas adult horses with fully mature immune systems readily resist pneumonia, suggesting that targeted immune stimulation during the critical early-life period could be protective. Live vaccine approaches and strategies designed to enhance innate immune activation show promise in preliminary work, offering potential pathways to overcome the historical challenges of active vaccination in young foals. Understanding these host–pathogen interactions at the cellular level is essential for developing practical preventative strategies that leverage foals' own immune capabilities rather than relying on passive transfer alone.
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Practical Takeaways
- •R. equi remains a significant cause of foal mortality with no available vaccine; understanding immune mechanisms is critical for developing prevention strategies
- •Foal age and immune maturity are key risk factors—young foals with naive immune systems require special management and monitoring protocols
- •Stimulating innate immune responses through live vaccines and immunomodulation may offer practical pathways to reduce pneumonia incidence in foals
Key Findings
- •Virulence associated protein A (VapA) in R. equi prevents macrophage phagolysosome acidification, enabling intracellular bacterial survival
- •Foal susceptibility to R. equi infection is directly associated with immaturity and naivety of both innate and adaptive immune systems
- •Adult horses with fully functional immune systems are resistant to R. equi pneumonia despite potential exposure
- •Live vaccines and innate immune response stimulation show promise for improving foal protection against R. equi infection