Cataracts are a common finding across at least 60 dog breeds, but very few genes have been implicated in causing them, much to the frustration of breeders and scientists. In a recent co-authored publication, we found a gene called CPT1A was linked to cataracts in specific Arctic breeds of dog, including the Siberian Husky and Samoyed. This research study represents the collaborative research of groups in the US, UK and Finland: Wisdom Panel™ US & UK, Canine Genetics Centre, Department of Veterinary Medicine, University of Cambridge, and the University of Helsinki. Let’s discuss a bit about what cataracts are, and then dive into what was discovered, and what it means for the breeds involved.
What are cataracts?
Cataracts are defined as opacities occurring in the lens of the eye or its capsule. Cataracts can be acquired, for example subsequent to injury or a metabolic disorder, or they can be hereditary in nature, usually genetic and developing early in life. The prevalence of cataract is age-related, with about 17% of the senior and geriatric mixed-breed dog population affected, and rates vary within purebreds. Unlike genetic eye conditions such as PRA that are simple recessive hereditary disorders, hereditary cataracts (HC) in dogs appear to have a more complex etiology suspected to involve many genetic risk factors and possibly environmental triggers. Hereditary cataracts often present differently than acquired ones, in that they are usually breed-specific, early-onset, and tend to be bilaterally symmetric. Per the American College of Veterinary Ophthalmology (ACVO):
“The prudent approach is to assume cataracts to be hereditary except in cases known to be associated with trauma, other causes of ocular inflammation, specific metabolic diseases, persistent pupillary membrane, persistent hyaloid, or nutritional deficiencies.”
Cataracts are often named based upon their appearance, cause (if known), and location in the lens or lens capsule, and can vary from pinpoint cataracts that cause minimal vision disruption to cataracts causing total blindness. Cataracts must be distinguished from lenticular sclerosis, a normal aging change seen in dogs over age 7, that causes a bluish haze to both eyes, and does not impair vision.
Current Genetic Hereditary Cataracts Tests
Only three forms of hereditary cataracts have genetic tests available, and two are variants within the same gene. Juvenile Cataract in Wirehaired Pointing Griffons is a recessive disorder caused by a deletion in FYCO1. The variant of HSF4 found in Australian shepherd dogs and breed derivatives (Miniature American shepherd, Miniature and Toy Australian shepherds, and crosses, such as Aussiedoodles) is a deletion, and is semi-dominant in inheritance, so dogs with one copy will typically be less severely affected than homozygous (2 copy) dogs. The second variant of HSF4 is autosomal recessive, and found in Boston terriers, French bulldogs, and Staffordshire bull terriers. Juvenile Cataracts of Wirehaired Pointing Griffons is available in MyDogDNA™ and Optimal Selection™ Canine tests.
Cataracts in Northern Breeds
In the US, research suggests that the prevalence of hereditary cataracts (HC) in Siberian Huskies and Samoyeds has increased significantly since the 1960’s, from under 1% to 6%. In Huskies and Samoyeds, a particular form of cataracts called posterior polar or posterior cortical/subcapsular (PPSC) is well known.
“In the Siberian Husky, cataracts begin in the axial posterior cortex at approximately one year of age. Progression is variable and vision impairment may occur. In cases with rapid progression, secondary lens-induced uveitis and glaucoma may be associated with partial cataract resorption.” (ACVO Blue Book)
Thankfully, PPSCs are generally smaller, slowly progressive or non-progressing (stationary) cataracts that interfere less with vision than other types of cataracts. This has led some to downplay the importance of PPSCs. However this study suggests that incidence of cataract is going up across purebred dog breeds, and a notable breed with low incidence is the Border Collie, which as a working herding breed would suggest that having them, even if small, isn’t optimal for best vision.
An important discovery in Northern Breeds
In this study, “Common variants in the CPT1A gene are associated with cataracts in Northern breeds of domestic dog,” almost 100 Siberian Huskies of both cases and controls for PPSC were initially genotyped, as well as 65 Alaskan Malamutes. An association was found on chromosome 18 for the Husky, but was not strongly associated with similar HC in Alaskan Malamutes. The association was found again in a larger set of approximately 300 Huskies, 50 Samoyeds, 50 Icelandic Sheepdogs, and 19 Norwegian Buhunds. A genome-wide association study (GWAS) examining over 35,000 variants narrowed the search down to 11 genes on chromosome 18.
A key risk marker in the CPT1A gene was discovered that is associated with PPSC in the Siberian Husky, Samoyed, Norwegian Buhund and Icelandic Sheepdog, all breeds of Arctic or northern descent. CPT1A has previously been shown to be expressed in the eyes and livers of humans and mice, and is involved in fatty acid metabolism, but this is the first study implicating the role of the CPT1A gene in retinal metabolism in dogs, and the first study to find an association with this gene and hereditary cataract risk. It was not found to be strongly associated with other forms of cataract in the same breeds, suggesting it’s specific to bilateral PPSC-type cataracts.
Although the variant has been mapped to the CPT1A gene, the mechanism by which this leads to cataract formation remains unclear, and other genetic risk factors and environmental influences are considered likely. One proposed theory is that functional deficiency of the CPT1A gene may lead to accumulation of toxins in the eye due to altered fatty acid metabolism.
The frequency at which this risk variant was found in Siberian Huskies is 88%, and although strongly associated with cataract formation risk, many unaffected Huskies with the risk variant were also observed, suggesting this variant is not the sole cause of HC in Arctic breeds.
Man’s best friend: Cataracts in Arctic Human Populations
Interestingly, CPT1A variants are found with similarly high frequencies in Canadian Inuit, Greenland Inuit, and Alaska Native populations, and is suspected to play a role in optimized metabolism for high-fat, high-protein traditional diets. Mitochondrial trifunctional protein (MTP) deficiency, which affects CPT1A activity and is found in indigenous Arctic human populations, is also associated with eye disease. Historically, northern dogs ate a similar diet to their owners, suggesting the CPT1A variation could arguably be a survival adaptation or positively selected variant for both species that comes with risks.
Next Steps for Huskies, Samoyeds, Buhunds and Icelandic Sheepdogs:
The CPT1A variant occurs with high frequency in Siberian Huskies and other northern breeds, possibly due to historical breed conditions, so any decisions to exclude dogs from breeding on the basis of a CPT1A test should also strongly consider effects on the diversity and health of the breed population as a whole, in addition to the other standard considerations of breeding merit.
The risk variant occurs in both unaffected and PPSC-affected dogs, but is found at higher rates in affected dogs, supportive of it having a significant role in risk, and would indicate use of caution in discarding all carriers and 2 copy dogs from breeding. The CPT1A variant is not likely the only influence in development of HC in these breeds, it should therefore be considered a predictive risk test rather than diagnostic test for PPSC, with other genetic or non-genetic causes involved. Further research as to what differentiates affected and unaffected dogs is needed.
In Siberian Huskies, a gene that causes blue eyes or odd eyes has been found in ALX4, which is also located nearby on chromosome 18. However, in this study, blue eyes was not associated with HC in the Huskies we studied.
At this time, no test is available for CPT1A, but we expect it will likely be available in MyDogDNA™ and Optimal Selection™ Canine tests starting in 2026.
References:
The University of Sydney, OMIA - Online Mendelian Inheritance in Animals: “OMIA:001758-9615 : Cataract, early onset, HSF4-related in Canis lupus familiaris (dog).” https://omia.org/OMIA001758/9615/ Retrieved May 2025.
Gelatt KN, Mackay EO. Prevalence of primary breed-related cataracts in the dog in North America. Vet Ophthalmol. 2005 Mar-Apr;8(2):101-11. doi: 10.1111/j.1463-5224.2005.00352.x. PMID: 15762923.
American College of Veterinary Ophthalmologists, ACVO Blue Book 2023: https://ofa.org/wp-content/uploads/2024/05/ACVO-Blue-Book-2023.pdf Retrieved May 2025.
Gelatt KN, Mackay EO. Prevalence of primary breed-related cataracts in the dog in North America. Vet Ophthalmol. 2005 Mar-Apr;8(2):101-11. doi: 10.1111/j.1463-5224.2005.00352.x. PMID: 15762923.
Gillingham MB, Hirschfeld M, Lowe S, Matern D, Shoemaker J, Lambert WE, Koeller DM. Impaired fasting tolerance among Alaska native children with a common carnitine palmitoyltransferase 1A sequence variant. Mol Genet Metab. 2011 Nov;104(3):261-4. doi: 10.1016/j.ymgme.2011.06.017. Epub 2011 Jun 28. PMID: 21763168; PMCID: PMC3197793.
Roomets E, Kivelä T, Tyni T. Carnitine palmitoyltransferase I and Acyl-CoA dehydrogenase 9 in retina: insights of retinopathy in mitochondrial trifunctional protein defects. Invest Ophthalmol Vis Sci. 2008 Apr;49(4):1660-4. doi: 10.1167/iovs.07-1094. PMID: 18385088.
