The small dog trend: impact of size on pet health

Smaller dog breeds are becoming more and more popular – for a number of reasons – but this popularity comes at a price. Jamie Freyer reviews the situation and discusses some of the most important factors that clinicians need to be familiar with.

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 Philip-pines, 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by 6% over the last ten years, while the number of giant dogs and medium dogs decreased by 17% and 8.6%, respectively (unpublished data) as shown in Figure 1.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. [...]