Burmese Head Defect (BHD)
Burmese head defect (BHD) is a craniofacial deformity which occurs in Burmese cat breed. Occurence of head defect in Burmese is connected to selected breed phenotype. The Burmese is a cat breed with an extreme brachycephalic phenotype. In the late 1970’s, a male Burmese cat in the USA with a more brachycephalic head type became a highly popular sire and his lineage became known as the “Contemporary” Burmese. More brahycephalic individuals were positively selected in the breed, and matings between two „Contemporary“ individuals produced 25% offspring with craniofacial defect.
Characteristics and Symptoms
Burmese head defect genetic mutation causes disruption of the normal developmental procceses in skull formation and facial features. In affected cats, a duplication of the tissue of the upper jaw is present, tohether with development of two hard palates and two sets of whisker pads. In addition, the region of the skull above the upper jaw does not form properly. Eyes and ears are absent, or malformed, and there is incomplete closure of the fontanelles of the skull. The brain appears to protrude from the skull, although it is generally covered by skin. These craniofacial malformations are severe and immediately apparent at birth. Affected kittens are generally born live and require euthanasia as the condition is incompatible with life.
Burmese head defect (BHD) is associated to a deletion in ALX1 gene (Aristaless-Like Homeobox protein 1). Defect is inherited in autosomal recessive manner, however, carriers of the mutation are more brachycephalic individuals than wildtype, thus the trait has also been described as co-dominant. When mating two carriers (heterozygotes) at conception each kitten has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier.
Although the frequncy of the mutation is higher in „Contemporary“ lines, it is detected in other Burmese breed lines also. A DNA test can be used to identify cats that are carriers (heterozygotes) of the mutation that causes Burmese head defect.
Lyons, L.A., Erdman, C.A., Grahn, R.A., Hamilton, M.J., Carter, M.J., Helps, C.R., Alhaddad, H., and Gandolfi, B. (2016). Aristaless-Like Homeobox protein 1 (ALX1) variant associated with craniofacial structure and frontonasal dysplasia in Burmese cats. Developmental Biology 409, 451–458.