Primary Open Angle Glaucoma Beagle Type (POAG)
Primary open angle glaucoma Beagle type is an eye disorder causing a build-up of pressure in the eye. Glaucoma is divided in two categories, primary and secondary glaucoma. Primary glaucoma is characterized by its onset without any other ocular cause, while secondary glaucoma appears when another cause is present, which triggers the glaucoma. Primary open angle glaucoma belongs in the group of primary glaucoma together with primary angle closure glaucoma (PACG) and primary congenital glaucoma (PCG). In Beagles, other than POAG, PACG has been identified as well. However, PACG is very rare and no researches or studies have been made and no DNA tests have been made for it. Primary open angle glaucoma in Beagles has similar conditions to glaucoma in humans that is leading cause of blindness worldwide. For this reason POAG in Beagles is used as an animal model for the human disease.
Fluid inside of the eye is called aqueous humor. It is produced behind the iris, in the ciliary body. This fluid flows through the eye and drains from the eye, maintaining the normal eye pressure. The drain network in the eye is called the drainage angle or drainage canals. Inadequate outflow of aqueous humor causes buildup of pressure, and consequently, glaucoma. Prolonged intraocular pressure results in damage of optic nerves, whose damage is irreversible. It can also lead to luxation of lens and narrowing of the iridocorneal angle. In average, the intraocular pressure starts to increase at 8 to 16 months of age. Eyes of affected dog may appear painful, pupils are red and dilated. Behavioral changes may appear as well, such as loss of appetite and eye scratching. POAG is a progressive disorder and although the symptoms occur in early age, their early identification is rare and is not made until the late stages of the disease. Like other forms of glaucoma, POAG can be treatable, if it is detected early enough. Unfortunately, due to its progressive nature, the owners do not detect POAG until blindness onset, final stage of POAG which is not treatable.
Primary open angle glaucoma Beagle type (POAG) is caused by a point mutation in the ADAMTS10 gene. Pedigree analysis of affected dogs show that POAG 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.