A dinucleotide mutation in the endothelin-B receptor gene is associated with lethal white foal syndrome (LWFS); a horse variant of Hirschsprung disease.

The study records a first-ever documented case of a horse born with Pelger-Huët anomaly (PHA), a rare genetic disorder that affects the neutrophils, the most common type of white blood cells. The horse, a 1.5-year-old Arabian male, showed symptoms consistent with PHA and similar findings were observed in the horse’s sire and his siblings.

Background

• The research revolves around a 1.5-year-old Arabian horse presented to the Louisiana State University Veterinary Teaching Hospital and Clinic with a deep trunk laceration. The condition triggered an initial investigation into the horse’s blood cell characteristics.

Initial Observations and Diagnosis

• Early complete blood count (CBC) results exhibited an inflammatory leukogram, indicating an ongoing inflammatory or infection process. This was characterized by a significant degenerative left shift, i.e., an abnormal increase in immature band neutrophils. No mature, segmented neutrophils were detected.
• Abdominal ultrasonography revealed the presence of excess fluid in the abdominal cavity, which consisted of a large quantity of neutrophils. These neutrophils displayed hyposegmentation, i.e., they had fewer segments than normal, and condensed mature chromatin structure.
• Neutrophil morphology on the initial blood smear matched with that observed in cells in the abdominal fluid, affirming the diagnosis of Pelger-Huët anomaly (PHA) in the young horse.

Confirmation of Congenital PHA

• The PHA condition was confirmed to be congenital, or hereditary, based on a series of blood smears showing persistent neutrophil hyposegmentation.
• Ruling out acquired causes of PHA was crucial to consolidating this diagnosis. Acquired PHA is usually associated with malignancies, or can be drug-induced. As no such factors were involved, the anomaly was deemed inborn.
• A supporting factor for the diagnosis was similar neutrophil hyposegmentation noticed on blood smears obtained from the colt’s biological father and his siblings.

Significance and Conclusion

• The documented case is the first known instance of congenital PHA in a horse. This notable finding impacts veterinary clinical pathology and genetics studies as it paves the way to furthering the examination of inherited white blood cell anomalies in animals.