This study was conducted at Oklahoma State University Center for Veterinary Health Sciences. Samples were collected from multiple private veterinary practices in Oklahoma and Texas.
Cardiotoxicity, Inflammation, and Immune Response after Rattlesnake Envenomation in the Horse
Article first published online: 1 NOV 2012
Copyright © 2012 by the American College of Veterinary Internal Medicine
Journal of Veterinary Internal Medicine
Volume 26, Issue 6, pages 1457–1463, November/December 2012
How to Cite
Gilliam, L.L., Holbrook, T.C., Ownby, C.L., McFarlane, D., Sleeper, M.M., Martin, S., Levis, K. and Payton, M.E. (2012), Cardiotoxicity, Inflammation, and Immune Response after Rattlesnake Envenomation in the Horse. Journal of Veterinary Internal Medicine, 26: 1457–1463. doi: 10.1111/j.1939-1676.2012.01022.x
Portions of this data were presented at the 2011 ACVIM Forum in Denver, Colorado
- Issue published online: 20 NOV 2012
- Article first published online: 1 NOV 2012
- Manuscript Accepted: 6 SEP 2012
- Manuscript Revised: 19 JUN 2012
- Manuscript Received: 14 MAR 2012
- ACVIM Foundation
- Center for Veterinary Health Sciences, Oklahoma State University
- Snake bite;
- Troponin I
Cardiac abnormalities are reported in rattlesnake-bitten horses. The prevalence and cause are unknown.
To detect cardiac damage in rattlesnake-bitten horses by measuring cardiac troponin I (cTnI) and evaluating ECG recordings for presence of arrhythmias, and explore causes of this cardiac damage by measuring venom excretion, anti-venom antibodies, and tumor necrosis factor alpha (TNFα).
A total of 20 adult horses with a clinical diagnosis of rattlesnake bite and 6 healthy adult horses.
In a prospective clinical study, bite site swabs, blood samples, and urine samples were collected at various time points from 20 horses with a clinical diagnosis of snake bite. Continuous ECG recordings were obtained on the 20 affected horses and 6 normal control horses using 24-hour holter monitors. Plasma samples were assayed for cTnI, serum samples were assayed for TNFα and anti-venom antibodies, and bite site swabs and urine were assayed for venom.
Forty percent of rattlesnake-bitten horses (8/20) experienced myocardial damage (increased cTnI). Seventy percent (14/20) experienced a cardiac arrhythmia. There was a positive correlation between cTnI and TNFα (P < .02). Horses with cTnI ≥2 ng/mL were more likely to have antibody titers >5,000 (P < .05). No correlations were found between venom concentration and cTnI, anti-venom antibody titers, TNFα, or presence of arrhythmias.
Conclusions and Clinical Importance
Cardiac abnormalities in this population of horses indicate that cardiac damage after rattlesnake bite is common. Rattlesnake-bitten horses should be monitored for signs of cardiac damage and dysfunction. Long-term follow-up should be encouraged to detect delayed cardiac dysfunction.
cardiac troponin I
tumor necrosis factor alpha
Rattlesnake envenomation in the horse, as well as in other species, can have devastating outcomes. The mortality rate reported in horses after rattlesnake envenomation varies from 9% to 25%, considerably higher than the mortality rates reported in dogs (1%) and people (<1%). The difference in these mortality rates is unknown and could be caused by differences in treatments among species or could indicate a difference in sensitivity to venom among species. The most common manifestations of rattlesnake envenomation in horses are mild-to-severe swelling around the bite site, tissue necrosis, and coagulopathy.[1, 2] Although cardiac abnormalities are not a common feature after snake bites, they have been reported in horses and can be important contributors to morbidity, future athletic potential, or mortality.[2, 6, 7] Cardiac abnormalities also have been reported in dogs and new world camelids after rattlesnake bite. Venom is a complex mixture of proteins and, although some snake venoms contain toxins that exert specific effects on the heart,[10, 11] the exact cause of cardiac toxicity after rattlesnake envenomation in the horse is unknown. There are no prospective studies evaluating cardiac abnormalities after rattlesnake envenomation in the horse, and the cause of this damage has not been investigated.
Myocardial damage in the horse may be detected by recording an ECG or measuring cardiac troponin I (cTnI). Cardiac troponin I is a sensitive tool for documenting myocardial damage in the horse whereas ECG is an insensitive indicator of myocardial damage.[13-15] Focal insult to the electrical system of the heart can result in arrhythmias in the absence of increases in cTnI. We hypothesized that horses bitten by rattlesnakes frequently experience cardiac damage which may or may not be detected upon physical examination. To detect both myocardial cell injury and electrical dysfunction, cTnI was measured and 24-hour continuous ECG was recorded.
Rattlesnake envenomation in horses commonly incites a profound local and systemic inflammatory response. It is unknown whether cardiac damage after rattlesnake bite is a consequence of the systemic inflammatory response, if it is caused by components of the venom itself, or a combination of both. Tumor necrosis factor alpha (TNFα) plays a role in the inflammatory response to rattlesnake envenomation. In addition to the body's own metalloproteinases, venom metalloproteinases process pro-TNFα to its active form resulting in a marked increase in circulating TNFα. In rats, TNFα has been implicated in cardiac damage after pit viper envenomation. TNFα has not been measured in horses bitten by rattlesnakes. We hypothesized that the degree of cardiac damage may be associated with increases in circulating TNFα concentrations or venom concentration or both.
Considering that all study horses lived in an area endemic to rattlesnakes, previous exposure to rattlesnake venom was possible. Ruling out previous exposure based on history was considered unlikely because of multiple previous owners of individual horses. We further hypothesized that pre-existing antivenom antibodies would be protective against the cardiotoxic effects of rattlesnake envenomation.
Materials and Methods
Horses with a clinical diagnosis of snake bite defined as the presence of a characteristic snake bite wound, acute and severe focal swelling, and an owner's witness of a snake bite or all were enrolled in this prospective clinical study. Each horse received a complete physical examination. Treatment of all horses was at the discretion of the attending veterinarian. Horses were maintained in the hospital for a minimum of 1 week for sample collection and returned at 1 month after presentation for 24 hours. If the bite site was visible, cotton swab samples of the site were placed in 1 mL of sterile saline, immersed in liquid nitrogen, and stored at −80°C until assays were performed. Thirty-four milliliters of whole blood was collected from the jugular vein of each horse at presentation, 24, 48, 72, 96 hours, 1 week, and 1 month after presentation for serum and plasma samples. Serum and heparinized plasma aliquots were immersed in liquid nitrogen and stored at −80°C until assays were performed. Urine samples were collected at each blood sampling time point. If horses did not urinate, they were sedated using xylazine (0.11 mg/kg), acepromazine (0.33 mg/kg), or a combination, and urine was collected by urinary catheterization. Urine aliquots were immersed in liquid nitrogen and stored at −80°C until assays were performed. A complete sample set was not obtained on every horse in the study. The study protocol was approved by the Institutional Animal Care and Use Committee at Oklahoma State University.
Cardiac Troponin I
Plasma samples were analyzed for cTnI using a 2-site sandwich colorimetric assay by use of a fluorometric analyzer1 with an analytical sensitivity of 0.03 ng/mL. The range of detection of the assay is 0–50 ng/mL. This assay has been validated for use in horses and has been used in previous studies of horses.[21, 22] Cardiac troponin I remains stable in serum samples stored at room temperature for 5 days and was not affected by up to 5 freeze-thaw cycles.
Holter Monitor Recording
Rattlesnake-bitten horses were fitted with a Holter monitor2 that recorded 24-hour continuous ECG at presentation, 1 week, and 1 month after presentation. Normal horses were fitted with a Holter monitor for one 24-hour period. Recordings were downloaded using Holter software,3 randomized, and interpreted by a blinded board-certified cardiologist who indicated the presence or absence of arrhythmia and defined the types of arrhythmias present.
Urine samples and bite site swab samples were assayed for venom using standard flurometric double sandwich ELISA technique. Horse antivenom antibodies4 and sheep antivenom antibodies5 were used as sandwich antibodies and alkaline phosphatase-labeled donkey antisheep antibodies6 were used as detection antibodies. The substrate was 4-methyumbelliferyl phosphate (4-MUP).7 Plates8 were read at 355 excitation/460 emission using a spectrophotometer.9 A standard curve was constructed for each plate and the limit of detection was ≥2 standard deviations above the negative control.
Tumor Necrosis Factor Alpha
Serum samples taken at presentation and 1 week, or 1 month, or both were assayed for antibodies against Crotalus atrox using standard ELISA techniques. Antiequine alkaline phosphatase antibody11 was used as a detection antibody. Plates were read at 405 nm on a spectrophotometer12 and titers were calculated based on the standard curve control. Titers ≥200 were considered positive.
Cardiac troponin I in bitten horses was compared with the normal range for horses established by the laboratory performing the assay. An exact binomial test was used to determine if bitten horses were more likely to have an increased cTnI than the normal population. A Fisher's Exact Test was used to evaluate the presence of arrhythmias in normal versus bitten horses as well as bitten horses with and without increased cTnI. Fisher's Exact Test also was used to evaluate the relationship of cTnI (increased/not increased above 2 ng/mL) with the presence of antibody titers >5,000. A Spearman's rank correlation coefficient was used to identify correlations between cTnI, TNFα, venom, and antivenom antibody titer.
Twenty horses were included in the study. Horses resided in the Texas Panhandle (12), West Texas (4), southeastern Oklahoma (2), north-central Oklahoma (1), and western Oklahoma (1). Fourteen horses were bitten on the muzzle, 2 on the distal limb, 1 on the head midway between the right ear and eye, and bite location was not recorded in 3 horses. Treatments were recorded in 15 horses and included corticosteroids (13/15), nonsteroidal anti-inflammatory drugs (14/15), antibiotics (13/15), and tetanus toxoid (2/15). Long-term follow-up was possible on 15 of the 20 horses. All 15 were alive at 1 month, but 1 horse died acutely at pasture 48 days after initial presentation. Other clinical signs reported included diarrhea (1 horse) and severe myonecrosis at the site of an IM injection (1 horse).
Forty percent of the horses had increased cTnI (8/20) (range, 0.08–59.58 ng/mL; reference range, 0.00–0.06 ng/mL). Seventy percent (14/20) of the bitten horses had an arrhythmia confirmed on at least 1 time point and 2 of 6 normal horses had arrhythmias (P = .16) (see Table 1). Three horses that had arrhythmias at presentation still had arrhythmias at 1 month. One horse only had an arrhythmia at the 1 month evaluation. All horses with increased cTnI (8) also had arrhythmias. Six horses had arrhythmias without increased cTnI, but horses with increased cTnI were more likely to have arrhythmias than horses with normal cTnI (P < .04). All 3 horses with arrhythmias present both at presentation and 1 month had increased cTnI concentrations.
|Arrhythmia||Bitten (n = 14)||Bitten + Inc. cTnI (n = 8)||Bitten + Norm. cTnI (n = 6)||Control (n = 6)|
|2 or more of the above||8||6||2||0|
TNFα was highly variable among horses. There was a positive correlation between TNFα concentrations and cTnI (P < .02).
Titers were measured at presentation and 1 week or 1 month or both in 15 horses. None of these horses had titers above assay background at presentation. Thirteen of 15 horses had positive antibody titers (>200). Manufacturers of a commercial rattlesnake toxoid vaccine (J.T., personal communication) consider titers >800 to be protective against the lethal effects of rattlesnake venom. Eleven of 15 horses had titers >800. When titers were <5,000, there was no positive correlation between antibody titer and cTnI (P > .05), but horses with a cTnI > 2 ng/mL were more likely to have markedly increased titers (>5,000) (P < .04) at 1 week or 1 month. Not all horses had samples available at 1 week and 1 month, but when both samples were available, the highest titer was used in statistical analysis.
Venom was detected in 5 of 9 bite site swabs and in urine of 13 of 19 horses on at least 1 time point. Venom excretion in the urine was variable and tended to occur on or after day 3 of presentation. The earliest time point postbite that venom was detected was within the first 24 hours, and venom was detected in the highest number of horses (7/13) at 4 days. There was no correlation between urine venom concentration and cTnI (P > .05), TNFα (P > .05), or anti venom antibody titers (P > .05).
A number of studies have utilized cTnI as a sensitive marker to document myocardial injury in the horse,[12, 21, 25-29] and its use is becoming more common in the clinical evaluation of horses bitten by rattlesnakes. The cTnI from the rattlesnake-bitten horses was compared with the established normal range for the laboratory performing the assay. Cardiac troponin I has not been measured in horses with non-snake bite-associated puncture wounds. To determine with certainty that the increase in cTnI is associated with rattlesnake envenomation and not simply wound-associated inflammation, cTnI would need to be measured in horses with similar wounds. Cardiac troponin I typically increases rapidly (within 2 hours), peaks around 12–48 hours, and remains increased as long as the injury continues. In humans experiencing an ischemic myocardial event, cTnI can remain increased for 10 days. The half life of cTnI in the horse has been reported to be 1–2 hours. Three of 8 horses had steadily increasing cTnI up to the 1-week sampling. The longer cTnI is increased, the more indicative it is of irreversible damage. An initial increase in cTnI can occur with mild damage because cytosolic troponin is released first, but a more marked and persistent increase is consistent with the release of structurally bound troponin proteins indicative of irreversible or ongoing myocardial damage.[23, 30] In dogs with heart disease, increased cTnI in 1 sample was not a good prognostic indicator, but evaluating trends in subsequent samples was a useful prognostic tool. A downward trend in subsequent samples was a good prognostic indicator, whereas persistently high concentrations of ≥ 1.0 ng/mL were indicative of a poor prognosis. The peak cTnI concentration has been correlated with infarct size in dogs, suggesting that the magnitude of cTnI increase may be helpful in assessing the extent of myocardial damage. The horse in our study with the highest cTnI died acutely 48 days postbite. This horse was presented 5 days postbite. Cardiac troponin I was measured on days 6–9 and concentrations ranged from 20.48 to 56.58 ng/mL.
Timing of the peak in cTnI was variable and occurred between 12 hours and 1 month postenvenomation (Fig 1). In 4 of 8 horses, the peak occurred at 7–30 days postpresentation. Although this was an unexpected finding, we suspect that cTnI appearance may be influenced by delayed venom absorption from damaged tissues. When horses are envenomated, the area around the bite site can have marked swelling, resulting in decreased blood flow to the area. This decreased perfusion may slow the absorption of venom into systemic circulation thereby postponing its cardiotoxic effects. Human snakebite victims can develop what is known as the recurrence phenomenon, which describes a recrudescence of symptoms of envenomation up to 1 week after being bitten. These individuals were treated with antivenom and signs of envenomation improved or disappeared and then recurred up to 1 week later.[33, 34]
Larger venom proteins may remain in circulation and be excreted, whereas smaller proteins may escape into the extravascular tissues and remain there for a longer period of time before being metabolized. Variable venom elimination may allow certain venom components to remain in the body longer, thus exerting their toxic effects at later time points in tissues such as the heart. Considering that 50% of the horses with biochemical evidence of cardiac injury demonstrated this delayed pattern of cTnI increase (7–30 days postenvenomation), clinicians should remain diligent in monitoring horses for myocardial damage after rattlesnake bite. Notably, others have documented horses with signs of cardiac disease 12 days to several weeks after envenomation.
When statistically evaluated as a whole, rattlesnake-bitten horses were not more likely to experience arrhythmias than normal horses hospitalized for Holter monitor data collection. This could have been caused by the small number of normal horses that were evaluated. It also could indicate that some arrhythmias noted in rattlesnake-bitten horses are not necessarily caused by the rattlesnake venom, but rather are a consequence of some factor associated with the bite such as stress or systemic inflammation. To better make this determination, a comparison should made with Holter recordings evaluated on horses with similar circumstances such as puncture wounds not associated with rattlesnake envenomation. Rattlesnake-bitten horses with increased cTnI were more likely to have arrhythmias than those with normal cTnI, but arrhythmias were seen in horses with normal cTnI. Thus, a normal cTnI cannot rule out the development of cardiac irritability or electrical disturbance after envenomation. Arrhythmias ranged in severity from occasional atrial premature complexes to frequent atrial premature complexes and paroxysmal atrial fibrillation. Ventricular arrhythmias were found in 2 horses; both had markedly increased cTnI. The horse with the highest cTnI that was found acutely dead 48 days postpresentation did not have ECG abnormalities, but only 1 24-hour Holter reading was obtained on this horse 1 week postpresentation. In general, arrhythmias were mild and included occasional atrial premature contractions, 2 couplets and rare sinus pauses. Only 1 horse had an arrhythmia auscultated on physical examination; therefore, continuous ECG monitoring may be necessary to detect intermittent arrhythmias after envenomation. Arrhythmias may not be evident immediately after rattlesnake envenomation. Horses with increased cTnI should receive long-term ECG follow-up to rule out the possibility of delayed development of arrhythmias. The clinical relevance of these arrhythmias should be determined by more extensive long-term follow-up including echocardiography and exercise stress testing.
Although there are many potential mediators of the systemic inflammatory response, TNFα has been specifically implicated as an important cytokine involved in rattlesnake envenomation. TNFα primarily is produced by macrophages in affected tissues. It can be beneficial or harmful depending on the amount produced and duration of time over which its production is sustained. High concentrations of TNFα over prolonged periods of time have been shown to be detrimental to cardiomyoctes. TNFα production may be amplified after pit viper envenomation by the action of venom metalloproteinases converting TNF substrates into their biologically active form therefore allowing more TNFα to be present in circulation. TNFα has been shown to have hypotensive effects in rats envenomated with Vipera aspis. Blocking the effects of TNFα by administering anti-TNFα antibody ameliorated cardiotoxic effects of venom in these rats. Exposing rat ventricular myocytes to TNFα resulted in electrophysiological changes. TNFα has been found to be detrimental in human cardiac patients with ischemia and reperfusion injury. TNFα causes endothelial cells to release many procoagulant factors that favor thrombosis and can result in disseminated intravascular coagulation. Additionally, TNFα is directly cytotoxic and can create a vascular leak.
In these snake-bitten horses, there was a positive correlation between TNFα and cTnI. Considering TNFα has been shown to be cardiotoxic in other species, there could be a direct causal relationship between these variables in the horses in our study. Alternatively, the higher TNFα and cTNI concentrations may merely reflect larger venom doses, and these 2 markers may not be directly related. The urine venom ELISA results, however, do not necessarily support this hypothesis. Urinary excretion of venom appears to be highly variable among horses, and urine sample data sets were incomplete. Thus, we cannot rule out that increased markers of systemic inflammation (TNFα) and myocardial injury (cTnI) reflect venom dosage.
We hypothesized that some horses may have pre-existing venom antibodies, which may be cardioprotective. We were unable to make this association. There are several possibilities for the positive correlation between increased cTnI and peak venom antibody titers when cTnI was >2 ng/mL and titers were >5,000. Higher venom antibody titers may indicate absorption of (and immune response to) a larger venom dose. As mentioned above, however, more precise sampling times and a complete data set may be necessary to make that correlation. Higher venom antibody titers also may be simply indicative of an individual's more marked immunological response to venom. This marked immune response could be associated with immune-mediated damage to the heart as occurs in conjunction with other forms of myocarditis such as viral or parasitic myocarditis. Serum antivenom titers did not persist long enough to conclude that horses maintain antibodies from natural exposure that provide long-term protection from subsequent envenomations.
Although venom was not detected in all bite site samples, this observation does not rule out envenomation. Depending on the amount of time that lapsed between the bite and presentation, bleeding or serous drainage could wash venom away from the wound or drying and desiccation could occur, leading to negative results. The amount of venom at the bite site did not correlate with the degree of myocardial damage. In fact, in at least 1 horse, the venom detected at the bite site was very high, whereas no venom was detected in the urine and there was no increase in cTnI, indicating that most of the venom was left on the skin and did not actually enter circulation.
Rattlesnake venom has been detected in the urine of a human rattlesnake bite victim. Excretion started at day 3 and continued until measurement was stopped at day 5. In our horses, the earliest time point postbite that venom was detected was within the first 24 hours and venom was detected in the highest number of horses (7/13) at 4 days. Venom was not detected in every sequential daily sample once it was initially detected, indicating that venom excretion may be intermittent. Because of this variable excretion, peak venom concentration could have been missed in some of our patients. Additionally, we had small numbers of horses with only 8 horses having increased cTnI. Urine was not collected sequentially in all 8 horses making a statistical comparison difficult. To better compare peak urine venom concentrations with cTnI, more frequent sample collection will be required. Although it is clear from our results that a single sample is unlikely to be adequate to make an estimation of venom dose based on urine venom concentrations, additional research will be required to determine ideal sample collection timing.
In the area where these samples were collected, horses could have been bitten by Crotalus viridis viridis (Prairie rattlesnake), Crotalus atrox (Western Diamondback rattlesnake), Sistrus milliarus streckerii (Pigmy rattlesnake), or Crotalus horridus horridus (Timber rattlesnake). Agkistrodon contortrix lacintus (broad-banded copperhead) also is endemic in the areas where 3 of these horses resided. There are various reports in the literature of the occurrence of cardiac damage after snakebite in the horse.[1, 2] The apparent discrepancy in mortality rates between these 2 studies may be influenced by the different snake species endemic to these regions. We cannot rule out the possibility that horses in this study that had myocardial damage (increased cTnI) were bitten by a different species of snake than those that did not have increased cTnI. In at least 1 study, cardiac tissue has been shown to be relatively resistant to the direct effects of the venom from 1 of the snakes endemic to our study areas (Crotalus atrox). Our venom assay was not able to differentiate among these species of snakes.
Treatment of all horses in this study was left to the discretion of the attending veterinarian. Most of the horses were treated with corticosteroids (13/15) and antimicrobials (13/15). The mouths of rattlesnakes have been shown to harbor a mixed population of bacteria, and although commonly accepted as standard of care in equine medicine, the use of antimicrobials in human snakebite victims is not deemed necessary. Corticosteroids have been used in the treatment of rattlesnake envenomation for decades. An increased mortality in dogs treated with corticosteroids postenvenomation has been reported, but other studies have not shown the same results. Although there is no evidence that corticosteroids increase mortality in the horse,[1, 2] our treatment groups lacked sufficient size to make this comparison. Dexamethasone has been shown to inhibit TNFα synthesis as has pentoxifilline. Additional investigation is needed to define the role of TNFα in snake venom-mediated cardiac damage, but decreasing TNFα production may prove beneficial in ameliorating the systemic effects of rattlesnake envenomation. This may give reason to reconsider the use of these drugs in the treatment of rattlesnake envenomation.
In conclusion, cardiac damage evidenced by increased cTnI occurs in horses bitten by rattlesnakes. Although this damage does not occur in all horses, repeated cTnI measurement is a valuable tool for detecting and monitoring cardiac damage. Continuous ECG recording may be useful in detecting snakebite-associated arrhythmias, particularly in those horses with increased cTnI. Snake bite-associated cardiac damage may be influenced by the envenomation dose and nature of the associated systemic inflammatory and immunologic responses, but we were unable to make these correlations in this study. The clinical outcome may be influenced both by individual variation in response to venom among horses as well as the individual snake species. Although arrhythmias in our study horses most frequently were mild, transient and did not cause clinical signs, more serious arrhythmias have been reported and long-term follow-up is warranted. Because of the often delayed increase in cTnI in our study population, we recommend that horses be monitored closely for 1 week and then rechecked at later time points (weekly up to 6 weeks) to rule out the possibility of permanent cardiac damage or dysfunction secondary to the rattlesnake bite. In horses with clinical evidence of cardiac injury, echocardiography and exercise stress testing could be warranted.
The authors acknowledge the following for their assistance with sample collection and data analysis: Amy Giuioli-Canida, Robert Carmichael, Quinley Koch, Jennifer Taylor (Red Rock Biologics), Mike Davis, Joe Atha, Angel Jordan, Tommy Wilson, Dustin Dersch, Will Evans, Stacey McLeod, and Lara Maxwell.
This study was funded by grants from the ACVIM Foundation and the Center for Veterinary Health Sciences, Oklahoma State University.
Conflict of Interest: Dr Gilliam performed a safety trial with Red Rock Biologics Rattlesnake Vaccine in horses. In exchange, Red Rock Biologics performed antibody titer assays on the horses in this study. Dr. Gilliam did not receive any financial compensation from Red Rock Biologics.
Stratus CS STAT Fluorometric Analyzer, Dade Behring Inc, Newark, NJ
Digitrak-Plus 24Hr, Philips Medical Systems, Andover, MA
Zymed Holter 2010 for Windows, Philips Medical Systems
Anti venin (Crotalidae) Polyvalent, Fort Dodge Laboratories, Inc, Fort Dodge, IA
Cro-Tab, University of Northern Colorado, Greeley, CO
Donkey anti-sheep IgG alkaline phosphatase labeled, Jackson ImmunoResearch, West Grove, PA
4-methylumbelliferyl phosphate, Sigma-Aldrich, St. Louis, MO
Immulon HB4X, Thermo Fisher Scientific Inc
SpectromaxM2 Microplate Reader, Molecular Devices, Sunnyvale, CA
Equine TNF alpha Screening Set, Endogen, Pierce, Rockford, IL
Anti-equine alkaline phosphatase antibody, Sigma, St. Louis, MO
THERMOmax microplate reader, Molecular Devices, Sunnyvale, CA
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