Fueled by concerns about the health of purebred dogs, and supported by the growing list of mutations known to cause genetic disease, there has been much discussion about whether purebred dogs are predisposed to health issues compared to dogs that are not purebred.
The latest addition to the discussion (Forsyth et al 2023) is a study coming out of the Dog Aging Project, which took advantage of a large database (27.5k dogs) including both mixed and purebred dogs with extensive information about the health and welfare of each individual.
The project summary states that "purebred dogs...did not show higher lifetime prealence of medical conditions compared to mixed breed dogs". Furthermore, "a higher proportion of purebred dogs than mixed-breed dogs had no owner-reported medical conditions".
In the purebred dog community, this was hailed as a decisive nail in the coffin of the "myth" that mixed breed dogs are healthier than purebreds. There was what I would characterize as glee in what they saw as confirmation of what most breeders have long asserted, despite warnings from the scientific community (and some smaller groups of breeders) that inbreeding is detrimental to health in dogs, just as it is in humans and other animals.
I was very interested in this paper, given that the information available to date supports the notion that inbreeding is detrimental to health. This study seems to present information that flies in the face of the expectations about dog health that come from an understanding of the genetics of animal breeding. So I gave it a careful review.
The project summary states that "purebred dogs...did not show higher lifetime prealence of medical conditions compared to mixed breed dogs". Furthermore, "a higher proportion of purebred dogs than mixed-breed dogs had no owner-reported medical conditions".
In the purebred dog community, this was hailed as a decisive nail in the coffin of the "myth" that mixed breed dogs are healthier than purebreds. There was what I would characterize as glee in what they saw as confirmation of what most breeders have long asserted, despite warnings from the scientific community (and some smaller groups of breeders) that inbreeding is detrimental to health in dogs, just as it is in humans and other animals.
I was very interested in this paper, given that the information available to date supports the notion that inbreeding is detrimental to health. This study seems to present information that flies in the face of the expectations about dog health that come from an understanding of the genetics of animal breeding. So I gave it a careful review.
Motivation of the study
The authors recognize that inbreeding in purebred dogs increases the risk of expressing genetic disorders produced by recessive mutations. This leads to the expectation that purebred dogs are more likely to be afflicted by genetic disorders than mixed breed dogs. However, they note that this is not always the case, and that "simpy being purebred may not necessarily be associated with increased disorder prevalence overall" (Forsyth et al 2023).
From this, they describe a study with goals to "estimate the lifetime prevalence of medical conditions among US dogs", and to "determine whether purebred dogs have a higher lifetime prevalence of specific medical conditions compared to mixed-breed dogs".
The authors recognize that inbreeding in purebred dogs increases the risk of expressing genetic disorders produced by recessive mutations. This leads to the expectation that purebred dogs are more likely to be afflicted by genetic disorders than mixed breed dogs. However, they note that this is not always the case, and that "simpy being purebred may not necessarily be associated with increased disorder prevalence overall" (Forsyth et al 2023).
From this, they describe a study with goals to "estimate the lifetime prevalence of medical conditions among US dogs", and to "determine whether purebred dogs have a higher lifetime prevalence of specific medical conditions compared to mixed-breed dogs".
What does "mixed breed" mean?
First of all, we must address the groups the study seeks to compare. "Purebred" dogs by definition can trace their ancestry back to a specific number of "founders". But a dog that is "not purebred" might be any of a diverse array of identifiable supopulations of dogs that we might expect to differ in general health. Among these would be "mixed breed dogs" with multiple identifiable breeds in their ancestry, crossbreed dogs that are the result of crossing two purebred dogs, dogs of identifiable type that were developed and remain outside a restrictive breeding program (e.g., the many livestock guarding breeds that are typical of a particular geographic region and have recognizable type), which might be described as land race, and pariah dogs, the so-called "village dog". The categories are not discrete or even universally recognized as useful subgroupings of dogs, but we need to address them in passing because most of the studies addressing the health of mixed breed vs purebreed dogs do not define what is specifically meant by "mixed breed", and we should expect that the genetic background of these individuals is likely to matter.
First of all, we must address the groups the study seeks to compare. "Purebred" dogs by definition can trace their ancestry back to a specific number of "founders". But a dog that is "not purebred" might be any of a diverse array of identifiable supopulations of dogs that we might expect to differ in general health. Among these would be "mixed breed dogs" with multiple identifiable breeds in their ancestry, crossbreed dogs that are the result of crossing two purebred dogs, dogs of identifiable type that were developed and remain outside a restrictive breeding program (e.g., the many livestock guarding breeds that are typical of a particular geographic region and have recognizable type), which might be described as land race, and pariah dogs, the so-called "village dog". The categories are not discrete or even universally recognized as useful subgroupings of dogs, but we need to address them in passing because most of the studies addressing the health of mixed breed vs purebreed dogs do not define what is specifically meant by "mixed breed", and we should expect that the genetic background of these individuals is likely to matter.
What is a "medical condition"?
The data for "medical condition" used in the study were not based on veterinary diagnosis. Rather, they used what they termed "ORMC", owner-reported medical condition. This could include whatever the owner considered a "medical condition". The authors report that "For purposes of the study reported here, we do not attempt to refine, combine or otherwise modify participant responses, and present the data as reported by the participant", and within these they filtered the data to the top 10 most commonly reported ORMC (Forsyth et al 2023).
I have listed these issues identified as "medical conditions" in this table (from Forsyth et al, Table 2). A quick scan down the list shows that there was a broad scope to the conditions listed by owners. They include things like fleas, patellar luxation, cataracts, urinary tract infection, broken toenail, corneal ulcer, kidney disease, lameness, seizures, and chocolate toxicity. You can see that this lists includes bacterial infections, parasites, injuries, organ failure, and neurological disorders, among others. This is a list of the array of potential reasons you might take your dog to the vet for some sort of diagnosis or treatment.
Now, if you remember the motivation of the study (above), it was whether it is true that purebred dogs are more likely than mixed breeds to suffer from a genetic disorder caused by inheritance of two copies of a recessive mutation.
The data for "medical condition" used in the study were not based on veterinary diagnosis. Rather, they used what they termed "ORMC", owner-reported medical condition. This could include whatever the owner considered a "medical condition". The authors report that "For purposes of the study reported here, we do not attempt to refine, combine or otherwise modify participant responses, and present the data as reported by the participant", and within these they filtered the data to the top 10 most commonly reported ORMC (Forsyth et al 2023).
I have listed these issues identified as "medical conditions" in this table (from Forsyth et al, Table 2). A quick scan down the list shows that there was a broad scope to the conditions listed by owners. They include things like fleas, patellar luxation, cataracts, urinary tract infection, broken toenail, corneal ulcer, kidney disease, lameness, seizures, and chocolate toxicity. You can see that this lists includes bacterial infections, parasites, injuries, organ failure, and neurological disorders, among others. This is a list of the array of potential reasons you might take your dog to the vet for some sort of diagnosis or treatment.
Now, if you remember the motivation of the study (above), it was whether it is true that purebred dogs are more likely than mixed breeds to suffer from a genetic disorder caused by inheritance of two copies of a recessive mutation.
A proper test of this would look specifically at disorders caused by recessive mutations. But looking at the list, there is no reason a priori to expect that a purebred dog is more or less likely to have a broken toenail or to have ingested chocolate than a mixed breed dog. By not restricting the list of disorders to those caused by recessive mutations, this study cannot tell us anything about the relative risks to purebred vs mixed breed dogs of suffering from a genetic disorder caused by a recessive mutation.
Many of the "medical conditions" on their list more appropriately fall under the description of "husbandry" issues, and more broadly described as "things that happen to dogs."
Somebody might want to ask if purebred dogs receive better care than mixed breed dogs, or if purebred owners are more likely to participate in a study or respond to a lengthy questionnaire.
Many of the "medical conditions" on their list more appropriately fall under the description of "husbandry" issues, and more broadly described as "things that happen to dogs."
Somebody might want to ask if purebred dogs receive better care than mixed breed dogs, or if purebred owners are more likely to participate in a study or respond to a lengthy questionnaire.
But the central question - are mixed breed dogs healthier than purebreds - remains an important one. Indeed, it critical to the defense of breeding within a closed gene pool, and justifying the high levels of inbreeding that result.
So here's the deal. We understand the science, so we understand why mixed breed dogs should be healthier than purebreds - if we consider disorders caused by single recessive mutations. (Read more about this HERE). Indeed, this science is the reason that most available DNA tests are for disorders caused by single recessive mutations. The science tells us why these things are true. If it happened that purebred dogs were LESS likely than mixed breed dogs to suffer from genetic disorders, then we would have to conclude that we don't have the science right, and there would be lots of scrutiny to figure out why we're wrong.
But all indications are that we DO have the science right. If somebody is trying to claim that purebred dogs are just as "healthy" as mixed breed dogs, you can know for a fact that they are not referring to recessive genetic disorders, or they are ignorant of the science.
So here's the deal. We understand the science, so we understand why mixed breed dogs should be healthier than purebreds - if we consider disorders caused by single recessive mutations. (Read more about this HERE). Indeed, this science is the reason that most available DNA tests are for disorders caused by single recessive mutations. The science tells us why these things are true. If it happened that purebred dogs were LESS likely than mixed breed dogs to suffer from genetic disorders, then we would have to conclude that we don't have the science right, and there would be lots of scrutiny to figure out why we're wrong.
But all indications are that we DO have the science right. If somebody is trying to claim that purebred dogs are just as "healthy" as mixed breed dogs, you can know for a fact that they are not referring to recessive genetic disorders, or they are ignorant of the science.
REFERENCES
Forsyth et al., 2023. Lifetime prevalence of owner-reported medical conditions in the 25 most common dog breeds in the Dog Aging Project pack. Frontiers in Veterinary Science 10: 1140417. DOI
10.3389/fvets.2023.1140417
Forsyth et al., 2023. Lifetime prevalence of owner-reported medical conditions in the 25 most common dog breeds in the Dog Aging Project pack. Frontiers in Veterinary Science 10: 1140417. DOI
10.3389/fvets.2023.1140417
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