Canine distichiasis is considered to be a presumed inherited eye disease (PIED) in dogs, with the American and English Cocker spaniels being some of the most frequently diagnosed breeds [4, 6, 19]. The prevalence found in this study (49.31 %) is considerably higher than that of earlier studies performed in this breed. The highest prevalence found in earlier investigations was 26 %, including both English and American cocker spaniels [4, 6, 19], which is about half of what was detected here. The population included in this study represents approximately one third of the Danish Cocker spaniel population and thus, can be assumed to be fairly representative for the entire population. There is, however, a possible bias in our data because the registration of eye results is a requirement for obtaining studbooks for any offspring. Thus many of the dogs registered with eye examinations results in the DKC database, are likely to have been tested because the owner wants to use the dog for breeding.
Many of the dogs included in this study are presumably intended for breeding, because they have been judged, and awarded prizes at dog shows before the eye examinations. It could be hypothesized that if the presence of long and dense eyelashes is rewarded in the show ring, and if this is related to an increased risk of distichiasis, this could unfortunately lead to a higher prevalence among show dogs compared to the entire population. However, we have no solid evidence that supports this hypothesis.
We found a significant association (p < 0.0001) between the breeding combination of the parents and the prevalence of distichiasis in the offspring. The relative risk (RR) of producing affected offspring was found to increase with the number of affected parents in the breeding combination. I.e., in offspring produced by one affected and one unaffected parent the relative risk was 1.3 times higher compared to the risk in offspring produced by two unaffected dogs. In offspring produced by two affected dogs the relative risk was 1.4 times higher compared to offspring from the previously mentioned mating combination and 1.8 times higher compared to offspring produced by two unaffected dogs. Hence, the risk of producing dogs that will develop distichiasis at some point in their lives was almost twice as high when mating two affected dogs. These observations together with other data showing that simple Mendelian inheritance is unlikely, lead us to conclude that distichiasis is most probably inherited as a threshold character. Thus, we disagree with earlier assumptions that canine distichiasis is inherited as a dominant trait [6, 15].
The heritability of distichiasis was estimated according to methods described for threshold characters () and found to be in the range of 0.22-0.51 depending on whether the estimate was based on the offspring from matings between one affected and one healthy parent or matings between two affected parents. The discrepancy between the two estimates might be explained by increased inbreeding within one of the breeding combinations. Heritability for distichiasis has also been estimated in Tibetan terrier . In this breed the heritability was estimated at 0.043, however, since the prevalence in the Tibetan terrier study population was much lower (11.43 % of 849 dogs) and since the heritability was calculated using estimates of additive genetic variation, the heritability estimates in the two populations are not directly comparable.
Although there have been no analyses of changing disease incidence with time, within the population, anecdotally, the proportion of affected animals has been increasing over the last few years. To date, there are no reported explanations of how, or why, the condition has become so widespread within some breeds. One suggestion, however, may be the increased use of affected dogs for breeding, or perhaps the selection of specific types of Cocker spaniels attending exhibitions, i.e., dogs with long and dense lashes. In this study, we have discovered that dogs diagnosed with distichiasis were used in 70.3 % of all breeding combinations over the last ten years. The most frequently used breeding combination was one affected and one unaffected dog (41.7 %), while using two affected dogs occurred in 28.6 % of matings. Another factor that could be contributing to the high incidence, is the way the sires are used for breeding within a specific population. Many sires, both affected and unaffected, produce far more offspring than the recommended limit of 105 puppies, which makes it quite difficult to measure the effect of preventive actions, and to control inherited diseases, such as distichiasis, within a population.
The grading scheme was introduced six months prior to this study, and since it is relatively new, no publications or statistics are available about distribution of the different levels of disease within the predisposed breeds. In this study, the majority of the graded dogs were only mildly affected, and moderate to severe cases were quite rare. This is consistent with the fact that distichiasis does not always cause clinical signs in the Cocker spaniel, and the identified distichiae are most frequently small and soft [6, 7]. Corneal ulcerations are also infrequently described in the literature [8, 19]. For now, there is no written standardisation for the characterisation of the different grades of disease severity. This may cause inconsistency in the distribution of the results, thus giving a misleading impression of the severity of the condition within a specific breed, as the grading is made solely on the subjective opinion of the eye scheme examiner. If the results are considered consistent, the scheme will be most applicable for identifying the general distribution of the disease within a population. Dogs with severe disease are excluded from breeding .
This study is to our knowledge the first study of prevalence and heritability of distichiasis in the Cocker spaniel. Since we have estimated a moderate to high heritability of the condition it will be possible to use selective breeding to reduce the incidence of disease. The high prevalence within this breed makes it impossible to exclude all affected animals without depleting the gene pool. The present breeding recommendations tries to overcome this by excluding only the severely affected. Thus, reducing the incidence of distichiasis must be regarded as a long time breeding goal.