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Genetic Welfare Problems of Companion Animals
An information resource for prospective pet owners
Yorkshire Terrier
Distichiasis
Related terms: distichia, abnormal eyelash growth
Outline: In distichiasis, the position and direction of growth of some eyelashes is abnormal so that they touch the surface of the eyeball. Depending on the extent of the number, the degree of contact, the bristliness of the eyelashes, and perhaps other factors, this can lead to abrasion, ulceration, inflammation and infection of eye tissues which can cause serious discomfort, pain and, in severe cases if left untreated, to partial blindness. It is said to be a common disease of Yorkshire terriers. It tends to occur from early in life and to persist throughout life. The genetics have not been determined but it seems likely that avoiding breeding from affected animals would lead to reduction in prevalence and elimination of the disease.
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Summary of Information
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1. Brief description
Distichiasis is an eye disease in which extra or abnormally positioned eyelashes cause irritation and damage to the surface of the eye. Eyelids are lined by Meibomian glands, the openings of which are between the normal line of eyelashes and the surface of the eye. In distichiasis, which is a common genetic abnormality in some breeds of dogs, some eyelashes grow out of the Meibomian gland openings and these tend to abrade and irritate the eyes. This can cause mild irritation and discomfort through to severe and painful disease if the cornea becomes damaged and infected. Many dogs with the condition do not go on to develop the disease.
2. Intensity of welfare impact
This varies from none to severe depending on whether the cornea becomes damaged, inflamed and infected.
3. Duration of welfare impact
Most affected animals show signs of the disease from early in their life and these persist unless treated. The severity of the disease may fluctuate with time as the abnormally positioned eyelashes that cause it go through cycles of growth and moulting.
4. Number of animals affected
Distichiasis is considered to be one of the most common genetic diseases of dogs and Yorkshire terriers are predisposed to develop it (Gray 2007, McGreevy 2010, Lundgren 2011). We are however unaware of data on the proportion of Yorkshire terriers that are affected with the disease.
5. Diagnosis
Distichiasis is diagnosed by careful examination of the eyelid margins of the Yorkshire terrier, using magnification if necessary. There is no genetic test for distichiasis in dogs.
6. Genetics
The genetic basis of distichiasis is unknown. The disease is influenced by abnormal position and direction of eyelash growth, the nature of the eyelash hair – whether thick and bristly or thin and soft, and possibly by the nature of the growth cycles of the eyelashes, ie how long they grow before being shed. It is likely that these aspects, all of which may influence the likelihood and severity of disease, are under the control of various genes or sets of genes.
7. How do you know if an animal is a carrier or likely to become affected?
The disease can often be detected from a young age through examination of the eye and eyelid. The genetics have not been elucidated but the fact that the disease is common in Yorkshire terriers suggests that there is a genetic link and that affected animals are more likely than non-affected animals to produce young that suffer from the disease.
8. Methods and prospects for elimination of the problem
As far as we are aware, there are currently no breeding schemes aimed at reducing the prevalence of, or eliminating, this disease. Animals with distichiasis ideally should not be used for breeding.
For further details about this condition, please click on the following:
(these link to items down this page)
- Clinical and pathological effects
- Intensity of welfare impact
- Duration of welfare impact
- Number of animals affected
- Diagnosis
- Genetics
- How do you know if an animal is a carrier or likely to become affected?
- Methods and prospects for elimination of the problem
- Acknowledgements
- References
1. Clinical and pathological effects
Distichiasis is an eye disease in which extra or abnormally positioned eyelashes cause irritation and damage to the surface of the eye (Lawson 1973). Dogs normally have eyelashes (cilia) only in the upper of their eyelids and these typically point away from the eye and do not touch it.
Another structure of the eyelid are the sebaceous Meibomiam glands whose secretions help normal eye function. The openings to these glands are on the inner rim of the eyelid, adjacent to the eyelashes. In distichiasis, eyelashes grow out of these Meibomian gland openings (Lawson 1973). These abnormal eyelashes tend to grow so that the hairs touch the surface of the eyeball (the cornea or the conjunctiva).
Figure 1. The diagram on the left is shows a normal eye: the eyelashes grow away from the eye. The diagram on the right illustrates eyelash growth out of the Meibomian gland, directed towards the eye, leading to abrasion and irritation of the cornea. (The image is the property of ‘Eye Care for Animals’- www.eyecareforanimals.com to whom we are grateful for permission to reproduce it here).
Whether or not these abnormal eyelashes cause a problem depends mostly on their structure: whether they are fine and soft or thick and hard. Fine, soft hairs seem to cause affected dogs few problems; thick, hard hairs more. The number of abnormally placed hairs varies greatly and the more there are the greater the likelihood of more severe consequences. Sometimes groups of hairs emerging together from a Meibomian gland opening become matted and may appear as a single hair. Renwick (2007) suggests most dogs with distichiasis do not need treatment and other sources of corneal and conjunctival irritation should always be looked for before diagnosing distichiasis as the cause.
It is the tissue of the eye, the cornea (the clear surface of the eyeball) and the conjunctiva (the tissues lining the eyelids and attaching to the eyeball) that are affected by the disease. These are very sensitive and the presence of an object rubbing on them is irritating. The first sign of this is increased blinking or holding the eye slightly closed. The irritation also often causes excessive tear production (Lawson 1973).
Figure 2. A moderate case of distichiasis - there is, as yet, no apparent damage to the cornea of the eye despite inverted eyelashes coming in to contact with the eye itself.
Figure 3. This dog is likely to be at a higher risk of corneal damage due to the thickness and number of hairs growing in the direction of the eyeball. In both cases there are signs of increased tear production, which may be the result of distichiasis. (Images property of Susan Jacobi at www.vision4pets.com, to whom we are grateful for permission for their reproduction).
More severe problems occur when the abnormal hair is thicker and harder and when the irritation has been of longer standing. The animal may involuntarily draw its eye into the socket. This may cause the eyelids to roll inwards (spastic entropion) leading to further irritation of the cornea, as more eyelashes are brought into contact with it, and escalation of the problem. The eyelids themselves may be held tightly closed which in itself is painful due to muscle fatigue.
Irritation of the conjunctiva commonly causes inflammation. The conjunctiva becomes red and swollen and at increased risk of desiccation and infection. Chronic irritation leads to the cornea becoming cloudy, scarred, pigmented, and ulcerated and, in response to this, blood vessels grow across it. When ulcerated, the cornea is very painful. Long-term corneal damage can lead to partial blindness.
Figure 4. More severe cases of distichiasis can occur in entropion (inversion of the edge of the eyelid), causing larger numbers of hairs come in to contact with the eye resulting in damage to the cornea and surrounding conjunctiva.
Figure 5. Distichiasis may also cause the dog to hold the eyelid closed, which can cause further pain and have further impacts on the welfare of the animal. (Images property of Susan Jacobi at www.vision4pets.com, to whom we are grateful for permission for their inclusion).
The condition is best treated by removing the abnormally located hairs under anaesthesia using cryotherapy or electroepilation. Treatment can be very effective but sometimes has to be repeated (Chambers & Slater 1984). Older methods of treatment involving surgery or cautery are no longer recommended.
2. Intensity of welfare impact
Depending on the nature and level of damage to the eye, the welfare effects of distichiasis on the affected Yorkshire terrier will range from none or mild and intermittent discomfort through to chronic (prolonged) discomfort and pain.
3. Duration of welfare impact
Most affected animals show signs of the disease from early in their life (less than three years of age) and these persist unless treated. The effects of the disease may be intermittent, varying in intensity as misdirected eyelashes grow and are shed.
4. Number of animals affected
Distichiasis is considered to be one of the most common genetic diseases of dogs and Yorkshire terriers are predisposed to develop it (Gray 2007, McGreevy 2010, Lundgren 2011). We are however not aware of data on the proportion of Yorkshire terriers that are affected.
5. Diagnosis
Distichiasis is diagnosed by careful examination of the eyelid margins, using magnification if necessary. For those dogs that are reluctant to keep still during examination, sedation or anaesthesia may be needed. Lawson (1973) found that use of the x25 lens of an operating microscope was necessary to detect half of the abnormal eyelashes present. It can be difficult to be certain whether a particular abnormal eyelash is the cause of the problems observed. Small, abnormally positioned wispy eyelash hairs may be present in dogs in cases when the cause of eye disease is found not to be these, but some other factor e.g. an infection or eye trauma. In such cases, it may be necessary to trial-treat by removing the abnormal eyelashes to observe what difference this makes. However, eyelashes, even if wispy, should not normally touch the surface of the eye and any tendency to do so is an abnormality likely to increase the risk of complications.
6. Genetics
The genetics of distichiasis have not been investigated in any detail and remain unknown. The disease is influenced by abnormal position and direction of eyelash growth, the nature of the eyelash hair – whether thick and bristly or thin and soft, and may be also by the nature of the growth cycles of the eyelashes, ie how long they grow for before being shed. It is likely that these aspects, all of which may influence the likelihood and severity of disease, are under the control of various genes or sets of genes.
There is no genetic test for distichiasis in Yorkshire terriers.
7. How do you know if an animal is a carrier or likely to become affected?
The disease can often be detected in affected animals from a young age through examination of the eye and eyelid. The genetics have not been elucidated but the fact that the disease is common in Yorkshire terriers suggests that there is a genetic link and that affected animals are more likely than non-affected animals to have affected offspring. If the eyes and eyelids are found to be normal on examination at two to three years of age then it is unlikely that the dog has this problem or will go onto develop it. Currently it is not known if carriers (ie animals that do not develop the disease themselves but that can pass it on to their offspring) exist.
8. Methods and prospects for elimination of the problem
As far as we are aware, there are currently no breeding schemes aimed at reducing the prevalence of, or eliminating, this disease. Dogs with distichiasis ideally should not be used for breeding. However, since this condition is considered to be common in Yorkshire terriers, limiting breeding to only unaffected individuals might limit the size of the breeding population significantly, narrowing the gene pool, and increasing the risk of other genetic conditions increasing in prevalence. In selecting against distichiasis, other genetic conditions need to be taken into account also. One approach would be to out-cross with unaffected dogs of other breeds
9. Acknowledgements
UFAW is grateful to Rosie Godfrey BVetMed MRCVS and David Godfrey BVetMed FRCVS for their work in compiling this section section and to Stephanie Kaufman for assistance in illustrating it.
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10. References
Chambers ED and Slatter DH (1984) Cryotherapy (N20) of canine distichiasis and trichiasis: an experimental and clinical report. Journal of Small Animal Practice 25: 641-659
Gray H (2007) Distichiasis. VIN Associate, accessed 24.11.2010.
http://www.vin.com/Members/Associate/Associate.plx?DiseaseId=2933
Lawson DD (1973) Canine distichiasis. Journal of Small Animal Practice 14: 469-8
Lundgren B (2011) Distichiasis. On-line Client leaflet. http://www.veterinarypartner.com/Content.plx?P=A&C=189&A=2684&S=0&EVetID=228974. Accessed 31.5.11
McGreevy P (2010) Disorder – Distichiasis On-line http://sydney.edu.au/vetscience/lida/dogs/search/disorder/88/Distichiasis. Accessed 31.5.2011.
Renwick P (2007) Eyelid surgery in dogs. In Practice 29: 256-271
© UFAW 2011
Credit for main photo above:
http://depositphotos.com/13265541/stock-photo-yorkshire-terrier-in-city-park.html ©Depositphotos.com/tandemich
