Increased proportion of apoptotic cells in cat kidney tissues infected with feline morbillivirus.

In order to study potential pathogenic mechanisms of feline morbillivirus (FeMV) in infected kidney cells, we performed a terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay and an immunofluorescence assay (IFA) with an anti-FeMV P protein antibody on a total of 38 cat kidney tissues, 12 of which were positive for FeMV. Among these samples, we detected significantly larger numbers of apoptotic cells in FeMV-positive tissues than in FeMV-negative tissues, and in these tissues, a substantial percentage of TUNEL-positive (TUNEL+) cells contained the FeMV P protein (mean, 37.4; range, 17.4-82.9), suggesting that induction of apoptosis may be an important mechanism for pathological changes associated with FeMV infection in cat kidney tissues.

Pathology and Proteomics-Based Diagnosis of Localized Light-Chain Amyloidosis in Dogs and Cats.

Amyloidosis is classified according to the amyloid precursor protein, and accurate diagnosis of the amyloidosis type may guide appropriate treatment. Immunohistochemistry and Congo red staining are the most frequently used methods used to distinguish types of amyloidosis, but problems with specificity and sensitivity indicate the need for an alternative diagnostic method. In this study, we evaluated laser microdissection-liquid chromatography-tandem mass spectrometry (LMD-LC-MS/MS) for the diagnosis of amyloid light-chain (AL) amyloidosis in animals. Plasmacytomas with amyloid deposits from 15 dogs and 2 cats were subjected to Congo red staining with or without potassium permanganate pretreatment, immunohistochemistry for kappa and lambda light chains, and LMD-LC-MS/MS. Congo red staining was diagnostic in 12 of 17 cases based on resistance to potassium permanganate pretreatment, but in 5 of 17 cases the pretreatment unexpectedly reduced Congo red staining or abrogated the birefringence and a definitive diagnosis could not be reached. Immunohistochemistry detected kappa or lambda light chains in 6 of 17 cases. With LMD-LC-MS/MS, immunoglobulin lambda light chain was detected in all 17 cases. The amyloid signature proteins ApoA-I, ApoA-IV, and ApoE were detected in 9, 1, and 3 of the 15 canine cases by LMD-LC-MS/MS, but not in the feline cases. In conclusion, LMD-LC-MS/MS consistently determined the amyloid type in all examined specimens, while Congo red staining after potassium permanganate treatment and immunohistochemistry were less sensitive tests.

Long-term outcomes of 54 dogs with tracheal collapse treated with a continuous extraluminal tracheal prosthesis.

OBJECTIVE: To describe the surgical placement of a continuous extraluminal tracheal prosthesis (CETP) and report the subsequent postoperative clinical outcomes in dogs with tracheal collapse. STUDY DESIGN: Retrospective case series. ANIMALS: Fifty-four dogs. METHODS: Medical records of dogs in which cervical and/or thoracic inlet tracheal collapse was diagnosed and treated by placement of a CETP between 2010 and 2017 were reviewed to evaluate postoperative complications, changes in respiratory function, and survival. Histological examinations of tracheal tissues performed in 2 dogs at 51 and 57 months after surgery were also reviewed. RESULTS: Fifty-three (98%) dogs survived to discharge. Postoperative complications included laryngeal paralysis (1 dog), disseminated intravascular coagulation (1 dog), and recurrent tracheal collapse (2 dogs). None of the dogs exhibited clinical evidence of tracheal necrosis. Preoperative dry, harsh cough resolved in 87% of the dogs after surgery. Goose honking cough was resolved in 25 of 26 (96%) dogs. Median follow-up time was 30 months (range, 16 days to 76 months). The survival rate at 36 months was 86% (CI: 75%-96%). On histological examination in 2 dogs, the tracheal tissue surrounding the prosthesis was well preserved and without evidence of chronic inflammation. CONCLUSION: Continuous extraluminal tracheal prosthesis placement in dogs with tracheal collapse resulted in low postoperative complication rates and good long-term outcomes. CLINICAL SIGNIFICANCE: Continuous extraluminal tracheal prosthesis placement provides a viable alternative surgical option for managing dogs with tracheal collapse.

Overexpression of heart-specific small subunit of myosin light chain phosphatase results in heart failure and conduction disturbance.

Mutations in genes encoding components of the sarcomere cause cardiomyopathy, which is often associated with abnormal Ca2+ sensitivity of muscle contraction. We have previously shown that a heart-specific myosin light chain phosphatase small subunit (hHS-M21) increases the Ca2+ sensitivity of muscle contraction. The aim of the present study was to investigate the function of hHS-M21 in vivo and the causative role of abnormal Ca2+ sensitivity in cardiomyopathy. We generated transgenic mice with cardiac-specific overexpression of hHS-M21. We confirmed that hHS-M21 increased the Ca2+ sensitivity of cardiac muscle contraction in vivo, which was not followed by an increased phosphorylation of myosin light chain 2 isoforms. hHS-M21 transgenic mice developed severe systolic dysfunction with myocardial fibrosis and degeneration of cardiomyocytes in association with sinus bradycardia and atrioventricular conduction defect. The contractile dysfunction and cardiac fibrosis were improved by treatment with the Rho kinase inhibitor fasudil. Our findings suggested that the overexpression of hHS-M21 results in cardiac dysfunction and conduction disturbance via non-myosin light chain 2 phosphorylation-dependent regulation. NEW & NOTEWORTHY The present study is the first to develop mice with transgenic overexpression of a heart-specific myosin light chain phosphatase small subunit (hHS-M21) and to examine the effects of hHS-M21 on cardiac function. Elevation of hHS-M21 induced heart failure with myocardial fibrosis and degeneration of cardiomyocytes accompanied by supraventricular arrhythmias.

Evaluation of endothelialization in the center part of graft using 3 cm vascular grafts implanted in the abdominal aortae of the rat.

In order to develop small-diameter vascular grafts, it is necessary to evaluate endothelialization, especially, in the center part at early stage. For implantation of vascular grafts of 1 cm in length to abdominal aortae of rat, endothelial cells can be formed easily by stretching anastomosis. We evaluated the endothelialization in the center part of vascular grafts by implanting vascular grafts using transgenic (TG) silk fibroin (SF) of 3 cm in length. Vascular grafts were prepared 1.5 mm in diameter and 1 and 3 cm in length using wild type (WT) SF and TG SF by braiding structure, respectively. The grafts were removed after 2 weeks or 3 months and evaluated pathologically. Endothelialization was not confirmed totally after 3 months of implantation. However, endothelialization in the center part of grafts was significantly higher in TG SF than in WT SF. No significant difference was found regarding tissue infiltration and internal diameter. The TG SF revealed migration of the endothelial cells into the center part of the vessels at the early stage. Also, tissue infiltration and remodeling is expected using SF. The 3 cm length vascular grafts can be evaluated as a new experimental system.

Molecular basis for clinical heterogeneity in inherited cardiomyopathies due to myopalladin mutations.

Abnormalities in Z-disc proteins cause hypertrophic (HCM), dilated (DCM) and/or restrictive cardiomyopathy (RCM), but disease-causing mechanisms are not fully understood. Myopalladin (MYPN) is a Z-disc protein expressed in striated muscle and functions as a structural, signaling and gene expression regulating molecule in response to muscle stress. MYPN was genetically screened in 900 patients with HCM, DCM and RCM, and disease-causing mechanisms were investigated using comparative immunohistochemical analysis of the patient myocardium and neonatal rat cardiomyocytes expressing mutant MYPN. Cardiac-restricted transgenic (Tg) mice were generated and protein-protein interactions were evaluated. Two nonsense and 13 missense MYPN variants were identified in subjects with DCM, HCM and RCM with the average cardiomyopathy prevalence of 1.66%. Functional studies were performed on two variants (Q529X and Y20C) associated with variable clinical phenotypes. Humans carrying the Y20C-MYPN variant developed HCM or DCM, whereas Q529X-MYPN was found in familial RCM. Disturbed myofibrillogenesis with disruption of α-actinin2, desmin and cardiac ankyrin repeat protein (CARP) was evident in rat cardiomyocytes expressing MYPN(Q529X). Cardiac-restricted MYPN(Y20C) Tg mice developed HCM and disrupted intercalated discs, with disturbed expression of desmin, desmoplakin, connexin43 and vinculin being evident. Failed nuclear translocation and reduced binding of Y20C-MYPN to CARP were demonstrated using in vitro and in vivo systems. MYPN mutations cause various forms of cardiomyopathy via different protein-protein interactions. Q529X-MYPN causes RCM via disturbed myofibrillogenesis, whereas Y20C-MYPN perturbs MYPN nuclear shuttling and leads to abnormal assembly of terminal Z-disc within the cardiac transitional junction and intercalated disc.

Pimobendan improves right ventricular myocardial contraction and attenuates pulmonary arterial hypertension in rats with monocrotaline-induced pulmonary arterial hypertension.

PURPOSE: The present study aimed to evaluate the therapeutic effect of pimobendan treatment for pulmonary hypertension (PH) in rats administered monocrotaline (MCT). METHODS: Fifty-four 12-week-old male Sprague-Dawley rats were injected with monocrotaline or saline solution. Serial echocardiography and right ventricular systolic pressure (RVSP) measurement via a cardiac catheter were performed. After injection of MCT, rats received oral pimobendan (MCT/pimobendan group) or no treatment (MCT group) until undergoing echocardiography and cardiac catheter insertion. RESULTS: Right ventricular systolic pressure in the MCT/pimobendan group was lower than that in the MCT group at 6 weeks. Right ventricle free wall (RVFW) myocardial systolic velocity (Sm) in the MCT group showed a reduction compared with the saline group at 2 weeks. RVFW Sm in the MCT/pimobendan group was preserved as compared with the saline group at 2 weeks. RVFW Tei index in the MCT/pimobendan group showed a reduction compared with the saline group and the MCT group at 2 weeks. Echocardiography in the MCT/pimobendan group showed improvement compared with MCT rats. CONCLUSIONS: Both a reduction in RVSP and improvement in myocardial contraction were demonstrated with administration of pimobendan in rats with PH induced by MCT. Echocardiography evaluation of systolic function seems to be useful for monitoring excess administration of pimobendan.

Pathological features of primary cardiac myxoid tumour in dogs: a review of 11 cases (2002-2022).

This report documents the pathological features of primary cardiac myxoid tumour (MT) in 11 dogs. Macroscopically, all the tumours were located in the tricuspid valve (TV), its septal leaflet being predominantly affected. Therefore, it appears that the TV is the most common site of occurrence for cardiac MT in dogs. Two gross anatomical types of canine valvular MT were evident. Seven of the 11 tumours were round or oval with a smooth or gently lobulated and glistening surface, while the other four were gelatinous, multilobulated and polypoid, with an irregular surface. Microscopically, in nine cases the tumours had an abundant myxoid matrix within which elongated spindle-shaped cells with no remarkable cytological atypia were sparsely embedded, suggesting a benign character (ie, myxoma). In the other two cases the tumours consisted of variably dense, haphazardly arranged, interlacing streams of anaplastic spindle-shaped or polygonal cells containing many mitotic figures, indicative of a malignant form of myxoma (ie, myxosarcoma). Isolated or clustered collections of myxoma cells (eg, cords, rings, syncytia) characteristic of human atrial myxoma were only rarely evident or lacking in all 11 cases, indicating that rarity or absence of such structural features may be specific to valvular MTs. Immunohistochemical findings were indicative of smooth muscle differentiation of the neoplastic cells. Tumour embolization to the intrapulmonary arteries and/or tumour implantation on the endocardium of the right heart chambers was evident only in the four cases of irregular-surfaced MT.

The anatomical basis of third-degree atrioventricular block in dogs with atrioventricular valve endocardiosis.

The cardiac conduction system was examined histologically in 13 canine cases of atrioventricular (AV) valve endocardiosis with third-degree AV block. In all cases, gross examination revealed marked thickening and distortion of the base of the central fibrous body (CFB) and varying degrees of endocardial thickening of the upper portion of the ventricular septum (VS) as well as marked thickening of the mitral and tricuspid valve leaflets due to myxomatous degeneration. Microscopically, the thickened and distorted CFB had encased or trapped, either partly or totally, the underlying penetrating and branching portions of the AV bundle. The myxomatous and/or fibrofatty tissue, which had proliferated at the base of the extensive CFB, protruded into or encroached on the AV bundle, causing severe (51-75%) to very severe (76% or more) reduction of the conduction fibres. The upper portions of the left and right bundle branches were involved in the endocardial thickening due to degenerative and fibrotic changes at the uppermost VS; however, both bundle branches were much less severely affected than the AV bundle, the degree of reduction of the conduction fibres ranging from mild (25% or less) to moderate (26-50%). These observations suggest that the sites most vulnerable to lesions in the AV conduction system are the penetrating and branching portions of the AV bundle, which would represent the anatomical basis for third-degree AV block in canine cases of AV valve endocardiosis.

Comparative effects of amlodipine and benazepril on left atrial pressure in dogs with experimentally-induced mitral valve regurgitation.

BACKGROUND: One of the purposes of treatment for dogs with mitral regurgitation (MR) is lowering left atrial pressure (LAP). There has been few study of the amlodipine in dogs with MR and amlodipine's effect on LAP has not been fully evaluated in a quantitative manner because of difficulties in directly measuring LAP. The objective of our study was to compare the short-term effects of amlodipine (0.2 mg/kg PO q12h) vs benazepril (0.5 mg/kg PO q12h), on LAP and echocardiographic parameters in five beagle dogs with experimentally-induced MR. LAP of eight dogs that has own control were measured using radiotelemetry system at baseline and again on days 1, 2, 3, 4, 5, 6, 7 of the drug administration. RESULTS: Mean LAP decreased significantly after amlodipine (11.20 ± 4.19 mmHg vs 14.61 ± 3.81 mmHg at baseline, p < .01) but not after benazepril treatment (13.19 ± 3.47 mmHg, p > .05). LAP was lower after 7 days of amlodipine treatment than after 7 days of benazepril treatment. Significant reduction was seen for the first time 4 days after the administration amlodipine. The rate of the maximal area of the regurgitant jet signals to the left atrium area (ARJ/LAA) of the amlodipine treatment was significantly lower (p < .05) after 7 days compared to baseline. Other echocardiographic parameters did not change significantly. CONCLUSIONS: LAP was significantly decreased after amlodipine treatment in dogs with surgically-induced MR but not after benazepril treatment. Although this study did not focus on adverse effects, amlodipine may be an effective drug for helping the patients with acute onset of severe MR, such as rupture of chordae tendinae or end stage patients were the LAP is likely to be elevated. Additional studies in clinical patients with degenerative mitral valve disease and acute chordal rupture are warranted because the blood-pressure lowering effects of amlodipine can decrease renal perfusion and this can further activate the RAAS.

Histological Features of the Atrioventricular Conduction System in Cats with High-Grade Atrioventricular Block.

The histopathological features of the atrioventricular (AV) conduction system of nine cats with high-grade atrioventricular block (H-AVB) were compared with those of 22 cases of third-degree atrioventricular block (3-AVB). All nine H-AVB cats had experienced syncopal attacks of variable severity and had been diagnosed electrocardiographically. The AVB, which was permanent in all cases, had been observed for 15-1,981 days (average 663 days) before death. Histological examination of the AV conduction system revealed moderate (25-50%) or severe (>50%) loss of conduction fibres associated with fibrous replacement in the regions of the branching portion of the AV bundle and the upper portion of the left bundle branch in almost all of the H-AVB cases and in all the 3-AVB cases. Comparison of lesion severity in each region in the H-AVB and 3-AVB cases revealed that the branching portion of the AV bundle was less severely affected in the H-AVB cases. This finding might explain why the H-AVB cases had intermittent block of AV conduction while the 3-AVB cats had permanent block.

Cardiac Inflammatory Myofibroblastic Tumours in Three Young Adult Cats.

Inflammatory myofibroblastic tumours (IMTs) of the heart are described in three young adult cats (case 1, 3.5 years old; case 2, 2.7 years old; case 3, 1.8 years old) that died due to pleural and pericardial effusions resulting from congestive heart failure. At necropsy, cardiac masses, measuring 4 × 3 × 2.5 cm (case 1), 3.5 × 2.5 × 2 cm (case 2) and 3 × 2.5 × 2 cm (case 3), were located mainly in the right auricle (case 1) and atrial septum (cases 2 and 3) with variable extension into the surrounding tissue. Histologically, the atrial masses in all three cats comprised proliferation of spindloid mesenchymal cells and an inflammatory infiltrate of plasma cells and lymphocytes in variable amounts of myxoid background. The spindloid cells were uniform and characterized by pale eosinophilic cytoplasm and a vesicular nucleus containing prominent nucleoli, with no remarkable cytological atypia or mitotic activity. Immunohistochemically, these cells were positive for vimentin and α-smooth muscle actin in all cases, supporting a myofibroblastic origin. The histopathological and immunohistochemical features were consistent with those of IMTs in humans and animals. There have been no previous reports of such tumours occurring in the heart of the cat.

Glycogen storage disease in a young cat with heart failure.

An 8-month-old domestic short-haired female cat presented with acute tachypnea, poor growth, hypothermia, and lethargy. Thoracic radiography showed cardiomegaly with mild pleural effusion, and transthoracic echocardiography identified dilatation of both atria and left ventricular systolic dysfunction. Although clinical signs improved temporarily with treatment, the cat died of pulmonary edema 135 days after the first visit. At necropsy, the heart was grossly enlarged. Microscopic examination of the heart identified severe vacuolization of cardiac muscle cells in histologic sections stained with hematoxylin and eosin. Examination of periodic acid-Schiff stained preparations of formalin-fixed heart tissue disclosed coarse granules within vacuoles that disappeared on predigestion with diastase, indicating that they were glycogen. On the basis of these findings, a necropsy diagnosis of glycogen storage disease type II (Pompe disease) was made. This report is the first case of a young cat with clinical signs closely resembling infantile Pompe disease of humans.

The Anatomical Substrate for Sick Sinus Syndrome in Dogs.

The hearts of 28 dogs, clinically diagnosed as having symptomatic sick sinus syndrome (SSS), were examined post mortem, with a particular focus on the sinus node (SN) region. The affected dogs were divided into two groups according to the findings of ambulatory electrocardiography: 16 dogs with severe sinoatrial (SA) block and/or sinus arrest (group A) and 12 dogs with long sinus or atrial pauses due to SA block and/or sinus arrest accompanied by atrial tachyarrhythmias (group B). The most significant histopathological changes found in both SSS groups were extensive destruction of the SN characterized by depletion of nodal cells with fatty or fibrofatty replacement and interruption of contiguity between the SN and the surrounding atrial myocardium. Furthermore, in group B, the SN lesions were combined with fibrosis of the atrial myocardium. The results of this investigation improve our understanding of the close relationship between the electrocardiogram findings and pathological alterations in each group. Because most human cases of SSS are due to degenerative fibrosis of the SN, the loss and disappearance of nodal cells with a corresponding increase in fatty or fibrofatty tissue, may be specific to canine cases of SSS.

Effects of enfonidipine hydrochloride in dogs with experimental supraventricular tachyarrhythmia.

It is required not to increase the ventricular rate and to preserve the ventricular systolic function in treating supraventricular tachyarrhythmia (SVTA). The objective of this study is to investigate whether or not Efonidipine hydrochloride (EH), a T and L dual type Ca(2+) channel blocker, suppresses the increasing ventricular rate without reducing the ventricular systolic function using canine SVTA models by rapid atrial pacing (RAP) method. Clinically healthy fourteen beagles were used. The 14 dogs were randomly assigned to the EH-administered group (EH group, n=7) and non-EH-administered group (control group, n=7). The EH group was orally-administered EH at 5 mg/kg SID during RAP. On the other hand, the control group was applied RAP without oral administration of EH. Duration of RAP was for 3 weeks for both groups. The ventricular rate for the EH group was significantly lower than that for the control group. The left ventricular- fractional shortening for the control group declined significantly compared to baseline. Those for the EH group did not show any changes over time and were significantly higher than the control group. The ratio between pre-ejection period and ejection for the EH group were significantly lower than those of the control group. In conclusion, the study demonstrated that EH suppresses the increasing ventricular rate without reducing the ventricular systolic function in canine SVTA model. Therefore, EH is expected to become a new treatment for canine SVTA.

Invasion of Pheochromocytoma from the Caudal Vena Cava to the Right Ventricular Cavity in a Dog.

Pheochromocytomas are catecholamine-secreting tumors that are composed of neuroectoderm-derived chromaffin cells. An 8-year-old miniature dachshund with abdominal distension was diagnosed with a neuroendocrine tumor with invasion from the caudal vena cava to the right ventricular cavity. The dog died due to hypotensive shock from the vagal reflex, and on autopsy, an extra-adrenal pheochromocytoma (paraganglioma) was diagnosed in the caudal abdomen. At autopsy, the tumor plug of the caudal vena cava was confirmed. To the best of our knowledge, this is the first case report that echo-captured the extension of pheochromocytoma in the right ventricle and shows it in a figure and video file.

Pathological Features of Persistent Atrial Standstill Syndrome in Three Dogs.

The hearts of three dogs, clinically diagnosed as having persistent atrial standstill syndrome (PAS), were studied post mortem. The most significant gross findings in the hearts of all three dogs were dilatation and marked reduction in the thickness of both atrial walls. Histopathologically, all three had widespread progressive loss of the atrial myocardium with replacement by fatty or fibrofatty tissue, consistent with atrial myopathy. The lesion mainly affected the upper half of both atria and was more severe in the epimyocardium and midmyocardium than in the endomyocardium. On the basis of these observations, it is proposed that the atrial myopathy commences in the upper regions of both atria and progresses downwards, as has been demonstrated electrophysiologically in PAS in humans, and extends from the epicardium towards the endocardium.

Selective vacuolar degeneration in dystrophin-deficient canine Purkinje fibers despite preservation of dystrophin-associated proteins with overexpression of Dp71.

BACKGROUND: Respiratory support therapy significantly improves life span in patients with Duchenne muscular dystrophy; cardiac-related fatalities, including lethal arrhythmias, then become a crucial issue. It is therefore important to more thoroughly understand cardiac involvement, especially pathology of the conduction system, in the larger Duchenne muscular dystrophy animal models such as dystrophic dogs. METHODS AND RESULTS: When 10 dogs with canine X-linked muscular dystrophy in Japan (CXMD(J)) were examined at the age of 1 to 13 months, dystrophic changes of the ventricular myocardium were not evident; however, Purkinje fibers showed remarkable vacuolar degeneration as early as 4 months of age. The degeneration of CXMD(J) Purkinje fibers was coincident with overexpression of Dp71 at the sarcolemma and translocation of mu-calpain to the cell periphery near the sarcolemma or in the vacuoles. Immunoblotting of the microdissected fraction showed that mu-calpain-sensitive proteins such as desmin and cardiac troponin-I or -T were selectively degraded in the CXMD(J) Purkinje fibers. Utrophin was highly upregulated in the earlier stage of CXMD(J) Purkinje fibers, but the expression was dislocated when vacuolar degeneration was recognized at 4 months of age. Nevertheless, the expression of dystrophin-associated proteins alpha-, beta-, gamma-, and delta-sarcoglycans and beta-dystroglycan was well maintained at the sarcolemma of Purkinje fibers. CONCLUSIONS: Selective vacuolar degeneration of Purkinje fibers was found in the early stages of dystrophin deficiency. Dislocation of utrophin besides upregulation of Dp71 can be involved with this pathology. The degeneration of Purkinje fibers can be associated with the distinct deep Q waves in ECG and fatal arrhythmia seen in dystrophin deficiency.

Changes in copper, zinc and cadmium distributions in the liver of Formosan squirrels with characteristic high copper accumulation.

We discovered previously that Formosan squirrels (Callosciurus erythraeus) accumulate copper (Cu) in their livers at levels averaging 1700 µg per dry g (approximately 420 µg per wet g). In the current study, we investigated the relationship between Cu accumulation and hepatic injury, and we determined the distribution and chemical form of Cu in the liver supernatant. In particular, we explored the role of metallothionein in the liver supernatant. We observed no significant differences in hepatic Cu concentration between squirrels that showed pathological changes in the liver and those that did not. Serum alanine aminotransferase activity did not increase with increasing hepatic Cu concentration. These results suggest that abnormal Cu accumulation in the livers of Formosan squirrels does not induce severe hepatic injury. We found that 26.7% of the Cu in the liver was distributed to the supernatant, and only 11.0% of the Cu in the liver was bound to metallothionein, suggesting that metallothionein in the hepatic supernatant does not contribute to detoxification of excess Cu in Formosan squirrels.

Long-term management of high-grade atrioventricular block using cilostazol in a cat.

CASE SUMMARY: A 12-year-old neutered female domestic shorthair cat was admitted for syncope. Clinical signs and electrocardiography revealed high-grade atrioventricular (AV) block. Treatment with cilostazol ameliorated the clinical signs and arrhythmia. However, the high-grade AV block recurred on several occasions. After 640 days, the cat presented again with clinical deterioration owing to reoccurrence of the arrhythmia and it died 11 days later. Histopathological examination revealed a loss of conduction cells within the His bundle. RELEVANCE AND NOVEL INFORMATION: To our knowledge, this is the first report of high-grade AV block treated with cilostazol in a cat. Treatment with cilostazol prolonged survival for 650 days without pacemaker implantation. Histological findings suggested that the AV block was related to fibrosis of the impulse conduction system.