The use of inhaled corticosteroids (ICs) could influence the respiratory microbiota. In animals with asthma it is, however, difficult to separate the immunomodulatory effects of ICs from their indirect effects via improvement of ventilation. Our objective was to determine if ICs alter the pulmonary microbiota independently from their effects on lung function, using a blinded, controlled trial in an experimental model of asthma exacerbation in horses. We treated horses with severe asthma with either bronchodilators alone, or in combination with ICs. Twelve horses in exacerbation received long-acting β2-agonist (LABA, salmeterol) or ICs/LABA (fluticasone/salmeterol) by inhalation, for 2 weeks. Lung function and bronchoalveolar lavages (BAL) were performed before and after treatment. 16S rRNA gene quantification and sequencing were performed on BAL fluid, using digital droplet PCR and the Illumina MiSeq platform. Data were processed using the software package mothur v. 1.44.2. In the LABA group, pulmonary bacterial load and the relative abundance of Actinobacteria and Verrucomicrobia phyla decreased with treatment (p < 0.05 for both), and β-diversity differed from baseline (p = 0.007). The relative abundance of families and genera belonging to the Bacteroidetes phylum increased with ICs/LABA (p < 0.05). Lung function significantly improved with both treatments, suggesting that treatment-related differences in pulmonary microbiota could be attributed in part to medication, not solely to change in ventilation. However, it is not clear if these changes are positive or detrimental to the lung environment. Furthermore, lung function following treatment was not perfectly identical between groups.

Differences in gut microbiota composition related to diet have been reported in horses, but characterization of specific microbial taxa remains limited, particularly at the species level. The objective of this study was to use long-read sequencing of the 16S rRNA gene to provide additional taxonomic insight into the intestinal microbiota in horses. Fecal samples were collected from 12 horses on pasture and from 6 of them after switching to a hay diet. Sequencing yielded low read counts per sample, and the analysis failed to detect statistical differences in alpha- and beta-diversity among dietary groups (p > 0.05). Species-level taxonomic resolution was not substantially enhanced using long-read sequencing, as only 3% of reads were assigned at the species level, and an additional 3% of reads were assigned at the genus level. The majority of reads (49%) were classified at the family level. Accordingly, in this dataset, long-read sequencing did not provide additional biological insight into diet-associated differences in the equine gut microbial community. This limited added value can be explained by the low sequencing depth obtained for several samples and the current incompleteness of reference databases for equine bacterial taxa, highlighting ongoing challenges in achieving high-resolution characterization of the equine gut microbiome.

Clinical signs of severe equine asthma can be controlled by feeding diets with fewer inhalable particles than conventional dry hay, such as steamed hay or pellets. We hypothesized that asthmatic horses would spend more time eating, less time resting, and would have fewer gastric ulcers when they are fed steamed hay compared to pellets. Nine asthmatic horses were housed indoors and fed dry hay to induce clinical exacerbation (baseline). They were then fed either alfalfa pellets or steamed hay for 4 weeks, in a crossover design. Horses were filmed in their stalls 18-22 h over a 24 h period during exacerbation, and the 1st and 4th weeks of each diet. Partial ethograms (time-budget and position) were recorded through the ZooMonitor application (Tracks software) using focal instantaneous sampling. Gastroscopies were performed before and after each diet. When horses were fed pellets, their time spent eating decreased by 33.8% during the 1st week (P < 0.001). Time spent eating did not change with steamed hay (48.1%, 47.8%, and 47.9%, P > 0.8, on exacerbation, 1st and 4th weeks). There was no association between diets and squamous gastric ulcers. Asthmatic horses spend more time eating when offered steamed hay. This suggests that steamed hay could allow a more physiological time-budget, an important welfare parameter that should be considered in managing horses with severe asthma, despite the equivocal improvement of lung function reported previously.

This study aimed to document antimicrobial resistance patterns of bacteria frequently isolated at a referral equine hospital between 2020 and 2022 and to compare these results to those of studies carried out in previous decades at the same reference center. Using the Kirby-Bauer method, 340 tests were completed on bacterial isolates and compared to 233, 255, and 396 tests carried out in 1986 to 1988, 1996 to 1998, and 2007 to 2013, respectively. Data were analyzed with X 2 tests for trends, followed by pairwise Fisher tests and Bonferroni corrections. Increasing resistance to at least 1 antibiotic was observed for 5 of the 6 bacteria studied, including increasing resistance to enrofloxacin for Escherichia coli, Pseudomonas spp., Enterobacter spp., and staphylococci, and to ceftiofur for Enterobacter spp. and staphylococci. Resistance of Actinobacillus spp. to ampicillin and penicillin decreased over time, whereas resistance of Streptococcus equi subsp. zooepidemicus to penicillin was not observed. Bacteria isolated from horses had increasing resistance to antibiotic classes considered critically important for human health. However, penicillin remained an appropriate 1st-line antibiotic for Actinobacillus spp. and Streptococcus equi subsp. zooepidemicus in our area.

The control of equine respiratory infections is a biosecurity challenge. Respiratory viruses are often rapidly detected using quantitative polymerase chain reaction (qPCR) on nasal swabs. In the past, some laboratories developed handmade techniques to increase the amount of nasal secretions collected, without comparing them with nasal swabs when qPCR replaced the use of viral culture. The objectives of this study were to compare nasal swabs and handmade foam cubes for i) the detection of a common equine herpesvirus (EHV-5) by qPCR, and ii) their tolerability. Forty-five polyester swabs and foam cubes were used to sample 9 horses 5 times. These were then analyzed by qPCR for EHV-5. Agreement of qPCR results (positive, suspect, negative) was assessed using the intraclass correlation (ICC) and the avoidance scores were compared using a proportional odds mixed model. The ICC showed moderate agreement (0.61, P < 0.001). Twenty-seven percent of the samples were positive or suspect with either swabs or foam cubes, whereas 18% were strictly positive with swabs and 11% with foam cubes. Avoidance scores were not statistically different. Both methods have an acceptable agreement. Handmade foam cubes did not provide additional diagnostic value compared to polyester swabs, which is the method presently recommended.

Background Fecal microbiota transplantation (FMT) has been used as a treatment option for horses (Equus caballus) with gastrointestinal diseases. Several preparation and conservation protocols to improve bacterial survival have been studied in other species. Methods This study aimed to evaluate the impact of oxygen exposure and different protectant solutions on bacterial viability before and after freezing using horse feces. Fecal samples from 10 healthy horses were aliquoted and diluted in cryoprotectant solutions containing antioxidants (n = 40) or 10% glycerol (n = 40). Half of the aliquots from each dilution condition were prepared inside an anaerobic chamber, while the other half were prepared under ambient air conditions. Each sample was also analyzed fresh and after freezing at −20 °C for 90 days. Bacterial viability was assessed using flow cytometry. A mixed linear model and the Friedman and Wilcoxon tests were used depending on data distribution. Results Freeze-thawing decreased bacterial viability by 47% (mean ± SD: 51 ± 27% before, 27 ± 8% after; p < 0.001). Glycerol was superior to the cryoprotectant after freezing (32 ± 8% glycerol, 24 ± 8% cryoprotectant; p < 0.001). Oxygen exposure did not affect viability (p = 0.13). There was no statistical difference between protectant solutions in fresh samples (p = 0.16). Conclusions Fresh FMT solutions may be better for treating horses with dysbiosis, but if freezing cannot be avoided, glycerol should be used to dilute feces.

Objective: This study aimed to document antimicrobial resistance patterns of bacteria frequently isolated at a referral equine hospital between 2020 and 2022 and to compare these results to those of studies carried out in previous decades at the same reference center. Procedure: Using the Kirby-Bauer method, 340 tests were completed on bacterial isolates and compared to 233, 255, and 396 tests carried out in 1986 to 1988, 1996 to 1998, and 2007 to 2013, respectively. Data were analyzed with X2 tests for trends, followed by pairwise Fisher tests and Bonferroni corrections. Results: Increasing resistance to at least 1 antibiotic was observed for 5 of the 6 bacteria studied, including increasing resistance to enrofloxacin for Escherichia coli, Pseudomonas spp., Enterobacter spp., and staphylococci, and to ceftiofur for Enterobacter spp. and staphylococci. Resistance of Actinobacillus spp. to ampicillin and penicillin decreased over time, whereas resistance of Streptococcus equi subsp. zooepidemicus to penicillin was not observed. Conclusion and clinical relevance: Bacteria isolated from horses had increasing resistance to antibiotic classes considered critically important for human health. However, penicillin remained an appropriate 1st-line antibiotic for Actinobacillus spp. and Streptococcus equi subsp. zooepidemicus in our area.

BACKGROUND:Steaming hay reduces respirable particles and is commonly used to feed horses with asthma. However, it showed inconsistent benefits in clinical studies. OBJECTIVES:(1) To assess the effects of steamed hay on lung function and airway inflammation in horses with severe equine asthma (SEA) in remission; (2) To compare these effects with a dry hay diet. STUDY DESIGN:Cross-over in vivo experiment. METHODS:Horses were fed steamed and dry hay for 4 weeks in a prospective, cross-over study, with a 4-week washout period. Lung function, bronchoalveolar lavage (BALF) cytology, and a 23-point weighted clinical score (WCS) were recorded before and after 4 weeks of hay feeding. A mixed linear model with post hoc tests was used. RESULTS:Resistance at 5 Hz (R5) increased over the 4-week period (time effect and post hoc end vs. baseline: p < 0.001), with no difference between treatments (mean [SD], kPa/L/s) (baseline dry: 0.065 [0.014]; end dry: 0.079 [0.019]; baseline steamed: 0.063 [0.009]; end steamed: 0.078 [0.014]). There was a significant increase in BALF neutrophil percentages over time (end vs. baseline: p < 0.001) (baseline dry: 6.7 [5.4]; end dry: 13.1 [6.0]; baseline steamed: 5.6 [2.6]; end steamed: 10.5 [4.3]). WCS did not change significantly (baseline dry: 2.6 [1.5]; end dry: 2.2 [1.2]; baseline steamed: 2.9 [1.5]; end steamed: 1.9 [1.3]). MAIN LIMITATIONS:This study involves small number of horses in a research setting. Hay dust content and particles in the breathing zone were not measured. CONCLUSIONS:Steamed hay induced a mild but significant deterioration of lung function and inflammation in horses with SEA. The lack of differences with dry hay could be due to the unexpectedly mild exacerbation during this study.

Abstract Background Reducing inhaled dust particles improves lung function in horses with severe asthma. Soaked hay is commonly used by owners, but its efficacy in improving lung function and inflammation has not been documented. Objectives To measure the effects of soaked hay and alfalfa pellets in horses with severe asthma. Animals Ten adult horses with severe asthma from a research colony. Methods Prospective controlled trial. Horses in clinical exacerbation were housed indoors and allocated to be fed either soaked hay (n = 5) or alfalfa pellets (n = 5) for 6 weeks. Soaked hay was immersed for 45 minutes and dried out hay was discarded between meals. Pulmonary function and clinical scores were measured before and after 2, 4, and 6 weeks. Tracheal mucus scores and bronchoalveolar lavages were performed before and after 6 weeks. Lung function was analyzed with a linear mixed model using log‐transformed data. Results Lung resistance decreased from (median (range)) 2.47 (1.54‐3.95) to 1.59 (0.52‐2.10) cmH2O/L/s in the pellets group and from 1.89 (1.2‐3.54) to 0.61 (0.42‐2.08) cmH2O/L/s in the soaked hay group over the 6‐week period for an average difference of 1.06 cmH2O/L/s for pellets (95% confidence interval [95% CI]: 0.09‐2.04, P = .03, not significant after correction) and 1.31 cmH2O/L/s for soaked hay (95% CI: −0.23 to 2.85, P < .001, significant). Conclusion and Clinical Importance Soaked hay can control airway obstruction in horses with severe asthma. The strict protocol for soaking and discarding dried‐out hay in this study could however be considered too great of an inconvenience by owners.

Both microbe–microbe and host–microbe interactions can have effects beyond the local environment and influence immunological responses in remote organs such as the lungs. The crosstalk between the gut and the lungs, which is supported by complex connections and intricate pathways, is defined as the gut–lung axis. This review aimed to report on the potential role of the gut–lung gut–lung axis in the development and persistence of equine asthma. We summarized significant determinants in the development of asthma in horses and humans. The article discusses the gut–lung axis and proposes an integrative view of the relationship between gut microbiota and asthma. It also explores therapies for modulating the gut microbiota in horses with asthma. Improving our understanding of the horse gut–lung axis could lead to the development of techniques such as fecal microbiota transplants, probiotics, or prebiotics to manipulate the gut microbiota specifically for improving the management of asthma in horses.

Background: Steaming hay significantly reduces respirable particles and provides a palatable alternative to dry hay for horses with asthma, but there are few prospective studies demonstrating the clinical efficacy of steamed hay. Objectives: To compare the efficacy of alfalfa pellets and steamed hay in improving lung function and inflammation of horses with severe asthma (SEA). Study Design: Controlled crossover study. MethodsTen horses with SEA were enrolled and nine completed the study. Horses were housed indoors and fed hay. Once in exacerbation, they were fed pellets and steamed hay for 4 weeks, in a crossover design. Weighted clinical scores and lung function were recorded weekly. Bronchoalveolar lavage fluid (BALF) cytology and mucus scores were recorded before and after each diet. Results: Based on linear mixed model (LMM) analysis, weighted clinical scores significantly improved over time (p < 0.001, no diet effect or time-diet interactions). With pellets, weighted clinical scores decreased from (median (interquartile range)) 13 (5.5) to 2 (1.5), while with steamed hay, they decreased from 10 (9.5) to 6 (8.5). With pellets, lung resistance decreased significantly from a baseline of (mean (SD)) 2.62 (0.77) cmH(2)O/L/s to 1.17 (0.66), 0.79 (0.54), 0.70 (0.20), 0.62 (0.18) on Weeks 1-4, respectively (LMM with post hoc tests, p < 0.001). With steamed hay, the resistance decreased significantly from a baseline of 2.34 (0.93) cmH(2)O/L/s to 1.38 (0.49) and 1.51 (0.66) on Weeks 1 and 2, respectively (p < 0.03). Neutrophils BALF decreased significantly with both diets (pellets: 40.2 (24.4) to 20.1 (11.0) %; steamed hay 30.9 (20.2) to 25.7 (17.6) %; LMM, p = 0.006). Main Limitations: A small number of horses in a research setting. Dust was not measured in the stalls. Conclusions: Clinical scores, lung function and BALF inflammation of horses with SEA improved with steamed hay and pellets, but the effect on lung function was more pronounced with pellets.

Understanding the importance of intestinal microbiota in horses and the factors influencing its composition have been the focus of many studies over the past few years. Factors such as age, diet, antibiotic administration, and geographic location can affect the gut microbiota. The intra- and inter-individual variability of fecal microbiota in horses complicates its interpretation and has hindered the establishment of a clear definition for dysbiosis. Although a definitive causal relationship between gut dysbiosis in horses and diseases has not been clearly identified, recent research suggests that dysbiosis may play a role in the pathogenesis of various conditions, such as colitis and asthma. Prebiotics, probiotics, and fecal microbiota transplantation to modulate the horse’s gastrointestinal tract may eventually be considered a valuable tool for preventing or treating diseases, such as antibiotic-induced colitis. This article aims to summarize the current knowledge on the importance of intestinal microbiota in horses and factors influencing its composition, and also to review the published literature on methods for detecting dysbiosis while discussing the efficacy of gut microbiota manipulation in horses.

Although horses with asthma share similar clinical signs, the heterogeneity of the disease in terms of severity, triggering factors, inflammatory profile, and pathological features has hindered our ability to define biologically distinct subgroups. The recognition of phenotypes and endotypes could enable the development of precision medicine, including personalized, targeted therapy, to benefit affected horses. While in its infancy in horses, this review outlines the phenotypes of equine asthma and discusses how knowledge gained from targeted therapy in human medicine can be applied to evaluate the potential opportunities for personalized medicine in equine asthma and to suggest avenues for research to advance this emerging field.

Abstract Background Altered innervation structure and function contribute to airway hyperresponsiveness in human asthma, yet the role of innervation in airflow limitation in asthma in horses remains unknown. Hypothesis To characterize peribronchial innervation in horses with asthma. We hypothesized that airway innervation increases in horses with asthma compared with controls. Animals Formalin‐fixed lung samples from 8 horses with severe asthma and 8 healthy horses from the Equine Respiratory Tissue Biobank. Ante‐mortem lung function was recorded. Methods Blinded case‐control study. Immunohistochemistry was performed using rabbit anti‐s100 antibody as a neuronal marker for myelinating and non‐myelinating Schwann cells. The number and cumulative area of nerves in the peribronchial region and associated with airway smooth muscle were recorded using histomorphometry and corrected for airway size. Results Both the number (median [IQR]: 1.87 × 10−5 nerves/μm2 [1.28 × 10−5]) and the cumulative nerve area (CNA; 1.03 × 10−3 CNA/μm2 [1.57 × 10−3]) were higher in the peribronchial region of horses with asthma compared with controls (5.17 × 10−6 nerves/μm2 [3.76 × 10−6], 4.14 × 10−4 CNA/μm2 [2.54 × 10−4], Mann‐Whitney, P = .01). The number of nerves within or lining airway smooth muscle was significantly higher in horses with asthma (4.47 × 10−6 nerves/μm2 [5.75 × 10−6]) compared with controls (2.26 × 10−6 nerves/μm2 [1.16 × 10−6], Mann‐Whitney, P = .03). Conclusions and Clinical Importance Asthma in horses is associated with greater airway innervation, possibly contributing to airway smooth muscle remodeling and exacerbating severity of the disease.

Bacterial imbalances are observed in intestinal diseases and fecal microbiota transplantation (FMT) has been used to restore the intestinal microbiota of horses. However, there is evidence that the current methods proposed for FMT in horses have limited efficacy. The objective of this study was to concentrate the bacteria present in the donor stool by centrifugation, and to test the effect in horses with antibiotic-induced dysbiosis. One healthy 11-year-old horse was selected as a fecal donor and 9 horses were given trimethoprim sulfadiazine (TMS) for 5 days to induce dysbiosis. Horses received either a concentrated FMT (cFMT, n = 3), fresh unconcentrated FMT (fFMT, n = 3), or 10% glycerol solution (vehicle, VEH, n = 3) by nasogastric tube for 3 days. Fecal samples were collected on Days 0, 4, 9, 11, and 21 for microbiota analysis (Illumina sequencing). The TMS significantly changed the bacterial composition of horses' feces (D0 versus D4). The composition of the cFMT and fFMT recipient horses was significantly different after transplantation compared to after antibiotic-induced dysbiosis (D4 versus D11), whereas the microbiota of the vehicle recipients was not, indicating that both protocols induced transient changes. However, preparation of FMT solutions markedly changed the original composition present in the donor's feces, with significant enrichment of Escherichia genus in the cFMT. Individual susceptibility to restoration of the microbiota was observed in horses, similar to what is known for other species. Our results suggest that concentrating bacteria should not be recommended in preparation of FMT solutions and that further research is required to improve current methods recommended to perform FMT in horses.

BACKGROUND The amount of thrombin generated reflects the endogenous thrombin potential (ETP), which depends on the balance of pro- and anticoagulant factors. The calibrated automated thrombogram (CAT) allows for the direct measurement of thrombin generation during the clotting process. OBJECTIVES (1) To describe the results of the CAT assay in horses, (2) to establish intra-assay and intra- and interindividual variation of thrombin generation in healthy horses, and (3) to compare in vitro low-molecular-weight heparin (LMWH) sensitivity between healthy and sick horses. The hypothesis for the last objective is that inhibition of thrombin generation in sick horses requires higher heparin concentrations. METHODS The plasma of 10 healthy mixed breed horses was used for the determination of normal thrombin generation parameters (lag time, time to peak, peak thrombin concentration, and ETP). Five of the healthy horses were compared with five horses with systemic inflammatory response syndrome (SIRS). In vitro heparin sensitivity was determined using LMWH. RESULTS The intra-assay variation was small (<5%) for all parameters. Relatively large intra- and interindividual variation were observed in healthy horses. Four of the five sick horses with SIRS had a thrombogram compatible with a hypercoagulable state. The in vitro heparin sensitivity test suggested decreased sensitivity to LMWH in hypercoagulable states. CONCLUSIONS The CAT assay could detect coagulopathy in horses. In vivo experiments are needed to confirm that it can be used to monitor responses to LMWH therapy.

Potomac horse fever (PHF) is an acute and potentially fatal enterotyphlocolitis of horses with clinical signs that include anorexia, fever, diarrhea, and laminitis. Its incidence is increasing despite a commercially available vaccine. PHF is caused by Neorickettsia risticii, and the recently rediscovered and classified N. findlayensis. PHF diagnosis is currently accomplished using serology or nested PCR. However, both methods cannot distinguish the two Neorickettsia species that cause PHF. Further, the current N. risticii real-time PCR test fails to detect N. findlayensis. Thus, in this study, two Neorickettsia species-specific real-time PCR assays based on Neorickettsia ssa2 and a Neorickettsia genus-specific real-time PCR assay based on Neorickettsia 16S rRNA gene were developed. The ssa2 real-time PCR tests differentiated N. findlayensis from N. risticii in the field samples for which infection with either species had been verified using multiple other molecular tests and culture isolation, and the 16S rRNA gene real-time PCR detected both Neorickettsia species in the samples. These tests were applied to new field culture isolates from three Canadian provinces (Alberta, Quebec, Ontario) and Ohio as well as archival DNA samples from suspected PHF cases to estimate the prevalence of N. findlayensis in different geographic regions. The results suggest that N. findlayensis frequently causes PHF in horses in Alberta and Quebec. The development of these tests will allow rapid, sensitive, and specific diagnosis of horses presenting with clinical signs of PHF. These tests will also enable rapid and targeted treatment and help develop broad-spectrum vaccines for PHF.

OBJECTIVE:To quantify dectin-1 expression in bronchoalveolar lavage fluid (BALF), create polyclonal antibodies against equine dectin-1 and localize it in tissues, and quantify fungal exposure in pastured and stabled asthmatic and nonasthmatic horses. SAMPLES:BALF samples from 6 controls and 6 horses with severe asthma. Stored lung and nasal wash samples. PROCEDURES:Dectin-1 expression was quantified by quantitative PCR (qPCR). Purified peptide from equine dectin-1 was used to generate polyclonal antibodies and was confirmed with immunological testing. Fungal exposure was quantified in BALF samples by counting fungal-like intracellular particles in phagocytic cells, by qPCR quantification of the "universal" 18S rRNA fungal gene, and by quantifying 36 specific fungi in equine and dust samples using qPCR assays. RESULTS:Equine dectin-1 was localized in tissues and cells, and functional isoforms were upregulated significantly in BALF after stabling. Pastured horses from both groups had low levels of fungi in BALF, and there was a significant increase in some specific fungi, most notably for Eurotium amstelodami, Wallemia sebi, and Aspergillus niger after stabling. However, stabled asthmatic horses had fewer phagocytized particles, less 18S rRNA signal, and fewer specific fungi compared to nonasthmatic horses. CLINICAL RELEVANCE:Stabling increases exposure to fungi, but asthmatic horses had fewer fungi reaching their lower airways, presumably resulting from congestion and narrowing of the airways. Exposure to fungi could contribute to airway inflammation by increasing dectin-1 functional isoforms, and exposure to indoor molds should be avoided.

This study describes the clinicopathological findings, diagnostic approach, treatment, and factors associated with non-survival of diarrheic horses admitted to 4 Canadian university teaching hospitals between 2015 and 2019. A total of 300 horses, ≥1-year-old, with acute diarrhea were included and represented 1.6% (300/18 481; range: 0.7 to 3%) of admissions during that period, 70% of the horses survived to discharge. Testing for enteropathogens was limited to a single fecal culture for Salmonella spp. in most cases. An enteropathogen was identified in 14% (42/300) of the horses, but in the hospital with higher testing rates enteropathogens were detected in 29% (16/55) of cases. Neorickettsia risticii was the pathogen most frequently detected (31%, 32/102). Antimicrobial drugs and plasma were administered to 57 and 8% of the cases, respectively. Laminitis occurred in 24/298 (8%) of the horses. A multivariable regression model identified an association between non-survival of diarrheic horses and colic signs, increased heart rate, packed cell volume, creatinine concentration, and decreased total protein concentration. A standardized approach for pathogen detection in diarrheic horses is not consistent among Canadian veterinary teaching hospitals, and testing for known pathogens is limited. Signs of colic, severe dehydration, endotoxemia, and hypoproteinemia are associated with non-survival of diarrheic horses.

Published studies describing the effects of bilateral radiographic projections on the detection of equine pulmonary lesions are currently lacking. The objectives of this retrospective, single center, observational study were to compare unilateral and bilateral thoracic radiographic projections for the detection of pulmonary lesions in a group of horses. Based on their clinical diagnosis, 167 adults and foals with bilateral thoracic radiographs were classified as having pneumonia (n = 88), inflammatory or diffuse pulmonary disease (n = 72), and pulmonary masses (n = 7). After an initial interrater repeatability test, right-to-left and left-to-right projections were anonymized and independently interpreted by a radiologist blinded to the clinical diagnosis. Scores were attributed for each pattern/lesion (alveolar, interstitial, bronchial, nodules/masses, cavitary lesions) and each quadrant. Agreement between scores from each projection was evaluated with Bland-Altman plots. Lesions identified on one side but not on the contralateral projection were considered discordant. There was no preferential lateralization of pulmonary lesions. The prevalence of discordance was 14.4%, 9.0%, and 4.2% for alveolar pattern, nodules/masses, and cavitary lesions, respectively. Up to nine horses (10.2%) with pneumonia could have been misdiagnosed. A pulmonary mass would have been missed in one case. For inflammatory or diffuse disease, discordance was slight, and the addition of contralateral projections had no impact on radiographic interpretation. In conclusion, in horses with pneumonia or neoplasia, bilateral projections, or adding at least one contralateral caudoventral view, increased the probability of identifying pulmonary lesions. In horses with inflammatory or diffuse disease, bilateral thoracic radiography provided no additional benefit.