May-Hegglin Anomaly

Other Names: MHA
Affected Genes: MYH9
Inheritance: Autosomal Dominant
Mutation: chr10:28120346 (canFam3): G>A
Breed(s): Carlin Pinscher, Pug

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Common Symptoms

May-Hegglin anomaly (MHA) is an inherited disorder of blood cells. This condition is diagnosed via microscopic examination of a blood smear and is characterized by three key features: reduced numbers of normal sized Platelets in the blood (thrombocytopenia), the presence of large platelets (macrothrombocytes), and abnormal neutrophils containing structures called Inclusion Bodies. This dog did not demonstrate clinical signs related to platelet abnormalities however, recognizing a congenital cause for macrothrombocytopenia is important because both thrombocytopenia and macrothrombocytosis can be associated with clinically important diseases. In addition, some blood analyzer machines can misinterpret macrothrombocytes as red blood cells, thus further complicating diagnoses. Though usually asymptomatic, humans with May-Hegglin anomaly have been reported to have issues with blood clotting leading to bruising and spontaneous bleeding. The Pug reported with this condition did not show increased incidence of Hemorrhage and blood clotting times were normal.

Testing Tips

Genetic testing of the MYH9 gene will reliably determine whether a dog is a genetic Carrier of May-Hegglin anomaly. May-Hegglin anomaly is inherited in an Autosomal Dominant manner in dogs meaning that they only need to inherit one copy of the mutated gene from either parent to develop the disease. Dogs identified to have the genetic Mutation are expected to have features of the disease, though symptoms can be very mild and dogs may appear healthy. Each pup that is born to a single affected parent has a 50% chance of inheriting one copy of the MYH9 gene mutation and having the disease. If both parents are affected, the chance of having affected offspring increases to 75-100%. Reliable genetic testing is important for determining breeding practices. In order to eliminate this mutation from breeding lines and to avoid the potential of producing affected pups, breeding of known carriers is not recommended. Dogs that are not carriers of the mutation have no increased risk of having affected pups.

There may be other causes of this condition in dogs and a normal result does not exclude a different mutation in this gene or any other gene that may result in a similar genetic disease or trait.


  • Flatland B, Fry MM, Baek SJ, Bahn JH, LeBlanc CJ, Dunlap JR, Carroll RC, Kosiba DJ, Millsaps DJ, Schleis SE. May-Hegglin anomaly in a dog. Vet Clin Pathol. 2011 Jun; 40(2):207-14. [PubMed: 21554370]