Genetic Welfare Problems of Companion Animals

An information resource for prospective pet owners

BostonTerrier

Boston Terrier

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. 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 genetic basis for the development of cataracts. Although there are many causes of cataracts in dogs including aging, nutritional deficiency or primary eye disease, some cataracts are inherited in certain breeds and can develop when the dogs are young or middle-aged. Hereditary cataracts are relatively common in dogs, and have been reported to affect over 60 breeds, including the Boston terrier, which has an increased risk of developing cataracts compared with the general dog population. Boston terriers suffer from two genetically distinct forms of inherited cataracts - those that develop early in life and those that develop later in life.


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. In particular, Boston terriers suffer from two genetically distinct forms of inherited cataracts - those that develop early in life and those that develop later in life.

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 disease 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

In both forms of cataracts affecting Boston terriers, dogs are born with normal lenses, which then start to degenerate over time leading to visual impairment and blindness later in life. In the early onset form of cataracts in Boston terriers, cataracts are visible by 15 months of age (compared with the late-onset form in which cataracts start to develop at 3-6 years of age) Cataracts may remain the same in size and shape over time, but they may also progress leading to further vision loss and eventually blindness. 

4. Number of animals affected

Overall 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). In one study in North America, 11.11% of Boston terriers had cataracts, which was one of the highest percentages within pure breed categories (Gelatt and Mackay 2005), suggesting that as a breed they are more prone to cataract development than other breeds or mixed breeds.

5. Diagnosis

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

Cataract development in the Boston terrier has two genetically distinct forms. An autosomal recessive mutation of the transcription factor gene, HSF4, is responsible for early-onset cataract development. A second form of cataract in this breed is more variable and develops later in life, and the genetic mutations for this form has yet to be identified.

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

Commercial genetic tests for the autosomal recessive HSF4 mutation (that causes early-onset cataracts) are available for the Boston terrier breed. Mutations causing late-onset cataracts in Boston terriers have not yet been fully identified and so the mode of inheritance is not known.

8. Methods and prospects for elimination of the problem

It is not advisable to breed from dogs with cataracts that have developed early in life (hereditary cataracts), especially from those breeds which are more susceptible to cataract development. For the recessive cataract-causing mutation, affected dogs and carriers should not be mated with other carriers since some of their offspring would be affected by the condition.

For the late-onset cataract development, which is likely to be a complex inherited disorder, it is advised to avoid breeding between moderately or severely affected individuals (i.e. dogs with rapidly progressive cataracts) or from dogs with severely affected relatives, including grandparents, siblings, previous offspring and siblings of parents.

 

For further details about this condition, please click on the following:
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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 also 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 the Boston terrier. In the Boston terrier, hereditary cataracts are always bilateral, affecting both eyes. 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 some cases, scar tissue may develop on the lens after surgery and/or cataracts may return after surgery. The mutation responsible for cataracts in Boston terriers exists in both eye lens, and usually the cataracts develop at the same time but may also occur at different times. 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

Cataracts are a progressive disease. They may remain the same size and shape, but may also become larger and more opaque leading to further vision loss and eventually blindness. In early-onset progressive cases, most of the lens is affected by 6 months, and obvious cataracts and vision loss usually occur between 9 and 15 months of age with further progression and maturation of the cataract (Mellersh et al 2007). In late-onset cases, cataracts usually develop at 3-6 years of age and progression may be slow (Curtis 1984). 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. Large cataracts or those with complications may be treated with surgery.

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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). In North America, 24.2 dogs per 1000 (2.4%) were diagnosed with cataracts between 1994 and 2003 (Gelatt and Mackay 2005). 1,693 out of 15,241 Boston terriers (11.11%) had cataracts, which was one of the highest percentages within pure breed categories (Gelatt and Mackay 2005), suggesting that as a breed they are more prone to their development than other breeds or mixed breeds

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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

To date only a single gene has been implicated in the development of hereditary cataracts in dogs – the transcription factor gene, HSF4. This gene has also been implicated in cataract formation in humans and mice (Mellersh 2014). In Boston terriers, a single mutation - a recessive nucleotide insertion to the HSF4 gene - is responsible for an early onset, bilaterally symmetrical and progressive form of hereditary cataract (Mellersh et al 2006). The same additive mutation causes cataracts in Staffordshire Bull terriers and French bulldogs. However, Boston terriers are also prone to develop a more variable and later onset cataract, and it has been confirmed that HSF4 gene is not associated with this form of cataract in this breed, and that the two forms are genetically distinct (Mellersh et al 2007). Mutations in the HSF4 gene have been excluded from involvement in many other dog breeds and thus the genetic basis of hereditary cataracts in dogs is likely to be complex and multi-genic (Mellersh 2014).

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

Commercial genetic tests are available to detect mutations of the HSF4 gene (early-onset cataract) in the Boston terrier, Staffordshire bull terrier and French bulldog (Mellersh 2014). In this 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.

There are currently no tests to detect the late-onset form of cataracts in the Boston terrier.

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8. Methods and prospects for elimination of the problem

The genetic basis of cataract formation is complex and elimination of the problem is therefore difficult. For the recessive cataract-causing mutation, a screening program could be developed to help eliminate the problem (Farrell et al 2015). A DNA test based on hair or saliva samples can be used to identify dogs that are clear, affected or carriers of the condition. Dogs that are clear of the mutation can be mated to other clear dogs or carriers. Affected dogs and carriers should not be mated with other carriers since some of their offspring would be affected by the condition.

For the late-onset cataract development, which is likely to be a complex inherited disorder, it is advised to avoid breeding between moderately or severely affected individuals (ie dogs with rapidly progressive cataracts) or from dogs with severely 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

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.

Curtis R (1984) Late-onset cataract in the Boston terrier. Veterinary Record 115: 577–578. doi:10.1136/vr.115.22.577

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

Mellersh CS (2014) The genetics of eye disorders in the dog. Canine Genetics and Epidemiology 1: 3. doi:10.1186/2052-6687-1-3

Mellersh CS, Graves KT, McLaughlin B, Ennis RB, Pettitt L, Vaudin M and Barnett KC (2007) Mutation in HSF4 associated with early but not late-onset hereditary cataract in the Boston Terrier. The Journal of Heredity 98: 531–3. doi:10.1093/jhered/esm043

Mellersh CS, Pettitt L, Forman OP, Vaudin M and Barnett KC (2006) Identification of mutations in HSF4 in dogs of three different breeds with hereditary cataracts. Veterinary Ophthalmology 9: 369–378. doi:10.1111/j.1463-5224.2006.00496.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

 © UFAW 2015


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