Intravenously administered plasma has been used most commonly in adult horses for treatment of endotoxemia and/or acute protein losing colitis. The purpose of this paper is to: (1) provide a review of previously published data and our experimental data on the use of hyperimmune plasma in treating endotoxemia in horses with colic and/or colitis; (2) speculate on new horizons for hyperimmune plasma therapy in horses; (3) review some of the additional indications (other than anti-endotoxic effect) for intravenous plasma therapy in adult horses.

Circulating endotoxemia has been reported in horses with naturally occurring gastrointestinal strangulation or interitis/colitis. 1.2. The efficacy of hyperimmune plasma containing antibodies to the core antigen of E. coli (J5) in the treatment of endotoxemia in horses and other species has been reviewed and results have been variable. 3.4. In an effort to further determine the efficacy of hyperimmune plasma in treating clinical endotoxemia, we completed a double-blind clinical trial using 25 adult horses with either large colon volvulus (LCV) (Group I), colitis (Group II), and anterior enteritis or small intestinal surgical disorders (Group III). In an effort to standardize the degree of illness in the horses certain clinical and laboratory criterion were developed for horses to enter the study. Using a double-blind design in each of the three groups, horses were randomly assigned to treatment with either plasma A or B at 2.5 ml plasma/lb. given intravenously either prior to surgery and/or within six hours of hospital admission. Plasma used in the study had been coded (A or B) with one of the plasma products having a titer of >40,000 to the lipopolysaccharide (LPS) core antigen (endotoxin) of E. coli (J5), while the other product had a titer of <1000. All plasma was acquired from horses housed on a single farm, and all horses were vaccinated and managed identically except for E. coli (J5) vaccination. All plasma was collected by centrifugation and frozen at ñ20 C prior to use. Horses entering the trial were also treated in a conventional manner depending on their disease (surgery for LCV or small intestinal strangulation and fluids, antimicrobial and anti-inflammatory therapy for all patients). Horses were evaluated for morality, duration of hospital stay, clinical evidence of laminitis and, in the enteritis cases, for time of disappearance of abnormal appearing neutrophils in the blood. At the termination of the study the code was revealed and analysis of variance was determined for the above variable to evaluate the significance of treatment (A or B). Furthermore, stratified analysis was performed within each group to evaluate whether the treatment was more efficacious for on condition than the others. Three horses (all with LCV) died and one horse with colitis developed laminitis. There was no significant difference in any of the variables, either inter- or intragroup for the treatments. Horses receiving the hyperimmune plasma did have a shorter hospital stay, 8.3 vs. 6.6 days, but this was not significant.

Hyperimmune plasma was not better than normal equine plasma in improving the variables of survival, incidence of laminitis, days of hospitalization or improvement in leukograms in these selected patients.This study differs from a previous study that showed a positive effect of hyperimmune plasma in horses with clinical endotoxemia. 5. The differences in the two studies may be explained by: chance, since the power of neither study was great, heterologous endotoxin challenge, duration of disease prior to plasma a administration; or from the tight selection criterion placed upon the horses in our study. This discrepancy in the two studies is actually consistent with reports in the literature for other species suggesting that hyperimmune plasma is not a panacea for endotoxemia and that clinical trial using patients with endotoxemia are complex. 4. The survival rate (88%) for all horses in our study seemed relatively high. This may have also been due to selection criterion, chance, or it may have been that the horses benefited from the plasma, regardless of LPS antibody titer. The benefit of fresh frozen plasma with or without LPS antibody might have been determined if a non-treatment group (no plasma) had been included. A non-treatment group was not included in the study for ethical reasons since there is a general feeling among clinicians in our hospital that equine plasma is beneficial in treating these diseases. Presumably, there may be factors in all fresh frozen plasma that could be beneficial in treating endotoxemia. Two plasma factors which may be beneficial are antithrombin III and fibronectin. 6.7. Antithrombin III provides 70% of the anticoagulation activity of plasma 8 and may decrease the severity of coagulopathies associated with endotoxemia. Fibronectin may enhance opsonophagocytosis of circulating particulate debris including endotoxin. 6 These two plasma proteins and other serum protein, e.g. protein C, alpha2 macroglobulin, may be equally or more important the LPS antibodies in plasma for treating septic shock. We have found that antithrombin III and fibronectin are stable and ñ20 C for at least 9 months in fresh frozen and stored equine plasma.

Passive immunotherapy in recent years has focused on LPS-antiserum. More recently, the focus of therapy has moved progressively away from the triggering endotoxin molecule to therapy directed against cytokines, e.g. Tumor Necrosis Factor and interleukin 1 (IL1). TNF is a polypeptide produced by macrophages that have been stimulated by LPS. It is TNF and its mediators that are responsible for many of the cellular events that may occur following endotoxemia. 9. Hyperimmune plasma containing antibodies against tumor necrosis factor (TNF) may soon be available for the equine practitioner and this may further improve therapy against septic shock. 10 Antibodies against TNF would appear to have advantage over LPS in that the antibodies are directed at a mediator more closely linked to cellular injury that is LPS and TNF production generally lags behind LPS by 2 hours which may permit therapy with TNF antibody in a clinical setting prior to peak TNF production. It is generally accepted that antibodies directed against LPS of TNF are best given prior to their release. Plasma with antibodies against both LPS and TNF would likely have an additive effect since LPS causes some physiologic disturbances not directly associated with TNF. Antibodies directed against TNF may not be the panacea in the therapeutic attack against endotoxemic shock though and, in human medicine, investigation has moved one further step further in the cascade with production of antibodies against IL1.

Albumin is also an important component of plasma that may be beneficial in treating endotoxic shock. Albumin has colloidal properties that should enhance and/or prolong the effect of polyionic crystalloid fluids in maintaining intravascular volume and in preventing edema formation. 11 Plasma therapy would, therefore, be indicated in patients with sever hypoalbuminemia that require fluid therapy. Albumin is considered a strong (independent) ion and in horses with severe loss of albumin, there is a compensatory increase in plasma bicarbonate and/or unmeasured anions. 12 The measured anion gap may therefore appear normal in spite of an increase in lactate (lactic acid) in horses with hypoalbuminemia. By adding purified equine albumin to hypoalbuminemic equine plasma, we have estimated that for 1 gram loss or gain of albumin/L plasma, there is approximately a 4 mEq change in other anions. In critically ill horses with severe hypoalbuminemia, the value of the normal anion gap should be reduced by approximately 4 mEq/L in order to more accurately estimate the degree of lactic acidosis.

In summary, we recommend intravenous plasma therapy for use in adult horses with endotexemia and/or those with severe volume depletion and hypoalbuminemia. Plasmaís benefit in treating endotoxemia could be attributed to many protein factors including LPS antibody, fibronectin and antithrombin III. Two to 5 liters of hyperimmune plasma is routinely recommended for horses with suspected strangulating lesions and/or with suspected endotoxemia from other gastrointestinal disorders. The plasma is generally administered intravenously over a 30 minute to 2 hour period. Larger amount may be need in some horses with severe protein losing enteropathy.