Horses transported more than 500 miles have a reduction in pulmonary macrophage function (responsible for clearance of small inhaled particles in the lung) for approximately three weeks, said Bonnie Rush, DVM, MS, Dipl. ACVIM, clinical instructor of equine internal medicine at Kansas State University, at the 50th annual American Association of Equine Practitioners (AAEP) Convention in Denver, Colo., Dec. 4-8, 2004. This is indicative of a reduction in how well the immune system can fight off invaders, and it is a good situation in which to use immunostimulants.

"The indications for immunostimulant therapy in horses are relatively specific, and these compounds are not intended to treat a broad spectrum of conditions," Rush explained. "The mechanism of action of non-specific immunostimulation is induction of macrophages to produce proinflammatory cytokines that drive a T-1-based (helper cells) immune system response. Immunostimulation therapy may not be effective in patients with acute, fulminating (suddenly occurring) infections, because the immune response is (already) maximally stimulated by the pathogen."

One preparation she discussed was non-viable Propionibacterium acnes, the immunostimulant activity of which has been recognized for more than 30 years. The first reported use in horses was more than a decade ago. She said that in equine medicine, P. acnes is labeled for treatment of chronic respiratory disease and is recommended for cases that are unresponsive or transiently responsive to conventional antibiotic treatment. "In addition," she said, "it is recommended for prophylactic administration before stressful events that may impair pulmonary defense mechanisms, including weaning and long-distance transport."

She said P. acnes also has been anecdotally recommended for treatment of endometritis, osteomyelitis, papillomatosis (warts), abdominal abscess, fistulous withers, and sarcoid skin tumors. Rush said in her experience, P. acnes was effective for treatment of viral papillomatosis, but its effects against sarcoid skin tumors were less consistent