INFANTILE ENTROPION IN SHEEP

Comparison of entropion outcome with and without intervention in Romane and Ile de France sheep breeds

BACKGROUND

Congenital entropion is the most frequent ocular disorder in newborn lambs of certain sheep breeds, which, if not treated, can result in complete blindness and death due to starvation.

OBJECTIVES

The aims of this study were to compare the spontaneous healing of entropion in two breeds and assess the outcome of cases with and without therapeutic intervention.

METHODS

A total of 158 entropion cases (119 Ile de France and 39 Romane) were investigated, and swab samples were collected from the cornea and conjunctiva of 73 of the affected lambs for bacteriological investigation. In addition, an ocular intervention was carried out in 123 affected animals.

RESULTS

The Romane breed developed entropion at an average age of 7 days compared to the Ile de France, which developed it at an average age of 1 day. Likewise, significant differences were found between bilateral and unilateral involvement in both breeds. Meanwhile, 22.1% of cases recovered spontaneously, and the highest rate of spontaneous recovery without intervention was observed in the Romane breed (66%). Bacteria isolated from ocular samples included Staphylococcus spp. (42.5%), Bacillus spp. (21.9%), Trueperella pyogenes (13.7%), Corynebacterium spp. (12.3%) and Escherichia coli (9.6%).

CONCLUSIONS

The results of the study showed that the onset time of entropion, bilateral involvement, the severity of the process and the need for re-treatment were higher in the Ile de France breed than in the Romane breed. Likewise, the Romane breed showed a higher degree of spontaneous recovery of entropion.

A genome-wide significant association on chromosome 15 for congenital entropion in Swiss White Alpine sheep

Entropion is a known congenital disorder in sheep presumed to be heritable but no causative genetic variant has been reported. Affected lambs show a variable inward rolling of the lower eyelids leading to blindness in severe cases. In Switzerland, the Swiss White Alpine (SWA) breed showed a significantly higher prevalence for entropion than other breeds. A GWAS using 150 SWA sheep (90 affected lambs and 60 controls), based on 600k SNP data, revealed a genome-wide significant signal on chromosome 15. The 0.2 Mb associated region contains functional candidate genes, SMTNL1 and CTNND1. Pathogenic variants in human CTNND1 cause blepharocheilodontic syndrome 2, a rare disorder including eyelid anomalies, and SMTNL1 regulates contraction and relaxation of skeletal and smooth muscle. WGS of a single entropion-affected lamb revealed two private missense variants in SMTNL1 and CTNND1. Subsequent genotyping of both variants in 231 phenotyped SWA sheep was performed. The SMTNL1 variant p.(Asp452Asn) affects an evolutionary conserved residue within an important domain and represents a rare allele, which occurred also in controls. The p.(Glu943Lys) variant in CTNND1 represents a common variant unlikely to cause entropion as the mutant allele occurred more frequently in non-affected sheep. Therefore, we propose that these protein-changing variants are unlikely to explain the phenotype. Additionally, WGS of three further disconcordant pairs of full siblings was carried out but revealed no obvious causative variant. Finally, we conclude that entropion represents a more complex disease caused by different non-coding regulatory variants.

Genome-Wide Association Identifies SLC2A9 and NLN Gene Regions as Associated with Entropion in Domestic Sheep

Entropion is an inward rolling of the eyelid allowing contact between the eyelashes and cornea that may lead to blindness if not corrected. Although many mammalian species, including humans and dogs, are afflicted by congenital entropion, no specific genes or gene regions related to development of entropion have been reported in any mammalian species to date. Entropion in domestic sheep is known to have a genetic component therefore, we used domestic sheep as a model system to identify genomic regions containing genes associated with entropion. A genome-wide association was conducted with congenital entropion in 998 Columbia, Polypay, and Rambouillet sheep genotyped with 50,000 SNP markers. Prevalence of entropion was 6.01%, with all breeds represented. Logistic regression was performed in PLINK with additive allelic, recessive, dominant, and genotypic inheritance models. Two genome-wide significant (empirical P<0.05) SNP were identified, specifically markers in SLC2A9 (empirical P = 0.007; genotypic model) and near NLN (empirical P = 0.026; dominance model). Six additional genome-wide suggestive SNP (nominal P<1x10^-5) were identified including markers in or near PIK3CB (P = 2.22x10^-6; additive model), KCNB1 (P = 2.93x10^-6; dominance model), ZC3H12C (P = 3.25x10^-6; genotypic model), JPH1 (P = 4.68x20^-6; genotypic model), and MYO3B (P = 5.74x10^-6; recessive model). This is the first report of specific gene regions associated with congenital entropion in any mammalian species, to our knowledge. Further, none of these genes have previously been associated with any eyelid traits. These results represent the first genome-wide analysis of gene regions associated with entropion and provide target regions for the development of sheep genetic markers for marker-assisted selection.

Genetic diseases of sheep and goats

Congenital malformations and inherited disorders constitute a substantial proportion of the afflictions seen in sheep and goats. Of these, malformations tend to be similar in both species, whereas the genetic diseases encountered to date, with the exception of a few, are different. Of the 28 genetic diseases of sheep and goats described in this review, 60% and 62.5%, respectively, are monogenic disorders. For a majority of the monogenic recessive disorders encountered in these species, the carrier state is not detectable at present, whereas in others, in which a biochemical lesion is known (dermatosparaxis, erythrocyte glutathione deficiency, globoid cell leukodystrophy and glycogen storage disease), the carrier state is detectable with the aid of enzyme and surface protein markers. The latter group and the dominant disorders (anury, cataract, glomerulonephritis, and lethal grey in sheep; gynecomastia and anotia-microtia complex in goats) are easy to eliminate through selective breeding. The polygenic disorders (entropion, epidermolysis bullosa, hereditary chondrodysplasia, and muscular dystrophy of sheep, and udder problems in goats) are more difficult to eradicate, because the mutant genes responsible for these traits generally do not declare themselves until inbreeding brings together a critical concentration to create a health crisis in some, whereas others, which are only short of a few of these mutant genes, might go totally unaffected and therefore undetected. Chromosome defects of the structural nature (translocations) seen in sheep and goats generally create meiotic disturbances, which in a majority of cases lead to subfertility, whereas sex chromosome aneuploids are generally sterile.(ABSTRACT TRUNCATED AT 250 WORDS)