From Pat B:

here is a common Cavalier condition (in probably 30-40% of Cavaliers) called idiopathic (because they don't know the cause) symptomless thrombocytopenia. The key word here is "symptomless." These Cavaliers do not need any treatment! Here are some articles to print out to give your vet. Your vet can also do a simple clotting test to feel reassured before he performs the neutering.

http://ackcsc.org/health/cavalierplateletissues.pdf

This is from the above article, and explains what this is:

Notes from the Educational Program 8/7/01 Presented to CKCS Club of Greater Atlanta “Asymptomatic thrombocytopenia and Macrothrombocytosis in the Cavalier King Charles Spaniel� By Club Member Dr. Kim Hamer, DVM

Terminology: 1. Platelets are also called thrombocytes. These are the blood cells that are responsible for normal blood clotting. 2. Thrombocytopenia is an abnormally low blood platelet count. 3. Macrothrombocytes are abnormally large blood platelets.

Normal blood platelet counts should be between 150-200,000. Dogs will have bleeding abnormalities if their counts are below 40,000. However, Cavaliers may have counts well below 40,000 with no problems. Approximately 30% of Cavaliers have macrothrombocytosis and/or thrombocytopenia.

This appears to be a congenital abnormality. The Cavalier does not experience any health problems despite these changes. It is thought that the large platelets of the Cavalier are able to provide the same function in lower numbers than that of other breeds. Because Cavalier platelets are so large, automated cell counters may mistakenly count them as white blood cells, artificially lowering the platelet count. Additionally, these cells may not be recognized when examined under the microscope on a blood smear because they may not look like the platelets of other breeds.

This condition should not be confused with immune-mediated thrombocytopenia, where the body attacks its platelets. Dogs with this disorder will be having SYMPTOMS: i.e. petechiae, bruising and bleeding. Remember, the Cavalier does not experience any health problems due to this condition, which has only been reported in our breed and no others!

Kim Hamer D.V.M. DeKalb Animal Hospital, (770) 938-3900
More info:

http://cavalierhealth.org/platelets.htm

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It is recommended absolutely not to treat a Cavalier with this condition with steroids, etc. as you would treat a dog with true immune mediated thrombocytopenia. The treatment has some pretty significant side effects. If your Cavalier has no symptoms (bruising, bleeding, etc.) I would not treat. The clotting test should help reassure you and your vet, and the vet should feel better after reading the articles. Over the past 15 years I have heard of many, many cases of this. In most of the cases, the vets freak out and begin treatment that isn't necessary and has powerful side effects. This is why all of these articles are posted in the Cavalier sites.

I had a copy of a letter from the University of Tennessee, which recently conducted a platelet study on Cavaliers. Here is part of that letter:


We have completed our study entitled 'Characteristics of Platelets in the
Cavalier King Charles Spaniel' and are excited about our results.
In our study, over 50% of the Cavaliers had a low platelet count, giant
platelets, or both. This platelet disorder does not appear to be associated
with problems such as bleeding. All dogs we examined were healthy except for those previously diagnosed with mitral valve disease.
Although the platelet disorder (which we will refer to as a Benign Inherited
Giant Platelet Disorder, or IGPD), seems to be transmitted through an
autosomal recessive mechanism, we do not have enough evidence to recommend against breeding affected dogs. There was not a direct relationship between IGPD and heart murmurs in the dogs. However, the search for specific associations among the two disorders (if one exists at all) will be difficult because each one has multiple clinical sub-types. For example, there are dogs with mitral valve disease that have a loud murmur yet no clinical signs, as well as dogs with IGPD that have giant platelets but do not have a low platelet count.

It is also possible that the search for any possible association between
IGPD and mitral valve disease will be made even more difficult by complex
genetic factors. In the future, genetic linkage studies may help identify
susceptibility for IGPD, and more importantly, mitral valve disease.

The research team at the University of Tennessee College of Veterinary Medic
ine and Medical Center are grateful for your willingness to participate in
this important study. Congratulate your pet on being part of a study worthy
of award recognition at the 2002 American College of Veterinary Internal
Medicine Forum in Dallas, Texas this past June. We hope that your
enthusiasm for the breed brings you enlightenment and enjoyment. Thank you
again.

Sincerely,
Sara M. Cowan, DVM
Principal Investigator