The small dog trend: impact of size on pet health

Introduction

All over the world, the population of small dogs is on the rise. Whilst smaller dogs have always been popular in some countries, such as Japan, Brazil, and the Philippines, their numbers are increasing in countries where preferences have historically leaned toward larger breeds 1 2. According to data from Banfield Pet Hospital, the largest veterinary chain in the United States, the proportion of small dogs seen in their clinics increased by 6% over the last ten years, while the number of giant dogs and medium dogs decreased by 17% and 8.6%, respectively (unpublished data) as shown in Figure 1. The British Kennel Club has also reported 3 that the average pet dog is approximately one inch (2.5 cm) shorter than 25 years ago, and that a growing discordance has been noted in breeds with both standard and miniature varieties, with the miniature subtypes increasing in popularity while their larger counterparts are less sought after. In a similar manner, studies in Australia have shown that recent trends lean toward shorter, smaller breeds with wider skulls 4. Several explanations for this shift have been postulated, including the fact that such dogs can remove a barrier to pet ownership, as they tend to cost less to keep, and that smaller pets are just more feasible in an urban setting. Smaller dog breeds can also be desirable for other reasons — for example, they are easier to transport, they often live longer, and of course they can make excellent companions. However, whatever the cause, it is important to consider the effect this trend towards smaller dogs will have on both veterinary care and the overall health of our canine companions. 

How can a breed be made smaller? 

In addition to favoring small breeds already in existence, there is a trend toward making already small dogs even smaller. There are a number of ways that the creation of “miniature” or “toy” varieties of a breed can be achieved, each with its own advantages and disadvantages. For example, smaller individuals of a breed can be mated together with the idea of gradually decreasing the size of a particular line. Breeding for a specific trait in this manner, particularly when the animals being bred share some degree of relation, is known as “line-breeding”. Breed purists tend to prefer this approach, as it keeps the genetics of the breed most similar to that of the parent breed. However, any sort of line-breeding will eventually change the fundamental genetic appearance or “genetic signature” of a breed, causing divergence from the other members of the breed (Figure 2). This method is quite slow, and can take multiple generations to pro-duce the desired effect. It can also result in what are known as “throwback” dogs, whereby unwanted larger offspring can crop up, sometimes many generations later, in the litters of breeds whose size has been de-creased in this manner. This type of genetic selection also decreases the diversity of the population, which can entrench genetic disease and have deleterious effects on the immune systems of the animals in question. 

A second method involves the introduction of other, smaller breeds to a line in order to facilitate a faster miniaturization. This is obviously the preferable method for those who are interested in expediting the size-reduction process, as it can result in smaller dogs from a single breeding. In this case, however, careful attention must be paid to the genetics of the dogs being added to the population, due to the risk of introducing new genetic diseases from the smaller breed. For example, merle coloration is a trait common to many of the herding breeds, and has become quite a popular color pattern (Figure 3). In recent years, it has been bred into a number of breeds that had not previously carried the gene, including the American Cocker Spaniel and the Chihuahua. Health problems such as deafness and microphthalmia can be seen in dogs homozygous for the merle gene (which has a dominant mode of inheritance), but there may be other genetic risks involved. Herding breeds often also carry the ABCB1 (also commonly known as MDR1) genetic mutation, which is associated with sensitivity to a number of medications such as ivermectin. Because these dogs can carry both merle and MDR1 genes, it would not be surprising if some of the breeds where merle has been introduced also exhibit adverse drug reactions, in a sort of human-induced “genetic hitchhiking”.

Animals created by mixing two pure breeds are sometimes known as “designer dogs” and breeders and owners of such dogs may believe that heritable disease is not an issue because they are not purebred. However, one recent study reported that approximately 40% of mixed breed dogs carried at least one risk factor for a genetically associated condition 5 and veterinarians should be aware that such problems are a real possibility in these crosses if the breeds in question have similar genetic origins. In addition to known genetic disease mutations, there are many diseases that appear to have an inherited component, but for which an underlying genetic cause has yet to be determined. For example, many cancers seem to be over-represented in certain breeds, and it is not known whether crossing with those breeds would also bring an increased risk of cancer in their offspring. 

Selection for the conformational trait chondrodysplasia is a third method that can be used to miniaturize a breed, and can be exploited to confer smaller stature and an appearance that many people find endearing. Chondrodysplasia is the shortening of a dog’s limbs caused by an abnormal development of its cartilage and bone. While some breeds are fixed for the trait, and therefore always carry it (such as the Dachshund, Basset Hound, and Corgi), other breeds carry it intermittently, or have had it introduced (Figure 4). However many people are unaware of the health issues associated with this longer, lower physique. Arthritis, intervertebral disc disease, hip dysplasia, and a number of other diseases that are seen commonly in chondrodysplastic breeds are closely associated with their conformation. Selecting for this trait can therefore result in the introduction of an entirely new set of health problems for a breed.

What problems can arise with smaller breeds?

As well as concerns such as genetic diseases, and the orthopedic problems linked to chondrodysplasia alluded to above, the trend towards miniaturization is linked to other issues. Firstly, as with anything that is in style, the more in demand something becomes, the more people want to take advantage of the trend for their own purposes. Unfortunately, irresponsible individuals may seize the opportunity to cater to fads such as miniaturization, attracted by the idea that it can be an easy money-maker. This can lead to practices that can have a negative effect on the health of the puppies they are selling. For instance, they may exploit unethical methods such as breeding unthrifty specimens to create smaller animals, selling puppies at an extremely young age (and possibly misleading owners about the actual age of the dog), or failing to provide the appropriate nourishment in order to stunt the growth of the animals. Some will even import very young toy-breed puppies from other countries, flying them thousands of miles at an age and condition that makes such travel extremely dangerous.

Secondly, an important consideration for veterinarians regarding the trend toward smaller dogs is the fact that many common diseases seen in small dogs are different than those of their larger brethren. One particularly telling difference is the disparity in oral health. Small dogs, for example, are more likely to suffer from retained deciduous teeth and periodontal disease than larger dogs. It has been hypothesized that this is due to their small mouths and crowded teeth, although research on the subject is still ongoing. Additionally, the smaller, more delicate jaws of these dogs have less bone, making them more susceptible to loose teeth, pathologic bone loss, and even the possibility of mandibular fractures during dental procedures. 

Oral health is not the only area of disparity. For example, tracheal collapse, which is virtually unheard of in larger dogs, is not uncommon in small and toy breeds, whilst one of the most common causes of canine heart disease, myxomatous mitral valve disease, is also much more common in small breeds 6 (Figure 5). The causes of lameness in small dogs are also very different from those of larger breeds. For example, differentials for hind limb lameness in small dogs include medial patellar luxation and Legg-Calvé-Perthes, in contrast to cruciate ligament tears and hip dysplasia which are abundant in the large-breed population. Dystocia due to maternal- fetal disproportion, and thus the need for cesarean section, is also more common in petite dogs. This is particularly serious in tiny dogs with large heads, such as many of the small brachycephalic breeds.

Another issue of concern in small animal medicine is adverse events related to commonly performed procedures. While the majority of oral and injectable medications are dosed in a size-dependent manner in veterinary medicine, small dogs have been shown to have a higher propensity for vaccine reactions 7. Toy breeds of dog may have more susceptibility to anesthetic-related com-plications, such as hypothermia, and may be more difficult to accurately monitor while under anesthesia 8. Even the most common causes of mortality differ between large and small dogs. For example, while larger dogs are more likely to die or be euthanized from causes such as neoplasia, musculoskeletal disease and gastrointestinal issues, the cause of death in smaller breeds is more likely to be urogenital, degenerative, cardiovascular, or metabolic disease 9.

The problems exhibited by small and toy dogs are only magnified in the micro-sized “teacup” versions of the breeds. With these dogs, their tiny, fragile bones, in combination with the canine tendency to be underfoot, can be a recipe for disaster. Fractures can occur after a short fall or jump, from being stepped or sat on, or even from rough play. Diminutively sized teacup dogs may also be predisposed to experiencing conditions such as persistent open fontanelles and hydrocephalus. Due to their miniscule bodies, miniature and toy breed dogs are also more apt to be dramatically affected by phenomena that may not be significant for a larger dog. For example, tiny dogs are more likely to suffer from severe dehydration secondary to diarrheal disease than large dogs, making veterinary care more urgent than it may be for a similar illness in a Labrador or German Shepherd. Toy and teacup dogs may also need to be fed multiple times a day, due to their propensity toward hypoglycemia. It should also be remembered that the occasional bite of “people food” for a toy dog is akin to a large breed dog being fed an entire cheeseburger, making obesity and nutrient deficiency or imbalances a very real concern for these pets (Figure 6). The majority of dog owners will not realize the importance of such distinctions, making education for these clients particularly important. 

The undesirable effects of decreased size may even be evident within a single litter. The smaller pups in a litter can have a decreased ability to maintain normothermia and normoglycemia, as well as having a reduced ability to suckle. This ineffective suckling can lead to decreased colostrum intake which in turn can result in a number of detrimental effects. Adequate absorption of colostrum ensures appropriate growth of puppies in early life, and also affects temperature and glucose regulation (Figure 7). Therefore, inadequate intake of colostrum can exacerbate problems already seen in these pups. Colostrum also plays a key role in increasing the size and surface area of the intestines during the early neonatal period 10. Decreasing the rate of intestinal growth can affect the pup’s ability to absorb nutrients. Because of the combined effects of these influences, the risk of neonatal mortality for these puppies is much higher, particularly in the first 48 hours of life. It has also been suggested that decreased consumption of colostrum in some species may lead to autoimmune disorders and other immune system dysfunction for the entirety of the animal’s life 11.

Small dogs and behavior problems

In addition to clinical health, behavior is a crucial aspect of every pet’s veterinary care, and can have an extreme impact on the human/animal bond. Behavior issues are one of the most common reasons that dogs are relinquished to shelters 12. It has been documented that there is an inverse relationship between a dog’s size and the number of problematic behaviors it exhibits 13. In one study that investigated undesirable behaviors in dogs in conjunction with height and weight, the regression coefficients for the 14 categories of behavior (stranger-, owner-, or dog-directed aggression; dog rivalry; stranger-directed or dog-directed fear; non-social fear; touch sensitivity; separation-related behavior; attachment and attention seeking; trainability; chasing; excitability; and energy level), were negative for all but trainability and stranger directed fear 13. Of the 22 other “miscellaneous” behaviors covered by the study, the majority also showed a negative regression with size, indicating that there is a significant correlation between bad behavior and small size in the dog.

These findings bring into question the cause of the relationship between bothersome behavior and smaller size: Are these behaviors genetically associated, or are they reinforced by the way we treat smaller pets? In this case, the effect of human behavior on that of our pets is likely quite substantial. It may be that owners tolerate different behaviors in small versus large dogs. For example, a large dog showing aggression or jumping up at people is clearly a cause for concern for most pet owners, but the same behavior may not be seen as problematic in a little dog. Indeed, some behaviors may even be seen as “cute”, and subsequently encouraged by the owner. In addition, a small dog may not be required to respond to commands, may be carried rather than expected to walk, and may be overprotected by its owners, which can interfere with its socialization with other people and dogs.

While environmental factors or “nurture” may be involved in the behavior of many small dogs, a genetic predisposition may also be involved. By breeding for size, humans may have inadvertently intensified these predilections toward problematic behaviors. There may also be physiologic causes for some of the “bad” behaviors seen in tiny dogs, e.g., toilet training may be difficult due to the minute capacity of the bladder, and therefore these dogs will need to be taken outside more frequently than their larger relatives. Regardless of the cause, the correlation between size and behavior is worth noting. Education will be key in ensuring that owners know what to expect, and how to combat these problems before they occur.

Where next?

It is particularly telling that a number of the breed and kennel clubs for toy breeds are addressing the issues surrounding miniaturization, warning potential buyers that it is not something to be desired. The Code of Ethics of the York-shire Terrier Club of America states that “A member’s advertising…shall not use terms such as ‘teacup’, ‘tiny specialists’, ‘doll faced’, or similar terminology”. The Chihuahua Club of America’s website also includes a statement about the misleading use of terms such as “teacup”, and declares that “We recognize that many Chihuahua fanciers do want the very small puppy. While they are adorable and can be perfectly healthy, the buyer should be cautioned as to the extra care that may be required with regard to their general health and well-being”, and the American Kennel Club (AKC) standard for the Jack Russell Terrier states that “any hint of achondroplasia” is a severe fault, and discourages any dog with such tendencies to be used in breeding for the show ring.

While the AKC states that it will not register or endorse “teacup” breeds, Britain’s Kennel Club goes one step further. It has issued a statement on “teacup puppies”, discussing the health issues that can be experienced by such animals, highlighting unscrupulous practices used by some breeders of such dogs, and noting that they do not recognize or register teacup dogs. The statement finishes by pointing out that the KC’s breed standards stress that “breeders should not exaggerate any characteristics, including those related to size, and puppy buyers are strongly advised to ensure that neither the puppy nor its parents look exaggerated in appearance, before they buy. Any departure from this could lead to serious health problems further down the line.” 

Encouraged by magazine photos of celebrities toting tiny “purse puppies”, prospective pet owners may be deceived into thinking one of these pint-sized dogs is an ideal canine companion. Pet ownership can appear easy and stylish when viewed from afar, and a miniature dog may seem like the perfect accessory. But despite their diminutive size, teacup dogs are still dogs, and require the same amount of care and upkeep as their larger counterparts. While their health concerns may be different than those of larger dogs, they are no less involved, and in some cases may even be more complex. In particular, they may have needs that are not anticipated by those unfamiliar with dogs; even those accustomed to larger canines may find that some of the issues faced by toy dogs are unexpected. Behavior issues may be more numerous, more severe, and may become ingrained more easily than in larger dogs. Certainly, it may be prudent for a veterinarian to discuss certain points with anyone who is considering buying, or has just purchased, a small breed puppy (Box 1). Suffice it to say, since the trend toward smaller and smaller dogs does not appear to be coming to an end anytime soon, veterinarians need to be able to lead their clients through the unique challenges posed by these pets.