Prevalence and Risk Factors for Obesity in Adult Cats from Private US Veterinary Practices

 
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Prevalence and Risk Factors for
Obesity in Adult Cats from Private
US Veterinary Practices
Elizabeth M. Lund, DVM, MPH, PhD*
P. Jane Armstrong, DVM, MBA*
Claudia A. Kirk, DVM, PhD†
Jeffrey S. Klausner, DVM
*                                                      †
 Veterinary Clinical Sciences Department                Advanced Research Department
College of Veterinary Medicine                         Hill’s Pet Nutrition, Inc.
University of Minnesota                                Science and Technology Center
St. Paul, MN                                           Topeka, KS

KEY WORDS: Epidemiology, population,                       Mixed, or Manx breeds, being fed a premi-
overweight body condition, feline                          um or therapeutic food, and concurrently
                                                           diagnosed with diabetes mellitus, neoplasia,
ABSTRACT                                                   oral disease, or dermatopathy. Practitioners
Using a cross-sectional study design, the                  can use these data to strongly advocate for
prevalence of overweight and obesity in                    the maintenance of feline patients at ideal
adult cats seen by US veterinarians during                 body condition.
1995 was determined. Risk factors for over-
weight and obesity were also determined                    INTRODUCTION
from the following variables: age, breed,                  The prevalence of obesity in domestic feline
gender, body condition score, food type,                   populations has been estimated from a limit-
reported concurrent disease, and geographic                ed number of studies. Results from these
region. Thirty-five percent of adult cats (n =             studies range from a combined overweight
8,159) were overweight or obese. From                      and obesity prevalence of 40% in a popula-
multivariate analyses, overweight cats were                tion of cats from a Danish veterinary
more likely to be neutered, male, eating a                 hospital1 to 25% in a population of adult cats
premium or therapeutic food, and to be con-                from a northeastern United States veterinary
currently diagnosed with disease of the oral               hospitals population.2 The relationship of obe-
cavity or urinary tract. Obese cats were                   sity to the development of disease in cats is
more likely to be of the Domestic Shorthair,               of much clinical interest, but has been the
Domestic Longhair, Domestic Mediumhair,                    subject of limited research. A cohort study of
This research was supported by the Mark Morris             disease risk in obese and overweight cats
Institute with a grant from Hill’s Pet Nutrition.          revealed associations with diabetes mellitus,
*Dr. Lund is now with DataDogs, Lake Elmo, MN.             lameness, and skin disease.3 More numerous
*Dr. Klausner is now with the Dean’s Office at the
University of Minnesota College of Veterinary              studies in canine populations have found
Medicine, St. Paul, MN.                                    relationships between canine obesity and
†Dr. Kirk is now with the Department of Small Animal
                                                           musculoskeletal disorders,4-6 cardiovascular
Clinical Sciences, Veterinary Teaching Hospital,
College of Veterinary Medicine, University of              problems,6 glucose intolerance and diabetes
Tennessee, Knoxville, TN.                                  mellitus,7,8 hypertension,9 immune dysfunc-

88                                                     Intern J Appl Res Vet Med • Vol. 3, No. 2, 2005
tion,10 and bladder11 and mammary cancer.12       ered in assigning a BCS in cats less than 6
Because of the known relationship of human        months of age because kittens often have
obesity to certain diseases, such as hyperten-    pendulous abdomens.)
sion, cardiovascular disease, diabetes melli-         All participating clinics were provided
tus, stroke, osteoarthritis, some cancers, and    with a training videotape on the use of the
premature mortality, it is speculated that sim-   BCS scale. Multiple body condition scores
ilar relationships may hold true for cats.13,14   over the year of data collection were aver-
Knowledge of obesity/overweight as a risk         aged to obtain a single BCS per animal.
factor for disease can heighten awareness and     Average BCS was used to place cats in either
target health screening of cats. With evidence    the overweight or obese category for this
from feline research studies as a tool, practi-   analysis. Obese animals were defined as hav-
tioners may be able to advocate more strong-      ing an average BCS greater than 4.5 and less
ly for obesity prevention and weight              than or equal to 5.0. Overweight animals
reduction plans for their clients’ pets.          were defined as having an average BCS
     The major objectives of this study were      greater than 3.5 and less than or equal to 4.5.
to determine the prevalence of overweight             Food type and form. Food categories
and obesity in adult feline populations seen      were Popular Dry, Premium Dry,
by private veterinary practitioners in the        Therapeutic Dry, Popular Canned, Premium
United States and to elucidate basic risk fac-    Canned, Therapeutic Canned, Homemade,
tors for overweight and obesity, that is, age,    Semi-moist, and Other. “Popular” described
breed, gender and neuter status, food type,       brands typically purchased in a grocery
concurrent disease, and geographic region         store, farm store, or large-format pet retail-
of residence. Such associations or risk fac-      er. “Premium” was used to define brands
tors are very important for identifying cats      typically purchased in a veterinary practice,
at risk for obesity and for identifying which     pet store, or large-format pet retailer.
diseases are more likely to afflict over-         “Therapeutic” referred to brands prescribed
weight and obese cats.                            and sold by veterinarians for the treatment
                                                  or prevention of disease. “Homemade”
MATERIALS AND METHODS                             referred to a complete diet prepared at home
The general study methods and population          from individual food ingredients. “Other”
from the National Companion Animal Study          included meat or other food products, com-
(NCAS) have been previously described in          mercial treats, or table scraps.
detail.15 Using a proprietary practice man-           The cat owner chose one major food
agement system (Advanced Veterinary               component (greater or equal to 60% of food
Systems [AVS], VetConnect Systems, Eau            by volume) and a minor food component
Claire, WI, USA), data collected included:        (remainder of food, if applicable).
date of birth, diagnoses (new and existing),      Additional food codes were added at subse-
breed, gender, neuter status, body condition      quent patient visits only if changes in feed-
score (BCS),16 major and minor food type          ing had occurred. Only major diet (greater
and form, and geographic region.                  than or equal to 60% of food fed by vol-
    Body condition score. Body condition          ume) was included in this analysis; semi-
scores were assigned as a whole number            moist food was combined with food in the
value from 1 to 5 by the veterinarian exam-       “Other” category due to the low frequency
ining the cat. A BCS of 1 indicated the ani-      of feeding semi-moist as a major food.
mal was excessively thin, 3 was ideal, and 5          Geographic region. West and East
was obese. The amount of fat cover over the       South Central were combined to form South
cats’ ribs and the abdominal contour were         Central; New England and Middle Atlantic
used in assessing body condition. (Criteria       were combined to Northeast; Pacific includ-
for assessing the abdomen were not consid-        ed Alaska and Hawaii.

Intern J Appl Res Vet Med • Vol. 3, No. 2, 2005                                              89
Diagnoses. New and existing diagnoses          lower cancer rates might be observed in
for a specific cat at each clinic visit were        Colorado. Since age is a predictor of cancer
recorded using a system for standardized            risk, the rates may reflect the older popula-
nomenclature.17 Individual diseases and dis-        tion in Florida compared with a younger
ease categories were included in the analy-         population in Colorado. Age, therefore, is
sis based on hypotheses informed by                 confounding the potential difference
literature review. Subjects were counted            between rates. Because the multivariate
once in a category for one or more diag-            model controls for confounding by age,
noses reported during the study that fell into      breed, and so on, the risk factors generated
that category.                                      are therefore independent predictors of obe-
Statistical Methods                                 sity. Variables in the multivariate models for
The adult cat population was defined as ani-        this study included age in years; 10 most
mals over 1 year of age; analyses were con-         prevalent breeds; gender; neuter status;
ducted on cats that had at least one diagnosis      major food type consumed (greater than or
(including “Healthy”) of any kind recorded          equal to 60% of food volume fed); concur-
over the study period (i.e., these cats consti-     rent disease; and geographic region of
tuted the population denominator for the            United States.
study) and had at least one recorded BCS.                Concurrent diseases and disease cate-
The SAS statistical program, 6th edition18 (SAS     gories included in the feline model included:
Institute, Cary, NC, USA), was used to gener-       arthritis, dermatopathy, diabetes mellitus,
ate frequencies, prevalence estimates, univari-     gastrointestinal disease, heart disease, hepat-
ate distributions, and multivariate analyses.       ic lipidosis, lameness, musculoskeletal dis-
                                                    ease, neoplasia, oral disease, and urinary
    Disease and disease category prevalence
                                                    disease (see Appendix). An interaction term
was estimated for obese and overweight cats
                                                    for gender and neuter status was also includ-
by BCS for all diseases included in the mul-
                                                    ed in all logistic models to account for the
tivariate analysis. Overall adult population
                                                    potential of differential risk for obesity for
prevalence estimates for obesity and over-
                                                    males versus females by neuter status. The
weight were generated using 2 methods:
                                                    referent category for food type was popu-
average BCS and reported diagnoses for
                                                    lar/dry; the West North Central region was
obesity and overweight (that is, when either
                                                    the referent category for geographic region.
“obesity” or “overweight” was entered as a
                                                    For all statistical tests, α = .05.
diagnosis for that visit by the veterinarian).
                                                         In the multivariate analysis, the preva-
    Univariate distributions were generated         lence odds ratio (OR) was used as an
to describe the prevalence of overweight and        approximation of relative risk. Relative risk
obesity by age, breed, gender, and neuter           is a measure of the strength of the associa-
status of the population. To control for con-       tion between a disease and potential risk fac-
founding of the relationship between risk           tors like age and breed. When the OR is
factors and overweight or obesity, a multi-         greater than one, the factor is found to be
variate analysis was conducted using full           associated with an increased risk of that dis-
logistic regression models.19 Confounders are       ease. For example, the OR for obesity and a
factors—for example, age or breed—that are          concurrent diagnosis of diabetes mellitus is
related to both the exposure being examined         2.2; an obese cat is more than twice as likely
and the outcome (overweight or obesity).            to be diagnosed with diabetes compared with
Confounders can distort assessment of the           an adult cat of normal/underweight body
relationship between the exposure of interest       condition (all other factors being equal)
and the outcome. For example, when com-             (Table 1). A relative risk estimate does not
paring state-specific cancer rates between          necessarily reflect a causal relationship (for
people living in Colorado versus Florida,           example, that obesity causes diabetes melli-

90                                                Intern J Appl Res Vet Med • Vol. 3, No. 2, 2005
Table 1. Multivariate Disease Models: Risk Factors for Overweight and Obese Adult Cats (N = 8,159)
                                 Risk Factors
                                   (P-value)                Odds Ratio (CI*)                β†
 Overweight Cats                   Male (.05)                  1.4 (1.0,1.8)               .31
 (n = 2,342)                      Neuter (.006)                1.4 (1.1,1.7)               .30
                           Premium foods (< .0001)             1.4 (1.2,1.5)               .31
                           Therapeutic foods (.002)            1.3 (1.1,1.6)               .24
                            Oral disease (< .0001)             1.8 (1.6,2.0)               .57
                            Urinary disease (.0002)            1.6 (1.3,1.9)               .46

 Obese Cats                Domestic Shorthair (.02)           10.5 (1.5,76.1)              2.4
 (n = 522)                Domestic Mediumhair (.03)           10.3 (1.3,78.7)              2.3
                           Domestic Longhair (.04)            8.0 (1.1,58.5)               2.1
                                   Manx (.04)                13.0 (1.1,151.1)              2.6
                               Mixed breed (.05)              8.1 (1.0,63.3)               2.1
                           Premium foods (
Table 2. Disease and Disease Category* Prevalence by Body Condition Category for Adult Cats
                                              Obese                   Overweight          Normal and
                                        (4.5
6.4%. The NCAS is
the first nationwide
study to provide
these important per-
centages. Of particu-
lar note is that more
than a third (41.0%)
of all castrated male
cats in this study
were overweight or
obese. Between the
ages of 5 and 11
years, over 40% of
all cats were over-
weight or obese. As        Figure 2. Prevalence of obesity and overweight by gender for adult cats.
expected, based on
previous research on
the effects of neuter-
ing on energy
requirements, intact
cats were underrepre-
sented in the over-
weight and obese
categories.20,21 The
data suggest that the
primary target of
overweight and obe-
sity prevention
should be young and
                           Figure 3. Prevalence of obesity and overweight by breed* for adult cats.
middle-aged,               *In order of decreasing overall breed frequency. DSH indicates domestic shorthair;
neutered cats, espe-       DLH, domestic longhair; DMH, domestic mediumhair.
cially males. Recent
canine data suggest
                                                           death of overweight and obese cats at an
that the ideal body condition range for health
                                                           earlier age than their lighter-weight counter-
maintenance for dogs is lower than previously
                                                           parts. A recent report on weight distribution
thought.22 In this study, euthanasia (end-of-life
                                                           in a large colony of cats suggests that this
decisions) due to incapacitating osteoarthritis
occurred earlier in dogs when body condition               observation is likely due to weight loss with
was not maintained within an ideal range.                  age.24 Our results suggest that it may be pru-
Although cats are not prone to developing                  dent to take steps to encourage maintenance
clinically apparent osteoarthritis as they age,            of adequate body weight in geriatric cats
arthritis is evident radiographically. Because
                                        23                 before significant weight loss is noted. This
of this and other disease risks, similar recom-            may require a change to a higher-caloric-
mendations may need to be extended to cats.                density food in aging cats even while their
     Data from this study also demonstrate                 BCS is still optimal.
that the prevalence of overweight or obesity                   Breed documentation is less certain in
diminishes in very old cats. A cross-section-              cats than in dogs, with the majority of cats
al study cannot determine whether this was                 being designated DSH, DMH, DLH or
due to weight loss in cats as they age or the              Mixed types without pedigree information

Intern J Appl Res Vet Med • Vol. 3, No. 2, 2005                                                          93
to make such categorization. Regardless,              indicate that the caloric density of premium
some useful breed-specific data emerged               foods as they are fed (in quantity or frequen-
from this study. Based on the multivariate            cy), may exceed the needs of many cats.
analysis, Manx cats were at risk for obesity.         Therapeutic foods were defined as brands
Conversely, Siamese cats, often anecdotally           prescribed and sold by a veterinarian for the
said to be prone to obesity, were not found           treatment or prevention of disease. This study
to be at risk for overweight or obesity.              did not collect data on the indication for being
                                                      fed a therapeutic food (for example, weight
    The disease associations documented
                                                      management). Presumably, some percentage
here are of particular interest as they pro-
                                                      of overweight or obese cats was being fed
vide additional evidence of the links
                                                      therapeutic foods for this reason.
between overweight or obesity and specific
diseases in cats. Both overweight and obese               Region did not emerge as a risk factor
cats are at greater risk of oral disease than         for adult cat obesity or overweight and
the rest of the adult cat population.                 could reflect the fact that the lifestyle of
Overweight cats are at increased risk for             cats across the geographic regions of the
urinary tract disease; however, a relation-           United States is fairly uniform and does not
ship between obesity and urinary tract dis-           influence the risk for obesity/overweight.
                                                      However, it is also possible that we were
ease as a category was not found, possibly
                                                      unable to discern any differences based on
due to a lack of statistical power because
                                                      lifestyle since data on whether cats lived
there were fewer obese cats and/or effects
                                                      predominately indoors or outdoors were not
of grouping many diseases into one catego-
                                                      collected. Previous studies have identified
ry. Obese cats are at increased risk for dia-
                                                      inactivity,3,26 indoor living,27 and apartment
betes mellitus, corroborating previous feline
                                                      dwelling3 as risk factors for feline obesity.
studies suggesting this association.3,25
                                                          We observed dramatic underreporting of
Obesity therefore joins gender (male-
                                                      overweight and obesity when comparing
neutered) as a risk factor for diabetes melli-
                                                      prevalence from BCS results to reported
tus.25 Obesity also increased risk for
                                                      diagnoses, especially for overweight cats.
dermatopathy and neoplasia in our study.
                                                      For example, the prevalence of obesity
The relationship between obesity and skin
                                                      defined by BCS for cats was 6.4% com-
disease supports the finding in an earlier
                                                      pared with 2.2% when defined by reported
cohort study of cats.3 An association of
                                                      diagnostic code. The difference between
obesity with neoplasia is interesting; how-           overweight prevalence in cats defined by
ever, risk for specific types of cancer would         BCS (28.7%) and reported diagnosis (1.4%)
require study designs more suited for the             was even greater. Underreporting of the
study of rare disease. Despite the greater            diagnosis of obesity and overweight may
prevalence of gastrointestinal disease in             reflect the perception of practitioners that
obese cats (Table 2), the multivariate analy-         obesity/overweight does not constitute a dis-
sis did not reveal a significant association          ease state, especially for animals in the
of obesity or overweight with this group of           overweight category. Alternatively, the
diseases.                                             knowledge that BCS was being collected for
    Feeding premium and therapeutic foods             this study may have reduced the frequency
was associated with increased risk of over-           with which overweight or obesity was
weight and obesity in this study. Premium             recorded as a diagnosis.
foods, defined for this study as nontherapeutic           Major strengths of the study reported
brands typically purchased from a veterinary          here include the size and geographic distribu-
clinic, pet store, or large-format pet product        tion of the population studied as well as the
retailer, are typically higher in caloric density     ability to generalize the results to private
than “popular” dry foods. Our results may             practice populations in the United States.

94                                                  Intern J Appl Res Vet Med • Vol. 3, No. 2, 2005
Although the data for this study were collect-            tionship. Misclassification of the outcome
ed during 1995, the scope and the results of              (overweight or obesity) by BCS must also
the study remain relevant to the contempo-                be considered as a source of differences
rary description and assessment of the obesi-             between our study results and others as the
ty problem of cats seen in private companion              assessment of body condition has not been
animal practice. Surveillance and monitoring              standardized across research studies.
of the health of US cats is especially relevant               In addition to providing important
in light of the current epidemic of human                 descriptive data on the problem of obesity
obesity. Despite the “age” of the data, the               in cats seen by practitioners in the United
NCAS provides knowledge that will enhance                 States, the factors reported in this study can
the health care of feline populations today.              help practitioners anticipate predisposition
     Limitations of the study include the use             to obesity/overweight. Feeding practices of
of a cross-sectional study design, a lack of              neutered pets may need to be more closely
standardized case definitions, and the                    guided by veterinarians, especially in
potential for underreporting of cases.15 A                regard to feeding calorie-dense premium
cross-sectional study is of limited use for               foods. Future studies should ideally include
discerning disease causality as well as risk              additional lifestyle factors such as
for rare diseases.28 For the results reported             indoor/outdoor status as variables to help
in this study, the lack of case definitions               fully model and predict obesity. A cohort
and the potential for disease misclassifica-              design would allow cats to be followed
tion must be considered. Grouping of indi-                over their lifetime, enabling determination
vidual diagnoses into system categories was               of which cats transition from overweight to
necessary to increase the efficiency of the               obesity as well as to elucidate whether
statistical analysis and to minimize error                obese cats are dying at younger ages than
from disease misclassification. The failure               their normal-weight cohorts. Finally, with
of this study to document a risk relationship             these study results, the ability of practition-
between overweight or obesity and an                      ers to communicate the importance of
uncommon disease, such as hepatic lipido-                 maintaining feline patients at ideal body
sis, does not prove or disprove such a rela-              condition will be strengthened.

APPENDIX. Diseases Diagnosed by Veterinarians in Study Cats
Dermatopathy (prevalent diagnoses > .05%): acne, alopecia (general, psychogenic, pustular), dandruff,
dermatitis (allergic, flea bite, food allergy, foxtail, fly bite, general, miliary, moist, paronychial, pruritic,
seborrheic), dermatophytosis, folliculitis, ingrown toenail, pyoderma (deep, general).
Gastrointestinal Disease (prevalent diagnoses > .05%): anal sac disease, chronic diarrhea, colitis (acute,
general), constipation, enteritis, gastritis, gastritis (acute, general), gastroenteritis (bacterial, eosinophilic,
general), inflammatory bowel disease (eosinophilic, lymphoplasmacytic, mixed), megacolon.
Heart Disease (prevalent diagnoses > .05%): congestive heart failure, cardiomyopathy (congestive, dilat-
ed, general, hypertrophic, taurine deficiency), heart disease, heart murmur (acquired, general), hyperten-
sion.
Musculoskeletal Disease (prevalent diagnoses > .05%): arthritis, cruciate ligament rupture, patellar luxa-
tion, soft tissue injury, osteomyelitis.
Neoplasia (prevalent diagnoses > .05%): adenocarcinoma, basal cell carcinoma, fibrosarcoma, lipoma,
lymphosarcoma, mammary tumor, mast cell tumor, squamous cell carcinoma, tumor-unspecified.
Oral Disease (prevalent diagnoses > .05%): cervical lesion, dental caries, dental calculus, dental calcu-
lus/gingivitis, fractured tooth, gingivitis, periodontal disease, ptyalism, stomatitis.
Urinary Tract Disease (prevalent diagnoses > .05%): acute cystitis (acute, bacterial, chronic, general),
bladder stone(s), feline urologic syndrome, obstruction, urinary tract infection, urolithiasis (calcium
oxalate, struvite, unspecified).

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