Equine blood reticulocytes: reference intervals, physiological and pathological changes

High-sensitivity haematology analysers detect equine blood reticulocytes, but their analysis and quantitation is largely unstudied. We assessed whether equine reticulocytes are affected by pathological or physiological factors, determined reference intervals, and examined reticulocyte and related, erythron parameters using 6-years’ data (Advia 2120) from our hospital archives. Data was categorised according to horse age (foals < 12 months), breed (hot-, warm-, cold-blooded), presence of anaemia and various diseases. In adults, reticulocytes (× 10⁹/L) were 20% lower in cold-blooded horses compared to others (4.14 ± 2.3, 25 vs 5.15 ± 3.3, 157; data shown as mean ± SD, n; p < 0.05.) There was no gender effect. In thoroughbreds, foals had 50% higher reticulocytes than adults (7.5 ± 4.5, 18 vs 5.0 ± 3.5, 56). We determined the first reference interval for healthy adult horses as 5.0 ± 3.2,182, 1.16–13.1. Reticulocyte counts were 120% higher with marked anaemia (11.0 ± 9.4, 6) but 36% with other diseases (6.9 ± 4.2, 74), versus reference mean. Erythrocyte size (MCV)/reticulocyte haemoglobin content (CHr) were 21/14% lower in foals versus adults and 12/37% higher with marked anaemia. Immature reticulocytes (IRF-H) were 60% lower in foals versus adult thoroughbreds, and 87% with marked anaemia. In conclusion, equine reticulocytes are only mildly increased by physiological parameters (breed, age, but not gender), various non-blood diseases (colic, dysproteinaemia) and marked anaemia. Reticulocyte counts are substantially lower in horses versus other species; their clinical use is limited to marked anaemias. This is consistent with the greater erythrocyte storage and buffering capacity of the spleen of the horse. Finally, this study supports the theory of a reticulocyte maturation role for equine spleen.