Pelger-Huët anomaly is an inherited, benign condition characterized by hyposegmentation of granulocytes (ie, neutrophils, eosinophils, basophils) that has been reported in dogs, cats, horses, rabbits, mice, and humans. Pelger-Huët anomaly has been identified in purebred and crossbreed dogs.1-6 Australian shepherd dogs, Australian cattle dogs, basenjis, border collies, German shepherd dogs, English foxhounds, and American foxhounds are overrepresented.3
The defining feature of Pelger-Huët anomaly is hyposegmentation of mature granulocytes. Nuclear shapes can be round, heart (Figure 1), oval (Figure 2), reniform (Figure 1), peanut (Figure 3), band (Figure 3), or bilobed; nuclei concurrently exhibit mature, coarse chromatin.2 Neutrophils from affected patients can be confused with immature forms (eg, band neutrophils, metamyelocytes, myelocytes), resulting in inappropriate interpretation of a left shift and confounding appropriate identification of the condition.3 Pelger-Huët neutrophils should be referred to as such and not misclassified as bands, metamyelocytes, or myelocytes, as these imply the presence of inflammation.1 Identification of hyposegmented eosinophils and basophils with clumped chromatin (Figure 4) can also help confirm suspicion of Pelger-Huët anomaly. No basophils were seen on the blood film in this case.
This mutation is an autosomal dominant trait with incomplete penetrance and has been demonstrated to be caused by the gene encoding the lamin-B receptor in humans and mice.2,3,7 In addition to granulocytes, megakaryocytes are also affected, suggesting that a stem cell defect can result in failure to lobulate.2
Heterozygous patients do not have any clinical signs of a disease process,3 making the heterozygous form of the Pelger-Huët anomaly an incidental finding during examination of a blood smear.2 The homozygous phenotype has not been reported in adult dogs, suggesting it is lethal in utero or these dogs die shortly after birth,3 similar to homozygous rabbits and cats, which have both been reported to exhibit skeletal deformities.8
The frequency of this condition is unknown in many veterinary species. One study demonstrated the Pelger-Huët anomaly has an incidence of 9.8% in Australian shepherd dogs, but these data may have a geographic bias (West Coast of the United States) and may be influenced by intensive inbreeding and line breeding.3 Further studies are required to assess the incidence of this anomaly in other dog breeds and veterinary species.