Congenital methemoglobinemia is an inherited blood disease in dogs. Affected dogs have an impaired ability to deliver oxygen to tissues of the body. Though affected dogs rarely have clinical signs requiring veterinary intervention, they may experience lifelong intolerance to exercise and display an intermittent or consistent blue or purple hue to gums, tongue, and skin, becoming more pronounced during exercise.
Genetic testing of the CYB5R3 gene will reliably determine whether a dog is a genetic Carrier of congenital methemoglobinemia. Congenital methemoglobinemia is inherited in an Autosomal Recessive manner in dogs. This means that a dog must receive two copies of the mutated gene (one from each parent) to develop the disease. In general, carrier dogs do not have features of the disease but when bred with another carrier of the same Mutation, there is a risk of having affected pups. Each pup that is born to this pairing has a 25% chance of inheriting the disease and a 50% chance of inheriting one copy and being a carrier of the CYB5R3 gene mutation. Reliable genetic testing is important for determining breeding practices. To eliminate this mutation from breeding lines and avoid the potential of producing affected pups, breeding of known carriers to each other is not recommended. Dogs that are not carriers of the mutation are not at 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.
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Zhou S, Tearle R, Jozani RJ, Winra B, Schaaf O, Nicholson A, Peaston A. Genetic cause for congenital methemoglobinemia in an Australian Pomeranian dog. J Vet Intern Med. 2019 Mar;33(2):868-873. doi: 10.1111/jvim.15435.