A Scoping Review of Published Research on the Relinquishment of Companion Animals

Abstract

Globally, large populations of companion animals are relinquished each year. The purpose of this scoping review was to identify all published research investigating companion-animal relinquishment to map out and evaluate research gaps, needs, and opportunities. A comprehensive search strategy was implemented in 4 online databases, identified citations were screened, and relevant articles were procured and characterized. From 6,848 unique citations identified, 192 were confirmed relevant, including 115 primary-research articles and 77 reviews and commentaries. The majority of these articles originated from the United States (131; 68.2%); 74 (38.5%) of them have been published since 2006. Among the primary-research articles, 84 (73.0%) investigated reasons for companion-animal relinquishment. The most commonly studied reasons were aggressive companion-animal behaviors (49; 58.3%); moving, rental, or housing issues (45; 53.6%); and caretaker personal issues (42; 50.0%). Only 17 primary-research articles investigated interventions to prevent companion-animal relinquishment. The quantity of research into reasons for relinquishment highlights an opportunity for future knowledge-synthesis activities in this area, including systematic review and meta-analysis. In comparison, the limited research into interventions identifies a priority for new research.

Keywords::

• companion-animal relinquishment
• companion-animal surrender
• companion animals
• scoping review

Small companion animals, including dogs, cats, and a variety of small mammals, are kept by humans throughout the world. The relationship between these nonhuman animals and humans has evolved steadily over centuries, with the last three decades probably being the most profound in terms of the recognition of and growth in the relationship between humans and animals (Case, 2008; Hines, 2003). This growing relationship between humans and animals has resulted in the now widely accepted and understood phrase, human–animal bond (HAB; Hines, 2003). Aligned with the increased attention and recognition of the HAB has been a notable amount of research dedicated to understanding the human-health benefits of our relationships with companion animals, including studies of the therapeutic, psychological, physiological, psychosocial, and physical benefits to humans of all ages (Barker & Wolen, 2008; Wells, 2009).

Along with these identified benefits, there are also some associated costs. Specifically, when the human–animal relationship breaks down or does not develop, animals within these situations can often find themselves relinquished by their caretakers. Although establishing accurate accounts for the number of animals relinquished each year is difficult due to the variety of ways animal relinquishment can be carried out (e.g., surrendered to a shelter, given to a friend or stranger, abandoned on the roadside), the number of animals relinquished annually around the globe is inarguably high, with often lethal consequences for the animals involved.

An annual survey of shelter statistics carried out by the National Council on Pet Population Study & Policy (NCPPSP) from 1994 to 1998, involving more than 1,000 shelters in the United States, revealed that 27.5% to 30.2% of the approximately 4 million companion animals entering shelters annually were relinquished by their caretakers (NCPPSP, 2009). Approximately 10% of animals entering the study shelters were returned to their caretakers annually, while 64% were euthanized (NCPPSP, 2009). Although the findings of the study cannot be used to estimate the overall number of companion animals relinquished or euthanized on an annual basis, the findings do highlight the prominence of companion-animal relinquishment and the inevitable fate for many of these animals.

In a similar survey of 90 shelters across Canada in 2010, 143,218 companion animals were admitted from a variety of sources (Canadian Federation of Humane Societies [CFHS], 2010). Of these animals, 11% were returned to their caretakers and 36% were euthanized (CFHS, 2010). These findings highlight that companion-animal relinquishment is an urgent issue that is not geographically isolated. In support of international recognition of an existing societal problem, research has been conducted around the world to investigate the extent of, and main reasons for, companion-animal relinquishment, particularly of dogs and cats.

Given the broad scope of the issue of companion-animal relinquishment and research published to date, there is a need to map out and assess the volume and characteristics of existing research on this topic to identify not only knowledge gaps but also areas that are already well understood and researched. Although traditional narrative reviews provide useful overviews of broad research fields, they have limited reliability to inform policy and decision making due to a lack of methodological transparency (e.g., rationale for study selection) and potential for inaccurate or biased conclusions (Sargeant, Torrence, Rajić, O'Connor, & Williams, 2006; Waddell et al., 2008). It is preferable to use systematic, transparent, and replicable research-synthesis methods to identify, appraise, and summarize research on broad or focused questions (Sargeant, Rajić, Read, & Ohlsson, 2006). The use of formal and structured research-synthesis methods has a long tradition in human health care and other sectors (Arksey & O'Malley, 2005; Higgins & Green, 2011; Tricco, Tetzlaff, & Moher, 2011) and has more recent applications in veterinary and agri-food public health (Bucher et al., 2012; Farrar, 2009; Ilic et al., 2012; Tusevljak et al., 2012).

Systematic reviews (SRs) are the most frequently used and established method to identify and summarize the global research evidence about intervention efficacy, risk factors, or diagnostic test accuracy (Egger, Smith, & Altman, 2001; Higgins & Green, 2011; Sargeant, Rajić, et al., 2006; Tricco et al., 2011). If data are sufficiently reported and comparable, SRs may include a meta-analysis (MA) to generate precise effect estimates from multiple studies investigating the same question rather than from a single study (Higgins & Green, 2011). Scoping reviews are used to investigate broader, policy-driven research questions and to identify areas with sufficient knowledge and those with major research gaps and needs (Arksey & O'Malley, 2005; Davis, Drey, & Gould, 2009). Scoping reviews can also be used to identify and prioritize research questions for subsequent SR–MAs (Arksey & O'Malley, 2005).

The aim of this study was to conduct a scoping review to map out and characterize the breadth and depth of existing global research about companion-animal relinquishment. The review objectives were to identify well-investigated aspects, research gaps and needs, and areas potentially suitable for SR–MAs. The results may be used to inform future evidence-based research directions as well as to inform policy decision making in this area at the local, national, or international level.

METHODS

Review Protocol, Team, Question, and Scope

We conducted the review by following a comprehensive protocol that was developed by the authors a priori and contained details on all methodology used and described herein. The review team consisted of nine professionals with topic (i.e., companion-animal relinquishment), epidemiologic, and research-synthesis method expertise. The review question was, “What is the current state of published research on the issue of companion-animal relinquishment?” The review scope included any primary research (qualitative or quantitative), reviews, or commentaries investigating or discussing the issue of companion-animal relinquishment. A list of key definitions used in this review, including companion-animal relinquishment, is included in Table 1.

Search Strategy

A comprehensive search strategy was developed and pretested on the basis of a review of an existing bibliography of relevant literature (n = 65 articles) previously developed by the principle author (JBC). Three alternative, database-specific search algorithms consisting of a combination of various population terms (e.g., pets, companion animals, dogs, or cats) and exposure terms (e.g., abandonment, surrender, or adoption) were developed by reviewing key descriptive terms in the titles and abstracts of the 65 known relevant articles. The searches were then implemented in MEDLINE, Scopus, CAB Direct, and PsychINFO to determine the proportion of the 65 articles captured by the various bibliographic databases.

The first three databases were chosen because they have been shown to include the majority of scientific veterinary literature coverage (Grindlay, Brennan, & Dean, 2012), while PsychINFO was also included to capture additional potentially relevant articles in this topic area. We selected the most inclusive of the three search algorithms, which captured the highest proportion (54/65, 83.1%) of the known relevant articles in all four databases, and implemented it in each database on October 30, 2011. An updated search was also conducted on March 21, 2012. The original search algorithm did not include the term euthanasia, so a repeated search to include this term was conducted on October 19, 2012.

TABLE 1 List of Key Definitions Used in the Review (Adapted From World Society for the Protection of Animals)

Term	Definition
Companion animals/pets	Defined as any animal species kept for companionship and enjoyment or a household animal, as opposed to livestock, laboratory animals, working animals, or sport animals, which are kept for economic reasons.
Companion-animal relinquishment	When a companion-animal caretaker voluntarily gives up possession or ownership of their pet. Includes pet surrender, abandonment, and euthanasia.
Companion-animal surrender	When the caretaker of a companion animal who is no longer wanted takes the animal to an animal shelter or municipal pound and legally surrenders all further claim to ownership of the pet.
Companion-animal abandonment	Distinguished from companion-animal surrender in that pets are discarded or left to fend for themselves rather than being formally surrendered to a facility.
Companion-animal re-relinquishment	When the recent adopter of a companion animal returns the animal to an animal shelter or municipal pound and legally surrenders all further claim to ownership of the pet.
Companion-animal euthanasia	When a caretaker relinquishes his or her companion animal, either to a shelter/pound or veterinary clinic, for the purpose of humane euthanasia.

No publication period, language restriction, or other filters were imposed on the searches. A Scopus web search and nine targeted Google searches were conducted on November 30, 2011, and February 7 to 10, 2012, respectively, to identify grey literature (e.g., reports). For pragmatic reasons, both searches were limited to the first 100 hits and were automatically sorted according to relevance by the search platforms' algorithms. The Scopus web search used the search algorithm described, while the Google queries were conducted using combinations of the terms pet, dog, or cat and relinquishment, abandonment, or surrender.

An additional Google search was not conducted at the time the search strategy was repeated to include the term euthanasia because of the practical limitations of repeating an identical Google search of the top 100 hits and because of the low number of citations (n = 44) originally identified through this search approach. The final search strategy was verified by hand-searching the reference lists of a pragmatic sample of approximately 20% of relevant articles, including 10 literature reviews and 30 randomly selected primary-research articles. In addition, targeted searches were conducted on the websites of six humane societies and organizations known to the authors to publish reports and other documents of potential relevance to this review.

Relevance Screening

Relevance screening was conducted by the review team on the titles and abstracts of identified citations using a pretested form. Each citation was reviewed by two independent reviewers. The screening form consisted of one question to determine whether citations described the issue of companion-animal relinquishment, caretaker expectations of companion-animal ownership, or reasons for acquiring a companion animal. Only citations describing companion-animal relinquishment were considered for inclusion at the data characterization and extraction (DCE) stage, while those describing caretaker expectations of companion-animal ownership or reasons for acquiring a companion animal were filtered into a separate category for future reference. Citations that primarily described animal welfare, animal behavior, HAB, or caretaker attachment and attitudes, without specific mention of the inclusion criteria, were excluded. Due to limited resources for translation, we also excluded any non-English articles.

Data Characterization and Extraction

A DCE form was applied on the full articles of relevant citations to confirm their relevance and to extract key information from the articles. Prior to its implementation, the form was sent to three stakeholders and one content expert in the area of companion-animal relinquishment for feedback. The three stakeholders held various roles within the Ontario Society for Prevention of Cruelty to Animals, and the expert was known to conduct research in the area of shelter medicine in the United States. One of the four provided feedback that was incorporated into the final version of the DCE form. Four members of the review team then used the DCE form to extract key characteristics from each article, including publication date and type; study objectives, focus, location, and target audience; study design and data collection methods; key definitions; populations studied; reasons for relinquishment; interventions against relinquishment; role of veterinarians; and reporting of results, strengths, weaknesses, and recommendations. Each article was reviewed by two independent reviewers.

The publication date variable was originally extracted as five time-period categories but was later collapsed to three time periods (before 2000, 2000–2005, and 2006–2012) with comparable numbers of articles to facilitate presentation and cross-tabulations. At this stage of the review, we decided to exclude two types of documents that were identified in grey literature searches and were originally considered for potential inclusion in the review: lay newspaper articles and website information that was not structured as a formal organizational report. We excluded these documents because they were not deemed relevant to the review focus on published research.

Review Management, Data Charting, and Analysis

Citations were uploaded to RefWorks (Thomson ResearchSoft, Philadelphia, PA), manually deduplicated, and imported into an electronic review management program (DistillerSR, Evidence Partners, Ottawa, ON, Canada). A-priori designed forms for every level of the review were pretested by multiple independent reviewers prior to use. Relevance screening was pretested on 20 purposively selected abstracts by seven reviewers; disagreements were discussed and reviewing proceeded when a kappa agreement of >0.8 was achieved.

DCE pretesting was conducted on six selected articles by five reviewers; major disagreements were discussed and the form was revised to improve clarity and relevance of the questions before use. Disagreements between reviewers at any stage of the review were resolved by the reviewing pair through consensus and, if required, by judgment of a third reviewer. Reviewer communication documents were circulated frequently about common disagreements and frequently asked questions.

DCE results were downloaded as Excel spreadsheets (Microsoft Corporation, Redmond, WA), formatted, and descriptively analyzed to determine the characteristics of the research available. Scoping-review evidence maps were created in Excel using descriptive tables and bubble charts (Ilic et al., 2012; Petersen, Feldt, Mujtaba, & Mattsson, 2008). The bubble charts were used to graphically represent cross-tabulations between the following variables: aspects of companion-animal relinquishment investigated by year of publication and study location and themes of companion-animal relinquishment investigated by year of publication and study location. Within each bubble chart, the size of the “bubbles” is proportional to the number of articles in each cross-tabulation category (Petersen et al., 2008).

We also developed a “heat map” table to visualize the cross-tabulation of reported reasons for companion-animal relinquishment by the following six variables: publication date, study location, study design, aspects of relinquishment investigated, companion-animal populations investigated, and study setting. In this table, cell values were shaded on a gray scale representing the percentage of articles in each cross-tabulation category out of the total number of articles investigating reasons for relinquishment (n = 84). The review team met after completion of DCE to discuss the findings and evidence charts and to identify and prioritize research strengths and gaps, potential SR–MA questions, and global and contextual (i.e., Canadian) research needs and opportunities.

RESULTS

Descriptive Characteristics of Published Research About Companion-Animal Relinquishment

The review summary is shown in Figure 1. From 6,848 unique citations screened for relevance, 602 were considered potentially relevant and procured as full articles. During DCE, 192 articles (115 primary-research articles and 77 reviews and commentaries) were confirmed as relevant (Figure 1). The main characteristics of all relevant articles are shown in Tables 2 and 3.

Most primary research about companion-animal relinquishment has been published since 2006, while most reviews and commentaries were published before 2000 (Table 2). The majority of published research was in the form of journal articles and was conducted in the United States (Table 2). A majority of the authors of primary-research articles did not specify a target audience or end users, while more than half of reviews and commentaries were targeted toward veterinary practitioners or clinicians (Table 2). Most articles did not provide definitions for relinquishment or related terms (Table 2).

The most commonly investigated aspects of companion-animal relinquishment were companion-animal surrender, euthanasia, and adoption (Table 3). Companion-animal re-relinquishment was investigated more often in primary-research articles than in reviews and commentaries, while companion-animal surrender, euthanasia, abandonment, and overpopulation were addressed more frequently in reviews and commentaries (Table 3). Reasons for relinquishment, prevalence of relinquishment, characteristics of relinquished companion animals, and characteristics of caretakers who relinquish companion animals were the most commonly investigated themes among primary-research articles (Table 3).

Reviews and commentaries mostly covered various interventions to prevent or control companion-animal relinquishment (Table 3). Cats were investigated in only half of primary-research articles; they were investigated in a much larger proportion of reviews and commentaries. Companion animals other than cats and dogs were investigated in a small percentage of all relevant articles (Table 3). The role of veterinarians in companion-animal relinquishment was discussed more often by authors of reviews and commentaries than by authors of primary-research articles (Table 3). Most authors recommended the use of one or more interventions, most frequently educational measures, to control or prevent companion-animal relinquishment (Table 3).

FIGURE 1 Scoping-review flowchart.

Characteristics of Primary Research Investigating Companion-Animal Relinquishment

The key aspects and themes of companion-animal relinquishment investigated among primary-research articles stratified by year of publication and study location are shown in Figure 2. Most aspects of and themes about companion-animal relinquishment were consistently investigated over time and across study locations (Figure 2). However, a relatively smaller number of articles (n = 15) from the United States and Canada investigated companion-animal abandonment. In addition, research on interventions to prevent companion-animal relinquishment has become an increased area of focus since 2000, compared with other research themes (Figure 2).

TABLE 2 Descriptive Characteristics of 192 Primary-Research and Review and Commentary Articles Investigating or Discussing the Issue of Companion-Animal Relinquishment

Question	All Articles (n = 192)		Primary Research (n = 115)		Reviews and Commentaries (n = 77)
	No.	%	No.	%	No.	%
Publication date
Before 2000	69	35.9	33	28.7	36	46.8
2000–2005	49	25.5	31	27.0	18	23.4
2006–2012	74	38.5	51	44.3	23	29.9
Document type
Journal article	155	80.7	101	87.8	54	70.1
Conference proceedings paper or abstract	14	7.3	6	5.2	8	10.4
Book chapter	10	5.2	1	0.9	9	11.7
Journal news article	5	2.6	0	0.0	5	6.5
Report	4	2.1	1	0.9	3	3.9
Thesis	4	2.1	4	3.5	0	0.0
Study location^a
United States	131	68.2	65	56.5	66	85.7
Europe	35	18.2	23	20.0	12	15.6
Australia	14	7.3	8	7.0	6	7.8
Canada	7	3.6	3	2.6	4	5.2
Other^b	18	9.4	17	14.8	1	1.3
Target end users/audience of the article^a
Not clear or not explicitly reported	139	72.4	108	93.9	31	40.3
Practitioners/clinicians	46	24.0	4	3.5	42	54.5
Service providers	9	4.7	2	1.7	7	9.1
Companion-animal caretakers/public	8	4.2	2	1.7	6	7.8
Policy and decision makers	4	2.1	3	2.6	1	1.3
Reporting of companion-animal relinquishment definitions
None provided	182	94.8	106	92.2	76	98.7
At least one definition provided	10	5.2	9	7.8	1	1.3

^a Multiple selections were allowed for these questions, so column percentages do not add up to 100%.

^b Other locations included Taiwan (_n = 5), Brazil (n = 3), New Zealand (n = 2), South Korea (n = 2), the Bahamas (n = 2), Japan (n = 2), and Israel (n = 1).

Specific characteristics, methods, and outcomes of primary-research articles investigating companion-animal relinquishment are shown in Tables 4 and 5. Most quantitative primary-research articles used a cross-sectional study design, and most primary-research articles used questionnaires for data collection, which was completed in humane societies, shelters, pounds, or animal rescues (Table 4). Among 84 studies that investigated reasons for companion-animal relinquishment, the vast majority of these reported various companion animal-related and caretaker-related factors, with the most consistently studied reasons being aggressive companion-animal behaviors; moving, rental, or housing issues; and caretakers' personal issues, such as relationship problems or a lack of time to care for the companion animals (Table 5).

TABLE 3 Primary Areas of Focus Among 192 Primary-Research and Review and Commentary Articles Investigating or Discussing the Issue of Companion-Animal Relinquishment

Question	All Articles (n = 192)		Primary Research (n = 115)		Reviews and Commentaries (n = 77)
	No.	%	No.	%	No.	%
Aspects of the companion-animal relinquishment issue investigated^a
Companion-animal surrender	126	65.6	73	63.5	53	68.8
Companion-animal euthanasia	111	57.8	50	43.5	61	79.2
Companion-animal adoption	81	42.2	52	45.2	29	37.7
Companion-animal abandonment	68	35.4	34	29.6	34	44.2
Companion-animal re-relinquishment	40	20.8	34	29.6	6	7.8
Companion-animal overpopulation	14	7.3	3	2.6	11	14.3
Primary themes of the investigation or discussion^a
Reasons for relinquishment	132	68.8	84	73.0	48	62.3
Prevalence of relinquishment	85	44.3	63	54.8	22	28.6
Characteristics of relinquished companion animals	76	39.6	59	51.3	17	22.1
Interventions for control of relinquishment	65	33.9	17	14.8	48	62.3
Characteristics of caretakers who relinquish companion animals	46	24.0	38	33.0	8	10.4
Companion-animal populations studied^a
Dogs	171	89.1	101	87.8	70	90.9
Cats	122	63.5	60	52.2	62	80.5
Other^b	18	9.4	14	12.2	4	5.2
Role of veterinarians discussed
Yes	107	55.7	45	39.1	62	80.5
No	85	44.3	70	60.9	15	19.5
Author-reported recommendations^a
Use of an intervention	144	75.0	85	73.9	59	76.6
Education	104	54.2	56	48.7	48	62.3
Community-based campaign	38	19.8	21	18.3	17	22.1
Legislation	29	15.1	11	9.6	18	23.4
Behavior modification, socialization, or training of companion animals	19	9.9	9	7.8	10	13.0
Behavioral, temperament, or suitability testing of companion animals	17	8.9	12	10.4	5	6.5
Spaying/neutering of companion animals	13	6.8	4	3.5	9	11.7
Increasing role/capacity of veterinarians in prevention and control of relinquishment	9	4.7	2	1.7	7	9.1
Companion-animal identification/microchipping	6	3.1	1	0.9	5	6.5
Other^c	24	12.5	12	10.4	12	15.6
More research should be conducted	67	34.9	48	41.7	19	24.7
No recommendations reported	21	10.9	8	7.0	13	16.9

Multiple selections were allowed for these questions, so column percentages do not add up to 100%.

^b Other studied companion animals included birds (n = 7), rabbits (n = 7), rodents (n = 4), reptiles (n = 3), fish (n = 1), potbellied pigs (n = 1), horses (n = 1), and other unspecified companion animals (n = 4).

^c Other recommended interventions reported by more than one article included providing additional support (e.g., financial loans) for caretakers to prevent relinquishment (n = 4), following up new adoptions (n = 3), temporary adoption and accommodation programs (n = 2), encouraging euthanasia of unwanted companion animals if they are unable to be rehomed (n = 2), and providing more assistance to shelters to increase the adoption of companion animals (n = 2).

FIGURE 2 Scoping-review evidence map of the key aspects and themes of companion-animal relinquishment investigated among 115 primary-research articles stratified by year of publication and study location. Panels A and B show the number of articles investigating key aspects of companion-animal relinquishment by year of publication and study location, respectively. Panels C and D show the number of articles investigating key themes of companion-animal relinquishment by year of publication and study location, respectively. The size of the bubbles is proportional to the number of articles within a given cross-tabulation category.

Reported reasons for companion-animal relinquishment are shown stratified by publication year, location, study design, setting, and companion-animal population in Tables 6 and 7. The distribution of reported reasons for relinquishment was fairly consistent across each of these variables (Tables 6 and 7). However, reports indicating that caretakers relinquished their companion animals because of their characteristics or because they did not want them were mostly published before 2000. In addition, caretaker health issues (e.g., allergies) were one of the most consistently investigated reasons for relinquishment among studies conducted in Europe (Table 7). General disobedience, unspecified behaviors, or other behaviors were the most consistently investigated companion animal-related reasons for relinquishment among cohort studies and studies investigating companion-animal adoption and re-relinquishment (Table 6).

TABLE 4 Methodological Characteristics of 115 Primary-Research Articles Investigating the Issue of Companion-Animal Relinquishment

Characteristic	No.	%
Study type (n = 110)^a
Cross-sectional	62	56.4
Cohort	32	29.1
Case-control	9	8.2
Controlled trial	4	3.6
Quasiexperiment	3	2.7
Data collection methods (n = 115)^b
Questionnaires	89	77.4
In person	42	36.5
Mailed	27	23.5
Telephone	21	18.3
Unspecified	15	13.0
Internet	8	7.0
Analysis of records or database	30	26.1
Qualitative interviews	7	6.1
Observations/field visits	5	4.3
Study setting (n = 115)^b
Humane society, shelter, pound, or animal rescue	84	73.0
Public places (e.g., retail stores, schools, universities)	18	15.7
Veterinary clinic	15	13.0
Caretaker homes	10	8.7
Not specified or other	4	3.5

^a Tabulated for primary quantitative and mixed-methods research only.

^b Multiple selections were allowed for these questions, so column percentages do not add up to 100%.

Only 17 primary-research studies were identified that investigated interventions against companion-animal relinquishment (Table 5). The most commonly reported types of intervention were educational programs such as behavioral advice, veterinary consultations, or a provision of resources for companion-animal caretakers or adopters (Table 5). Most primary-research studies investigating interventions have been conducted since 2006 (n = 8, 47.1%) and in the United States (n = 11, 64.7%), and they primarily investigated outcomes in dogs (n = 16, 94.1%).

Authors of most primary-research articles reported their results as prevalence outcomes (e.g., percentage of positive or negative outcomes), and nearly all provided a sample size for at least some of the outcomes (Table 5). However, some authors did not provide all of their results in a format sufficient for extraction and potential use in SR–MAs (e.g., raw data were not reported or data were presented only graphically; Table 5). Most authors reported at least one strength and one limitation of their studies (Table 5).

TABLE 5 Outcomes and Reporting Characteristics of 115 Primary-Research Articles Investigating the Issue of Companion-Animal Relinquishment

Characteristic	No.	%
Reasons for companion-animal relinquishment investigated (n = 84)^a^,^b
Companion-animal related	72	85.7
Aggression	49	58.3
General disobedience, unspecified, or other companion-animal behavior problems	41	48.8
Destruction, hyperactivity, and noisiness	36	42.9
House soiling	35	41.7
Fearfulness and escaping	28	33.3
Unfriendly or does not get along with people or other companion animals	27	32.1
Companion-animal characteristics (e.g., age, gender, breed)	26	31.0
Companion-animal health or illness	26	31.0
Separation anxiety	10	11.9
Caretaker related	68	81.0
Moving, rental, or housing issues	45	53.6
Personal issues (e.g., relationship problems, lack of time, lack of responsibility)	42	50.0
Economic reasons	35	41.7
Health issues (e.g., allergies, illness, or death)	35	41.7
Unwanted companion animal (e.g., too many companion animals, gift, or from unwanted litter)	33	39.3
Caretaker characteristics (e.g., age, education)	9	10.7
Interventions investigated for the prevention or control of companion-animal relinquishment (n = 17)^c
Education program (e.g., behavioral advice or consultation)	8	47.1
Behavioral assessment program	3	17.6
Sterilization program	3	17.6
Socialization/training program	2	11.8
Fee-waived adoption program	1	5.9
Author investigation of any of the following issues in addition to companion-animal
relinquishment (n = 115)^a
Expectations of companion-animal guardianship	19	16.5
Knowledge of companion animals	18	15.7
Reason/motivators for acquiring companion animals	15	13.0
Formats used to report study results (n = 110)^a^,^d
Prevalence outcome	102	92.7
Sample size	102	92.7
Measure of variability (e.g., standard deviation, standard error, confidence intervals)	85	77.3
Continuous outcome	43	39.1
At least one outcome reported in a nonextractable format	28	25.5
Measure of association (e.g., odds ratio, relative risk)	26	23.6
Reporting of study strengths and limitations (n = 115)^a
At least one strength reported	71	61.7
At least one limitation reported	79	68.7

^a Multiple selections were allowed for these questions, so column percentages do not add up to 100%.

^b Tabulated only for studies that investigated reasons for companion-animal relinquishment.

^c Tabulated only for studies that investigated interventions to prevent or control companion-animal relinquishment.

^d Tabulated for primary quantitative and mixed-methods research only.

TABLE 6 Scoping-Review Evidence Map Identifying the Number of Studies Investigating Companion Animal-Related Reasons for Companion-Animal Relinquishment Among 84 Primary-Research Articles

Characteristic^a	Aggr.	Disob., Unspec., Other	Dest., Hyper., Noisy	House Soiling	Fear., Escap.	Not Friendly	Pet Char.	Health, Illness	Separ. Anxiety	Total

DISCUSSION

This review provided a structured, robust, and transparent identification and synthesis of the key characteristics of worldwide published research articles about companion-animal relinquishment. The review identified 115 primary-research articles that predominantly represented studies conducted in the United States and Europe. Establishing a global perspective on the reasons for companion-animal relinquishment is valuable to gain a broader understanding on the topic, including the similarities and differences in reasons for relinquishment among the various countries in which it has been studied.

TABLE 7 Scoping-Review Evidence Map Identifying the Number of Studies Investigating Caretaker-Related Reasons for Companion-Animal Relinquishment Among 84 Primary-Research Articles

Characteristic^a	Moving, Housing Issues	Person. Issues	Econ.	Health Issues	Unwant.	Caretaker Char.	Total

Unfortunately, very few studies were identified from several countries and regions around the world, including Canada, South America, and Asia. This may, in part, be because of the language restrictions placed on the current scoping review. Given the widespread nature of companion-animal relinquishment globally and the potential for geographic differences in the reasons for relinquishment, it may be worth prioritizing further primary research in regions where gaps in information currently exist.

This review showed that companion-animal abandonment was investigated in a relatively smaller number of articles from the United States and Canada compared with other countries. This may suggest that geographical differences in stray animal populations or socioeconomic conditions determine what aspect of the issue of companion-animal relinquishment is at the forefront for a particular region. A plausible reason for the lack of primary research on companion-animal relinquishment in many regions may have to do with the complexity of the issue and the recognition that it is a challenging area to research. For example, ready access to funding for research may be lacking because much of the funding is directed to humane organizations to provide housing and care for homeless companion animals.

There is also diversity in the types of facilities involved in companion-animal relinquishment, which include but are not limited to shelters, rescues, humane societies, and municipal pounds. Some are government-run organizations and others are independent, privately owned entities, which complicates identification of and access to these organizations, thus limiting the generalizability of findings. There is also diversity in the range of stakeholders, forms of relinquishment (euthanasia, surrender, abandonment), and sources of animals. Despite these challenges, gathering evidence-based information and relaying this information to stakeholders is vital to addressing companion-animal relinquishment globally.

Another significant challenge to researching companion-animal relinquishment is a lack of standardization in measuring outcomes. Efforts to standardize intake forms and shelter databases have been attempted. One example is the Asilomar Accords, adopted by a number of American states, where organizations utilize similar data collection methods and definitions to ensure consistent reporting across agencies (Asilomar Accords, 2004). A similar effort to have reliable data collection with standardized forms at shelters has been encouraged by the NCPPSP, also in the United States.

The NCPPSP's mission is to gather reliable data to characterize the problem of unwanted companion animals and to use these data to recommend programs. Studies conducted by the NCPPSP have emphasized the importance of data collection and synthesis in pet population research. For example, Salman, New, Scarlett, Kass, & Ruch-Gallie (1997) emphasized the need for accurate data to characterize the companion-animal surplus problem before attempting to solve the problem. Having a system in place that provides consistent and reliable data on companion-animal intake and disposition at shelters is necessary to evaluate adequately and broadly the success of implemented programs (Salman et al., 1997).

Perhaps the lack of standardized and reliable data collection may explain why only a small number of intervention studies (n = 17) were identified in this review. This is of interest in terms of future research, given that 75% of all identified articles included author recommendations for use of an intervention to address companion-animal relinquishment. This may explain the relative increase in research on interventions to prevent companion-animal relinquishment since 2000. It is possible that by focusing on the outcome of companion-animal relinquishment, the current review may have failed to identify a number of potential intervention studies that would indirectly affect companion-animal relinquishment. For example, if an intervention study looked only at modification of canine aggression and used a level of behavioral modification as its outcome measure (and not specifically relinquishment), it would have been excluded from this review.

Accepting this possible limitation, the current review still indicates very little intervention research has been conducted using companion-animal relinquishment as a final outcome measure. A Canadian study on management of interdog aggression in shelters suggested that shelters may be more likely to provide rehabilitation for aggressive dogs if a scientifically validated program was available (Orihel, Ledger, & Fraser, 2005). Understanding the impact of interventions on companion-animal relinquishment would greatly assist the animal welfare sector in focusing its resources on evidence-informed initiatives with known impacts on companion-animal relinquishment.

Education was the intervention most commonly recommended by authors in the companion-animal relinquishment literature. However, this recommendation appears to be founded on a limited evidence base, as only seven intervention studies have investigated the efficacy of education to reduce or prevent companion-animal relinquishment. Yet the existing observational research supports this recommendation.

Research has shown education of caretakers to be protective against relinquishment (Diesel, Brodbelt, & Pfeiffer, 2010; Diesel, Pfeiffer, & Brodbelt, 2008; Patronek, Glickman, Beck, McCabe, & Ecker, 1996). Further research examining the role and impact of education both before and after acquisition of a companion animal is necessary to help inform the development of evidence-based educational initiatives that will have an impact on companion-animal relinquishment. Areas for further study include the effects of education as well as the format of the educational material provided (e.g., in-person vs. written formats), the target population (e.g., caretakers before and after acquisition), and duration (e.g., length and number of training sessions).

Research about companion-animal relinquishment appears to be on the rise, with a notable proportion (44%) of the primary-research articles identified in this review having been published since 2006. This may suggest a burgeoning interest in understanding the issues surrounding companion-animal relinquishment. This interest may mirror the evolving relationship between companion animals and humans (Case, 2008). Case (2008) suggests that the current investment of time, emotion, and money in animals kept solely for companionship has never been higher. Although the relationship between companion animals and humans is changing, including a greater appreciation for and promotion of the public health benefits of keeping companion animals (Hodgson & Darling, 2011), this changing relationship may be driving a greater need to understand companion-animal relinquishment and the factors contributing to successful and unsuccessful human–animal relationships.

It is interesting to note that primary research to date has focused its attention mainly on understanding the reasons for companion-animal relinquishment, including animal and caretaker characteristics. Aggression was the most common animal-related reason investigated in the primary-research articles. This is not surprising, given the public health and safety concerns that arise from caring for an aggressive animal (Quirk, 2012). In addition, shelters face liability issues if they allow the adoption of an aggressive companion animal. Therefore, it is important for future intervention research to focus on prevention and modification of aggressive behaviors.

Housing issues, including rental and moving issues, were the most commonly investigated caretaker-related reasons among the primary-research articles. However, health issues were among the most consistently investigated reasons in European studies. Future research should investigate geographical differences in reasons for companion-animal relinquishment. Given the quantity and distribution of worldwide research investigating reported reasons for companion-animal relinquishment, this area of focus appears to be ideal in terms of prioritization for subsequent SR–MAs, which can be used to investigate sources of heterogeneity in the outcomes and to guide future decision making about preventing companion-animal relinquishment.

It is not surprising that the published primary research about companion-animal relinquishment focused mainly on dogs (87.8%), while only 52.2% investigated cats. This may reflect society's long history with dogs as human companions. It may be generally perceived that researching dogs is a higher priority than is researching cats due to their relative roles in society. Only 14 primary-research studies investigated relinquishment of other companion-animal species such as birds, rabbits, rodents, and reptiles. The focus of research on these species is reflective of companion-animal ownership in Western society, where the majority of research into companion-animal relinquishment appears to have taken place.

For example, a 2008 survey revealed that 56% of Canadian households have at least one dog or cat, whereas only 12% have fish, 5% have birds, 2% have rabbits and hamsters, and 1% have lizards, horses, guinea pigs, snakes, frogs, turtles, ferrets, or gerbils (Perrin, 2009). In the United States, 37.2% and 32.4% of households keep dogs and cats, respectively, while 3.9% of households keep birds (American Veterinary Medical Association, 2007). Given these statistics and the welfare issues surrounding cat overpopulation and feral and free-roaming cats (Griffin, 2001), there is a need for more primary research involving cats. Further, mass euthanasia of homeless cats in shelters is a major concern (Griffin, 2001), further supporting the need for additional research in this area.

The role of veterinarians was discussed in nearly 40% of primary research and 65% of reviews and commentaries. This highlights the perceived importance of veterinarians and veterinary associations becoming involved in addressing the issue of companion-animal relinquishment. In contrast to primary research, reviews and commentaries tended to focus on the need for interventions, possibly explaining why the educational role of veterinarians was discussed more often in these articles. The results of a study on the relinquishment of dogs to shelters in the United States indicated that behavioral advice received from a veterinarian was associated with a lower risk for relinquishment (Patronek et al., 1996). Examining the importance of a veterinarian's role should be considered for future research on interventions to prevent companion-animal relinquishment.

The role of shelters and animal control agencies is also an important area to target in future research on intervention programs. For example, shelter programs that utilize behavioral assessments and preadoption and postadoption behavior training can improve the adoptability of dogs and reduce the rate of return of adopted dogs (Bollen & Horowitz, 2008; Luescher & Medlock, 2009). Investigating the efficacy of different sources of caretaker education, such as animal shelters, veterinarians, pet stores, and breeders, and the relative roles of these stakeholders in reducing companion-animal relinquishment is likely to be beneficial.

In addition, researchers should consider investigating the efficacy of collaborative approaches to public education campaigns among stakeholders. Avanzino (1991) has described an example of a coalition approach. This approach has resulted in successful campaigns to educate the public on the benefits of spaying and neutering companion animals in the San Francisco Bay area, Los Angeles, and San Diego (Avanzino, 1991).

Most primary-research studies on companion-animal relinquishment have been observational or survey-based studies. In this scoping review, 73% of primary-research studies investigated reasons for companion-animal relinquishment. With this type of research, there is a concern for social desirability bias; that is, caretakers may not reveal their real reason for surrender. Human emotions and the social aspect of companion-animal relinquishment must be considered when researching this issue because caretakers may provide what they believe is a socially acceptable reason for surrendering their companion animals (DiGiacomo, Arluke, & Patronek, 1998).

Qualitative studies may at least partially assist in overcoming this limitation. Building relationships with relinquishers allows them to feel safe freely sharing their stories and broadens our understanding of the issue of relinquishment. For example, DiGiacomo et al. (1998) used qualitative methods to interview 38 relinquishing caretakers to provide a detailed view of the relinquisher's perspective. Only four qualitative studies examining companion-animal relinquishment were identified, suggesting an opportunity for further qualitative research in this area to gain a more in-depth understanding of the issues, barriers, and challenges associated with relinquishment.

Outcome measures reported across primary-research articles were varied. Most studies reported results as prevalence outcomes (93%), while a smaller proportion (24%) reported associations (odds ratios or risk ratios). This reflects the types of studies conducted (i.e., most were cross-sectional studies reporting prevalence outcomes at one point in time). Cross-sectional studies are not suitable for investigating causes or risks for a particular outcome to occur, whereas case-control studies and cohort studies, which report associations, are more suitable for investigating risk or the causality of the association between the variable of interest and the outcome. These types of studies should be prioritized for future research to build stronger evidence for establishing causal relationships between risk factors and companion-animal relinquishment.

One quarter (26%) of all primary-research articles reported at least some primary outcomes in a format that is not extractable and useable for subsequent SR–MAs. This limits the ability for this evidence to be further synthesized, appropriately assessed and interpreted by end users, and used to support policy and decision making. Future primary-research studies investigating companion-animal relinquishment should follow standardized reporting guidelines for observational studies, as specified by the Strengthening the Reporting of Observational Studies in Epidemiology statement (Von Elm et al., 2007).

One potential limitation of this review is that unpublished or grey literature (e.g., shelter or humane society reports) might have been missed by the search if it was not indexed in the databases, search engines, or websites covered in this review. For logistical reasons, we were only able to screen the first 100 hits in the Scopus web and Google searches, which could have missed some additional, relevant grey literature articles. In addition, we did not perform an updated Google search to include the term euthanasia, leading to the possibility that grey literature articles specific to this topic may have been overlooked. Currently, there is limited empirical evidence or consensus about the utility of Internet searches in scoping reviews and SRs and the potential limits that should be imposed on them given the potential for thousands of hits (Eysenbach, Tuische, & Diepgen, 2001; Higgins & Green, 2011). Overall, we believe that our comprehensive search verification strategy supports the likelihood that most, if not all, relevant articles were identified.

A second limitation is that articles in languages other than English were excluded at the DCE stage because of limited resources for translation. A total of 66 non-English articles were excluded, which might have contributed to the relatively low number of identified studies from non-English countries and regions. It is not known to what extent these excluded articles would have been confirmed as relevant. Previous SR–MAs of veterinary and agri-food public health topics have identified only a small proportion of relevant studies in languages other than English (Mederos et al., 2012; Olivry & Mueller, 2003; Tusevljak et al., 2012). It is possible that including these 66 articles would not have significantly changed the key results of the review, other than increasing representation from non-English geographic regions. Although beyond the resources of the current research team, further screening and evaluation of the 66 non-English articles investigating companion-animal relinquishment should be considered.

CONCLUSION

Scoping reviews are a knowledge-synthesis method used to explore and describe the breadth of literature available and to identify concepts in a broad area of research, evidence that is available, and research gaps (Arksey & O'Malley, 2005). They can also provide the preliminary groundwork for subsequent SR–MAs and guide additional primary research (Arksey & O'Malley, 2005). Although the current scoping review required considerable time and resources to complete, the understanding gained from this review on the current state of research relating to companion-animal relinquishment provides valuable information for guiding future research projects and determining the best use of future research resources.

The global research identified in this review should be regularly updated and maintained to identify research gaps and needs and to prioritize focused questions for SR–MA. The results indicate that a majority of the primary research on companion-animal relinquishment has investigated the reported reasons for relinquishment. These outcomes should be prioritized for SR–MA.

Most authors of the reviews and commentaries identified discussed interventions and strategies for reducing companion-animal relinquishment. Only a small number of primary-research articles investigated interventions; thus, future studies should focus on identifying and measuring the effectiveness of interventions such as public education. The social complexity of the issue and the impact of the HAB on companion-animal relinquishment also underscore the need for qualitative research in this area.

ACKNOWLEDGMENTS

We thank Mai Pham for developing and conducting the search strategy and conducting relevance screening, and Drs. Malcolm Weir and Sanja Ilic for assistance conducting relevance screening and DCE.

FUNDING

Funding for this project was provided through the Nestlé Purina PetCare Canada Chair in Communications.