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© 2006 American College of Veterinary Ophthalmologists
Veterinary Ophthalmology
(2006)
9
, 5, 341–349
Blackwell Publishing Inc
CLINICAL ARTICLE
Prevalence of feline cataract: results of a cross-sectional study of 2000
normal animals, 50 cats with diabetes and one hundred cats following
dehydrational crises
David L. Williams* and M. Fred Heath†
*
Queen’s Veterinary School Small Animal Hospital and
†
Farm Animal Epidemiology and Informatics Unit, Department of Clinical Veterinary Medicine,
University of Cambridge, Madingley Road, Cambridge, England, CB3 0ES, UK
Abstract
Objective
In this study 2000 normal cats, 50 cats with diabetes and 100 cats with a
history of dehydrational crises were examined ophthalmoscopically to determine
presence of cataract.
Materials and methods
The cats examined were predominantly from veterinary hospital
populations but also from re-homing facilities and breeding catteries. Prevalence of
cataract was determined for different age groups (year cohorts). The age at which
prevalence of cataract was 50% (C
50
) was determined indirectly from a fitted prevalence
curve as previously described. C
50
was determined for animals of different genders and
different breeds as well as for those with diabetes and histories of dehydrational episodes
related to chronic renal failure, chronic vomiting or chronic diarrhea.
Results
The mean
±
standard deviation of C
50
for all normal cats in the study was
12.7
±
3.4 years. All cats over 17.5 years were affected by some degree of lens opacity.
C
50
for cats with diabetes was 5.6
±
1.9 years (significantly different from normal cats at
P
< 0.0001). For cats with a history of dehydrational crises C
50
was 9.9
±
2.5 (difference
from normal cats nearing statistical significance at
P
= 0.06).
Conclusion
The study yields novel findings regarding the prevalence of age-related
cataract in normal cats together with cats with diabetes and history of previous
dehydrational episodes in which prevalence of cataract is increased.
Key Words:
age-related, cataract, dehydration, diabetes, epidemiology, feline, lens
Address communications to:
D. L. Williams
Tel.: +44 1223 232977
Fax: +44 1223 232977
e-mail:
doctordlwilliams@aol.com/
dLw33@cam.ac.uk
INTRODUCTION
We have previously reported the prevalence of cataract in
2000 dogs,
1
providing data for the prevalence of lens opaci-
fication in dogs at different ages. In this study we undertook
the same size study for the cat but also sought to assess
cataract prevalence in cats with diabetes and in those with a
history of previous dehydrational episodes. Cats are generally
accepted to have a lower prevalence of cataract than dogs.
2
While age-related cataract is reported in the dog, feline lens
opacification is generally noted as either congenital or
secondary to nutritional abnormalities, trauma or uveitis.
Specifically, diabetic cataracts have been reported as rare in
cats in contradistinction to the situation in dogs,
3
apart from
occasional cases in young kittens.
4
However, as we noted in
our previous canine study, such anecdotal statements have
yet to be supported or disproved by large-scale epidemiologi-
cal studies. Here we seek to provide a cross-sectional survey
to provide firm evidence of the prevalence and nature of age-
related cataracts in cats.
Before we start, however, we might ask, from a compara-
tive perspective, whether a large study to determine cataract
prevalence in the cat is warranted. Globally it has been esti-
mated from the most recently published studies based on
surveys in 2002, that 22.8 million people are blinded by
cataract in the developing world.
5
Several large cross-sectional
population-based studies show that around 75% of people
over the age of 75 have sight-impairing lens opacification.
6
Definitive documentation of the later onset of age-related
cataract in the cat compared with the dog is, we suggest, the
first step to defining what it is in the cat lens that delays the
onset of age-related cataract, and renders these opacities less
342
© 2006 American College of Veterinary Ophthalmologists,
Veterinary Ophthalmology
,
9
, 341–349
sight-threatening than is the case in canine age-related cata-
ract. Such future work could have significant implications
for the prevention of human age-related cataract. This pre-
liminary study to determine the prevalence of cataract in cats
of different ages forms a basis for studies to identify what
factors are associated with the slower development of
cataract in cats compared with the rate of age-related lens
opacification in dogs.
The human ophthalmic literature is well served with
large cross-sectional population-based studies of ocular
disease. The Beaver Dam study, for example, involves ocular
examination of 4926 people from one geographic area in
Wisconsin to determine prevalence
7
and incidence
8
of vari-
ous ocular conditions including cataract, while in the Blue
Mountains study over 3600 people from a region west of
Sydney in Australia were subject to detailed ophthalmic
examination with similar epidemiological end points.
9
Per-
forming a geographic population-based survey of the pet
animal population is exceptionally difficult because census
data for animals do not exist. It is possible, however, to
examine cats presenting for nonocular conditions in first
opinion and referral clinics as well as those normal animals
housed in catteries and re-homing facilities, and thus to
survey a relatively large number of animals from an ophthal-
mologically unremarkable population. Having undertaken
such a cross-sectional study of normal cats what questions do
we propose to ask of the data?
In his review of human age-related cataract, Taylor noted
five ‘D’s as cataractogenic factors: daylight, diet, dehydra-
tion, diabetes and don’t know!
10
We cannot in this study
population assess the influence of daylight and diet, and will
seek to cover those areas in future reports of studies cur-
rently in progress in various species and population groups.
However, the cat provides an excellent species for the deter-
mination of cataract prevalence in animals affected by both
diabetes and dehydration: we postulate that cats with a history
of diabetes or dehydrational episodes will have an increased
prevalence of lens opacities. To determine this requires a
normal population against which to compare the animals
with diabetes and previous dehydration; the 2000 normal
cats in this study provide just such a control group. In our
canine study we specifically excluded animals with catarac-
togenic systemic conditions such as diabetes and hypo-
calcemia. In this study we sought to examine 2000 similarly
normal cats but also to assess cataract prevalence in animals
with diabetes and with previous dehydrational episodes.
Taylor’s ‘don’t know’ is most likely to be predominantly a
genetic predisposition to increased age-related cataract.
11
In
our canine study we showed a breed-related difference in
prevalence of age-related cataract, with the C
50
value for dif-
ferent breeds being related to their longevity. Is the same
association seen in the cat? Although Michel’s previous work
provided longevity data for different breeds of dog, there is
no definitive data on the average lifespan of different cat
breeds but for a paper on cemetery records for cats from the
Far East in the mid 1980s,
12
data which are probably not of
great relevance to the cat population in this study. Anecdotal
reports from cat breeders give some indication of the expected
lifespan of specific breeds. http://www.petplanet.co.uk/
petplanet/breeds/catbreedprofile.asp, for instance, states:
‘The Persian Longhair may be expected to live for about 10–
12 years while Siamese often live well into their late teens
and it is not unheard of for them to live into their twenties’.
Such anecdotal information can hardly, however, be con-
sidered scientifically reliable. We have thus, for these two
breeds, undertaken to provide an age structure for pet popu-
lations seen in a group of first opinion veterinary clinics.
While this cannot be considered as a definitive measure of
longevity, it seeks to provide some confirmation of the state-
ment on relative longevity obtained from the Internet.
This study only provides preliminary results for 2000 nor-
mal cats of different breeds and for 50 diabetic animals and
one hundred cats with a history of previous dehydrational
episodes. Yet we hope that these results for small numbers of
cats will demonstrate the value of using a spontaneous
animal model in the investigation of factors important in
age-related cataractogenesis, and will form the basis for
future research in companion animal age-related cataract.
MATERIALS AND METHODS
Animals
Cats from a number of populations were examined ophthal-
moscopically as detailed below. The predominant group of
normal animals was drawn from cats examined in eight first
opinion clinics visited regularly by DLW (1045 cats). Three
hundred and eighty-one normal cats were taken from the
hospital population at the Queen’s Veterinary School
Hospital, Department of Clinical Veterinary Medicine,
University of Cambridge, while 504 cats were examined at
the Wood Green Animal Shelter, a large re-homing center
where most cats were placed following geriatric owners
entering residential care or where owners could not care for
the animal after marital break-up. Forty Siamese cats and 30
Persian cats were examined in breeding catteries. The fact
that a substantial number of the apparently normal animals
examined were drawn from hospitalized populations is a
potential failing in the study, as will be discussed further
below, but no animals in this group were hospitalized for
cataractogenic conditions and were otherwise ophthalmo-
logically unremarkable.
The 50 diabetic animals and one hundred animals with a
history of dehydration were drawn from the first opinion
and referral hospital populations. Diagnostic criteria for
diabetes included persistent blood glucose elevated above
10
µ
13
and a circulating fructosamine equaling or exceed-
ing 400
µ
.
14
Eighteen animals had insulin resistant diabetes
associated with acromegaly. Animals included in the
dehydrated group had a history of chronic renal failure (85
cats), diarrhea (8 cats) or vomiting (17 cats) necessitating
intravenous fluid therapy at least three times in the previous
12 months.
© 2006 American College of Veterinary Ophthalmologists,
Veterinary Ophthalmology
,
9
, 341–349
-
343
Ophthalmic examination
All animals were examined were screened using distant
direct ophthalmoscopy at 0D and direct ophthalmoscopy at
+10D. Pupils were dilated only by examining the animal in
the dark, rather than by pharmacological intervention, in
order to ensure that the investigation was entirely noninva-
sive and thus ethically valid, as owner consent was not avail-
able for a proportion of the animals examined. As discussed
below this may be considered as a significant methodological
failing of the study, yet pupil dilation sufficient to examine
the majority of the lens was achieved merely by examination
in the dark. The status of both lenses (either unaffected, with
nuclear sclerosis or with frank cataract) was documented and
if cataract was present the lens was examined by portable
slit-lamp biomicroscopy (Kowa SL-14 slit lamp, Kowa,
Tokyo, Japan). The position and extent of the lens opacity
was documented graphically and scored using a scale from 0
(clear lens or nuclear sclerosis without opacity) to 10 (mature
cataract). To avoid interobserver error only one ophthalmo-
logist (DLW) examined all cats.
Statistics
The prevalence of cataract at different ages for the specific
population sampled was derived from the cross-sectional data
obtained in the study as described below. The proportion of
animals affected in each year group was plotted to obtain a
graph of prevalence proportion against age cohort. The pre-
valence proportion of an age cohort for an irreversible condition
such as cataract is an estimate of the cumulative probability
of onset up to and including that age. If it is assumed that the
age at onset is normally distributed, by fitting a normal cumu-
lative distribution curve to these data, using Microsoft Excel
software, the mean and standard deviation of the underlying
normal distribution of age of onset can be extracted. The
age at which cataract prevalence was 50%, here termed C
50
,
given the similarity to LD
50
in toxicological studies, is the
mean from the cumulative probability curve. C
50
for cataract
for different subgroups may be compared by Student’s
t
-test,
using the means and standard deviations extracted from
the fitted curve with the degrees of freedom based on the
number of age cohorts used to determine the curve.
RESULTS
The median age of the 2000 cats examined was 8.3 years.
The age structures of the four populations examined (refer-
ral hospital), first opinion clinic and nonhospital (re-homing
center/breeding kennels) populations were not statistically
different apart from a higher number of younger cats in the
referral hospital population (Fig. 1). The overall cataract
prevalence showed a C
50
of 12.7
±
3.4 years. The cataract
prevalence by year group of these different populations was
somewhat different, however, with C
50
of 12.1
±
2.7, 13.5
±
3.5 and 14.5
±
3.6 in referral hospital, first opinion clinic
and nonhospital populations, respectively (Fig. 2). However,
the nonhospital population was difficult to model, as the
prevalence of cataract in the first year of life was greater than
zero, the age structure of this population being significantly
different from the others at
P
= 0.02.
There were only sufficient data for analysis of C
50
values
for male and female neutered animals rather than entire cats
and for the Domestic Short-haired, Persian and Siamese
breeds. The prevalence of age-related cataract in female and
male neutered cats differed as is shown in Fig. 3, mirroring the
difference in the age structure of the two gender populations
(Fig. 4). C
50
for the female neutered cats was 12.9
±
3.2 years
and for the male neutered cats was 11.7
±
2.8, these not
being statistically significantly different. Cataract prevalence
for Persian and Siamese cats is shown in Fig. 5. The age
Figure 2. Cataract prevalence by year group in different populations
examined. Black: all cats examined; red: veterinary hospital referral
population; blue: first opinion clinic population; green: nonhospitalized
population.
Figure 1. Age structure of the populations sampled. Black: All cats
examined; red: veterinary hospital referral population; blue: first
opinion clinic population; green: nonhospitalized population.
344
© 2006 American College of Veterinary Ophthalmologists,
Veterinary Ophthalmology
,
9
, 341–349
structures of populations of Persian, Siamese and Domestic
Short-haired cat breeds in a first opinion clinic population of
2388 cats are shown in Fig. 6.
Prevalence curves of different cataract types are shown in
Fig. 7. C
50
for nuclear sclerosis is 14.6
±
4.1 years, for nuclear
opacities is 16.5
±
3.2 years, for cortical cataract 14.9
±
3.7
years and for posterior subcapsular cataract, 17.4
±
4.6 years.
These cataract groups were not easy to model, as the preva-
lence did not reach 100% at any age group.
The majority of cataracts were small linear opacities in the
posterior cortex and, although older animals generally had
more pronounced opacities, very few had mature lens opa-
cities. A number of cats also exhibited congenital cataracts or
opacities associated with trauma or intraocular inflammation.
Figure 6. Age structure of three cat breeds examined in first opinion
clinic population of 2388 cats. Black: Domestic Short-haired; red:
Siamese; blue: Persian.
Figure 7. Prevalence curves for different cataract types. Black: nuclear
sclerosis; red: posterior cortical cataract; green: nuclear cataract; blue:
posterior polar subcapsular cataract.
Figure 3. Prevalence of cataracts in cats examined by gender.
Figure 4. Age structure of cats examined by gender.
Figure 5. Cataract prevalence in three cat breeds examined. Black:
Domestic Short-haired; red: Siamese; blue: Persian.
© 2006 American College of Veterinary Ophthalmologists,
Veterinary Ophthalmology
,
9
, 341–349
-
345
Six representative ophthalmic photographs of different types
of cataract are shown in Fig. 8. Note that these photographs
were taken focusing on the opacities, so where these were
posterior cortical the iris may appear out of focus because it
lies in a more anterior focal plane.
Prevalence of cataracts in the diabetic and dehydrated
populations is shown in Fig. 9. C
50
for cataract in diabetic
cats was 5.6
±
1.9, differing from that of the normal popula-
tion with a high statistical significance at
P
< 0.001, although
the unusual distribution of these data rendered modeling of
the C
50
value difficult. Of cats with diabetes all but two
animals had some degree of lens opacification. Twenty-two
of these cats had linear posterior cortical opacities similar
to those seen in the normal, aged cats. Twenty-six had more
pronounced cortical cataracts or posterior subcapsular
plaques. Photographs of cataracts in six representative
diabetic cat eyes are shown in Fig. 10. C
50
for cataract in
dehydrated cats was 9.9
±
2.5 years, differing from that in
the normal population with a
P
-value nearing significance
(
P =
0.06). All of the dehydrated cats had posterior cortical
linear opacities similar to those seen in normal, aged cats,
although on occasion these were multiple. Photographs of
cataracts in six representative eyes from dehydrated cats are
shown in Fig. 11.
DISCUSSION
The data accrued in this survey demonstrate that prevalence
of cataract in the general feline population increases with
age and that by the age of 17 years all cats were affected by
some degree of lens opacity. This compares with the age of
13.5 years for all dogs to be affected, as shown in our previ-
ous study of 2000 dogs. While the dog has an average C
50
of 9.4
±
3.3 years, the cat has a significantly later C
50
of
12.7
±
3.5 (
P =
0.0005). This difference is not particularly
surprising, given that the lifespan of cats is significantly
greater than that of dogs.
15,16
Age-related cataract would
seem to occur at a similar proportion of lifespan for these
different species, although, clearly, considerable further
research is required to substantiate such a hypothesis. This
difference in age at cataract onset probably reflects the
longevity of these two different species; age-related cataract
would appear to occur at a specific fraction of the total
lifespan of the individual. It is undisputed that, on average,
cats live longer than dogs, so this difference in mean C
50
is
not unexpected. Such a correlation between the C
50
value
and lifespan was found in our previous study of 2000 dogs
and does exist between the Persian and Siamese breeds in
this study. The attempt at making this correlation is ham-
pered by the lack of definitive longevity data for different cat
breeds. Our evaluation of population ages of cats seen in
several of the clinics visited during this survey aimed to estimate
maximum lifespan in Domestic Short-haired, Persian and
Siamese cats. The very rapid decline in older Persians may
Figure 8. Cataracts in normal-aged cats.
(a) 15-year-old, male neutered Domestic
Short-haired cat with three linear posterior polar
opacities. (b) 17-year-old, female spayed Burmese
cat with nuclear and posterior cortical dot opacities.
(c) 13-year-old, female spayed Domestic
Short-haired cat with nuclear sclerosis and hazy
nuclear opacities. (d) 17-year-old, male neutered
Siamese cat with posterior cortical dot opacities.
(e) 12.5-year-old male neutered Domestic
Short-haired cat with posterior cortical short linear
and dot opacities. (f) 20-year-old female spayed
Domestic Short-haired cat with pronounced linear
posterior cortical and capsular opacities.
Figure 9. Prevalence of cataracts in normal, diabetic and dehydrated
cats. Normal: black; diabetic red; dehydrated: blue.
346
© 2006 American College of Veterinary Ophthalmologists,
Veterinary Ophthalmology
,
9
, 341–349
Figure 10. Cataracts in diabetic cats. (a) 18-year-old, male neutered Domestic Short-haired cat, diabetic for 3 years with pronounced nuclear and
posterior cortical opacities after pharmacological mydriasis – dense focal paracentral opacity is corneal. (b) 15-year-old, female spayed Domestic
Short-haired cat, diabetic for 2 years with nuclear sclerosis and posterior cortical opacities. (c) 12-year-old, female spayed Domestic Short-haired cat,
diabetic and acromegalic for 6 months with pronounced anterior and posterior opacities and nuclear sclerosis. (d) 11-year-old, male neutered
Domestic Short-haired cat, diabetic for 18 months with posterior subcapsular focal opacity, posterior cortical dot opacities, perinuclear plaque
opacity and early nuclear sclerosis. (e) 13-year-old, female spayed Domestic Short-haired cat, diabetic for 2 years with posterior subcapsular plaque
and nuclear sclerosis. (f) 14-year-old, female spayed Domestic Short-haired cat, diabetic for 6 months with early nuclear sclerosis and posterior
cortical hazy opacities.
Figure 11. Cataracts in dehydrated cats. (a) 13-year-old, female spayed Domestic Short-haired cat with recurrent episodes of diarrhea for 2 years,
requiring intravenous fluid therapy on four occasions with posterior cortical dot opacities. (b) 10-year-old, male neutered Domestic Long-haired cat
w
ith chronic diarrhea for 3 months requiring intravenous fluid therapy on three occasions with posterior cortical dot opacities. (c) 12-year-old,
female spayed Domestic Short-haired cat with persistent episodes of vomiting for 2 years requiring intravenous fluid therapy on four occasions with
two linear posterior cortical opacities. (d) 13-year-old, female spayed Domestic Short-haired cat with chronic renal failure requiring intravenous fluid
therapy on three occasions with one linear posterior cortical opacity. (e) 14-year-old, male neutered Domestic Short-haired cat with chronic renal
failure for 2 years requiring intravenous fluid therapy on three occasions with posterior cortical and subcapsular opacities. (f) 15-year-old, female
spayed Domestic Short-haired cat with chronic renal failure for 2 years, requiring intravenous fluid therapy on three occasions with area of posterior
cortical hazy opacity.
© 2006 American College of Veterinary Ophthalmologists,
Veterinary Ophthalmology
,
9
, 341–349
-
347
be associated with the high prevalence of polycystic kidney
disease (PKCD) in this breed and highlights the difficult in
estimating true longevity: are cats dying of PCKD really
dying of old age? Such questions must be postponed for a
research paper on feline longevity and are peripheral to the
current research. What is more relevant is that, while the
cataract prevalence curve of Domestic Short-haired cats lies
close to that of the Persian breed (Fig. 5), the population age
structure shows the longevity of the Domestic Short-haired
cat to be closer to the Siamese. The explanation to this may
be that the Domestic Short-haired is less of a specific breed
than a cross-breed. Resolution of this conundrum may have
to wait until we have more concrete longevity data based on
age at death in a substantially sized population, rather than
the data on living aged cats as presented here.
Different types of cataract have different C
50
values,
possibly reflecting different etiopathologic pathways in the
development of nuclear, cortical and subcapsular cataract,
as first conjectured by Chylack in the mid-1980s.
16
Photo-
oxidation of crystalline proteins is widely held to be a key
factor in age-related cataractogenesis, as discussed more
fully in a companion paper in this issue of
Veterinary
Ophthalmology
.
17
The fact that nuclear proteins are present
in the eye prior to birth leads one to expect that they would
be most affected of all the lens tissue by photo-oxidative
change. That said, the nuclear opacities in this study are less
common and generally occur later (i.e. with a greater C
50
value) than do posterior cortical opacities. Nuclear sclerosis
is less commonly seen in the cat than in the dog and does not
necessarily seem to be the precursor to nuclear opacification
as it is in the canine lens. Further work on this will include a
longitudinal study of the development of lens opacities in a
smaller population of older cats. This is currently under
development. The cortical opacities seen in aged cats are
most commonly small linear opacities in the central and
paracentral posterior cortex, probably representing opacifi-
cation of individual lens fibers.
The data on lens opacities in normal cats are an essential
backdrop to investigation of cataracts in animals with
diabetes and previous dehydrational episodes. Both of these
diseased populations are made up of older cats and, without
the prevalence curve for normal animals, it would be impos-
sible to assess whether a cataract in a specific individual was
related to its disease or merely a function of its age. Figure 9
demonstrates that cataracts in both these diseased popula-
tions do occur at an earlier age than do lens opacities in com-
paratively aged normal cats.
Many of the cataracts in the diabetic cats were small linear
opacities similar to those seen in the majority of normal,
aged cats, but 56% of the diabetic cataracts were more
pronounced posterior cortical opacities or posterior polar
subcapsular plaques, as shown in Fig. 10. The data showing
that almost all diabetic cats have lens opacities is contrary to
findings reported to date in the literature. Salgado
et al.
, in
the only large survey comparing diabetic cataracts in dogs
and cats, state specifically that cataract is rare in diabetic cats,
in distinction to the case in dogs.
3
We have shown here that
lens opacities are common in diabetic cats, occurring in the
vast majority of animals. How can this substantial difference
be explained? We suggest that this disparity is related to
the appropriation methods of the two studies. Salgado
et al.
analyzed diabetic cats and dogs using a retrospective survey
of case records. Small cataracts may well have been missed
using such a survey scheme; none of the diabetic cataracts
here were mature blinding opacities and few of the diabetes-
associated lens changes in our series (Fig. 10) would have
been noted without a specific close ophthalmic examination.
The fact that we have noted lens opacities in almost all of the
diabetic cats does not mean that the lens changes in diabetic
animals from these two species are to be seen as identical or
equivalent. Cataracts in diabetic cats resemble cataracts in
diabetic humans much more closely than do the mature
cataracts characteristically seen in diabetic dogs.
18
Mechanisms
of diabetic cataractogenesis in the cat may involve processes
other than the sorbitol generation classically considered
responsible for the rapid development of the mature cataract
characteristic of the diabetic dog.
19
Other biochemical
processes such as protein glycation and the formation of
advanced glycation end products (AGEs) through the Mail-
lard reaction are considered important in human diabetic
cataractogenesis,
20,21
and may also occur in the cat. Indeed,
research from the Zurich group comparing the ratios of
NADPH reductases and sorbitol dehydrogenase in the dog
and cat lens suggested that sorbitol would accumulate at a
faster rate in the cat than the dog lens; the difference in
diabetic cataractogenesis between the two species cannot
be explained by differences in enzyme ratio.22 Further work
from the same group working under Professor Bernard
Speiss demonstrated posterior cortical opacities in feline
lenses cultured in high glucose concentrations though these
occurred in young but not old cats.23 In that study canine
lenses developed equatorial vacuoles but not the characteris-
tic mature cataract seen in 80% of dogs within 16 months of
diagnosis in one recent study.24 Clearly there is considerable
work to be done on the pathogenesis of diabetic cataract in
the dog and cat and the differences between the two species.
Nonetheless, the findings in the present study are important
in showing that almost all diabetic cats do have some signs of
lens opacification, although substantially different from that
seen in dogs.
Figure 9 also shows that cats with a history of previous
dehydrational episodes requiring intravenous fluid therapy
develop opacities in the lens at an earlier age than do normal
cats. Again, as these cats are usually older animals, the base-
line data on 2000 normal cats is needed, to compare against
cataract prevalence in the dehydrated population. As noted
above, dehydration is one of Taylor’s five ‘D’s as factors pre-
disposing to age-related cataractogenesis. Harding has par-
ticularly championed this as a major influence on the high
prevalence of cataract in the tropical developing world,25
suggesting that increased lens protein carbamylation may be
responsible for the cataract formation with dehydration.26
348
© 2006 American College of Veterinary Ophthalmologists, Veterinary Ophthalmology, 9, 341–349
Other groups have not considered dehydration as important:
those promoting daylight as the key factor in cataractogene-
sis in tropical areas may fail even to recognize it – Robman
does not mention it in a recent survey of cataractogenesis
from an Australian perspective, where clearly dehydration is
of little importance.27 Consider India, however, and the
picture is very different. Zodpey et al., working on a hospital-
based population in Nagpur, Maharashtra State, show a
relative risk of 3.1 of developing cataract in dehydrated
individuals28 while Minassian et al., working on a similar
central Indian population, estimated that dehydration from
diarrhea or heatstroke might account for around a third of
cataract cases in the affected area.29 A second study on a dif-
ferent group, both geographically and from a socioeconomic
perspective, strongly confirmed these findings with an esti-
mated 38% of blinding cataract cases arising from repeated
dehydrational crises.30 It was in view of this research that
we sought to investigate cataract prevalence in dehydrated
cats.
We chose to take a historical perspective on the diagnosis
of dehydration sufficiently severe and persistent or recurrent
to cause lens changes similar to those seen in the Indian
studies in humans. Thus, cats with three or more episodes of
dehydration requiring intravenous fluid administration were
included in the study. This necessarily resulted in a hetero-
genous case population; cats with chronic renal failure,
chronic diarrhea and chronic vomiting were all included and
are not separated in the dehydrated group data. This could
clearly be seen as a failing of the study; a wide variety of
concurrent diseases was present in the population, some
of which could have cataractogenic potential. Specifically,
it may be that uremia is in itself cataractogenic,31 that hypo-
calcemia associated with renal failure can be responsible for
cataract formation,32 or that oxidative stress, seen in chronic
renal failure, is an important factor.33,34 Oxidative stress and
reduced levels of intra-erythrocytic glutathione has been
suggested as one of the causes of anemia associated with
chronic renal failure,35 but to our knowledge the association
with cataracts has not previously been linked to intralenticu-
lar oxidative stress. The fact that we have seen similar cata-
racts in cats with chronic renal failure, chronic diarrhea and
chronic vomiting, suggests that it is the common factor here,
dehydration, which is the factor responsible for the cataract
formation noted in these cats. The fact that the lens opacities
are the same in form as those seen in older cats, suggests that
dehydration is merely an acceleration of processes already
giving rise to age-related cataract; however, the confirma-
tion of such a hypothesis requires further clinical research.
We aim to undertake a more comprehensive study of cataract
formation in dehydrated cats, specifically using a case-control
methodology rather than a cohort analysis as documented
here. The difference in cataract prevalence between the
dehydrated and normal groups nears, but does not reach,
statistical significance at P = 0.06. We hope that a case-
control methodology with a larger population size will
allow significance to be reached. Nevertheless, here we have
shown that increased cataract prevalence is seen in cats with
a history of recurrent dehydration, a finding which is novel
in a nonhuman population.
CONCLUSION
We have sought in this cross-sectional study to document
cataract prevalence in a sizeable cat population, which, while
having the disadvantage of being drawn mostly from a veteri-
nary hospital setting, does provide concrete evidence with
regard to the prevalence of cataract in the aging cat as well as
showing increased numbers of cataracts in cats with diabetes
and previous dehydrational episodes. It is hoped that this
work will serve as a foundation for further in-depth studies
of lens opacification in companion animal species.
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