Prevalence and heritability of distichiasis in the English Cocker spaniel

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 (14]) 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 18]. 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 16].

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.