Objective:To quantify gastrointestinal motility at 3 anatomic locations of the large colon in healthy horses fed 2 diets. Methods:12 American Quarter Horses were enrolled in a prospective crossover study. Horses received either a complete pelleted diet fed at 1.35% body weight (BW) per day or Coastal Bermudagrass hay fed at 2% BW with 0.5% to 0.7% BW of complete pelleted feed. Each diet was fed for 13 days prior to motility assessment. Two ultrasonographers recorded the frequency of contractions at the sternal flexure of the large colon, left ventral colon, and cecum for 4 consecutive, 1-minute periods at 2 hours after the morning and evening feedings. A contraction was defined as movement of the large intestinal wall that exceeded 2 cm. Data were analyzed using the Wilcoxon signed-rank and Kruskal-Wallis tests with Bonferroni correction. Results:Time of day did not affect the frequency of large colon contractions at any site. At the sternal flexure of the large colon, horses fed the complete pelleted feed exhibited a greater frequency of contractions compared to those receiving hay with supplementation. No significant differences in contraction frequency at the left ventral colon or cecum were observed across diets or time points. Conclusions:These results suggest that diet may affect the frequency of contractions at specific anatomic locations of the large colon in healthy horses. Clinical Relevance:Diet can affect large colon motility in healthy horses, emphasizing the need for veterinarians to consider dietary factors when assessing gastrointestinal health and function.

Clodronate disodium (CD), a bisphosphonate, modulates bone metabolism. Though extra-label use in juvenile horses is anecdotally reported, impacts on skeletal development are unknown. The objective was to determine the effects of CD on systemic markers of bone turnover in yearling horses undergoing exercise, hypothesizing that biomarkers of bone resorption would decrease while biomarkers of bone formation would not change and that repeat CD treatments would have a greater effect. To test this, 32 yearling Quarter Horses were used in a 168-d trial. Horses were stratified by age (500 ± 13 d), BW (336 ± 26 kg), sex (n = 16 geldings; n = 16 fillies), and initial bone optical density and randomly allocated to one of four treatment groups receiving either 1.8 mg/kg BW CD (Osphos) or isovolumetric saline (placebo). Investigators were blinded to treatments that included control (CON; n = 8), single-dose CD (1X; n = 8; d84), two-dose CD (2X; n = 8; d0 and 84), and four-doses CD (4X; n = 8; d0, 42, 84, and 126). Horses were housed individually in stalls and fed to meet nutrient requirements. Horses exercised 5 d/wk using a free stall exerciser in a phase-based progressive workload; Phase I (d0-84) simulated sales preparation and Phase II (d85-168) mimicked an early training program. Blood was collected on d0, 42, 84, 126, and 168 before treatment injections, when applicable. Serum was analyzed for receptor activator of nuclear factor κB ligand (RANKL), tartrate resistant acid phosphatase 5 b (TRAP5b), c-terminal crosslinks of type I collagen (CTX-1), bone-specific alkaline phosphatase (BAP), and procollagen type I n-terminal propeptide (PINP) via commercial ELISA or EIA. Data were analyzed using PROC MIXED of SAS with a baseline covariate for BAP. A treatment×time interaction was noted for osteoclastic TRAP5b (P = 0.03), decreasing in 4X from d0 to 126 and returning to baseline at d168, and decreasing in 2X to d84 whereas in CON and 1X it increased or remained the same over time. Serum CTX-1, a type I collagen degradation marker, increased over time (P < 0.01) in all treatments. Osteoblastic BAP increased (P < 0.01) from d42 to 84 and remained elevated until d168. There was no change in PINP (P = 0.35) or osteoclast differentiation signal RANKL (P≥0.24). The results indicate that CD administration in horses undergoing low-intensity exercise reduces a serum biomarker of osteoclast number and activity without affecting serum biomarkers of bone formation or resorption.

Extra-label bisphosphonate (BP) use in juvenile horses is anecdotally reported, primarily for analgesic effects, despite the limited scientific understanding of biologic impacts on skeletally immature horses undergoing exercise. The objective of this study was to determine the effects of clodronate disodium (CD), a form of BP, on endocrine regulators of calcium. Thus, 32 yearling Quarter Horses were stratified by age (500 ± 13 d), body weight (BW; 336 ± 26 kg), sex (n = 16 geldings, n = 16 fillies), and initial bone optical density into 1 of 4 treatment groups for a 168-d trial. The experimental period was divided into 2 exercise phases to model industry standards. Investigators were blinded to treatment, and all horses received iso-volumetric intramuscular injections of either 1.8 mg/kg BW CD (OSPHOS) or saline (placebo) on days 0, 42, 84, and 126. Treatments consisted of control (CON; n = 8), 1-dose (1X; n = 8; day 84), 2-dose (2X; n = 8; day 0, 84), and 4-dose groups (4X; n = 8; days 0, 42, 84, 126). Serum samples were collected, and physical measurements were recorded including BW, wither height, hip height, body length, and heart girth circumference (HG) every 42 d, prior to treatment administration. Serum samples were analyzed for growth hormone (GH), calcitonin, parathyroid hormone (PTH), and ionized calcium (Ca2+). Data were analyzed using MIXED and CORR procedures of SAS. All physical measurements increased over time (P ≤ 0.01) but were not affected by treatment (P ≥ 0.62). Similarly, there was no effect of treatment for GH (P = 0.44), but concentrations tended to decrease over time (P = 0.09). A treatment × day interaction was observed for PTH (P = 0.05) where concentrations increased following a second CD dose. Specifically, concentrations increased on day 84 in 4X and on day 126 in 2X following the second treatment with CD whereas 1X and CON remained unchanged. Despite the increase in PTH, there was no effect on calcitonin (P ≥ 0.33). Ionized calcium concentrations decreased over time (P < 0.01) with no effect of treatment (P = 0.26). Parathyroid hormone was negatively correlated with serum Ca2+ (r = -0.28, P < 0.01), whereas calcitonin was not correlated with serum Ca2+ (r = 0.06, P = 0.48) nor PTH (r = -0.13, P = 0.12). According to these results, CD has no effect on growth morphometrics, but its use transiently increases PTH concentrations after 2 doses.

The objective was to evaluate dietary Saccharomyces cerevisiae fermentation product (SCFP) on joint inflammation and cartilage metabolism in exercising yearlings challenged with intra-articular lipopolysaccharide (LPS), hypothesizing dietary SCFP (TruEquine®C, Diamond V Mills, Inc.) would ameliorate joint inflammation and increase cartilage metabolism. Thirty Quarter Horse yearlings were stratified by bodyweight (BW), age, sex, and randomly assigned to dietary treatments (n = 10/treatment): control (0), 46, or 92 mg/kg BW/d SCFP. Treatments were top-dressed to 1% BW/d concentrate void of added microbials. Horses were stalled (3.6 m × 7.3 m), offered ad libitum Coastal bermudagrass hay, and exercised 30 min/d, 5 d/wk. On days 0, 21, 42, and 56, wither height, hip height, heart girth, body length, body condition scores (BCS), and BW were recorded. On day 46, one radial carpal joint received 0.8 mL of a 0.5 ng LPS solution or sterile lactated Ringer's solution (LRS) in the contralateral joint. Synovial fluid was collected pre- (0) and 6, 12, 24, and 336 hours post-injection and analyzed for prostaglandin E2 (PGE2), carboxypropeptide of type II collagen (CPII), and collagenase cleavage neopeptide (C2C) via commercial ELISA, and chemokines (CCL2, and CCL11) and cytokines (TNF α and IL-10) via multiplex platform. Rectal temperature (RT), heart rate (HR), respiration rate (RR), and carpal circumference (CC) were recorded prior to arthrocentesis. Data were analyzed using PROC MIXED of SAS. By day 56, growth parameters increased (P < 0.01), BCS did not change (P = 0.39), and BW had a treatment × d interaction (P = 0.02) where control tended to be heavier than 92 mg/kg BW on day 56 (P = 0.07). Clinical parameters (RT, HR, RR, CC) were uninfluenced by diet (P ≥ 0.29) but varied over time (P ≤ 0.03). Treatments did not influence cartilage metabolism (CPII, C2C, and CPII:C2C) (P ≥ 0.46) or logPGE2, logCCL2, CCL11, or logIL-10 (P ≥ 0.23). There was a treatment × h interaction for CCL11 (P = 0.04) where control was greater than SCFP groups at h 6. LogIL-10 had a treatment × h interaction where 46 mg/kg BW was lower than control and 92 mg/kg BW at h 12 (P = 0.05). There was a main effect of treatment for TNF α (P = 0.04) where 92 mg/kg BW was lower than 46 mg/kg BW and tended to be lower than control. Results indicate that SCFP didn't influence cartilage metabolism or PGE2, though SCFP may ameliorate inflammatory cytokines and chemokines following an acute, intra-articular insult.

Perceived chondroprotective and anti-inflammatory benefits of bisphosphonates in the juvenile horse has led to extra-label use without supportive data regarding intra-articular effects on cartilage metabolism and inflammation. Thirty-two yearling Quarter Horses were stratified into 4 treatment groups by age (500 ± 13 d), BW (336 ± 26 kg), sex (n = 16 female; n = 16 male) and initial bone optical density for a 140-d study. The study consisted of two exercise phases: Phase 1 (d 0-84) emulated sales preparation and Phase 2 (d 99-140) mimicked early exercise training. Horses were housed individually (3.6 m × 7.3 m stalls) and fed to meet nutritional requirements. All horses received iso-volumetric intramuscular injections of 1.8 mg/kg BW clodronate disodium (CD) (OSPHOS) or saline (vehicle) on d 0, 42, 84, and 126. Specifically, the treatment groups consisted of the saline control (0×; n = 8), and clodronate injected once (1×; n = 8; d 84), twice (2×; n = 8; d 0, 84), or four times (4×; n = 8; d 0, 42, 84, 126). Horses underwent an intra-articular lipopolysaccharide (LPS) challenge post treatment administration on d 126. Synovial fluid was collected prior to LPS injection (h 0) and at 6, 12, 24, and 336 h post injection. Synovial fluid concentrations of carboxypropeptide of type II collagen (CPII), collagenase cleavage neopeptide (C2C), and prostaglandin E2 (PGE2) were determined by ELISA. Intra-articular LPS increased CPII, C2C, PGE2 and CPII: C2C (P ≤ 0.01). There tended to be a treatment × time interaction in CPII (P = 0.06) where CPII was greatest in 2× with 1× and 0× being the lowest, 4× concentrations were between this range at 24 h post-injection. Likewise, CPII: C2C tended to be influenced by treatment (P = 0.06) with 2× and 4× having greater synthesis: degradation ratio than 1× and 0×. There was a treatment × time × carpus interaction (P = 0.02) in which 1×, 2× and 4× CD groups had greater synovial PGE2 concentrations at 6 h post-injection in the LPS joint compared to 0×. The results of this study indicate that administration of CD increases intra-articular inflammation (PGE2) but does not affect cartilage degradation (C2C) and only tends to increase cartilage synthesis (CPII) according to biomarkers measured.

Thirty mature Quarter Horse geldings were used in a completely randomized 32-d study to test the hypotheses that supplemental live Saccharomyces cerevisiaeCNCM; I-1077 improves apparent digestion, stabilizes the fecal pH, reduces gut permeability, maintains microbial communities, and decreases inflammation in horses fed a high-starch diet. Horses were stratified by body weight (BW), age, and body condition score (BCS) to one of two treatments: concentrate formulated with 2 g starch · kg BW-1 · meal-1 (control (CON); n = 15) or the same concentrate top-dressed with 25 g/d S. cerevisiae CNCM I-1077 (treatment (SC); n = 15; 8 × 108CFU). Horses were fed individually in stalls every 12 h. Between meals, horses were housed in dry lots with ad libitum access to water and Coastal bermudagrass hay. On days 0 and 32, BW and BCS were recorded, and blood was collected before feeding and 2, 8, 16, and 24 h postmeal on day 32 to analyze serum D-lactate. Fecal samples were collected on days 0, 16, and 32 at 8, 16, and 24 h postmeal for fecal pH and starch content. Intake and fecal production were recorded over 4 d to measure digestibility on days 28-31. Whole blood total bacterial counts and 16S fecal microbiota rRNA sequencing were performed at days 0, 16, and 32. Results revealed an increased ∆BW in SC horses compared with CON horses (P = 0.03), with no change in BCS (P = 0.97). D-lactate tended to be greater in SC horses on day 32 at 16 and 24 h postmeal compared with CON horses (P = 0.10). Concentrations of TNFα and LogCCL2 decreased from day 0 to day 32 regardless of dietary supplementation (P ≤ 0.02). Fold change of percent reads from day 0 in whole blood bacterial 16S rRNA did not differ between groups. Fecal starch was undetectable, and there were no differences in intake or apparent digestibility. Fecal pH tended (P = 0.07) to be lower in CON at 0 h on day 32 (6.03 ± 0.06) than on day 16 (6.14 ± 0.06). Additionally, pH tended (P = 0.09) to be lower in CON (6.03 ± 0.06) than in SC (6.16 ± 0.06) at 0 h on day 32. Supplementation of S. cerevisiae CNCM I-1077 maintained Bacteroidales and reduced acidosis-like bacteria like Streptococcus and potential pathogens like Enterobacteriaceae, Stenotrophomonas, and Rhodococcus at day 16 (P < 0.05). Furthermore, supplementation increased fibrolytic bacteria at day 32, such as Ruminococcus, Fibrobacter, and Succinivibrio (P < 0.05). These results indicate S. cerevisiae CNCM I-1077 increases BW and promotes a more diverse microbiome when horses are fed ad libitum hay and a high-starch concentrate.

Abstract The non-nitrogenous bisphosphonate clodronate disodium (CD) inhibits bone resorption and is approved for horses ≥ 4 yr of age. Extra-label use in juvenile exercising horses is a concern due to potential interference with normal bone growth and development. The objective was to determine the effects of single and repeated doses of CD on bone optical density (radiographic bone aluminum equivalence; RBAE) and lameness score (LS) in juvenile, exercising horses. Quarter Horses [n = 32 (16 geldings, 16 fillies); 500 ± 13 d of age; body weight (BW) = 336 ± 26 kg]) were used to test the hypothesis that horses receiving CD would have greater RBAE and reduced LS, and additional doses of CD would accentuate these effects. In a 168-d trial, horses were housed in individual stalls, and fed 1% BW/d concentrate and ad libitum Coastal bermudagrass hay. Horses were exercised 5 d/wk following a phase-based progressive workload using a free stall exerciser; Phase I (d 0 to 84) mimicked sales preparation whereas Phase II (d 85 to 168) simulated early training. Horses were stratified by age, BW, sex, and initial RBAE into control (CON; n = 8), single dose CD (1X; d 84; n = 8), 2-doses CD (2X; d 0 and d 84; n = 8), and four-doses CD (4X; d 0, d 42, d 84, d 126; n = 8). On injection d (0, 42, 84, and 126), horses were administered either 1.8 mg/kg BW CD (OSPHOS) or isovolumetric saline. On d 0, 84, and 168, dorsal-palmar and lateral-medial radiographs of the left third metacarpus and metatarsus were captured. An aluminum step wedge was included in each image and raw images were analyzed (Quantity One Basic, BioRad) to determine total bone RBAE at 1 cm distal to the nutrient foramen. On d 0 and d 168, horses were evaluated by a veterinarian unaware of treatment group and assigned a LS according to the American Association of Equine Practitioners Lameness Scale. The RBAE and LS data were analyzed via PROC MIXED and PROC GENMOD, respectively, in SAS. For each view, RBAE increased from d 0 to d 84 and remained increased to d 168 (P ≤ 0.02), regardless of treatment, likely due to progressing exercise protocol. On d 0, 29 horses were LS 0, 1 horse was LS 1, and 2 horses were LS 2; LS did not differ among treatment groups (P = 0.61). On d 168, 5 horses were LS 0, 13 horses were LS 2, 11 horses were LS 3, and 1 horse was LS 4; LS did not differ among treatments (P = 0.16). Based on these results, the hypothesis that single or repeated CD administration would increase RBAE and decrease lameness in juvenile, exercised Quarter Horses was rejected. Funding: AFRI#2021-67015-34079

Abstract Concerns over the extra-label use of bisphosphonates in skeletally immature horses has extended to include its likely re-release from bone into circulation in response to stressors. Therefore, the objective of this study was to be first to quantify the re-release of clodronate in response to controlled stressors. Yearling Quarter horses (n = 32) were stratified by age, body weight (BW), sex, and initial bone optical density into four treatment groups for a 168-d trial. Treatments consisted of control (CON; n = 8), single-dose (1X; d 84; n = 8), 2-dose (2X; d 0 and 84; n = 8), and four-dose groups (4X; d 0, 42, 84, and 126; n = 8). All horses received iso-volumetric intramuscular injections of either 1.8 mg/kg BW clodronate disodium (OSPHOSÒ) or saline (placebo) on d 0, 42, 84, and 126. Horses were housed in individual stalls (3.6 m × 7.2 m). Diets were formulated to meet nutrient requirements including concentrate offered every 12-h and ad libitum coastal bermudagrass (Cynodon dactylon) hay and water. Horses were exercised 5 d/wk with a progressive workload, and maximum exercise intensity was reached on d 120, verified via heart rate and blood lactate. On d 120, blood samples were collected prior to the start of the exercise bout (pre), and 0-, 0.5-, 1-, 3-, 12-, and 24-h post exercise. Both tuber coxae of each horse were biopsied on d 168 with blood samples collected prior to (pre), and 0, 12, 24, 48, 72, 168, and 336 h post biopsy. Clodronate was quantified in plasma from exercise and biopsy samples using liquid chromatography tandem mass spectrometry. Exercise and biopsy serum were analyzed for cortisol, a marker of stress, while biopsy serum was analyzed for substance P, a pain marker. Data were analyzed using SAS PROC MIXED. Exercise did not result in clodronate re-release as there was no treatment x time interaction or time effect (P ≥ 0.44), but clodronate was dose-dependently greater in treated groups throughout the exercise collection period (P < 0.01). A treatment x time interaction (P ≤ 0.01) was observed for clodronate concentrations surrounding bone biopsy in which concentrations decreased at 168 h post biopsy before increasing at 336 h in 4X horses. Substance P increased over time (P < 0.01) following biopsy, regardless of treatment. Despite decreasing over time (P ≤ 0.01), there were similarly no treatment differences (P ≥ 0.46) nor treatment ´ time interactions (P ≥ 0.78) for cortisol following either exercise or bone insult. In conclusion, submaximal exercise did not result in re-release of clodronate; however, direct bone insult disrupted clodronate concentrations in horses receiving 4 administrations of bisphosphonate despite no treatment impact on stress or pain indicators.

OBJECTIVE To describe the indications for and surgical technique of mastectomy of mares and to describe the outcome of 10 mares that underwent mastectomy in a retrospective case series. ANIMALS 10 mares having disease of one or both mammary glands. CLINICAL PRESENTATION Medical records (1995 to 2022) from 2 university teaching hospitals were searched to identify mares that had undergone unilateral or bilateral mastectomy. Data regarding history, signalment, diagnostic tests, preoperative treatment, surgical procedure, and postoperative management were reviewed. Follow-up information was obtained by interviewing the owners by telephone. RESULTS One (n = 4 mares) or both mammary glands (6 mares) were excised for the following reasons: chronic bacterial mastitis (4), neoplasia (3), lymphangiectasia (1), pythiosis (1), and lymphoid hamartoma (1). None of the mares experienced intraoperative complications. The surgical site was closed primarily in 2 mares and left unsutured in 8 mares. Both sutured wounds developed a seroma, and 1 dehisced. The owners reported that the surgical wound, whether sutured or unsutured, was healed within 3 months. All mares returned to use for their intended purpose, but 3 mares were euthanized 2 to 4 years after surgery due to progression of disease. One mare drowned 1 year after discharge. CLINICAL RELEVANCE Mastectomy can be an effective treatment for mares suffering from disease of one or both mammary glands when the mare is refractory to medical treatment.

Abstract Anthelmintics are commonly used in the equine industry to reduce parasite load, but there is a growing concern about anthelmintic resistance, especially in young horses where there is limited information available. Twenty-four, 2-yr-old Quarter Horses (825 ± 21 d of age, initial BW 409 ± 6 kg, 12 fillies and 12 geldings) originating from a single herd were used in a 42-d study to investigate the effectiveness of a commercial anthelmintic (fenbendazole) on fecal egg counts (FEC), hypothesizing that there would be a decrease in FEC following anthelmintic administration. Horses were housed individually (3.7m × 11m) and offered ad libitum Coastal Bermudagrass hay and water and fed a pelleted concentrate offered at 0.75% (as-fed) of their BW. During the 42-d study, BW was obtained and BCS were assigned in 21-d intervals. Fenbendazole was administered orally on d 0 and 14 dosed on BW per manufacturer label. Fresh fecal samples were collected on d 0 and 14, before anthelmintic administration, and on d 28 and 42. Fecal egg floats were conducted using the Modified Wisconsin Method, and eggs per gram (EPG) were determined in duplicate. A fecal egg count reduction test (FECRT) was calculated, beginning on d 14. Outliers were determined as values ± 2 standard deviations of the mean, and data were analyzed using PROC MIXED of SAS (v9.4) with the main effect of time. Horses increased in BW (P ≤ 0.01) over time, but BCS did not change (P = 0.25). Mean FEC increased over time (P ≤ 0.01), beginning on d 14, and remained increased to d 42 compared to baseline. However, there was a trend (P = 0.07) for mean FEC to be less on d 42 (189.70 ± 29 EPG) when compared with d 14 (251 ± 28 EPG). The calculated reduction of FEC were below the targeted 95% or greater and included d 14 (1.04%), d 28 (35%), and d 42 (41%). The increased shedding of eggs did not negatively impact BW or BCS. However, we reject the hypothesis since FEC increased following anthelmintic treatment, indicating the need for consistent monitoring of anthelmintic efficacy in young horses.

Abstract Antibiotics are commonly given to horses to treat known bacterial infections or as prophylaxis against the development of infection. Diarrhea is a recognized adverse effect of antimicrobial administration in horses (antimicrobial associated diarrhea, AAD). In veterinary referral centers, AAD has a reported incidence of 22 to 94% (McGorum, 2010), while horses with AAD are 4.5 times more likely to die compared with horses with other types of colitis (Woods and Cohen, 1999). Although all antibiotics have potential to induce diarrhea, some have increased risk due to low oral absorption, biliary excretion, or enterohepatic recycling. The role of dysbiosis in the gut microbiota is a key feature in the pathogenesis of AAD. Disruption of commensal bacteria of the hindgut allows for pathogen overgrowth and alteration in microbial metabolic function. To date, 16S rRNA sequencing has been used to characterize the effects of penicillin, trimethoprim sulfa, doxycycline, erythromycin, ceftiofur, enrofloxacin, oxytetracycline and metronidazole on the fecal microbiome in healthy horses (Costa et al., 2015; Arnold et al., 2020; Liepman, 2022). Regardless of mechanism of action, antibiotics typically reduce diversity (species richness and evenness) and alter the bacterial community structure of the fecal microbiome, even when animals maintain normal health status and fecal character (Arnold et al., 2021). These effects typically require at least 30 days before the microbiome returns to baseline (Gomez et al, 2023). In horses with colitis, changes in the microbiome are often significant. Horses with AAD demonstrate the most severe disruption of the fecal microbiome compared with horses with Salmonella or undifferentiated colitis. These changes are evident from the phylum to species level, and include important phyla such as Bacteroidetes, Firmicutes, Fibrobacter and Verrucomicrobia (Arnold et al., 2020; Liepman, 2022; Costa et al., 2015). Alterations in the microbiota result in functional metabolic changes that are clinically manifested by diarrhea. Potential changes include alterations in the metabolism of bile acids, carbohydrates including short chain fatty acids, amino acids, lipids and proteins. There is also evidence that antimicrobials increase intestinal permeability via effects on tight junction proteins and by compromising the barrier function provided by the mucus layer between the enterocytes and gut lumen. Recent work comparing horses on antimicrobials that did and did not develop diarrhea confirm that horses with diarrhea have depletion of Verrocomicrobia and metabolites related to gastrointestinal barrier function (Arnold et al., 2021).

Abstract Intra-articular lipopolysaccharide (LPS) is an established model for inducing robust, transient inflammation and increasing cartilage metabolism for up to 24 h following administration. Currently, there is limited information evaluating the long-term effects of this gram-negative endotoxin on the intra-articular environment of young horses. Therefore, the objective of this study was to determine the lasting effects of a single administration of intra-articular LPS on synovial biomarkers of inflammation and cartilage metabolism over 35 wk. Quarter Horse yearlings [n = 10; body weight (BW) = 409 ± 24 kg; 619 ± 16 d of age; fillies, (n = 7) and geldings (n = 3), originating from a single herd were used to test the hypothesis that intra-articular inflammation and cartilage metabolism would not be elevated at 35 wk post-injection. Each horse had one randomly selected radiocarpal joint injected with 0.8 mL of a 0.5 ng LPS solution derived from Escherichia coli O55:B5 (INFL; Sigma-Aldrich, St. Louis, MO) and the contralateral joint received sterile lactated Ringer’s solution (CON) as a control. Synovial fluid was obtained on 0 (before injection of LPS), 2, and 35 wk following arthrocentesis. Synovial fluid samples were analyzed in duplicate for carboxypropeptide of type II collagen (CPII), collagenase cleavage neopeptide (C2C), chondroitin sulfate 846 epitope (CS846), and prostaglandin E2 (PGE2) by commercial ELISA (IBEX Pharm, Montreal, Quebec, CA and Arbor Assays, Ann Arbor, MI). Biomarkers were analyzed using PROC MIXED of SAS v9.4 with repeated measures (time). The model included treatment (CON, INFL), time (week), and treatment × time interaction as fixed effects. Synovial biomarkers PGE2, CPII, and CS846 changed over time (P < 0.01) regardless of LPS administration, increasing from 0 to 2 wk and declining to below baseline concentrations at wk 35. There was a treatment × time interaction (P ≤ 0.01) in which INFL joints had decreased catabolic C2C concentrations compared with CON joints at wk 35. Therefore, LPS administration did not increase cartilage metabolism and inflammation levels at 2 or 35 wk post-LPS induction, indicating no negative effects compared with the contralateral controls. This study supports using intra-articular LPS in young horses without negative long-term effects.