BackgroundPheochromocytoma is the most common adrenal medullary neoplasm of domestic animals, but it is rare in horses. Antemortem diagnosis in horses is difficult, with clinical signs often being vague or non‐specific.ObjectiveThe objective of this study was to describe the clinical, laboratory, and pathologic findings of pheochromocytoma in horses.AnimalsThirty‐seven horses diagnosed with pheochromocytoma based on postmortem examination from 2007 to 2014.MethodsRetrospective case series.ResultsPheochromocytoma was identified in 37/4094 horses during postmortem examination. Clinical signs consistent with pheochromocytoma had been observed antemortem in only 7 cases, with the remainder being incidental findings. Colic was the most common presenting complaint (13 of 37 cases) and tachycardia was noted in 95% of cases (median heart rate of 86 bpm in clinical cases). Hyperlactatemia (median, 4.9 mmol/L) and hyperglycemia (median, 184 mg/dL) were the most common clinicopathologic abnormalities. Hemoperitoneum caused by rupture of pheochromocytoma was noted in 4/7 clinical cases. Concurrent endocrine abnormalities (eg, thyroid adenoma, adrenal hyperplasia, pituitary pars intermedia hyperplasia or adenoma, parathyroid C‐cell carcinoma) were found in 27/37 horses, with 8/37 horses having lesions consistent with multiple endocrine neoplasia syndrome as described in humans.ConclusionsPheochromocytoma was diagnosed in 0.95% of horses presented for necropsy. The majority of these were incidental findings, but pheochromocytoma was thought to contribute to clinical findings in 19% of cases, and multiple endocrine neoplasms were commonly seen. Usually an incidental finding at necropsy, pheochromocytoma may cause acute death from intraperitoneal exsanguination and should be considered in horses presenting with colic, tachycardia, and hemoperitoneum.

BackgroundAssessment of blood compatibility, typically by tube agglutination (TUBE) and hemolysis crossmatch or, less commonly, by blood typing and alloantibody screening, often is performed before blood transfusion in horses. In contrast, gel column (GEL) and immunochromatographic strip (STRIP) techniques are preferred for compatibility testing in dogs and cats.ObjectiveTo determine the accuracy of novel and standard crossmatch and typing methods.AnimalsThirty‐eight healthy horses, previously blood typed and alloantibody screened.Methods TUBE and GEL crossmatches were performed on 146 different recipient‐donor pairs with 56 incompatible TUBE crossmatches. Crossmatches were compared by nonparametric area under the curve of receiver operating characteristic (AUC‐ROC) analyses. Horses also were blood typed by the novel immunochromatographic Ca typing STRIP.ResultsCompared to TUBE crossmatch, GEL had excellent accuracy for agglutination (AUC‐ROC = 0.903), but marginal accuracy for hemolysis (AUC‐ROC = 0.639). Compared to macroscopic TUBE, microscopic TUBE had excellent accuracy for agglutination (AUC‐ROC = 0.912). The predicted crossmatch compatibility based on blood type and alloantibody assay showed excellent accuracy compared to TUBE and GEL (AUC‐ROC = 0.843 and 0.897, respectively). However, there were more recipient‐donor pairs identified as incompatible by both TUBE and GEL than predicted by blood type and antibody screen, suggesting the presence of unidentified alloantibodies. A Ca typing STRIP exhibited 100% sensitivity and specificity for the 35 Ca+ and 3 Ca‐ horses tested.Conclusions and Clinical RelevanceGel column crossmatch and Ca typing immunochromatographic strip are simple and accurate methods to evaluate clinical blood compatibility.

BackgroundCalculation of desired whole blood transfusion volume relies on an estimate of an animal's circulating blood volume, generally accepted to be 0.08 L/kg or 8% of the animal's body weight in kilograms.ObjectiveTo use packed cell volume before and after whole blood transfusion to evaluate the accuracy of a commonly used equation to predict packed cell volume after transfusion in small ruminants and South American camelids; to determine the nature and frequency of adverse transfusion reactions in small ruminants and camelids after whole blood transfusion.AnimalsFifty‐eight small ruminants and 22 alpacas that received whole blood transfusions for anemia.MethodsRetrospective case series; medical record review for small ruminants and camelids that received whole blood transfusions during hospitalization.ResultsMean volume of distribution of blood as a fraction of body weight in sheep (0.075 L/kg, 7.5% BW) and goats (0.076 L/kg, 7.6% BW) differed significantly (P < 0.01) from alpacas (0.103 L/kg, 10.3% BW). Mild transfusion reactions were noted in 16% of transfusions.Conclusions and Clinical RelevanceThe generally accepted value of 8% for circulating blood volume (volume of distribution of blood) is adequate for calculation of transfusion volumes; however, use of the species‐specific circulating blood volume can improve calculation of transfusion volume to predict and achieve desired packed cell volume. The incidence of transfusion reactions in small ruminants and camelids is low.

Background Prognosis associated with lymphoma in horses is poorly characterized, and treatment is often palliative. Long‐term outcome after chemotherapy for horses with lymphoma is not well documented. Objective To report long‐term outcome of horses with lymphoma treated with chemotherapy. Animals Fifteen equids. Methods Retrospective case series. Medical record search and call for cases on the ACVIM listserv for horses treated with chemotherapy for lymphoma. Results Fifteen cases with adequate data were identified. Complete remission was achieved in 5 horses (33.3%), partial response was achieved in 9 equids (60%), and stable disease was achieved in 1 horse. Overall response rate was 93.3% (14/15). Overall median survival time was 8 months (range, 1‐46 months). Nine horses experienced a total of 14 adverse effects attributable to chemotherapy. Adverse effects were graded according to the Veterinary Cooperative Oncology Group common terminology criteria for adverse events grading system (grade 1 alopecia, n = 2; grade 1 neutropenia, n = 2; grade 1 lymphopenia, n = 3; grade 1 lethargy, n = 1; grade 2 neurotoxicity, n = 1; grade 2 colic, n = 1; grade 1 hypersensitivity, n = 1; grade 2 hypersensitivity, n = 2; grade 5 hypersensitivity, n = 1). Higher grade adverse effects most commonly were associated with doxorubicin administration (n = 4), including 1 horse that died 18 hours post‐administration. Conclusions and Clinical Importance Chemotherapy can be used successfully for treatment of horses with lymphoma. Adverse effects, most commonly mild, occurred in approximately two‐thirds of treated horses.

Background Antemortem definitive diagnosis of lymphoma in horses is often difficult. Thymidine kinase 1 (TK1) assay is a potentially useful biomarker for lymphoma in horses. Hypothesis/objectives To report the clinical performance of a commercially available TK1 assay for diagnosis of lymphoma in horses. We hypothesized that there would be no association between serum TK1 activity and a diagnosis of lymphoma in horses. Animals Forty‐two hospitalized horses, 14 with a definitive diagnosis of lymphoma, 4 with other neoplasia, and 24 with inflammatory disease. Methods Retrospective medical record review, groups were compared via Kruskal‐Wallis and Mann‐Whitney tests, and logistic regression was performed. Results Median (range) TK1 was 3 U/L (0.4‐17.7 U/L) in horses with lymphoma and 3.9 U/L (0.8‐94 U/L) in horses without lymphoma (P = .59). There was no significant difference in total protein between horses with and without lymphoma (6.6 g/dL [5.5‐8.3 g/dL] vs 6.6 g/dL [4.7‐10.4 g/dL]; P = .83). There was no significant difference in fibrinogen between horses with and without lymphoma (447 [100‐1364] mg/dL vs 433 [291‐2004] mg/dL; P = .47). On logistic regression, serum TK1 activity was not associated with a diagnosis of lymphoma (odds ratio, 0.97; 95% confidence interval, 0.9‐1.05, P = .48). Conclusion and Clinical Importance Serum TK1 values were not predictive of lymphoma diagnosis in this cohort of horses.

Background: Acute colitis is a serious cause of morbidity and death in horses. Recent studies have compared clinical features of coronavirus and salmonellosis, but no study has compared clinical features of enteric salmonellosis, coronavirus, and neorickettsiosis. Hypothesis/Objectives: To identify risk factors for laminitis and nonsurvival to discharge in horses with enteric salmonellosis, coronavirus, or neorickettsiosis. Animals: Eighty-five horses hospitalized for acute colitis from 2011 to 2019. Methods: Retrospective case series. Medical record review (2011-2019) of adult (≥2 years) horses with colitis. Primary outcomes were laminitis and survival to discharge. Multivariable logistic regression was performed to assess association between variables and the development of laminitis. Stepwise Cox regression was performed to assess association between variables and survival. Results: Seventeen of 85 (20%) horses developed laminitis during hospitalization.Neorickettsiosis cases (11/26, 42%) were more likely to develop laminitis than coronavirus (0/16, 0%) cases (odds ratio [OR] 24.48; 95% confidence interval [CI]: 1.33-451.74, P = .03). There was no significant difference in laminitis between salmonellosis and neorickettsiosis cases (OR 0.27; 95%

Idiopathic hypocalcemia in Thoroughbred (TB) foals causes tetany and seizures and is invariably fatal. Based upon the similarity of this disease with human familial hypoparathyroidism and occurrence only in the TB breed, we conducted a genetic investigation on two affected TB foals. Familial hypoparathyroidism was identified, and pedigree analysis suggested an autosomal recessive (AR) mode of inheritance. We performed whole-genome sequencing of the two foals, their unaffected dams and four unaffected, unrelated TB horses. Both homozygosity mapping and an association analysis were used to prioritize potential genetic variants. Of the 2,808 variants that significantly associated with the phenotype using an AR mode of inheritance (P<0.02) and located within a region of homozygosity, 1,507 (54%) were located in a 9.7 Mb region on chr4 (44.9-54.6 Mb). Within this region, a nonsense variant (RAPGEF5 c.2624C>A,p.Ser875*) was significantly associated with the hypoparathyroid phenotype (P allelic = 0.008). Affected foals were homozygous for the variant, with two additional affected foals subsequently confirmed in 2019. Necropsies of all affected foals failed to identify any histologically normal parathyroid glands. Because the nonsense mutation in RAPGEF5 was near the C-terminal end of the protein, the impact on protein function was unclear. Therefore, we tested the variant in our Xenopus overexpression model and demonstrated RAPGEF5 loss-of-function. This RAPGEF5 variant represents the first genetic variant for hypoparathyroidism identified in any domestic animal species.

Third‐degree atrioventricular block (AVB) and primary inflammatory myocarditis are uncommon findings in horses. The horse of this report presented for collapse at rest and was found to have multiple cardiac arrhythmias, most notably 3rd‐degree AVB. The horse was subsequently diagnosed with eosinophilic myocarditis on necropsy, a rare form of myocarditis not previously reported in horses. Despite extensive testing, an etiologic agent could not be identified, illustrating the difficulty in identifying a specific cause of myocarditis in horses.