Genetic Welfare Problems of Companion Animals

An information resource for prospective pet owners

Miniature Schnauzer 

Miniature Schnauzer

Hereditary Cataract

Related terms: Primary cataract

VeNom term: Cataract (VeNom code: 521).

Related conditions: Diabetes mellitus; Lens-induced uveitis; Lens luxation; Glaucoma

Outline: A cataract is a disorder that affects the lens of the eye in which the lens becomes progressively cloudier. Cataracts cause blurred vision and eventually, when the entire lens becomes cloudy, they can cause blindness. Cataracts are not usually directly painful but the loss of vision and eventual blindness can cause confusion and possibly anxiety in dogs and may make them more prone to injury. In Miniature Schnauzers, cataracts can be seen as early as 8 weeks in life, with more obvious changes occurring by 4 months of age, and they can affect both eyes. Owners may not recognise the problem or seek veterinary advice early on until the symptoms progress. Cataracts can be associated with other conditions, such as persistent inflammation of the eye or increased fluid pressure within the eyeball, both of which are painful and can cause permanent damage and irreversible vision loss. Large cataracts or those with complications can be removed via surgery to improve the quality of life for the dog.

Cataracts are a genetically complex condition and there is some evidence of a recessive genetic basis for the development of hereditary cataracts in the Miniature Schnauzer, which has an increased risk of developing cataracts compared with the general dog population. A recessive mutation causing cataracts means that two copies of the gene are needed for dogs to be affected, and that dogs which only have one copy are not affected but they may pass the gene to their offspring.

 


Summary of Information

(for more information click on the links below)

1. Brief description

A cataract is disease of the eye, in which the lens of the dog’s eye becomes opaque and loses transparency; appearing cloudy or bluish-grey. Cataracts develop due to disturbances in the protein structure of the lens, where proteins and protein fibres clump together and prevent light from passing clearly through the lens. This causes blurred vision and loss of vision in dogs, and may lead to blindness if the entire lens becomes cloudy.

A cataract is not usually a painful condition in itself, but there may be associated conditions which are painful, including lens-induced uveitis which is common in dogs with cataracts. Uveitis is a condition where the middle layer of the eye, called the uvea, becomes inflamed causing loss of vision and pain ranging from mild aching to intense discomfort. Dogs with cataracts may also develop glaucoma (an increase in the fluid pressure of the eyeball) secondary to lens-induced uveitis or may develop lens luxation (in which the lens is displaced from its normal position). In both cases, these associated conditions can be painful and cause permanent blindness..

2. Intensity of welfare impact   

Cataracts are the leading cause of blindness in dogs. There are several stages of cataracts – incipient (in which there is mild blurring to the lens), immature (in which vision is obstructed), mature (the entire lens is cloudy/opaque) and hypermature (in which the lens is shrunken in size). Early diagnosis and treatment of this condition is recommended.

Cataracts themselves are not thought to be painful, and the symptoms generally relate to the degree of vision loss, which in itself can cause confusion and may make affected dogs more prone to injury. Secondary complications of cataracts can lead to painful conditions, such as lens-induced uveitis, secondary glaucoma and lens luxation. Cataracts can be treated via surgery, which is invasive but tends to be effective. In some cases, scar tissue may develop on the lens and/or cataracts may return after surgery.

3. Duration of welfare impact

Dogs are born with normal lenses, which then start to degenerate over time leading to visual impairment and blindness later in life. Cataracts may remain the same in size and shape over time, but they may also progress leading to further vision loss and eventually blindness. In Miniature schnauzers, cataracts can be seen as early as 8 weeks in life, with more obvious changes occurring by 4 months of age.

4. Number of animals affected

Overall in the UK, 20.3 dogs per 1000 animals (2%) were diagnosed with cataracts between 2009 and 2013 (Vet Compass Infographic: http://www.rvc.ac.uk/vetcompass/learn-zone/infographics/canine). Miniature schnauzers are more prone to cataract development than other breeds or mixed breeds.

5. Diagnosis

Once formed, cataracts are easily identifiable by a cloudy or blueish-grey mass in the dog’s eye, and an examination of the eye by a veterinary surgeon will confirm the presence of a cataract. In older dogs, cataracts must be distinguished from nuclear sclerosis, which is the natural change of the lens density in ageing animals and which does not substantially affect vision.

6. Genetics

An autosomal recessive mutation is responsible for early-onset cataract development in this breed. However, the specific gene involved has not yet been identified.

7. How do you know if an animal is a carrier or likely to become affected?

The mutation for early-onset cataracts is autosomal recessive, and therefore it affects both males and females, and dogs can carry one copy of the mutation gene without being affected by the condition (carriers). If a dog has two copies of the gene, it will develop cataracts. Both affected dogs and carriers will pass these genes onto their offspring.

DNA tests are not currently available for hereditary cataracts in Miniature schnauzers, and further research is need to identify the genetic nature of the condition. It is recommended to regularly get dogs’ eyes checked by a veterinarian from 6 months.

8. Methods and prospects for elimination of the problem

For early-onset cataracts, where a simple mutation of a single gene is responsible, a screening program could be developed to help eliminate the problem. In the interim, it is not advisable to breed from affected dogs or from dogs of predisposed breeds with affected relatives, including grandparents, siblings, previous offspring and siblings of parents.


For further details about this condition, please click on the following:
(these link to items down this page)


1. Clinical and pathological effects

A cataract is disease of the eye, in which the lens of the dog’s eye becomes opaque and loses transparency; appearing cloudy or bluish-grey. They cause blurred vision and eventually, when the entire lens becomes cloudy, they can cause blindness in affected individuals. There are many causes of cataracts in dogs including old age, nutritional deficiencies (eg deficiency of antioxidants) or diseases of the eye. Some cataracts are inherited in certain breeds and can develop in young dogs, with diagnosis as early as 8–12 weeks of age. Dogs with inherited cataracts are born with normal eye lenses, which then proceeds to degenerate over time, leading to visual impairment and then blindness later in life.

In cataract formation, the normal transparency of the lens is disrupted by the destabilisation of proteins, called crystallins. These crystallins abnormally clump together and bind to other proteins in the lens and this causes cloudiness and prevents lights from passing clearly through the lens, resulting in loss of vision. Since new cells form on the outside of the lens, older cells are compacted into the centre of the lens resulting in the cataract. Heat stress, oxidation or exposure to heavy metals can also cause this reaction. Genetic mutations in the crystalline protein structure can cause structures known as amyloid fibrils to clump together, causing misting of the lens. Hereditary cataracts are common in dogs, and have been reported to affect 60 breeds, but are more common in specific breeds, such as Miniature schnauzers. In this breed in particular, cataracts primarily affect the back surface of the lens and forms beneath the lens capsule (the sac or membrane which encloses the lens and holds it in place); this is termed “posterior subcapsular cortex” (Rubin et al 1969). This type of cataract can typically create “halo” effects and glare around lights. Another common cause of cataracts is the disease diabetes mellitus (Beam et al 1999), as it leads to excess sugars and water in the lens that cause loss of transparency of the lens.

There are important complications of cataracts, including lens-induced uveitis which is common for dogs with cataracts. In this condition, the middle layer of the eye, called the uvea, becomes inflamed causing loss of vision and pain ranging from mild aching to intense discomfort. Uveitis can be difficult to treat effectively and the long-standing inflammation may result in traumatic rupture of the lens capsule. Over time, dogs with cataracts may develop glaucoma secondary to lens-induced uveitis or may develop lens luxation. Glaucoma is caused by an increase in fluid pressure in the eyeball, due to the fluid not draining away properly. Glaucoma can damage the nerve fibres of the eye. In lens luxation, the lens shifts out of its normal position in the eyeball, being displaced either to the front or back of the eyeball. In both cases, the condition can be painful and cause permanent blindness.

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2. Intensity of welfare impact

Cataracts are the leading cause of blindness in dogs. Cataracts can occur in one or both eyes and there are several stages of cataracts – incipient (in which there is mild blurring to the lens), immature (in which vision is obstructed) and mature (the entire lens is cloudy/opaque). Some mature cataracts progress to hypermature cataracts where because of loss of water and proteins the lens becomes shrunken in size. Early diagnosis and treatment of this disease is recommended.

Cataracts themselves are not thought to be painful, and the symptoms generally relate to the degree of vision loss, that in itself can cause confusion and anxiety in dogs and make them more prone to injury. People with cataracts report issues with light sensitivity, double vision and seeing colours. Secondary complications of cataracts can lead to painful conditions, such as lens-induced uveitis and secondary glaucoma. These conditions may irreparably damage the eye resulting in permanent vision loss. Cataracts themselves can be treated via surgery which is invasive but effective in most cases. In some cases, scar tissue may develop on the lens after surgery and/or cataracts may return after surgery. If the mutation exists in both eye lens, cataracts may also occur in the other eye at the same time or at different times. It is important to note that blind dogs may be able to adequately adjust to their vision loss, and this should be assessed for each individual dog. Before surgery careful consideration needs to be given, weighing the possible benefits of the intervention ie the chances of success of the surgery and the improvement it will bring to the life of the animal against the possible costs – the pain, discomfort and risk associated with the surgery and possible complications thereafter.

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3. Duration of welfare impact

Dogs are born with normal lenses, which then start to degenerate over time leading to visual impairment and blindness later in life. Cataracts may remain the same in size and shape over time, but they may also progress leading to further vision loss and eventually blindness. For Miniature schnauzers, the mean age at which cataracts were diagnosed was 5.4 years, ranging from 2.2 to 8.6 years (Park et al 2009), and this was significantly earlier in life than for other small breeds. However, in Miniature schnauzers, cataracts can be seen as early as 8 weeks in life, with more obvious changes occurring by 4 months of age. Owners may not recognise the problem or seek veterinary advice early on until the symptoms progress.Return to top

4. Number of animals affected

In the UK, 20.3 dogs per 1000 animals (2%) were diagnosed with cataracts between 2009 and 2013 (VetCompass Infographic: http://www.rvc.ac.uk/vetcompass/learn-zone/infographics/canine) and in North America, 24.2 dogs per 1000 (2.4%) were diagnosed with cataracts between 1994 and 2003 (Gelatt and Mackay 2005).

In a study of 244 dogs with cataracts in University of Tennessee during 2001-02, 54 breeds were affected but Miniature schnauzers had significantly higher odds for developing cataracts compared with mixed breed dogs (Adkins and Hendrix 2005). In North America, 1714 Miniature schnauzers out of 34,385 (4.98%) had cataracts (Gelatt and Mackay 2005), and they were more prone to cataract development than other breeds or mixed breeds. In a study of small breeds, 7.5% of all dogs that had cataracts were Miniature schnauzers (Park et al 2009).

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5. Diagnosis

Cataracts are easily identifiable by a cloudy or blueish-grey mass in the dog’s eye, and an eye examination by the veterinary surgeon (or specialist veterinary ophthalmologist) can confirm the presence of a cataract. Dogs may be referred to a specialist veterinary ophthalmologist for monitoring and treatment. In older dogs, cataracts must be distinguished from nuclear sclerosis, which is the natural change of the lens density in ageing animals and which does not affect vision. Cataracts are usually classified by their age of onset (congenital, juvenile, senile), anatomic location, cause, degree of opacification (incipient, immature, mature, hypermature), and shape.

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6. Genetics

Cataract development in Miniature schnauzers has been attributed to a simple autosomal recessive gene (Rubin et al 1969), although the specific gene involved has not yet been identified. Genetic analysis of lens-specific proteins (ie affecting crystallins and membrane proteins) in Miniature schnauzers revealed no evidence of mutations (Zhang et al 1991), such as those found in other breeds (see Boston terrier cataract page on this website).

Since the mutation is autosomal recessive, it can affect both males and females, and dogs can carry one copy of the mutation gene without being affected by the condition (carriers). If a dog has two copies of the gene, it will develop cataracts. Both affected dogs and carriers will pass these genes onto their offspring.

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7. How do you know if an animal is a carrier or likely to become affected?

In a recessive cataract-causing mutation, the dog which inherits one copy of the abnormal gene will not show the condition but will be a carrier and may pass the gene to any offspring. When two carriers produce offspring, 25% of their offspring will be affected, and 50% will carry the mutation without developing the condition ie will be carriers.

DNA tests are not currently available for hereditary cataracts in Miniature schnauzers, and further research is need to identify the genetic nature of the condition. It is recommended to regularly get dogs’ eyes checked by a veterinarian from 6 months.

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8. Methods and prospects for elimination of the problem

For the recessive cataract-causing mutation, a screening program could be introduced to help eliminate the problem (Farrell et al 2015). This would involve screening for dogs that are affected, carriers and clear of the condition, by used of DNA samples taken from hair or saliva.  However, this test is currently not available for cataracts in Miniature schnauzers and further research is required to identify the specific gene responsible for cataract development.

It is not advisable to breed from dogs with cataracts that have developed early in life or those that have rapidly progressed (ie those that are likely to be of a hereditary form), especially from those breeds which are more susceptible to cataract development. It is also advisable to avoid breeding from dogs of predisposed breeds with affected relatives, including grandparents, siblings, previous offspring and siblings of parents.

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9. Acknowledgements

UFAW thanks Dr Emma Buckland (BSc, PhD), Dr David Brodbelt (MA VetMB PhD DVA DipECVAA MRCVS) and Dr Dan O’Neill (MVB BSc, MSc, PhD, MRCVS) for their work in compiling this section.

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10. References

Adkins EA and Hendrix DVH (2005) Outcomes of Dogs Presented for Cataract Evaluation: A Retrospective Study. Journal of the American Animal Hospital Association 41: 235–240. doi:10.5326/0410235

Beam S, Correa MT and Davidson MG (1999). A retrospective-cohort study on the development of cataracts in dogs with diabetes mellitus: 200 cases. Veterinary Ophthalmology 2: 169–172

Farrell LL, Schoenebeck JJ, Wiener P, Clements DN and Summers KM (2015) The challenges of pedigree dog health: approaches to combating inherited disease. Canine Genetics and Epidemiology 2: 3. doi:10.1186/s40575-015-0014-9

Gelatt KN and Mackay EO (2005) Prevalence of primary breed-related cataracts in the dog in North America. Veterinary Ophthalmology 8: 101–11. doi:10.1111/j.1463-5224.2005.00352.x

Park SA, Yi NY, Jeong MB, Kim WT, Kim SE, Chae JM and Seo KM (2009) Clinical manifestations of cataracts in small breed dogs. Veterinary Ophthalmology 12: 205–210. doi:10.1111/j.1463-5224.2009.00697.x

Rubin LF, Koch SA and Huber RJ (1969) Hereditary cataracts in miniature schnauzers. Journal of the American Veterinary Medical Association 154: 1456–8

Zhang RL, Samuelson DA, Zhang ZG, Reddy VN and Shastry BS (1991) Analysis of eye lens-specific genes in congenital hereditary cataracts and microphthalmia of the miniature schnauzer dog. Investigative Ophthalmology & Visual Science 32: 2662–5 

© UFAW 2015


Credit for main photo above:

By Psiara at pl.wikipedia [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], from Wikimedia Commons