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.

There are side effects to the use of P. acnes, especially after the first administration. They include fever, anorexia, and lethargy within 12-24 hours of administration. She noted that subsequent injections usually elicit milder reactions.

Another preparation she discussed was the bacillus Calmette-Guerin (BCG) vaccine, which was developed from an attenuated (less virulent) strain of Mycobacterium bovis. She said live BCG, whole-inactivated BCG, and mycobacterial cell wall fractions have been used as non-specific immunostimulants and vaccine adjuvants for decades.

Rush noted that mycobacterial cell wall products are used in horses to treat infectious respiratory disease and sarcoid skin tumors. She reported a study where this product was given to horses with respiratory disease resulting from stress, transportation, bacteria, and/or viral infections. The study noted that within seven days after administration of a single dose, 83% of treated horses were clinically normal, but only 36% of untreated horses were free from clinical signs.

A new product was introduced in late 2004 called Settle; it is a mycobacterial cell wall fraction labeled to treat endometritis in mares caused by Streptococcus zooepidemicus, said Rush. She said the product could be used intrauterine or IV, and that in studies it showed a reduction in bacteria and fluid in seven days (clinical cure of 75% at seven days).

Interferon alpha (IFN-alpha) is an endogenous (originating within the organism) immunostimulant with antiviral, immunomodulatory, and antiproliferative activity, noted Rush. She said oral administration of IFN-alpha reduces pulmonary inflammation in racehorses with chronic inflammatory airway disease. Rush said interferon administration is not beneficial in treating acute, fulminating viral respiratory infection in horses. She added that high-dose parenteral IFN-alpha has seen used to treat horses with West Nile virus.

Granulocyte-colony stimulating factor (G-CSF) is an endogenous immunomodulator that has been used to treat neonatal infection and "may aid in the treatment of endotoxin-induced neutropenia (a decrease in the number of white blood cells) in septicemic foals," she said. G-CSF might enhance immunity in post-operative patients by promoting the number and function of circulating neutrophils (phagocytic white blood cell that has a fast response time to infection), she said.

About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

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