Feline primary erythrocytosis (PE), more commonly referred to as polycythemia in veterinary medicine, is a rare myeloproliferative disorder that results in the increased mass of red blood cells. It is caused by an erythropoietin-independent expansion of a single hematopoietic stem cell. This disorder, which also occurs in humans, is considered a form of myeloproliferative cancer.
In humans and in dogs, the excessive erythropoiesis is caused by known mutations in the erythropoietin signaling cascade. In cats, the mutation is not confirmed though extrapolation from humans suggests that JAK2, with a possible focus on exon 12, could be worthwhile for further exploration.
The case series from the United Kingdom included eighteen cats. The most common presenting signs were seizures and mentation changes (n=10). This presentation agrees with previous reports of affected animals demonstrating severe signs, such as seizure activity, requiring intensive early support. Diagnosis of PE is typically based on a marked increase in packed cell volume (PCV) of greater than 48%. In these cases the median PCV at presentation was 70% (range 64-83%) with a total protein (TP) concentration of 76 g/l (range 51-90 g/l).
Eight of the cats were female and 10 were male, all were neutered. The median age was 5 yr 7 mos and the majority were domestic shorthair cats (15/18). The most common presenting signs were neurological followed by gastrointestinal signs with the median average duration of clinical signs of 2 days.
The key treatment for a cat patient presenting with acute PE is phlebotomy. This procedure usually rapidly improves clinical signs. The average amount of blood using phlebotomy removed in this case series was 19.5 ml/kg, leading to a reduction of PCV of approximately 15%. If phlebotomy is needed too frequently or leads to side effects (such as hypoferritaemia, hypoproteinemia, sedation risks, venous thrombosis and patient stress), medical therapy using hydroxyurea (HU) should then be considered. One side effect of the treatment with HC can be methaemoglobinemia, therefore, using lower doses with more regular frequency is associated with reduced incidence of symptomatic methaemoglobinemia. Beginning with a daily or alternate day dose with maintaining on a long-term dose at less than 20-30 mg/kg HU PO per day (or alternate day dosing) is recommended as a result this case series.
Prior study literature recommends an aim for a stable PCV of less than 50% with treatment. In this case series, only 7/16 of patients achieved this status, though 6/9 remaining cats were asymptomatic long term. The authors state that the results of this study (the average stable PCV was higher in cats without clinical signs) don’t suggest a new therapeutic target level, so a PCV <50% may be an unnecessary low goal. Two cats achieved remission which was defined in human medicine as clinical and haematological resolution without intervention for at least 3 months.
The authors state their results suggest feline PE has a good prognosis for survival. Cats can maintain a good quality of life with survival times of up to, and possibly beyond, 5 years. (VT)