Vegan versus meat-based dog food: Guardian-reported indicators of health

Andrew Knight; Eason Huang; Nicholas Rai; Hazel Brown

doi:10.1371/journal.pone.0265662

Abstract

Alternative pet foods may offer benefits concerning environmental sustainability and the welfare of animals processed into pet foods. However, some worry these may compromise the welfare of pets. We asked 2,639 dog guardians about one dog living with them, for at least one year. Among 2,596 involved in pet diet decision-making, pet health was a key factor when choosing diets. 2,536 provided information relating to a single dog, fed a conventional meat (1,370 = 54%), raw meat (830 = 33%) or vegan (336 = 13%) diet for at least one year. We examined seven general indicators of ill health: unusual numbers of veterinary visits, medication use, progression onto a therapeutic diet after initial maintenance on a vegan or meat-based diet, guardian opinion and predicted veterinary opinion of health status, percentage of unwell dogs and number of health disorders per unwell dog. Dogs fed conventional diets appeared to fare worse than those fed either of the other two diets. Dogs fed raw meat appeared to fare marginally better than those fed vegan diets. However, there were statistically significant differences in average ages. Dogs fed raw meat were younger, which has been demonstrated to be associated with improved health outcomes. Additionally, non-health related factors may have improved apparent outcomes for dogs fed raw meat, for three of seven general health indicators. We also considered the prevalence of 22 specific health disorders, based on predicted veterinary assessments. Percentages of dogs in each dietary group considered to have suffered from health disorders were 49% (conventional meat), 43% (raw meat) and 36% (vegan). Significant evidence indicates that raw meat diets are often associated with dietary hazards, including nutritional deficiencies and imbalances, and pathogens. Accordingly, the pooled evidence to date indicates that the healthiest and least hazardous dietary choices for dogs, are nutritionally sound vegan diets.

Citation: Knight A, Huang E, Rai N, Brown H (2022) Vegan versus meat-based dog food: Guardian-reported indicators of health. PLoS ONE 17(4): e0265662. https://doi.org/10.1371/journal.pone.0265662

Editor: Simon Clegg, University of Lincoln, UNITED KINGDOM

Received: September 8, 2021; Accepted: March 6, 2022; Published: April 13, 2022

Copyright: © 2022 Knight et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: Our data analysed are accessible at https://rebrand.ly/2020/pet-food-consumers-study.

Funding: This research and its publication open access was funded by food awareness organisation ProVeg International (https://proveg.com). AK received this award ID: Oct2019- 0000000286. However, this funder played no role in study conceptualisation, design, data collection and analysis, preparation of the resultant manuscript nor decisions relating to publication. We are grateful for their financial support.

Competing interests: The authors have declared that no competing interests exist.

Introduction

In 2018, the global pet population was estimated to include 471 million dogs, and 373 million cats [1] (p. 4). Pet food sales internationally were worth Euro 131.7 billion in 2014 [2]. The UK pet food market was expected to reach GBP 2.8 billion by the end of 2019, having risen 17% over the previous five years [3], and US pet food and treat sales were also rising, being valued at USD 42.0 billion by 2020 [4].

A market of such size drives considerable research and product development, and between January 2013 and October 2014, over 6,000 new petfood products (3,000 dry and 3,200 wet pet foods), as well as 4,000 new pet snacks, were launched globally [5] (in [6]). Some of the new products being developed include raw meat diets, in vitro meat products, and diets based on novel protein sources, including terrestrial plants, insects, yeast, fungi and seaweed. Some of this development may be driven by significant recent concerns about the environmental sustainability of animal agriculture, and of traditional pet foods based on animal produce [7–11]. This market is already large, and is growing fast. The vegan pet food market was worth USD 8.7 billion globally by 2020, and its forecast valuation by 2028 has been estimated at USD 15.7 billion–a compound annual growth rate of 7.7% [12].

However, concerns exist that the imposition of human petfood preferences may be suboptimal for the welfare of dogs. Claiming the existence of “almost insurmountable challenges–biological, legal and downright practical–facing anyone attempting to shoehorn dogs and cats into a vegan dietary system”, Loeb [13] stated, “it’s fairly clear that feeding a dog a vegan diet is not recommended.”

How valid are such concerns about the nutritional suitability for dogs, of vegan diets? There are two clear routes to assess this. The first involves examining steps taken by petfood manufacturers to ensure the quality and nutritional soundness of their products. These were recently examined in a survey of 29 companies producing meat-based (19) and plant-based (10) pet foods [14]. Although there were limited areas in which practices could be improved, most manufacturers had acceptable or superior standards at nearly all stages examined, throughout the design, manufacturing, transportation and storage phases, with plant-based diets slightly superior to meat-based diets overall.

However, the most important test is always the effect(s), if any, on the animals themselves. This is why feeding trials are considered the gold standard to ensure the nutritional soundness of new formulations [15, 16]. The health status of dogs maintained on different diets has been the subject of limited studies to date, some of which we’ve reviewed elsewhere [17]. In 1987, Yamada and colleagues [18] reported the results of a study of eight dogs divided into two groups, maintained on commercial animal or vegetable protein (VP)-based diets. Each comprised around 30% protein, and other macronutrients and energy contents were closely matched. Six weeks of rest was followed by four hours daily of enforced running at 12 km/h for two weeks. This was followed by one week of recovery. Blood tests indicated that the VP-based dogs experienced marked anaemia following this relatively severe exercise regime. This was theorised to be due to changes in circulating lipid levels (reduction of free cholesterol), resulting in lowered erythrocyte resistance to haemolysis.

In 2009, Brown and colleagues [19] reported conflicting results, from a marginally larger study of 12 sprint-racing Siberian Huskies, fed either a commercial meat-based diet recommended for active dogs (n = 6), or a meat free (although not vegan) diet formulated to the same nutrient specifications (n = 6). The dogs were fed these diets for 16 weeks, including 10 weeks of competitive racing. Blood tests were conducted on four occasions, and veterinary health checks on three occasions. All dogs were assessed as being in excellent physical condition, and none developed anaemia or other detectable health problems.

Additionally, in 2014 Semp [20] reported a study of vegan companion animals in Austria, Germany and Switzerland. A questionnaire was completed by 174 dog and 59 cat guardians, some of whom had both species. Participating dogs had eaten vegan diets for six months to seven years, with a mean of 2.83 years. Clinical examinations and blood tests were conducted on 20 randomly selected dogs. No diet-related clinical abnormalities were detected. Haematological (complete blood count) and biochemical (liver, kidney, and pancreatic) parameters were assessed, as well as levels of magnesium, calcium, iron, total protein, folic acid, vitamin B12, and carnitine. The serum total protein of all dogs was within normal ranges. No significant differences were evident in any tested parameters, compared to dogs fed a conventional diet. Not even the 10% (2/20) of dogs fed a homemade supplemented diet showed any significant deviations.

However, the relatively small numbers included in these samples limits their predictive value for wider dog populations. By 2021, no large-scale study of dogs had been published, describing how health indicators vary between dogs maintained on vegan, meat-based, or indeed, other diets. Accordingly, we designed a survey to explore this. Our null hypothesis was that guardian-reported canine health indicators would not significantly vary with diet type. The success of new pet foods under development also depends on the views of consumers. We also sought to determine the importance of pet health as a purchasing determinant, to a large group of dog guardians. Results of some survey parts were recently reported (palatability of different diets; [21]), or are the subject of related, forthcoming studies.

Methodology

We designed a survey for dog or cat guardians using the ‘Online surveys’ platform (https://www.onlinesurveys.ac.uk). Guardians were asked to provide information about themselves and one dog or cat resident within their household for at least one year. Guardians were asked about the main ingredients within their pet’s normal diet. They were asked to identify whether the diet was based on conventional, raw or in vitro meat, insects, fungi or algae, or whether it was a vegetarian, vegan or some ‘other’ diet. Respondents could select only one option. Vegetarian diets were explained as including eggs or milk, but not meat, and vegan diets as eschewing any animal products. Where animals were fed a prescription or therapeutic diet, guardians were asked to base answers on the diet in use prior to the commencement of the therapeutic diet. Guardians were also asked about any treats/snacks/scraps or supplements provided.

Our survey also inquired about human demographics (continental region, urban or rural location, educational qualifications achieved, occupation, household income, age categories in 10 year age bands with the exception of bands for 18–19 and ‘70 or older’, gender, and respondent diet). Information was also obtained about animal demographics. These included role (companion or working animal), age (with any year entry up to ‘over 25’ possible), sex/neuter status, canine breed (toy, small, medium, large and giant), activity level, health status, reaction to meals, factors of importance to guardians when choosing pet food, and information sources guardians relied on.

Guardians provided information about seven general indicators of health, and about the prevalence of specific health disorders, for the previous year, or the year prior to the commencement of a therapeutic diet if one was currently used. Specifically, guardians were asked to report the frequency of veterinary visits, and medication use (other than routine vaccinations and treatments for external or internal parasites, such as fleas, ticks, lice, heartworm and intestinal worms, or treatments associated with neutering operations, or microchipping). Guardians were asked to report whether their dog had progressed onto a therapeutic diet, after initial maintenance on another diet type. They were asked to report their own opinion of their dog’s health status, and also to report what they believed their veterinarian’s assessment to be. Guardians were asked to “Think about your veterinarian. Which of the following would most likely describe their opinions about your animal’s medical condition over the previous 12 months?” Possible answers for both their own opinions, and for the reported assessments of their veterinarians, ranged from no problems/routine preventative healthcare, to seriously ill. If veterinarians reportedly considered dogs to be suffering from health disorder(s), guardians were asked which disorder(s) these were, from among 18 disorders indicated to be among the most common disorders experienced by companion dogs [22–26]. Guardians were able to select multiple disorders, and to provide details of additional disorders by selecting ‘other’. Details for each ‘other’ entry were examined, with these entries then reclassified into 18 existing or four new disorder types, giving a total of 22 possible health disorders.

When analysing health disorders, cases were excluded, where veterinary visits had not occurred at least once in the previous year, or where guardians were unsure of the assessments of their veterinarians. The remaining subset comprised guardians who had recently seen their veterinarians, and were sure of their health assessments. This subset was used to calculate the percentage of unwell dogs, and the average number of cases of disorder, per dog. It was also used to calculate the prevalence of the 22 specific health disorders.

Potentially confounding factors

Health status may be affected by age, sex, desexing (neutering) status and breed [25–28]. Hence, we sought to ascertain differences between major dietary groups, in age, sex and neuter status. We decided not to attempt to account for the possible effects of certain additional factors, on health outcomes. Breed, for example, can affect health status [27, 28]. However, we were concerned that small numbers within breed groups would limit our ability to statistically analyse subsequent results, and so ultimately elected not to discriminate by breed within this study. We also chose not to exclude dogs who received treats regularly, expecting most to receive such treats.

Survey pilot and distribution

Our survey piloting and distribution were described in Knight and Satchell [21]. The ‘Online surveys’ platform we chose to use complies with the UK General Data Protection Regulation, following the UK Data Protection Act 2018, and was used by 88% of UK higher education institutions by 2019 [29], including our University of Winchester.

We piloted our survey to 25 respondents in April 2020. Improvements were then made to both survey structure and questions. With respect to structure, changes were made to the ordering of survey parts, to minimise inadvertently biasing answers to questions about health. These survey sections were moved toward the beginning, to eliminate chances that answers might be affected by prior answers about pet diet choices, particularly where unconventional diets were used, e.g., if a guardian reporting use of an unconventional diet might subsequently be more likely to consciously or unconsciously downplay any health problems. Similarly, changes were made to the ordering of questions about veterinary opinions about animal health. In general, the variable most likely to be dependent, was positioned prior to any possibly corresponding independent variable. Various questions were also clarified and simplified. The final survey steps were those in Fig 1.

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Fig 1. Survey parts.

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The final survey was made available from May–December 2020. It was widely advertised through social media to dog and cat interest groups. Paid Facebook advertising and several volunteers were utilised to increase survey exposure. Facebook advertising demographics were unlimited, other than to include terms relating to dogs and cats. In anticipation of lower levels of unconventional diets, and the need to achieve group numbers sufficient for statistical analysis, volunteers and the authors made some efforts to reach unconventional pet food interest groups, as well as conventional dog and cat interest groups. However, by careful wording choice, no bias for or against any particular diet choice was implied within advertising materials, or within the survey questions or explanatory text.

Statistical analysis

We reported demographic results for survey respondents, and for their dogs. With respect to mean dog ages, we used T-tests to explore differences between dietary groups. When significant differences were found, effect size interpretations were provided using the Cohen’s d statistic, with small, medium or large effects interpreted when |d| was close to 0.2, 0.5 and 0.8, respectively [30]. With respect to sex/neuter status, where chi-squared results indicated the existence of significant differences, we provided effect size interpretations using the Cramer’s V statistic, with small, medium or large effects interpreted when V was close to 0.2, 0.5 and 0.8, respectively [31]. Where such significant differences existed, we then compared each main dietary group combination, calculating p-values. Where these indicated significant differences between dietary groups, we provided odds ratios, indicating relative differences in the likeliness of outcomes, between dietary groups.

After initial examination of dog diets, we limited further analysis to dogs maintained on three main diets: conventional meat, raw meat, and vegan pet food. We excluded smaller dietary groups to avoid potentially substantial differences in variances between dietary groups of small and larger sizes, which could adversely affect our statistical analysis.

We investigated the impacts of these three main diet types on dog health. Guardians provided information about seven general indicators of health, and about the prevalence of 22 specific health disorders, as described previously. Similarly to our analysis of sex/neuter status, for the main dietary groups identified, we investigated associations between diet type and the seven general health indicators using chi-squared tests, reporting test statistics and p-values. Where chi-squared results indicated the existence of significant differences, we similarly provided effect size interpretations using the Cramer’s V statistic, with small, medium or large effects interpreted when V was close to 0.2, 0.5 and 0.8, respectively [31]. Where such significant differences existed, we then compared each main dietary group combination, calculating p-values. Where these indicated significant differences between dietary groups, we provided odds ratios, indicating relative differences in the likeliness of outcomes, between dietary groups.

For most general health indicators, our data were categorical. However, assessments or opinions of health status were ordinal. These data were coded into levels 1 to 4 (indicating no health problems, up to seriously ill, respectively). We then used Kruskal-Wallis tests to explore differences between dietary groups. Dunn’s pairwise tests were carried out to explore differences within the three pairs of dietary comparisons, with the Bonferroni correction for multiple tests applied to p-values.

When investigating the significance of the association between diet and the number of disorders suffered by unwell dogs, we initially conducted an ANOVA test. Finding a significant difference, we calculated the effect size using eta squared. Following Cohen [32], we interpreted a small, medium or large effect depending on the proximity of eta squared to 0.01, 0.06 or 0.14 respectively. We also included a test for homogeneity of variance. Finding a lack of homogeneity, we then conducted a Games-Howell post hoc comparison to test the significance of apparent differences between dietary groups.

As well as exploring associations between diets and the seven general health indicators, we also explored associations with the 22 specific health disorders. Binomial response models provided odds ratios and p-values indicating differences between dietary groups. We used software packages R-studio and SPSS, v26. Significance was interpreted when p < 0.05.

Ethical approval and data availability

Our research complied with the University of Winchester Ethics Policy [33] (approval reference RKEEC200304_Knight).

Results

Of 4,060 respondents to our combined cat and dog survey, 4,057 confirmed they met the survey conditions (18 years or older, with answers relating to one dog or cat resident within their household, for at least one year). The following results are limited to the 2,639 dogs and their guardians who responded. Results concerning 1,418 cats and their guardians are the subject of a related, forthcoming study.

Dog guardians

Of the 2,610 human respondents who provided their sex, 92% (2,412) identified as females, 7% (194) as males, and 0% (4) as other. Most age brackets from 18 to 70+ were well represented, other than the extreme ends where numbers were low. The majority of the 2,639 total respondents identified their geographical region as the UK (71%, 1,884) or Europe (15%, 398), with North America (6%, 150) and Australia/New Zealand/Oceania (4%, 117) being the next most prevalent continental regions. A minority (18%, 488/2,639) worked in the pet or veterinary industries. The most common diet reportedly followed by these 2,639 survey respondents was omnivorous (40%, 1,066), followed by vegan (22%, 586), reducetarian (omnivore reducing animal product consumption) (21%, 567), vegetarian (10%, 266) and pescatarian (consuming fish but no other meats) (5%, 134).

Importance of health to guardians.

Of the 2,612 respondents who indicated their involvement in pet diet decision-making, 95% (2,489) were primary decision-makers, 4% (107) played some lesser role, and 1% (16) played no role. Those 99% (2,596) playing at least some role were asked which factors were important when choosing pet diets. Among 13 options including ‘other’, health and nutrition was considered the most important factor, being of importance to 94% (2,453) of 2,596 respondents to this question. These 2,596 individuals were asked which health and nutrition factors were important to them. Maintenance of pet health was considered the most important factor among five health and nutrition options including ‘other’. It was cited as important by 90% (2,211) of 2,449 respondents to this question.

The importance of health was similarly highlighted by the 1,370 respondents who used a conventional meat formulation as their dog’s normal diet, and the 830 who used a raw meat formulation. These combined 2,200 respondents were asked whether they would realistically choose alternative diets, if these offered their desired attributes. The alternatives offered for consideration were vegetarian and vegan diets, as well as those based on laboratory grown meat, insects, fungi and algae. Of 2,181 who answered this question, 44% (955) confirmed they would realistically choose such alternative diets. ‘Confidence about pet health’ was the second most important among 14 desired attributes (including ‘other’), that any alternative diet would need to provide. It was cited as essential by 83% (789) of these 955 respondents, after ‘Confidence about nutritional soundness’ (84%, 805).

Dogs

Diets.

2,639 dog guardians responded, each describing a single dog. 2,612 indicated the main diet their dog was maintained on. 2,536 dogs were jointly maintained on the three main diets identified. These were conventional meat (1,370–54%), raw meat (830–33%) and vegan (336–13%) diets (Fig 2). Smaller dietary groups were excluded from further analysis. The largest excluded group was dogs reportedly fed vegetarian diets (n = 35). We also excluded 41 dogs reportedly maintained on fungi- (1) and insect-based diets (6), laboratory-grown meat (7), mixtures of other dietary types (17) and diets listed as ‘unsure’ (10). These groups were excluded due to low numbers, lack of clarity concerning the main ingredient type, or current unavailability of these sources as canine maintenance diets (as distinct from treats, snacks or supplements). Included within the set of 2,612 were 46 dog diets identified as ‘other’. These were examined and reclassified into conventional meat, mixture or unsure, depending on further details provided in textual answers.

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Fig 2. Three main diets fed to 2,536 dogs.

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As mentioned, we chose not to exclude dogs who received treats regularly, expecting most would fall within this group. This turned out to be true, with 76% (1,935) of these 2,536 dogs receiving treats/snacks/scraps at least once daily. Treats provided to these 2,536 dogs were most commonly vegetables or fruit (1,315), commercial treats (1,174), dental/oral bars or chewable sticks (1,129), human food prepared at home (901), and raw meat or bones (758). Some dogs received more than one kind of treat.

Thirty seven percent (926) of these 2,536 dogs were also regularly offered dietary supplements other than treats/snacks/scraps. These included products for joint health (558), fatty acids (e.g., omega-3 fatty acids) (364), probiotics or prebiotics (349), vitamins (235), minerals (198), digestive enzymes (130), amino acids (101) such as taurine, and other products. Some dogs received more than one kind of supplement.

Ages.

Considering the 2,536 dogs fed the three main diets, guardians were unsure of dogs’ ages in two cases. Ages of the remaining 2,534 dogs are indicated in Fig 3. The mean ages in years, were: overall– 6.18, raw meat– 5.52, conventional meat– 6.31, vegan– 7.30. Differences between all diet groups were significant, and of small to medium size (Table 1).

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Fig 3. Ages of 2,534 dogs fed three main diets.

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Table 1. Age differences between 2,534 dogs fed three main diets.

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Sex/Neuter status.

The sex/neuter status of these 2,536 dogs is provided in Table 2 and Fig 4. Females comprised around 47%, and males around 53% of this sample. A chi-square test of independence showed a significant association between diet type and sex/neuter status, overall: χ2 (6) = 57.23, p < 0.05. The effect size was small (Cramer’s V = 0.106). Within this sample, dogs fed vegan diets were slightly more likely to be female, than either of the other two dietary groups. However, sex and diet type were not statistically significantly associated: χ2(2) = 3.9468, p = 0.139.

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Fig 4. Sex/neuter status of 2,536 dogs fed three main diets.

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Table 2. Sex/neuter status of 2,536 dogs fed three main diets.

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Statistically significant differences were apparent however, with respect to desexing. The odds of being sexually intact were significantly different between all dietary groups (Table 2). Dogs fed vegan diets were less likely, and dogs fed raw meat were more likely, to be sexually intact, than conventionally fed dogs. Dogs fed raw meat were more than twice as likely to be sexually intact, than dogs fed vegan diets. Additionally, the odds of being sexually intact were significantly different between males and females, with males significantly more likely to be sexually intact (Table 3).

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Table 3. Differences in odds of being sexually intact, between males and females, for 2,536 dogs fed three main diets.

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General health indicators

The results in this section consider the 2,536 dogs in the three main dietary groups.

Number of veterinary visits.

After excluding 16 ‘unsure’ responses, 2,520 guardians reported frequency of veterinary visits within the last year (Fig 5, Table 4). Routine health checks are normally conducted annually, whereas multiple veterinary visits within a single year may sometimes indicate a health problem. We were interested in those dogs who saw veterinarians more than once in the previous year. A chi-square test of independence showed that there was a significant association between diet type and veterinary visits more or less than once in the last year: χ2 (2) = 84.75, p < 0.05. The effect size was small (Cramer’s V = 0.183).

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Fig 5. Veterinary visits of 2,520 dogs fed three main diets, in the last year.

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Table 4. Veterinary visits of 2,520 dogs fed three main diets, in the last year.

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There were significant differences (p < 0.05) in the likelihood of seeing veterinarians more than once in the previous year, between all dietary groups (Table 5). Dogs fed vegan diets were less likely, and dogs fed raw meat less than half as likely, to meet this criterion, than conventionally fed dogs. Dogs fed vegan diets were more likely to meet this criterion, than dogs fed raw meat.

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Table 5. Likelihood of seeing veterinarians more than once in the previous year, for 2,520 dogs fed three main diets.

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Medication use.

All 2,536 guardians provided information about medication use in the previous year (Fig 6, Table 6). A chi-square test of independence showed a significant association between diet type and medication use: χ2 (2) = 56.002, p < 0.05. The effect size was small (Cramer’s V = 0.149). There were significant differences (p < 0.05) in the likelihood of medication usage in the previous year, between dogs fed vegan and conventional diets, and between dogs fed raw meat and conventional diets, but not between dogs fed vegan and raw meat diets. Dogs fed vegan and raw meat diets each had a lower risk of meeting this criterion, compared to conventionally fed dogs.

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Fig 6. Medication use in 2,536 dogs fed three main diets.

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Table 6. Medication use in 2,536 dogs fed three main diets.

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Progression onto a therapeutic diet.

All 2,536 guardians provided information about whether or not their dog progressed onto a therapeutic diet, after initial maintenance on one of the three main diets (Fig 7, Table 7). A chi-square test of independence showed a significant association between initial diet type and subsequent progression onto a therapeutic diet: χ2 (2) = 35.659, p < 0.05. The effect size was small (Cramer’s V = 0.119). There were significant differences (p < 0.05) in likelihood of subsequent progression onto a therapeutic diet, between dogs initially fed raw meat and conventional diets, and between dogs initially fed vegan diets and raw meat, but not between dogs initially fed vegan and conventional diets. Dogs initially fed raw meat were less than one fifth as likely to meet this criterion, as dogs initially fed conventional diets, and dogs initially fed vegan diets had more than three times the risk of this outcome, compared to those initially fed raw meat.

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Fig 7. Subsequent progression onto a therapeutic diet in 2,536 dogs maintained on an initial diet as specified.

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Table 7. Subsequent progression onto a therapeutic diet in 2,536 dogs maintained on an initial diet as specified.

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Reported veterinary assessments of health status.

2,074 dogs saw a veterinarian at least once in the previous year (Table 4). After excluding 12 ‘unsure’ respondents, the 2,062 remaining guardians were reportedly sure of the assessments of their veterinarians regarding the health status of their dogs (Fig 8, Table 8). A chi-square test of independence showed a significant association between diet type and reported veterinary assessment: χ2 (6) = 16.770, p = 0.0101. The effect size was small (Cramer’s V = 0.064).

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Fig 8. Guardian-reported veterinary assessments of the health status of 2,062 dogs fed three main diets.

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Table 8. Guardian-reported veterinary assessments of the health status of 2,062 dogs fed three main diets.

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After coding into 1 to 4 (indicating no health problems (1), up to seriously ill (4), respectively), significant differences existed between dogs fed vegan and conventional meat diets, but not between other dietary groups. A Kruskal-Wallis test provided very strong evidence of a difference (p = 0.002) between the means ranks of at least one pairing (Table 9). Dunn’s pairwise tests were carried out for the three pairs of groups. There was very strong evidence (p = 0.002, adjusted using the Bonferroni correction) of a difference between dogs fed a vegan and a conventional meat diet. There was no evidence of differences between dogs fed vegan and raw meat diets, or for dogs fed raw or conventional meat diets (Table 10).

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Table 9. Differences in guardian-reported veterinary assessments of the health status of 2,062 dogs fed three main diets.

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Table 10. Pairwise comparison of guardian-reported veterinary assessments of the health status of 2,062 dogs fed three main diets.

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When comparing each main diet group combination and calculating odds ratios, there were significant differences (p < 0.05) in the likelihood that reported veterinary assessments of health status would indicate poorer health, between dogs fed vegan and conventional diets, but not between the other dietary groups (Table 11). Dogs fed vegan diets were less likely to meet this criterion, than conventionally fed dogs.

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Table 11. Likelihood of guardian-reported veterinary assessments indicating poorer health, in 2,062 dogs fed three main diets.

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Guardian opinions of health status.

After excluding six ‘unsure’ responses, 2,530 guardians reported their own opinions of the health status of their dogs (Fig 9, Table 12). A chi-square test of independence showed a significant association between diet type and guardian opinion: χ2 (6) = 52.875, p < 0.05. The effect size was small (Cramer’s V = 0.102).

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Fig 9. Guardian opinions of the health status of 2,530 dogs fed three main diets.

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Table 12. Guardian opinions of the health status of 2,530 dogs fed three main diets.

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After coding into 1 to 4 (indicating no health problems (1), up to seriously ill (4), respectively), statistical analysis indicated significant differences between dogs fed vegan and conventional diets. A Kruskal-Wallis test provided very strong evidence of a difference (p < 0.0001) between the means ranks of at least one pairing (Table 13). Dunn’s pairwise tests were carried out for the three pairs of groups. There was very strong evidence (p < 0.0001, adjusted using the Bonferroni correction) of differences between dogs fed vegan and conventional meat diets, and between dogs fed conventional meat and raw meat diets. There was no evidence of a difference between dogs fed vegan and raw meat diets (Table 14).

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Table 13. Differences in guardian opinions of the health status of 2,530 dogs fed three main diets.

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Table 14. Pairwise comparison of guardian opinions of the health status of 2,530 dogs fed three main diets.

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When comparing each main dietary group combination and calculating odds ratios, there were significant differences (p < 0.05) in the likelihood guardians would assess their dogs as having worse health, between dogs fed vegan and conventional diets, and between dogs fed raw meat and conventional diets, but not between dogs fed vegan and raw meat diets (Table 15). Dogs fed vegan and raw meat diets were both less likely to meet this criterion, compared to conventionally fed dogs.

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Table 15. Likelihood of guardian opinions indicating poorer health, in 2,530 dogs fed three main diets.

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Specific disorders

2,074 dogs saw veterinarians at least once in the previous year (Table 4). After excluding 12 cases in which guardians were unsure what veterinary opinions would be, guardians were reportedly sure of the opinions of 2,062 veterinarians (Table 8). 1,123 of these dogs were considered entirely healthy. The remaining 939 dogs were considered to suffer from one or more disorders. In eight cases (conventional meat– 3, raw meat– 4, vegan– 1), details were not provided or veterinarians reportedly considered dogs to be ‘healthy’, ‘old’, or variations of these–i.e. not truly unwell. These dogs were excluded. The remaining 931 dogs were analysed. In 161 of these cases, details of ‘other’ disorders were reported. These were examined, and then reclassified into the 18 existing, or four new disorder types. In total, respondents reported that these 931 dogs were considered by their veterinarians to be suffering from 1,477 cases of 22 specific disorders (S1 Table).

For five disorders, guardians had the option to provide additional information. With respect to skin/coat problems, further information was provided about 140 of these 147 cases. The most common causes, in order, were atopic/allergic dermatitis (inflamed skin due to allergies), moist dermatitis, and pruritis (itchiness) of unspecified origin. With respect to mobility problems, further information was provided about 123 of these 135 cases. The most common causes, in order, were osteoarthritis/arthritis, and a variety of ’other’ causes. With respect to dental/oral problems, further information was provided about 109 of these 110 cases. The most common causes, in order, were dental calculus/plaque/tartar, gingivitis, and a variety of ‘other’ causes, particularly damaged, broken or worn teeth. With respect to body weight problems, all 80 respondents described whether dogs were over- or underweight. Eighty five percent (68) of reported cases were overweight, and 15% (12) were underweight. With respect to eye problems, further information was provided about 57 of these 58 cases. The most common causes included eye ulcers and related conditions such as dry eye and entropion, conjunctivitis, infections, blindness/vision loss and cataracts.

Percentage of unwell dogs and average number of disorders per unwell dog.

In addition to these 931 dogs with 1,477 cases of 22 specific disorders, respondents reported that the remaining 1,123 dogs were considered by their veterinarians to be healthy. Overall, 45% were suffering from at least one disorder, and the average number of disorders per unwell dog was 1.59 (Table 16).

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Table 16. 1,477 occurrences of 22 specific disorders, in 2,054 dogs fed three main diets.

https://doi.org/10.1371/journal.pone.0265662.t016

Percentage of unwell dogs.

A chi-square test of independence showed that there was a statistically significant association between diet type and the number of unwell dogs: χ2 (2) = 15.65, p < 0. 0001. The effect size was small (Cramer’s V = 0.087). There were significant differences (p < 0.05) in the likelihood of being considered unwell between dogs fed vegan and conventional meat diets, and between dogs fed raw and conventional meat diets, but not between dogs fed vegan and raw meat diets. Dogs fed both vegan and raw meat diets had less risk of this outcome compared to conventionally fed dogs (Table 16).

Number of disorders per unwell dog.

The number of disorders per unwell dog ranged from one to eight (Table 17). An ANOVA test revealed a significant difference between the number of disorders suffered by unwell dogs, depending on diet type (F = 3.953 (2, 928), p = 0.02). The effect size was small (eta squared = 0.008). A test of homogeneity of variances indicated that variances were not homogenous. Accordingly, a Games-Howell post hoc comparison test was used. This revealed that unwell dogs fed a raw meat diet suffered less disorders than unwell dogs fed a conventional meat diet. This difference was statistically significant (p = 0.022). Unwell dogs fed a vegan diet did not significantly differ in the number of disorders suffered, compared to unwell dogs fed conventional or raw meat diets.

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Table 17. Number of disorders per unwell dog, among 931 unwell dogs fed three main diets.

https://doi.org/10.1371/journal.pone.0265662.t017

Prevalence of 22 specific disorders.

The prevalence of these 22 specific disorders in these 2,054 dogs is indicated in S2 Table and Fig 10.

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Fig 10. Prevalence of 22 specific disorders or affected bodily systems in 2,054 dogs fed three main diets, based on reported assessments of veterinarians.

Vertical axis order reflects overall prevalence of disorders (combining all diets).

https://doi.org/10.1371/journal.pone.0265662.g010

Differences between dietary groups.

Based on probability of occurrence, the 10 most common disorders found within each dietary group are listed in Table 18. Some significant differences in the prevalence of certain disorders between dietary groups were detected. There are indicated in S3 Table and summarised in Table 19.

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Table 18. The 10 most common disorders or affected bodily systems among 2,054 dogs fed three main diets, and overall, based on reported assessments of veterinarians.

https://doi.org/10.1371/journal.pone.0265662.t018

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Table 19. Disorders or bodily system effects with significantly different prevalences between dietary groups, among 2,054 dogs, based on reported assessments of veterinarians.

https://doi.org/10.1371/journal.pone.0265662.t019

Discussion

Importance of health to guardians

Our results affirmed the importance of pet health to guardians. Among 2,596 respondents, health and nutrition was the factor considered most important in purchasing decisions. These results concur with other studies. ‘Health & Nutrition’ was the most important among 24 pet food characteristics ranked by 2,181 pet guardians, in a US-based study from 2015 to 2016 by Schleicher and colleagues [34].

It was noteworthy that 44% of our respondents feeding conventional or raw meat-based diets, stated they would realistically consider alternatives. These results were similar to those of Dodd and colleagues [35], who surveyed 3,673 primarily Canadian and US pet guardians. They found that 35% (1,083/3,130) of responding guardians who did not already feed a plant-based diet to their dog or cat, indicated interest in doing so. Our respondents reported that the most important attributes such an alternative diet would need to provide, were ‘confidence about nutritional soundness’, closely followed by ‘confidence about pet health’ (cited as necessary by 84% and 83% of these respondents, respectively). Dodd and colleagues found that the most important attributes such an alternative diet would need to provide, were ‘further evidence of nutritional sufficiency’ (45% - 269/599), followed by veterinary approval (20% - 122/599), and greater availability (20% - 117/599).

Canine demographics

Health outcomes for animals fed different diets may be affected by age, sex and desexing (neutering) status [26, 36], and so it was important for us to understand whether these varied between different diets, and how they compared to normal dog populations, in our sampled dogs.

All three of the dietary groups we studied (conventional meat, raw meat, vegan) appeared to have had an age distribution broadly representative of normal dogs [26, 37], with the exception of the first five years of life, in which there was a higher percentage of dogs fed raw meat diets, and a lower percentage fed vegan diets (Fig 3). This resulted in significant differences between the average ages of these dietary groups. Within our studied sample, on average, the youngest dogs were those fed raw meat, and the oldest dogs were those fed vegan diets, with statistically significant differences between all dietary groups. Given that younger dogs generally have fewer health problems, this may have positively influenced the general health outcomes of dogs fed raw meat diets. In contrast, dogs fed vegan diets could be expected to have relatively worse health outcomes. Due to their shorter lifespans, a single canine year of life equates to many years of human life [38], from a veterinary healthcare perspective.

Females comprised just under half and males just over half of our 2,536 dogs (Table 2). This was consistent with a 2016 study of 22,333 UK dogs [26], which found just over half to be male. Variations in sex distributions between dietary groups within our sample were not statistically significant. Statistically significant differences did exist with respect to desexing. Around three quarters of all dogs in our sample were desexed (Table 2). This differed from the findings of O’Neill and colleagues [26], who found 45% of all dogs to be desexed. Within our sample, male dogs were more likely than females to be sexually intact (Table 3). This was consistent with the findings of O’Neill and colleagues, who found desexing to be slightly less common for males. We also found that desexing was most common for dogs fed vegan diets, and least common for dogs fed raw meat diets. Significant differences existed between all dietary groups in this respect (Table 2). Lowered desexing rates in guardians feeding raw meat diets may relate to the reduced likelihood of such guardians visiting veterinarians (Fig 5). Such guardians may be less likely to receive, or to comply with, veterinary advice, and routine preventative healthcare advice commonly includes desexing recommendations.