Platelet Counts In Cavaliers - What's Normal?

Cavalier Health Articles

Written by Myra Ehrman, RN, BA, BS, MScN

Original Article Written by Myra Ehrman, RN, BA, BS, MScN, On behalf of the Health Education Committee, CKCSCC. Originally Published in The Cavalier King Charles Spaniel Club of Canada Newsletter, "Quotes", in November, 1995. First of all, what are platelets? The Miller-Keane Encyclopedia and Dictionary of Medicine and Nursing says the following: platelet is a small disk or platelike structure, especially the smallest of the formed elements in blood. Blood platelets (called also thrombocytes) are disk-shaped, non-nucleated blood elements with a very fragile membrane; they tend to adhere to uneven or damaged surfaces. They are principally concerned with coagulation of blood and the contraction of a blood clot. They are formed in red bone marrow and the rate of their formation seems to be governed by the amount of oxygen in the blood and the presence of nucleic acid derivatives from injured tissue. The Merck Veterinary Manual, Seventh Edition, adds that platelets are fragile and counts should be performed within 2 hours of blood collection. Excessively low values are the most common cause of bleeding disorders in companion animals. There has been some question as to whether Cavalier King Charles Spaniels typically have a low platelet count. A study was done at the Veterinary College in Uppsala, Sweden to determine how often thrombocytopenia, or low platelet counts, occurred in the Swedish Cavalier population. Clinically normal Cavaliers with no history of bleeding problems were examined, and blood samples were collected. In the Swedish study, the counts were done both by manual visual inspection by a technician and by an automated cell counting system. It was thought an incorrectly low platelet count resulted from the automated cell counting system failing to differentiate between large platelets and small erythrocytes (red blood cells). Cardiac murmur was found in 52 of the 102 dogs in the study. No difference in platelet counts was found between Cavaliers with detectable cardiac murmurs and those free from a murmur. The question was raised as to whether there is a correlation between the high prevalence of cardiac valve disease and low platelet counts in the breed, but no definitive associations were found. Another study was done at the Royal Veterinary College in London, U.K. That study reported adequate platelet numbers, but many of the platelets observed were unusually large in size. No conclusion was reached as to whether the large size of the platelets represented a disease process, or rather a breed abnormality. This study also showed the automated platelet counts to be much lower than the manual counts, and the differences in the counts for Cavaliers to be more significant than the differences for the other breeds. When compared with non-Cavalier dogs, the platelet size of Cavaliers was much greater than dogs of other breeds. Some platelets were reported to be as big as erythrocytes. The conclusions of this study were that Cavaliers probably have platelets which are large in size and which could lead to an erroneous diagnosis of thrombocytopenia when automated blood analysis is used. Dr. I.B. Johnstone, Department of Biomedical Sciences, University of Guelph has concluded, "It looks like the low platelet counts frequently reported in this breed are 'artifacts' of the methodology used to count platelets." Most commercial laboratories will use an automated counting system, which determine blood cell types on the basis of size and volume. As the Cavaliers have platelets which are larger than usual, these counting systems may not recognize platelets as being platelets. Dr. Johnstone goes on to say, "Visual counts (not widely used in commercial laboratories because of the time involved in doing them) seem to indicate that these dogs have normal numbers of platelets. Another example of where automation is not necessarily an improvement over manual techniques." Dr. Michael O'Grady, cardiologist at the Ontario Veterinary College in Guelph has concurred with Dr. Johnstone's assessment. The moral of the story is, if you have a sick Cavalier, where your vet has concluded some abnormal bleeding has taken place and where he/she wants to do blood work, speak up. We know enough now to suggest the vet order a manual inspection to properly evaluate the platelet count. Not all vets know everything there is to know about each and every breed, and we can help keep them informed. Disclaimer: Your veterinarian is the most qualified person to answer all of the questions you have about your pet's health. Nothing in this article should be construed as medical advice regarding any individual animal’s condition. © Copyright April 11, 1999. Contact Myra Ehrman at marcrest@sympatico.ca for permission to re-use material. Dr. O'Grady has also suggested we start monitoring ages and causes of death of our Cavaliers on an annual basis, to determine if there are other life-threatening problems besides mitral valve disease besetting our breed. The actual dogs and the owners would be completely anonymous, but the data collection could be done as a log maintained in the receptionist's area. He suggested Cavalier owners in different regions of the country approach our vets to request their participation. Thus far, a vet in Calgary and Toronto have been asked to participate, and a member will check in P.E.I. for us. If you have approached a vet in your region who is willing to take part, please notify me and I will do the annual follow ups. Please photocopy and enlarge the sample below to use.