Excerpt from a letter from the Dept. of Veterinary Clinical Studies, University of Edinburgh

Thrombocytopenia

“This condition is certainly recognized in Cavaliers in a number of countries, but the significance is still under dispute.  Some investigators believe it is a harmless anomaly seen in CKCS (is they have fewer platelets, but they are large and work just as well as larger numbers of smaller ones), whereas other investigators believe it is a significant problem and many CKCS live on a knife-edge and may develop a clinically-apparent bleeding disorder at any time.  Clearly more work is needed, and again, this will require a prospective study whereby large numbers of Cavaliers are followed over a period of time.”

Virginia Luis Fuentes MA VetMB CertVR DVC MRCVS

Idiopathic, Asymptomatic Thrombocytopenia in Cavalier King Charles Spaniels:  11 Cases (1983 - 1993)

The medical records of 11 Cavalier King Charles spaniels with idiopathic, asymptomatic thrombocytopenia and large-to-giant platelets were identified from a 10-year retrospective search using the Veterinary Medical Data Base at Purdue University.  Eight of the dogs had been treated with various immunosuppressive drugs.  Six of the treated dogs remained thrombocytopenic, one was not reevaluated, and one developed a normal platelet count.  The underlying etiology of idiopathic, asymptomatic thrombocytopenia in Cavalier King Charles spaniels has not been identified, but this condition could represent a congenital macrothrombocytopenic disorder. J Am Anim Hosp Assoc 1997;33:411-5.

From the Journal of the American Animal Hospital Association

Linda E. Smedile, DVM

Doreen M. Houston, DVM, DVSc

Susan M. Taylor, DVM

Klaas Post, DVM, MVSc

Gene P. Searcy, DVM, PhD

From the Departments of Veterinary Internal Medicine (Smedile, Houston, Taylor, Post) and Veterinary Pathology (Searcy), Western College of Veterinary Medicine, University of Saskatchewan, 52 Campus Drive, Saskatoon, Saskatchewan S7N 5B4, Canada.

Doctor Smedile’s current address is the Broadview Animal Hospital, 34 Ten Rod Road, Rochester, New Hampshire  03867

Doctor Houston’s current address is Veterinary Medical Diets, Inc., 67 Watson Road S, Unit 3, Guelph, Ontario N1H 6H8, Canada.

Introduction

Idiopathic, asymptomatic thrombocytopenia and enlarged platelets have been reported in Cavalier King Charles spaniels (CKCSs). (1.2)  Eksell reported a 31% incidence of thrombocytopenia (defined as a manual platelet count of less than 100x109/L) in 102 clinically normal CKCSs. (1)   Enlarged platelets were identified in an investigation of 10 clinically normal CKCSs. (2)  Platelet size in this investigation was determined by measuring the diameter of 100 consecutive platelets per dog with an eyepiece graticule using ethylenediaminetetraacetic acid (EDTA)-treated blood smears.l  A bimodal platelet-size distribution was observed , with 44.5% of the platelets being normal size and 30% being twice normal size.  The manula platelet counts (greater than 200x109/L) in these cases were normal, but the automated platelet counts were low; the authors postulated that the enlarged platelets may have resulted in the erroneously low automated platelet counts.  Buccal mucosal bleeding times were measured in two cases and were normal. (2)  The objective of this study was to describe the clinical and clinicopathological features of CKCSs presented to veterinary teaching hospitals with idiopathic, asymptomatic thrombocytopenia.

Criteria for Selection of Cases

A 10-year retrospective search of medical records from 29 North American veterinary colleges was conducted using the Veterinary Medical Data Base at Purdue University.  Selection criteria included any CKCS with thrombocytopenia (defined as a platelet count of less than 200x109/L) (3) seen between January 1, 1983 and July 31, 1993.  Specific diagnostic codes included in the search were thrombocytopenia, thrombocytopenia purpura, congenital thrombocytopenia, thrombocytopenia due to infection, thrombocytopenia due to drug, autoimmune thrombocytopenia, and thrombocytopenia due to unknown etiology.  The universities identified by the search were contacted, and copies of the medical records were requested.

Results

Eighteen cases of thrombocytopenia in CKCSs were identified.  The medical records for 12 of these 18 cases were obtained and reviewed.  Eleven of the cases were determined to have idiopathic, asymptomatic thrombocytopenia.  The criteria for selection of this diagnosis were that the thrombocytopenia had no identified etiology and there was no evidence of abnormal hemostasis.  The 12^th case had been diagnosed with immune-mediated hemolytic anemia (IMHA) and experienced transient thrombocytopenia attributed to disseminated intravascular coagulation (DIC).  This case was not included in the retrospective study.

Details of age, sex, presenting complaint, initial platelet count, quantity of megakaryocytes in the bone marrow, and diagnoses made are summarized in Table 1.  The platelet counts ranged from 34 to 121x109/L (mean platelet count, 68.9x109/L).  Vaccination status was reported to be current in five cases and was unknown in six cases.  None of the cases were known to have been vaccinated within two months of the thrombocytopenia diagnosis.  At initial and subsequent evaluation, 10 of the cases had no history or clinical signs suggestive of a bleeding disorder.  Case no. 9 developed transient petechiation for five weeks after initial presentation (platelet count, less than 10x109/L).  Platelet counts were performed manually in nine cases, with an automated counter used in one case (case no. 8) and an estimation made from a blood smear in one cases (case no. 5).  In all 11 cases, the platelets were described as large or giant, based on subjective assessment of size on microscopic examination of air-dried EDTA whole blood smears.  In the initial complete blood cell counts, other cell lines were normal in all cases except case no. 8, which had a mild, nonregenerative anemia (hematocrit of 29%).  Bone-marrow cytology was performed in five cases, and the results are reported in Table 1.  Various additional diagnostic tests were performed to investigate the presenting complaints as well as to explore the etiology of thrombocytopenia [Table 2].  Abnormal test results are reported in the diagnosis column of Table 1.  All other test results were within normal limits.  No underlying etiology for the thrombocytopenia was identified in any one of the 11 cases.

Table 1

Idiopathic, Asymptomatic Thrombocytopenia in 11 Cavalier King Charles Spaniels

* NR=not reported

 F=female; M=male; FS=spayed female; MC=castrated male

 Quantity of megakaryocytes in the bone marrow; NE=not evaluated

Eight cases were treated with prednisone alone or in combination with other drugs for suspected immune-mediated thrombocytopenia (ITP) [Table 3].  Seven of the treated cases were reevaluated at various times for periods ranging from 10 days to three years.  The platelet counts in six of the cases fluctuated but remained below normal.  Case no. 9 had a normal platelet count (by estimation from a blood smear) three weeks after the addition of weekly gold salt (i.e., aurothioglucose; 1 mg/kg body weight) injections to the dog’s oral prednisone (0.5 mg/kg body weight, once daily) therapy.  Gold salts and prednisone were continued, and the platelet count fell to 105x109/L five months later; 10 months later, it was estimated to be within normal range.  After three months of medical therapy, a splenectomy was performed on case no. 3.  Histopathological examination of the spleen revealed extramedullary hematopoiesis and moderate lymphoid depletion.  The postsplenectomy platelet count increased to 185x109/L, but it decreased to less than 100x109/L one week later and remained low at subsequent rechecks.  Five (case nos. 3, 4, 6, 9, 11) of the treated cases developed mild-to-severe, nonregenerative anemias (hematocrit range, 15% to 35%) during treatment.

The 11 cases in this study initially were presented for a variety of clinical problems and had idiopathic, asymptomatic thrombocytopenia with large-to-giant platelets identified during routine laboratory evaluation.  There was no apparent age or sex predilection.  No temporal correlation between vaccination and thrombocytopenia was established.  Four cases had evidence of mitral valvular endocardiosis, a commonly recognized problem in

the CKCS; (4) no correlation between mitral valve endocardiosis and thrombocytopenia has been established.  Five of the cases developed nonregenerative anemias which were assessed to be secondary to immunosuppressive therapy.  Prior to therapy, bone-marrow erythropoiesis was evaluated in four cases and was normal.  Concurrent IMHA was not identified in any of these cases.

The etiology of the thrombocytopenia identified in CKCSs is unknown. Thrombocytopenia in dogs can result from platelet sequestration, increased platelet consumption, decreased platelet production, or increased platelet destruction. (5)  Platelet sequestration is relatively uncommon and is considered unlikely in these CKCSs, as non of the dogs had detectable splenomegaly. (5)  Platelet consumption due to DIC

was considered unlikely in the CKCSs evaluated since there was no evidence of severe, progressive, systemic disease.(5)  Only one of the five CKCSs which had bone-marrow analyses performed had megakaryocyte hypoplasia, making decreased platelet production an unlikely mechanism for the thrombocytopenia. (5) 

Table 2

Case No.                                             Other Diagnostic Tests

Platelet destruction due to ITP is a common cause of thrombocytopenia in dogs; definite diagnosis of ITP is difficult and frequently is made by exclusion of other conditions. (6)  Immune-mediated thrombocytopenia is suspected when there is a normal-to-increased number of megakaryocytes in the bone marrow and when potential causes of platelet sequestration, consumption, and decreased production have been eliminated systematically. (5)  Several assays have been designed to attempt to detect canine antiplatelet antibody and confirm immune-mediated destruction of platelets, but the low specificity of the available assays makes them of limited use in clinical practice. (7,8) Shortened platelet life span has been documented in many human patients with ITP, but platelet life span is not evaluated routinely in dogs. (9)  Immune-mediated thrombocytopenia  was suspected in several cases in this study when no underlying etiology could be identified and when bone-marrow analyses in four of five cases evaluated revealed normal or increased numbers of megakaryocytes.  Seven of eight CKCSs in this study did not respond to immunosuppressive therapy, however, making a diagnosis of ITP less likely.  The role of gold salt therapy in the resolution of the thrombocytopenia in case no. 9 is not known.  Gold salts are not used routinely in the treatment of ITP, and thrombocytopenia has been reported to develop in dogs receiving long-term (45 to 72 months), high-dose (2.4 to 3.6 mg/kg body weight per day) gold salt therapy. (10)

The asymptomatic nature of the thrombocytopenia in the CKCSs in this report suggest adequate platelet function.  Clinical evidence of hemorrhage commonly does not occur until the platelet count is less than 50 x 109/L. (6) Perhaps the larger-than-normal platelets in the dogs with platelet numbers less than 50x109/L had increased functional capacity; platelet function has been speculated to depend more on total platelet mass (i.e., platelet number x platelet volume) than on actual platelet number. (11)  Mucosal bleeding time was evaluated in case no. 11 (platelet count, 48x109/L) and was normal.

The large-to-giant platelets subjectively identified in CKCSs have been measured manually in one report and confirmed to be enlarged. (2)  Increases in platelet size can be artifactual or real.  Artifactual changes in platelet size and shape can occur when anticoagulant, temperature, storage time, osmotic conditions, and degree of activation are varied. (12,13)  Platelet size is increased during accelerated thrombopoiesis, as seen during recovery from experimentally induced thrombocytopenia disorders are thought to arise from abnormal megakaryocytopoiesis. (16)

A congenital macrothrombocytopenic disorder could explain the thrombocytopenia and the enlarged platelets observed in the CKCSs in this study.  Congenital macrothrombocytopenia has not been reported in dogs, but several rare, congenital macrothrombocytopenic disorders have been described in humans. (16)  Well-recognized disorders include the Bernard-Soulier syndrome, (17) Fechtner syndrome, (18) May-Hegglin anomaly, (19) Montreal platelet syndrome, (20) gray platelet disorder, (21) and Mediterranean macrothrombocytopenia. (22)  Individuals with these syndromes may have prolonged bleeding times and can be asymptomatic (22) or experience mild-to-severe hemorrhagic episodes. (16, 21)  An asymptomatic congenital macrothrombocytopenia also has been reported in the Wistar Furth rat. (23)

Conclusion

The recognition of idiopathic, asymptomatic thrombocytopenia in the CKCS is important.  Although the number of cases identified in this study is low, previous reports indicated that the incidence of idiopathic, asymptomatic thrombocytopenia may be as high as 31% in this breed. (1) Determination of the prevalence of the disorder and investigation of the possible mode of inheritance are indicated.  Most affected dogs have remained asymptomatic with no clinical evidence of abnormal hemostasis.  Therapeutic intervention may not be warranted when a case is identified; certainly treatment with immunosuppressive agents has not been effective in most cases.  Further studies, such as investigations of platelet structure, function and life span, and megakaryocyte structure, should help to define the underlying pathophysiology of idiopathic, asymptomatic thrombocytopenia in this breed.

Acknowledgments

This study was part of the M.Vet.Sc. degree requirement for Dr. Smedile and was funded by the Companion Animal Health Fund at the Western College of Veterinary Medicine.  The medical records search was performed by the Veterinary Medical Data Base at Purdue University.