Epidemiology of surgical castration of dogs and cats in the United States

Rosalie Trevejo College of Veterinary Medicine, Western University of Health Sciences, Pomona, CA 91766

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 DVM, PhD, DACVPM
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Mingyin Yang Banfield, The Pet Hospital, Applied Research & Knowledge Team, 8000 NE Tillamook St, Portland, OR 97212.

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Elizabeth M. Lund Banfield, The Pet Hospital, Applied Research & Knowledge Team, 8000 NE Tillamook St, Portland, OR 97212.

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Abstract

Objective—To estimate the prevalence of surgical castration among dogs and cats evaluated at private US veterinary hospitals and to determine the influence of sex, age, breed, geographic location, and prepaid wellness plan enrollment on the likelihood of castration.

Design—Retrospective period prevalence study.

Animals—320,172 cats and 1,339,860 dogs examined at 651 hospitals during 2007

Procedures—Univariate and multivariate analyses were used to compare prevalence among subpopulations for each species.

Results—The overall prevalence of castration was 82% in cats and 64% in dogs. Prevalence increased significantly with age in both species. Among cats, males were slightly more likely to be castrated than females (prevalence ratio [PR] = 1.03) and mixed breeds slightly less likely than purebreds (PR = 0.99). Among dogs, males were less likely to be castrated than females (PR = 0.93) and mixed breeds more likely than purebreds (PR = 1.19). Prevalence was lowest in dogs in the Southeastern United States (61%). Dogs and cats on a wellness plan were more likely to be castrated than those not on a plan (PR = 1.33 and 1.18, respectively). Among commonly reported dog breeds, pit bull-type dogs (27%) and Chihuahuas (46%) were least likely to be castrated.

Conclusions and Clinical Relevance—Many young adult (1- to < 4-year-old) dogs (32%) were uncastrated, signaling a need to promote earlier castration. Outreach efforts should be directed toward owners of pets least likely to be castrated, such as male dogs, dogs of specific breeds (ie, pit bull-type and Chihuahua), and dogs in the Southeastern United States. Additional research is needed to evaluate the potential impact of wellness programs on an owner's decision to have his or her pet castrated.

Abstract

Objective—To estimate the prevalence of surgical castration among dogs and cats evaluated at private US veterinary hospitals and to determine the influence of sex, age, breed, geographic location, and prepaid wellness plan enrollment on the likelihood of castration.

Design—Retrospective period prevalence study.

Animals—320,172 cats and 1,339,860 dogs examined at 651 hospitals during 2007

Procedures—Univariate and multivariate analyses were used to compare prevalence among subpopulations for each species.

Results—The overall prevalence of castration was 82% in cats and 64% in dogs. Prevalence increased significantly with age in both species. Among cats, males were slightly more likely to be castrated than females (prevalence ratio [PR] = 1.03) and mixed breeds slightly less likely than purebreds (PR = 0.99). Among dogs, males were less likely to be castrated than females (PR = 0.93) and mixed breeds more likely than purebreds (PR = 1.19). Prevalence was lowest in dogs in the Southeastern United States (61%). Dogs and cats on a wellness plan were more likely to be castrated than those not on a plan (PR = 1.33 and 1.18, respectively). Among commonly reported dog breeds, pit bull-type dogs (27%) and Chihuahuas (46%) were least likely to be castrated.

Conclusions and Clinical Relevance—Many young adult (1- to < 4-year-old) dogs (32%) were uncastrated, signaling a need to promote earlier castration. Outreach efforts should be directed toward owners of pets least likely to be castrated, such as male dogs, dogs of specific breeds (ie, pit bull-type and Chihuahua), and dogs in the Southeastern United States. Additional research is needed to evaluate the potential impact of wellness programs on an owner's decision to have his or her pet castrated.

Pet overpopulation is a serious problem that results in animal abandonment and relinquishment, places a strain on animal shelters and shelter personnel, and leads to the euthanasia of unwanted pets.1–3 Population control of dogs and cats is recognized by the AVMA as a primary welfare concern of American society.4 In addition, there are behavioral and other health benefits to castrating (spaying or neutering) dogs and cats as opposed to leaving them sexually intact, such as decreased urine spraying in male cats and decreased incidence of certain neoplasms of the reproductive tract (ie, mammary gland and ovarian or uterine tumors) in female dogs and cats.5 There are many potential reasons pet owners may choose not to have their pet castrated, including but not limited to cost of the procedure, desire to breed their pet, personal beliefs (eg, procedure is painful or unnecessary), concern about potential detriments (eg, resulting obesity, surgical complications, and increased risk of prostatic neoplasms in castrated male dogs5,6), or simple neglect. Some US counties have imposed restrictions such as requiring breeders without a kennel license to obtain a dog breeding license and placing annual limits on the number of litters per dog per household.7,8 However, such regulations are ineffective when owners ignore the laws or certain species (ie, cats) are exempted.

Population-based data on the prevalence of castration among pet dogs and cats are essential for establishing a baseline by which temporal and regional trends can be evaluated. Such data would permit the evaluation of the effectiveness of client education programs, introduction of regulations pertaining to the castration of pets, innovative approaches to making castration services available or more affordable for those living on a low income, and other pet population control strategies. In addition, the identification of factors associated with castration would be useful for developing focused outreach and public education campaigns to more effectively target and motivate pet owners that are at highest risk of not having their pets castrated. The objectives of the study reported here were to estimate the prevalence of castration in dogs and cats evaluated at private veterinary hospitals in the United States and to identify factors associated with the prevalence of castration.

Materials and Methods

Animals and study setting—Data used for the study were obtained from the electronic medical records of all cats and dogs examined at US veterinary hospitals owned by Banfield, The Pet Hospital, between January 1, 2007, and December 31, 2007. During this period, Banfield operated 651 primary-care hospitals in 45 states (no hospitals in Alaska, Hawaii, Maine, North Dakota, Wyoming, or the District of Columbia). Staff in all hospitals used the same proprietary software program to support electronic medical records.a All patient information was collected and stored in an electronic format that was uploaded daily into a central data warehouse. Patients were identified by unique patient identification numbers.

Data selection—To be included in the study, dogs and cats were required to have on record an estimated age (last encounter date minus recorded birth date), reported breed, sex, reproductive (castration) status, and status of enrollment in a prepaid wellness plan.b The reported breed was based on the owner's statement or visual assessment by the hospital personnel entering the medical record data. For dogs and cats with multiple visits during 2007, only data from the last encounter were selected for analysis.

Records for cats with an estimated age ≥ 30 years or with a stated breed that was incompatible with the feline species, such as a canine breed, were excluded from the analysis. Dogs with an estimated age ≥ 25 years or with a stated breed that was incompatible with the canine species, such as a feline breed, were similarly excluded. Animals with an unknown castration status or unknown sex were also excluded as well as those with no birth date and those with spay or neuter dates that preceded the birth date.

Descriptive analysis—Distributions of demographic characteristics of the dog and cat populations were calculated for the following: sex (male vs female) and castration status (sexually intact vs castrated), age category (0 to < 6 months, 6 months to < 1 year, 1 to < 4 years, 4 to < 10 years, and ≥ 10 years), reported breed status (mixed breed vs purebred), geographic region of clinic, and participation status in a wellness plan (prepaid plan that covers preventive care such as physical examinations, vaccinations, fecal parasite testing, and castration [puppy and kitten plans only]). Cats with breed reported as domestic shorthair, domestic longhair, domestic medium hair, mixed, calico, or red tabby were classified as mixed breed and all other cat breeds as purebred. Similarly, dogs with breed reported as mixed breed, hybrid breed (eg, cockapoo), or other breed that was not recognized by a local or international organization (eg, American Kennel Club) were classified as mixed breed and all other breeds as purebred. Dogs reported as both purebred and pit bull were classified as pit bull—type because it is possible that other breeds (eg, American Staffordshire Terrier), which maintain their own separate breed designation, may have nevertheless been recorded as pit bull in some instances.

Six US geographic regions were used for analysis: Northwest (Idaho, Montana, Oregon, and Washington), Southwest (Arizona, California, Nevada, and Utah), North Central (Iowa, Illinois, Minnesota, Nebraska, South Dakota, and Wisconsin), South Central (Arkansas, Colorado, Kansas, Louisiana, Missouri, New Mexico, Oklahoma, and Texas), Northeast (Connecticut, Delaware, Indiana, Kentucky, Massachusetts, Maryland, Michigan, New Hampshire, New Jersey, New York, Ohio, Pennsylvania, Rhode Island, Virginia, Vermont, and West Virginia), and Southeast (Alabama, Florida, Georgia, Mississippi, North Carolina, South Carolina, and Tennessee).

Statistical analysis—The period prevalence of castration was defined as the number of unique castrated cats or dogs divided by the total number of unique cats or dogs with known castration status that visited a Banfield hospital in 2007. The prevalence of castration for dogs and cats was calculated by sex, age category, reported breed status, geographic region, and participation status in wellness plans. Age was also analyzed as a continuous variable, and the mean ages for the castrated and uncastrated populations were compared by use of a 2-sample t test.

For the univariate analysis, potential associations between castration status and sex, age, reported breed status, geographic region, and wellness plan status were evaluated by use of the χ2 test for independence and the χ2 test for linear trend (age as categorical variable) for each species. For the multivariate analysis, the large number of dog records (> 1.3 million) precluded analysis of the full data set given the amount of computer memory available. Therefore, the multivariate analysis for the dog population was performed on a subset of the data. The subset was comprised of a computer-generated random sample of the dog population, yielding a sample size of 371,309 (rounded to 400,000) when the following criteria were applied: population castration prevalence of 64%, estimated prevalence within < 1% of population prevalence, and 99.9% confidence level. Data for the cat population were analyzed in a separate multivariate model.

Given the high prevalence of the outcome (castration) and large sample size, a robust Poisson model was used to obtain prevalence ratio estimates adjusted for the other covariates.9,10 This approach is reportedly a better approximation of the prevalence ratio than logistic regression when the outcome is not rare. The robust method provides a sandwich estimator of the variance, which corrects for the overly large estimates of the standard errors obtained by nonrobust Poisson regression. All variables for which a value of P < 0.10 was obtained through univariate analysis were entered into the multivariate model. Pearson product-moment correlations between all possible pairs of variables were examined for evidence of collinearity between pairs of variables. The contribution of variables to the model was evaluated in a backward stepwise fashion, with comparison of the residual deviance between full and nested models used to evaluate the contribution of variables to the model. The χ2 goodness-of-fit test was used to assess the fit of the models to the data. The contributions of biologically plausible 2-way interactions to the model were evaluated in the same manner. The fit of the model with age as a categorical variable (coded 1 to 4) was compared with that of a model involving age midpoints for those categories (0.5, 2.5, 7, and 20 years) by means of goodness-of-fit testing.

Data were analyzed by use of standard software.c–e Values of P < 0.05 were considered a strong indication of a systematic influence (not chance variation). The association between predictor variables and castration status was expressed as the prevalence ratio and 95% confidence interval.

Results

Animals—During the study period, the records of 320,172 cats and 1,339,860 dogs were deemed eligible for inclusion in the study on the basis of the study criteria. Records of the following animals were excluded: 458 cats with an estimated age ≥ 30 years, 10 cats with a stated breed that was incompatible with the feline species, 1,434 dogs with an estimated age ≥ 25 years, and 196 dogs with a stated breed that was incompatible with the canine species. Also excluded were records of 1,129 cats and 1,115 dogs with unknown castration status or sex and 12 cats and 63 dogs with spay or neuter dates that preceded the birth date.

Overall, 262,912 (82.1%) cats and 861,300 (64%) dogs had been castrated. Demographic characteristics of the cat and dog populations were summarized (Table 1).

Table 1—

Demographic data for dogs and cats examined at 651 private veterinary hospitals in the United States during 2007.

VariableNo.(%) of catsNo.(%) of dogs
Age group  
0 to < 6 mo54,744 (17)205,662 (15)
6 mo to < 1 y30,855 (10)150,985 (11)
1 to < 4 y98,000 (31)475,747 (36)
4 to < 10 y90,883 (28)397,121 (30)
> 10 y<*45,690 (14)110,345 (8)
Reproductive status  
Sexually intact male26,661 (8.3)267,979 (20)
Sexually intact female30,599 (9.6)210,581 (16)
Castrated male132,268 (41)425,413 (32)
Spayed female130,644 (41)435,887 (33)
Breed status†  
Mixed278,853 (87)396,422 (30)
Purebred41,319 (13)943,438 (70)
Geographic region  
North Central28,350 (9)100,916 (8)
Northeast79,144 (25)253,138 (19)
Northwest26,994 (8)78,070 (6)
South Central61,312 (19)314,463 (23)
Southeast71,098 (22)336,002 (25)
Southwest53,274 (17)257,271 (19)
Wellness plan in 2007  
Had a wellness plan in 2007139,326 (44)730,606 (55)
Did not have a wellness plan in 2007180,846 (56)609,254 (45)
Wellness plan when castrated Had a wellness plan when castrated48,305 (15)261,457 (20)
Did not have a wellness plan when castrated214,607 (67)599,843 (45)
Total320,1721,339,860

Maximum age for cats was 30 years and for dogs was 25 years. †Mixed breeds include hybrids such as cockapoo. Purebred includes breeds recognized by a national or international organization (eg, American Kennel Club).

Univariate analysis—The prevalence of castration in cats increased significantly with age (χ2 test for linear trend; P < 0.001; Table 1). The mean age of castrated cats (5.2 years) was significantly higher than that of uncastrated cats (1.5 years). The prevalence of castration was significantly higher in male cats than in female cats, although the difference was small (83% vs 81%, respectively; Table 2). In addition, purebred cats and cats on wellness plans in 2007 were significantly more likely to be castrated than were mixed-breed cats and cats not on a wellness plan. The prevalence of castrated cats differed significantly between geographic regions (χ2 test for independence; P < 0.001), with the highest prevalence in the Northwest (86%) and the lowest in the Northeast (80%) United States.

Table 2—

Factors associated with castration in dogs (n = 1,339,860) and cats (320,172) examined at 651 private veterinary hospitals in the United States during 2007, as identified via univariate analysis.

 CatsDogs
VariableNo.(%) castratedPR (95% CI)P valueNo. (%) castratedPR (95% CI)P value
Age group*      
 0 to < 6 mo23,390 (43)Referent36,477 (18)Referent
 6 mo to < 1 y23,730 (77)1.80 (1.78–1.82)< 0.00183,687 (55)3.13 (3.09–3.16)< 0.001
 1 to < 4 y85,333 (87)2.04 (2.02–2.06)< 0.001325,884 (68)3.86 (3.83–3.90)< 0.001
 4 to < 10 y86,331 (95)2.22 (2.20–2.25)< 0.001320,615 (81)4.55 (4.51–4.60)< 0.001
 ≥ 10 y*44,128 (97)2.26 (2.24–2.28)< 0.00194,637 (86)4.84 (4.79–4.88)< 0.001
Sex      
 Female130,644 (81)Referent435,887 (67)Referent
 Male132,268 (83)1.03 (1.02–1.03)< 0.001425,413 (61)0.91 (0.91–0.91)< 0.001
Breed status      
 Purebred†34,519 (84)Referent572,774 (61)Referent 
 Mixed228,393 (82)0.98 (0.98–0.98)< 0.001288,526 (73)1.20 (1.20–1.20)< 0.001
Geographic region      
 North Central24,153 (85)Referent71,696 (71)Referent
 Northeast63,708 (80)0.94 (0.94–0.95)< 0.001166,131 (66)0.92 (0.92–0.93)< 0.001
 Northwest23,270 (86)1.01 (1.00–1.02)< 0.00155,587 (71)1.00 (1.00–1.01)0.469
 South Central49.706 (81)0.95 (0.95–0.96)< 0.001197,335 (63)0.88 (0.88–0.89)< 0.001
 Southeast57,563 (81)0.95 (0.94–0.96)< 0.001205,000 (61)0.86 (0.85–0.86)< 0.001
 Southwest44,512 (84)0.98 (0.97–0.99)< 0.001165,551 (64)0.91 (0.90–0.91)< 0.001
Wellness plan in 2007      
 No139,602 (77)Referent334,294 (55)Referent
 Yes123,310 (89)1.15 (1.14–1.15)< 0.001527,006 (72)1.31 (1.31–1.32)< 0.001

Maximum age for cats was 30 years and for dogs was 25 years. †Breeds recognized by a national or international organization (eg, American Kennel Club).

— = Not applicable. CI = Confidence interval. PR = Prevalence ratio.

The prevalence of castration in dogs increased significantly with age (χ2 test for linear trend; P < 0.001; Table 1). The mean age of castrated dogs (4.7 years) was significantly higher than that of uncastrated dogs (2.2 years). The prevalence of castration was significantly higher in female dogs than male dogs. In addition, mixed-breed dogs and dogs on wellness plans in 2007 were significantly more likely to be castrated than were purebred dogs and dogs not on wellness plans (Table 2). The prevalence of castrated dogs differed significantly between geographic regions (χ2 test for independence; P < 0.001), with the highest prevalence in the Northwest and North Central (71% each) and the lowest in the Southeast (61%) United States.

Influence of dog breed—The prevalence of castration was calculated for the 10 most commonly reported dog breeds that were recorded as purebred in the medical record (Table 3). There was considerable variation between the breeds, with Golden Retriever and Beagle most likely to be castrated (74%) and pit bull—type and Chihuahua the least likely to be castrated (27% and 46%, respectively).

Table 3—

Prevalence of castration among top 10 reported purebred* dog breeds at 651 private veterinary hospitals in the United States during 2007

BreedNo. (%) castratedNo. (%) not castratedTotal
Labrador Retriever62,058 (72)23,719 (28)85,777
Chihuahua32,309 (46)37,799 (54)70,108
Shih Tzu31,943 (57)24,270 (43)56,213
Yorkshire Terrier23,750 (51)22,700 (49)46,450
Pit bull-type10,691 (27)29,287 (73)39,978
Golden Retriever25,150 (74)8,761 (26)33,911
Dachshund20,878 (66)10,842 (34)31,720
Boxer17,172 (58)12,393 (42)29,565
Beagle19,980 (74)7,154 (26)27,134
German Shepherd Dog15,526 (58)11,108 (42)26,634
Total259,457 (58)188,033 (42)447,490

Breeds recognized by a national or international organization (eg, American Kennel Club). †Dogs reported as purebred American Pit Bull Terriers (APBTs) were classified as pit bull-type since it is possible that other breeds (ie, American Staffordshire Terrier), which maintain their own separate breed designation, may have nevertheless been recorded as APBTs in some instances.

Multivariate analysis—Almost all variables in the univariate analysis were significant (P < 0.001 for all except one of the geographic regions for dogs), so all were entered into the multivariate model. Results agreed closely with those of the univariate analysis, with the exception of a few geographic differences (ie, the prevalence of castration in cats in the Northeast and Southwest no longer differed significantly from that in North Central United States; Table 4). All other pairwise comparisons for geographic region were significantly different for cats and dogs, except between dogs in the Northeast and Southwest (data not shown). The addition of 2-way interaction terms contributed little to the fit of the dog and cat models. The fit of the model containing age as a categorical variable was significantly better than that in which age midpoints were used instead. However, results of χ2 goodness-of-fit testing indicated that neither the final dog (P < 0.001) nor cat models (P < 0.001) were a good fit for the data.

Table 4—

Factors associated with castration in dogs (n = 1,339,860) and cats (320,172) examined at 651 private veterinary hospitals in the United States during 2007, as identified via multivariate analysis.

 CatsDogs
VariablePR (95% CI)P valuePR (95% CI)P value
Age group*    
 0 to < 6 moReferentReferent
 6 mo to < 1 y1.76 (1.74–1.78)< 0.0012.94 (2.89–2.99)< 0.001
 1 to < 4 y2.03 (2.01–2.05)< 0.0013.68 (3.62–3.75)< 0.001
 4 to < 10 y2.23 (2.20–2.25)< 0.0014.40 (4.33–4.48)< 0.001
 ≥ 10 y2.29 (2.27–2.31)< 0.0014.70 (4.62–4.78)< 0.001
Sex    
 FemaleReferentReferent
 Male1.03 (1.03–1.04)< 0.0010.93 (0.93–0.93)< 0.001
Breed status    
 Purebred†ReferentReferent
 Mixed0.99 (0.98–0.99)< 0.0011.19 (1.18–1.19)< 0.001
 Geographic region    
 North CentralReferent Referent 
 Northeast0.98 (0.97–0.99)0.4270.96 (0.95–0.96)< 0.001
 Northwest1.00 (1.00–1.01)< 0.0011.03 (1.02–1.04)< 0.001
 South Central0.98 (0.97–0.98)< 0.0010.94 (0.93–0.94)< 0.001
 Southeast0.97 (0.96–0.97)< 0.0010.89 (0.88–0.89)< 0.001
 Southwest0.99 (0.99–1.00)0.3210.95 (0.95–0.96)< 0.001
Wellness plan in 2007    
 NoReferentReferent
 Yes1.18 (1.17–1.18)< 0.0011.33 (1.33–1.34)< 0.001

See Table 2 for key.

Discussion

In the present study, the epidemiology of castration of dogs and cats evaluated in private veterinary hospitals was elucidated. These populations serve as potential contributors to the animal shelter population. Previous studies11–13 have shown the problem of pet overpopulation and the attendant massive numbers of dogs and cats euthanized each year in animal shelter settings. Although Ohio animal control agencies report a decreasing trend in the proportion of dogs euthanized, the reverse is true for cats there, and pit bull-type dogs make up a disproportionate share of dogs euthanized.12 In Louisiana from 1988 through 1990, 36.4% of animals relinquished to animal control were from unplanned litters.11 Surveys13,14 of US households have found that most litters are unplanned, particularly litters of kittens. For households with litters of kittens and puppies, cost and failure to recognize signs of estrus (heat) have been cited as the most common reasons for not having females spayed before pregnancy.13 Two case-control studies15,16 conducted to compare households that relinquished dogs and cats to an animal shelter with current dog- and cat-owning households found that sexually intact dogs and cats accounted for a disproportionate share of relinquishments.

To the authors' knowledge, there have been no other reports on the epidemiology of castration in dogs and cats evaluated in private practice settings. A United Kingdom survey of cat-owning households reported a prevalence of castration of 92% among cats ≥ 6 months old and 66% among cats 6 to 12 months old; cats ≥ 10 months old, with indoor access, vaccinated in the last year, or registered with a veterinary practice were more likely to be castrated than their counterparts.17 According to a US household survey,18 the overall prevalence of castration was 80% among cats, with respondents who reported an annual income ≥ $75,000 and who were ≥ 40 years old most likely to have their cats castrated. A survey14 of recent adopters of a pet dog or cat (within the previous year) found that 48% of adopters of uncastrated pets had not yet had their pet castrated; respondents < 35 years old and residing in the South were least likely to have their pet castrated. Among Florida households in 1999, 90% of owned cats were castrated, although only 11% of households at which free-roaming cats were fed had had any of the unowned cats castrated.19 A study20 of Texas animal control and licensing agencies revealed an overall prevalence of castration of 27% among dogs and 33% among cats in 1997; females, urban, and licensed animals were more likely to be castrated than were males, rural, or sheltered animals. Among animals adopted from Louisiana animal control centers, adult female cats were most likely to be returned for castration, followed by adult male cats in 1988 through 1990; adopters of male puppies and adult dogs were least likely to return to have their pet castrated.11 In addition, adopters of female dogs suspected to be of mixed breed were more likely to have the dog spayed than were adopters of female dogs that appeared to be purebred.

In the present study, as expected, the prevalence of castration in dogs and cats examined at private veterinary hospitals across the United States increased significantly with age. However, many young adult dogs (1 to < 4 years) of reproductive age were uncastrated (32%). Pet owners might be motivated to have their pets spayed or neutered earlier through campaigns promoting the benefits of castrating pets before the onset of sexual maturity, as recommended by spay-neuter program guidelines.5,21 The lower prevalence of castration in dogs than in cats, particularly males, specific breeds (ie, Chihuahua and pit bull—type), and those in the Southeast, supports the need for targeted education and public outreach efforts that take into account differences between dog and cat owners, owners of specific dog breeds, pets' sex, and geographic differences. Veterinarians in private practice can use these data to gain insight into their own patient population and to effectively advocate for castrating animals most at risk of going uncastrated. In addition, data such as these can be used to help develop and promote spay and neuter programs that are tailored to the needs of specific communities.

Also in the present study, pets on prepaid wellness plans in 2007 were more likely to be castrated than those that were not. However, wellness plan status may not be directly associated with prevalence of castration because a pet may have been spayed or neutered in an earlier year when it was not necessarily on a wellness plan. Rather, owners' willingness to prepay for wellness care may be an indicator of the overall degree of care those owners provide for their pets, regardless of whether the pets were on a wellness plan at the time of castration. The high proportion of dogs (45%) and cats (67%) that were not on wellness plans at the time of castration was likely a reflection of the number of dogs and cats that were spayed or neutered before their first visit to a study hospital (ie, spayed or neutered at another private hospital or upon adoption from an animal shelter). Because only the puppy and kitten wellness plans in the study hospitals include castration, enrollment would not be expected to influence a client's decision for a pet > 6 months of age. In the shelter setting, prepayment for castration services does not appear to be a large incentive, as approximately 40% to 60% of adopted dogs and cats are not returned for the procedure after it has already been paid for.11,22 Additional studies are needed to determine the proportion of adoptive owners who prepay for castration at 1 facility but elect to have the procedure performed elsewhere and to determine whether prepayment is an effective motivator for pet owners to have their pets castrated.

An advantage of the animal population in the present study was that it represents a large population of pet dogs and cats in the United States. On the basis of national marketing data on pet ownership, the study population comprised approximately 1.9% of the US dog population and 0.3% of the cat population.23 A very small proportion of dogs (0.2%) and cats (0.5%) were excluded by the study criteria. Although the authors are not aware of any studies in which the demographics of the Banfield patient population were compared with those of the general US pet population, demographic data for dogs and cats examined at other private US veterinary practices during 1995 have been reported.24 The present study population was similar with respect to the distributions of mixed-breed dogs and cats, sex, and castration status. However, the median age in the present study (2.6 years for dogs and 3.0 years for cats) was younger than that reported for the other study (4.8 years for dogs and 4.3 years for cats). It is possible that the puppy and kitten wellness plans offered by Banfield pet hospitals might have attracted clients with younger pets. In addition, many Banfield pet hospitals opened recently, which might have attracted new owners of younger animals.

Some limitations of the present study should be considered. Although the software used for entry of medical record data is standardized across all Banfield pet hospitals, data entry is still subject to human error. For instance, a mixed-breed dog that resembles a particular breed could potentially be reported as a purebred. Unless the animal's pedigree is known, the reported breed is likely based on visual identification, which may not necessarily correspond with results of breed identification through DNA testing.25 For instance, pit bull-type dogs were among the most common dogs reported as purebred, although the designation pit bull has been challenged as being vague and for being applied to various mixes of pit bull types.26 Estimation of age by use of the given birth date and last encounter date is another potential limitation, given the reliance on owners to accurately report the birth date. In addition, chronological age rather than physiologic age was used as a variable, and chronological age does not taken into account lifespan differences associated with the size (ie, breed) of dogs.27 Lastly, the use of 6 large geographic regions in the present study may not have been sufficiently detailed to allow detection of more localized geographic differences in the prevalence of castration.

Given the large number of observations included in the analyses, it is possible that very small and potentially unimportant differences would yield significant findings as a result of random error. In addition, no data were available on other factors that may be associated with the likelihood that a pet is castrated, such as owner characteristics (eg, age, gender, income, race or ethnicity, or education level), animal housing (eg, indoor vs outdoor), and function of animal (eg, pet, show, or hunt). Such explanatory variables might have improved the fit of the multivariate model and would be important to consider for future studies. For instance, a survey28 of attitudes toward responsible pet ownership found that female respondents were significantly more likely than males to state that they would have their dog or cat castrated. Studies29,30 have shown that Hispanic pet owners are less likely to have their pet dogs and cats castrated than are pet owners of other race or ethnicity groups. In addition, previous surveys14,18 have found that older respondents and those reporting higher incomes are most likely to have their pets castrated than their counterparts. It was beyond the scope of this study to determine the reason the prevalence of castration was higher among cats than dogs. Two possible reasons are that the procedure may generally be less expensive in cats than in dogs and that motivation to have a pet castrated may be associated with the tendency of some cat owners to allow their cats unsupervised outdoor access.

Regardless of the aforementioned limitations, the present study yielded population-based data on the prevalence of castration in the private practice setting and a foundation for further elucidation of factors that influence whether a pet is castrated. Significant differences were evident in the prevalence of castration in subpopulations of dogs and cats. These findings can be used to develop specifically targeted educational messages and spay-neuter programs to various categories of pet owners to encourage them have their pets castrated and to do so prior to the onset of sexual maturity. Health educators can use these data to develop targeted client education materials for use by private practitioners and to effectively formulate educational messages for dissemination to the general public through public service announcements, community events, printed advertisements, and other means. Such messages should attempt to address the many potential reasons owners of certain categories of pets are less likely to have their pet castrated. Perceptions such as the lower valuation of cats than dogs,13 the affront to masculinity by having male dogs castrated, and the cost of the procedure relative to the benefit may all play a role to some degree.14 Future research should help quantify the effect of such factors and identify others that serve as a barrier to increasing the prevalence of castration and reducing pet overpopulation.

a.

PetWare, Banfield, The Pet Hospital, Portland, Ore.

b.

Optimum Wellness Plans, Banfield, The Pet Hospital, Portland, Ore.

c.

S-Plus 2000, Professional Release 3, MathSoft Inc, Seattle, Wash.

d.

Epi Info 6, version 6.04d, CDC, Atlanta, Ga.

e.

SAS, version 9.2, SAS Institute Inc, Cary, NC.

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