Equine immunostimulant therapy


Definitive diagnostic criteria should be used to aid in the selection of available agents.

By definition, immunomodulation means that an alteration to immune function is induced. Methods of immunomodulation include immune suppression or immune enhancement, termed immunostimulation. Mechanisms of immune suppression involve chemotherapeutic or corticosteroid therapies, while immune stimulation refers to the use of an agent that will enhance immune activity.

In equine veterinary medicine we are fortunate to have several USDA-approved immunostimulant therapeutic options. This discussion will briefly review the mechanisms of immune stimulation and several of the available products that are currently used in equine veterinary practice.

Immunostimulant therapy is indicated for use in horses with or at risk of developing infectious disease. In particular, immunostimulant therapy is indicated for those individuals undergoing immune suppression due to severe stress or chronic illness (Figure 1). Although the general health-care goal is to provide host protection at the time of pathogen challenge, sometimes the pendulum will tilt on the side of challenge, and host immunity is not adequate for pathogen clearance. Therefore, antimicrobial agents and (in some instances) immunostimulant therapy can aid in pathogen clearance.

Figure 1 The balance of health and disease is maintained by increasing health status and reducing disease challenge. If the scales are tipped to an increased disease challenge or reduced health status, then host disease more likely will occur. Immunostimulants have a positive effect on helping to maintain host health under certain circumstances.

Pathogen challenge of the respiratory system and other organ systems, including the central nervous system, may benefit from immunostimulant therapy. Under normal physiologic conditions, host immunity is capable of protecting and clearing pathogenic challenges. Under conditions of stress, altered respiratory function or iatrogenically administered immunosuppressive therapy, host resistance to infectious disease diminishes. Immunostimulant therapy may provide prophylactic protection from pathogenic challenge, or in some settings of chronic disease immunostimulant therapy may aid in clearance of persistent pathogen challenge. Immune activation is characterized by the induction of macrophage activity and cytokine synthesis. Effectiveness of immune activation has been reported for up to approximately one week, long-term cellular activation has not been demonstrated with any of the available immune stimulant preparations.

Immune stimulation therapy is targeted at enhancement of endogenous mechanisms of pathogen clearance. Immunostimulants act primarily by activating macrophages, or other antigen presenting cells such as B lymphocytes or dendritic cells. Antigen presenting cells are located in lymph nodes, liver, spleen, lung (pulmonary intravascular macrophage) and bone marrow. Following microbial internalization, cellular activation is induced in a tightly controlled manner so that programmed host immune activation occurs. Immune stimulation is a highly regulated response so host effects are not counter-productive to the host, such as worsened illness, pyrexi, and/or depression.

Immunomodulator agents:

Inactivated Propionibacterium acnes:

In equine medicine, Propionibacterium acnes (EqStim, Neogen Inc.) is recommended for treatment of chronic, infectious respiratory disease that is incompletely responsive to conventional antimicrobial therapy. P. acnes is also recommended for prophylactic administration prior to stressful events that may impair pulmonary defense mechanisms, such as weaning and long-distance transport. When treating for bacterial challenge P. acnes is recommended as an adjunct treatment to antibiotic therapy, not as a stand-alone therapy. Treatment requires a series of three intravenous injections over a period of one week approximately.

In addition to equine respiratory disease, P. acnes is recommended for treatment of endometritis, osteomyelitis, papillomatosis (warts), abdominal abscess, fistulous withers and sarcoid skin tumors. Administration of P. acnes is typically effective for management of viral papillomatosis in young horses (Figure 2), whereas, efficacy for treatment of sarcoid skin tumors (intralesional and intravenous) is less consistent.

Figure 2 A yearling Quarter Horse filly presenting with facial equine papillomatosis (warts).

The mechanism of P. acnes immune activation results from host-bacterial interaction. It has been well established that bacterial DNA binds to specific pathogen associated molecular protein receptors (PAMPs); in particular, bacterial CpG DNA binds with the PAMP termed Toll-like receptor (TLR)-9 (Figure 3). From this interaction an intracellular cascade of signal transduction is induced that culminates in up-regulation of cellular cytokines and other inflammatory proteins that aid the host in pathogen clearance.

Figure 3 An important endogenous signaling cascade is induced with host exposure to foreign proteins and pathogens. The signaling cascade of bacteria, such as Propionibacterium acnes, involves bacterial DNA interacting with host-cell receptors. This cascade occurs through the Toll-like receptor-9 signaling pathway. This signaling cascade then works through NF-kB, a nuclear transcription factor that results in upregulation of inflammatory mediators and cytokines.

Inactivated Parapoxvirus ovis:

Parpoxvirus ovis (Zylexis™, Pfizer Animal Health) is a non-specific immunomodulator that contains a purified, highly concentrated viral strain that is inactivated and packaged in a freeze-dried form. In a respiratory challenge model of equine herpes virus (EHV) infection, P. ovis was shown to reduce the severity of nasal discharge when compared with placebo-treated horses. The response of this treatment was reported to occur rapidly after treatment and with responses observed in less than 24 hours. Similar to other immunostimulant compounds P. oxis treatment requires a series of three treatments (intramuscular) over approximately 10 days.


Several mycobacterial products have been marketed for use as immunostimulant agents. Original investigations that determined the effectiveness of mycobacterium as an immunostimulant preparation used bacillus Calmette-Guerin (BCG) vaccine originating from a strain of Mycobacterium bovis that was attenuated following serial passage in culture. In equine practice, live BCG, whole inactivated BCG and mycobacterial cell-wall fractions have all shown non-specific immunostimulant effectiveness. Specific products include: Equimune (Bioniche) for the treatment of infectious respiratory disesase; Regressin, (Bioniche) for the management of sarcoid skin tumors and most recently Settle® (Bioniche, Belleville, ON, K8N 5J2) for the management of endometritis in mares.

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Equimune® is a purified cell-wall extract labeled for single-dose administration to aid in the management of horses suffering from viral respiratory disease resulting from equine herpesvirus (EHV) rhinopneumonitis or equine influenza. Equimune has been shown to increase cellular interleukin-1, which has a direct effect on enhanced cell-mediated immune responses and antibody production. Clinical evidence of respiratory disease was reduced in treated horses in a randomized, double-blind study. It is strongly recommended to avoid repeated administration of this agent due to the potential for adverse pulmonary reactions. Pulmonary lesions that were reported and suspected to result as a complication of repeated administration of this agent included multifocal granulomatous pneumonitis, bronchiolitis and progressive pulmonary fibrosis.

Regressin®-V is an emulsion of mycobacterial cell wall fractions which have been modified to reduce toxic or allergic effects. The product is labeled for use in the management of equine sarcoid skin tumors, infiltration at the base of the tumor frequently results in tumor regression. Treatment may need to be repeated and remission is not guaranteed. However, in treated animals, in some instances even untreated sites have been reported to undergo lesion remission.

Settle® is mycobacterial cell-wall extract emulsion that has been formulated for the management equine endometritis. In an experimental model of Streptococus equi endometritis, Settle® was administered intravenously or by the intrauterine route. Mares treated with Settle (by either route) demonstrated clearance of bacterial challenge. Settle has been shown to work well alone and in combination with standard therapies to clear endometritis.


Interferon-alpha is a naturally produced protein that has antiviral activity. Synthesis of this protein is induced by viral infection, and is an early, nonspecific antiviral defense mechanism. Interferon-alpha aids with nonspecific immunity via enhanced leukocyte-killing activity. Interferon-alpha induces an antiviral state in target host cells by stimulating production of cytokines that inhibit viral protein synthesis and degrade viral RNA. In mice, administration of interferon-alpha stimulates leukocytes to produce inflammatory mediators that activate the cell responses effective with enhanced phagocytosis.

Interferon-alpha can be used therapeutically in various ways, low dose (30-100 U) oral administration or parenterally at higher doses (3-5 x 106 U). Immune effects induced following oral administration include: reduced inflammation in the lower respiratory tract of racehorses with pulmonary inflammation. This treatment reduces exudate and pulmonary mucous production, reduced leukocyte counts in respiratory fluid aid in the restoration of pulmonary function, leading to improved athletic performance. Interferon-alpha administration is particularly beneficial for inflammatory airway disease; diagnosis of IAD should be made with specific pulmonary examination, including hematology, endoscopy and bronchoalveolar lavage cytologic examination.

Orally administered interferon-alpha is effective due to local epithelial lymphoid-associated (oropharangeal-associated lymphoid tissue) tissues rather than via enteral absorption. Once swallowed, interferon-alpha is degraded by digestive enzymes and cannot be detected in peripheral blood. Lymphocytes exposed to interferon-alpha transfer enhanced biologic effects to other local naive lymphocytes via paracrine activity. Once oral lymphocytes initiate this cascade, lymphocyte activation continues even in the absence of additional drug administration. Cell-to-cell transfer of the antiviral state to naive cells permits low to undetectable concentrations of interferon-alpha to produce potent antiviral activity, and possibly represents a major mechanism for amplification natural interferon-alpha activity. Lymphocytes then enter general circulation and communicate this antiviral capability to cells at distant sites. This mechanism allows the biologic effects of interferon-alpha to reach tissues accessible to mobile white blood cells, in which penetration of interferon-alpha is poor, such as the epithelium of the respiratory tract, gastrointestinal tract and eye.

Although this treatment is effective under various conditions, it is important to note that patients can become unresponsive to interferon-alpha therapy after repeated administration due to production of anti-interferon-alpha antibodies or reduced host sensitivity to exogenously administered interferon. The effect of anti-interferon-alpha antibody production that limits the effectiveness of interferon has been reported in humans and calves.

An additional use of interferon-alpha is a high-dose (3-5 x 106 U) intravenous administration. When used in high doses, this treatment can be beneficial for horses suffering from certain viral diseases, such as West Nile virus encephalitis. Although not labeled for this use, this is a specific therapeutic treatment for people suffering from WNV encephalitis.

In this author's experience, this is an appropriate therapeutic for use in WNV-affected horses. In our hospital population, we have observed favorable clinical responses in treated horses.

Final thoughts

Immunostimulant therapy is indicated for the clinical management of equine patients under a variety of settings that include prevention and treatment of various infectious diseases. Definitive diagnostic criteria should be used to aid in the appropriate selection of available agents. Immunomodulation is indicated for prevention or as an adjunct to antimicrobial therapy for infectious disease.

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