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Cataracts are defined as any partial or complete opacity of the lens. Gelatt and MacKay (2005) reported that primary or hereditary cataracts (HC) have been found in 97 dog breeds and are the leading cause of blindness in the species. Cataracts may also have a congenital or nutritional origin, be secondary to trauma or another disease (for example, diabetes), or be related to aging.
Description of the
Disease
Cataracts can affect the entire lens (i.e., diffuse cataract) or just a particular region such as the nucleus, anterior cortex, or posterior cortex (Fig. 1). Congenital cataracts often appear as Y-shaped lines or as three, small opacities occurring along sutures that result from incomplete development of the lens during gestation. In the Belgian Shepherd, HC usually occur as a non-progressive, triangular opacity in the posterior cortex (ACVO 2017). Such cataracts typically do not have a significant effect on vision (Bjerkls 2006). Complete cataracts (i.e., blindness) were reported in only 25 cases from among 21,737 eye exams conducted on Belgian Shepherds between 1991 and 2016 (ACVO 2017).
Figure 1. Anatomy of the lens. Source: Kyselova (2010)
Age-related cataracts are presumed to be caused by damage to crystallin proteins in the lens from exposure to ultraviolet light over the lifetime of the dog. A study in the UK found that half of all dogs showed signs of age-related cataracts by 10 years old and no dogs older than 13.5 years were free of the disease (Williams et al. 2004). Although not strictly a hereditary disease, the onset of age-related cataracts is related to the life spans of purebred dogs, with large breeds having earlier onset than small breeds (Williams et al. 2004). Lenticular sclerosis, the bluish haze commonly observed in the eyes of older dogs, is considered a normal condition and is distinct from age-related cataracts (Ward 2009).
HC Prevalence in the
Belgian Shepherd
Using data from the Canine Eye Registration Foundation
(CERF) and Orthopedic Foundation for Animals (OFA) eye exams, the American
College of Veterinary Ophthalmology (ACVO) periodically publishes
breed-specific statistics for many eye diseases in their AVCO Blue Book. The 10th Edition (2017) of the
Blue Book reports that a total of 21,737 eye exams were performed on the four
varieties of the Belgian Shepherd between 1991 and 2016. Of this number, 84% of
the eye exams were classified as normal (i.e., no eye disease was noted). Cataracts of all types were discovered during
1,983 exams: 48% classified as HC and 52% due to other causes (Table 1). There are not large differences in HC
incidence among Groenendaels (4.2%), Malinois (4.6%), and Tervuren (4.5%). Too
few Laekenois have been examined to estimate the prevalence of cataracts in
that variety. The average incidence of HC among the combined varieties is 4.4%
(Table 1).
Variety
N
Normal
Not HC
HC
Population
% HC
Laekenois
159
131
16
0
0.00
Groenendael
5854
5000
209
248
4.24
Malinois
2767
2445
122
128
4.63
Tervuren
12957
10738
683
577
4.45
Total
21737
18314
1030
953
4.38
Table 1. Numbers of CERF and OFA eye exams conducted on Belgian Shepherds in the U.S., 1991-2016, and rates of hereditary cataracts (HC) among the four varieties of the breed. Source: ACVO Blue Book, 10th Edition (2017).
The 2017 Blue Book further categorizes eye disease
statistics according to three time periods: 1991-1999, 2000-2009, and
2010-2016. The rate of HC among eye
exams performed during 1991-1999 was 4.2%, for 2000-2009 was 4.6%, and for
2010-2016 was 3.6%. These data are insufficient to indicate a significant increasing
or decreasing trend in HC rate for the Belgian Shepherd.
The prevalence of HC among Belgian Shepherds in Europe may
be similar to that of the US population.
A total of 5,579 eye exams were performed on Belgian Shepherds
registered with the Finnish Kennel Club between 1991 and 2016 (KoiraNet
database). HC was found in 4.1% of the
exams (5.3% if suspected HC exams are included in the estimate).
Mode of Inheritance
There has been very little research on the mode of inheritance for HC in most dog breeds. However, there are a few exceptions. A recessive mode of inheritance has been proposed for the Miniature Schnauzer, Boston Terrier, Staffordshire Bull Terrier, and American Cocker Spaniel, while adominant mode with incompletepenetrance has been suggested for the Golden Retriever and Australian Shepherd (Bjerkls 2006, Mellersh 2014).
At present, only a single gene has been linked to HC. Mutations in HSF4 cause HC in the Staffordshire Bull Terrier, Boston Terrier, and Australian Shepherd (Mellersh 2014). The HSF4 gene has been specifically excluded from affecting the Groenendael and Tervuren (Mellersh 2014).
Diagnosis
In the US and several European countries, it is customary to
have breeding stock regularly examined for eye disease by a board-certified
veterinary ophthalmologist. Because eye diseases can develop any time during a
dog’s life, the ACVO recommends an eye exam be performed no more than 12 months
prior to a breeding. Since the gene(s)
causing HC in the Belgian Shepherd and most other breeds have not yet been
identified, cataracts are presumed to be hereditary when dogs of the same breed
have cataracts that share similarities in appearance, location, and age of
onset; and when these breed-specific characteristics of cataracts are observed
less frequently in most other breeds (ACVO 2017).
Prevention and
Treatment
Until the genes causing HC have been identified in the Belgian Shepherd, there is no way for a breeder to predict or prevent onset of the disease in individual dogs. The most conservative course of action is to have all breeding stock and as many of their offspring as possible regularly examined for eye disease. Affected dogs should be prevented from breeding, thus reducing the frequency of HC-associated locus and nonallelic if they arise from different gene sequences of different loci.">alleles in the Belgian shepherd population as a whole.
Although there are surgical treatments available for
cataracts in dogs, the disease rarely progresses enough in severity among
Belgian shepherds for surgery to be considered.
Involvement of the
Belgian Shepherd Health Project
Recent advances in genetic research technology and
analytical methods have led to a greater understanding of many heritable
diseases in dogs. Unfortunately, the
genetic basis of canine HC has not received a great deal of study beyond a few
breeds such as the Boston Terrier, Staffordshire Bull Terrier, and Australian
Shepherd. Research on these breeds seems to indicate that HC follows relatively
simple modes of inheritance that may only involve one or very few genes. With the capability of researchers to look
into the genome of a dog more deeply every year, it is conceivable that
HC-associated genes in the Belgian Shepherd could be discovered with a
relatively economical research program.
The Belgian Shepherd Health Project (BSHP) has been
facilitating genetic testing for dogs of our breed in the US for more than a
year. More recently we have been
encouraging collaboration on health and genetic diversity topics among Belgian
Shepherd communities in North America and Europe. We presently have more than 300 dogs in our
database linking individual genetic test results to registration and ancestry
information. All of the data has been
voluntarily submitted by breeders and dog owners. The BSHP will soon be
publishing online health surveys for owners to report on the health status of
their dogs. By compiling genetic data with ancestry, registration, and health
condition information into a single database, the BSHP is shouldering much of
the data management effort necessary for dedicated studies on the health and
diversity of our breed. These tasks
would otherwise have to be performed by paid staff at research organizations,
thus raising the cost of this work.
The BSHP is encouraging breeders and owners of dogs affected
by HC to have a genetic test performed on the dog and to enroll the dog in the
BSHP database. Embark DNA has already
expressed an interest in investigating HC in the Belgian Shepherd if a
sufficient number of affected dogs can be recruited for a study. In the meantime, the BSHP has been searching
for affected dogs using publicly accessible online databases. At present, we
have identified 85 Belgian Shepherds diagnosed with posterior, cortical
cataracts, the most common type of HC afflicting the breed. The objective of this effort is to discover
the mode of inheritance for HC based on pedigree data alone.
Concluding Remarks
It is surprising that so little research has been focused on
the genetic basis of canine HC, even though it is reported to be the leading
cause of blindness in dogs. Fortunately for our breed, HC is rarely progressive
and vision is not significantly affected in most dogs. Nevertheless, the disease does have important
consequences for Belgian Shepherds. The ACVO makes the unequivocal
recommendation that no dog of any breed that has partial or complete cataracts
should be used for breeding unless the examiner finds sufficient cause to
suspect that the case is not hereditary.
OFA, following the guidance of the ACVO, will not issue an eye certificate
for any dog having cataracts, regardless of their size or effect on vision
unless the examiner suspects the cause to be other than hereditary. Two AKC-affiliated Belgian breed clubs, the
BSCA and ABMA[2],
have ethics rules that restrict members from breeding a dog without an OFA or
CERF eye certificate. Belgian Shepherd breeders in the US and Canada have
voluntarily maintained a high level of compliance in regard to this standard.
Therefore, most of these dogs are lost to the Belgian Shepherd breeding
population in North America. Given that some large sub-populations within the
breed have exceedingly high levels of inbreeding, excluding even 5% of dogs
otherwise eligible for breeding is an impediment to maintaining genetic
diversity in the Belgian Shepherd.
The problem is different in Europe. Eye exams are not obligatory and are rarely performed in Belgium, France, Germany, or the Netherlands, among other European countries. The lack of general eye disease screening in these countries has two unfortunate consequences. The first is that the prevalence of HC cannot be known in these Belgian Shepherd populations. Secondly, dogs carrying HC alleles will continue to be bred, certainly maintaining the disease in these populations, and possibly leading to increasing its prevalence in the future.
Acknowledgements
I wish to thank Annabelle Weaver Cambier, Edrie Greer, Dr.
Sarah Maxwell, Carilee Moran, and Thea Van Neikerk for their editorial
assistance on earlier versions of this article.
Gelatt, K.N. & MacKay, E.O. (2005). Prevalence of
primary breed-related cataracts in the dog in North America. Veterinary Ophthalmology, 8(2),
101-111. Retrieved from https://doi.org/10.1111/j.1463-5224.2005.00352.x
Kyselova, Z. (2010). Different experimental approaches in
modelling cataractogenesis. Interdisciplinary
Toxicology, 3(1), 3-14. Retrieved from https://doi.org/10.2478/v10102-010-0005-3
Mellersh, C. S. (2014). The
genetics of eye disorders in the dog. Canine
Genetics and Epidemiology, 1(3). Retrieved from https://doi.org/10.1186/2052-6687-1-3
Williams, D.L., Heath, M.F., & Wallis, C. (2004). Prevalence of canine cataract: Preliminary results of a cross-sectional study. Veterinary Ophthalmology, 7(1),29-35. Retrieved from https://doi.org/10.1111/j.1463-5224.2004.00317.x
[2] I could not find a statement in ABTC or ABLA ethics rules explicitly requiring members to perform health screenings before breeding a dog.
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