Hereditary Vitamin D-Resistant Rickets (HVDRR)
Hereditary Vitamin D-Resistant Rickets (HVDRR) is a hereditary disorder affecting the Pomeranian dog breed, caused by insufficient vitamin D intake. Generally, canine rickets can be divided into two forms, type I and type II. In type I Vitamin D-dependent rickets, a renal enzyme needed for vitamin D conversion is inactive, while in type II vitamin D-resistant rickets there is an end-organ resistance to the active hormone. The form of hereditary vitamin D-resistant rickets which if affecting Pomeranians belongs to the group type II. The disorder is characterized by hypocalcemia, secondary hyperparathyroidism, hypomineralization of bones rickets and in some cases alopecia.
Characteristics and Symptoms
In the HVDRR affected dogs, the intestinal absorption of calcium is insufficient to meet the calcium demands of a growing skeleton. The first symptoms appear early, around 4 months of age when puppy seems to tire easily when playing. Physical examination reveals lateral bowing of both forelimbs and thickening of the distal radii. With time, alopecia is becoming more generalized. Serum analysis shows hypocalcemia, as well as hyperphosphatemia and increased alkaline phosphatase. Radiography shows decreased mineralization. Based on results which are revealing hypocalcemia, hyperparathyroidism, and the skeletal abnormalities, veterinarians can diagnose a vitamin D-dependent form of rickets. Treatment with oral calcium supplementation was relatively inefficient, because total serum calcium levels were persistently low, despite a consistent increase in oral calcium supplementation doses. Altogether, clinical signs include hypocalcemia, secondary hyperparathyroidism, joint pain, lameness, bone malformations, alopecia, spontaneous fractures, and limb deformities. Due to spontaneous fractures, which can occur anywhere in the body, affected dogs are commonly euthanized upon owners’ request.
Hereditary Vitamin D-Resistant Rickets (HVDRR) in Pomeranians is caused by a mutation in the VDR gene. HVDRR is inherited as an autosomal recessive disorder. Healthy parents of an affected dog are obligate heterozygotes and therefore carry one mutant allele. Heterozygotes have no symptoms. At conception, each cub 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.
LeVine DN, Zhou Y, Ghiloni RJ, Fields EL, Birkenheuer AJ, Gookin JL, Roberston ID, Malloy PJ, Feldman D. (2009) Hereditary 1,25-dihydroxy vitamin D-resistant rickets in a Pomeranian dog caused by a novel mutation in the vitamin D receptor gene. J Vet Intern Med 23:1278-83.