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Lymphocytosis is a blood disorder of cats defined as an absolute number of circulating lymphocytes greater than reference range of 7 x 109/L or >7000/µl, and below 15,000/µl (the reference cut-off for a diagnosis if leukemia).

Absolute lymphocyte counts are greater in kittens than adults[1]. Young kittens - absolute lymphocyte count increases from birth to a maximum at approximately 12-14 weeks of age of 10.5 x 109/l; value is within reference range for adults by 16-20 weeks of age.

Causes include:

- Physiological lymphocytosis due to stress - absolute lymphocyte count can rise to >10,000/µl and is much greater than the relatively mild increase in mature neutrophils that accompanies this type of lymphocytosis.
- Neoplasia - with lymphosarcoma or lymphocytic Leukemia, peripheral blood lymphocyte count can range from lymphopenia to counts >100,000 lymphocytes/µl. Significantly high counts (i.e., > 30,000/µl) are uncommon.
- Antigenic stimulation due to
- chronic inflammation - often with a suppurative component such as pyometra and pyoderma
- immune mediated diseases, acquired and autoimmune
- Ehrlichia spp infection
- Hypoadrenocorticism - lymphocytosis is accompanied by mild eosinophilia and the absence of neutrophilia or monocytosis in a sick and stressed animal[2].
- Hyperthyroidism - approximately 10% of cats with hyperthyroidism have lymphocytosis.
- Methimazole is associated with lymphocytosis in 7% of cats treated for hyperthyroidism.

Treatment must be directed at the primary disease causing lymphocytosis.


  1. Jain NC (1986) Schalm's veterinary hematology. 4th ed. Philadelphia: Lea & Febiger, pp:109-111,132-134
  2. Breitschwerdt EB, et al (1987) Monoclonal gammopathy associated with naturally occurring canine ehrlichiosis. J Vet Intern Med 1:2-9