Eyes (Brachycephalic Ocular Syndrome)

© Therese Rodin

Background: Pugs’ eye problems can be summarized in the term “brachycephalic ocular syndrome” (BOS) (ufaw.org.uk: 2011). The reason why Pugs suffer from this syndrome is, as the term expresses, that they are brachycephalic. As we saw in the section about BOAS, a syndrome is “a condition in which there are deviations and injuries in several of the body’s organs and which have a common cause” (Definition from Karolinska University hospital, my translation from Swedish).

BOS is serious both because it means suffering for the afflicted and because in principle the whole Pug breed is affected. Ufaw writes that all Pugs probably have BOS to some degree, which is confirmed by Barbara Braus, ECVO-certified veterinary eye specialist, who says that all Pugs have entropion (personal communication). (Entropion is part of BOS.) Entropion is one of the causes of pigmentous keratitis (corneal pigmentation). There is a study that included 130 AKC Pugs of which 87.6 percent had pigmentous keratitis as can be seen in the graphics of Cassie Smith below:

The statistics from the Genetic Committee of the American College of Veterinary Ophthalmologists (ACVO) do not look that bad. Between 1991 and 2015, on average 43.7% of the Pugs whose data were submitted to ACVO had normal eyes. The rest had some eye disease. Below is a summary of the results regarding the eye diseases they take up in their statistics that affect Pugs to a greater extent.

Amount of dogs:
1991 – 1999
2000 – 2009
2010 – 2015
entropion 21.8% 19.9% 16.1%
distichiasis 11.7% 8.3% 5.2%
pigmentous keratitis 18.6% 22.9% 51.7%
persistent pupil
5.5% 10.1% 11.5%
normal eyes 42.7% 46.8% 41.6%

The feature of the Pug that creates BOS is that the eyes are situated too shallow in the skull, which means that they are protruding and thus create an appearance where the eyes look round instead of almond-shaped, which is the normal form (Wallin Håkansson et al. 2014: 5ff.). This leads to a number of different complications of varying degrees depending on the degree of brachycephaly (Tierklinik Hofheim; ufaw.org.uk):

  • protruding eyes (exophthalmos)
  • too large eyelids in relation to the size of the eye (macroblepharon, also called macropalpebral fissure)
  • difficulty in closing the eyelids completely (lagophthalmia)
  • the rolling-in of the lower eyelid in the corner of the eye (lower medial entropion)
  • nose fold hair that reaches the cornea (trichiasis)
  • eyelashes that grow in the wrong place and reach the eye (distichiasis)
  • too little and/or bad tear fluid
  • formation of dark pigment in the eye caused by external irritation (pigmentous keratitis)
  • running eyes (epiphora)
  • dry eyes (keratikonjunktivis sicca)
  • wounds and/or inflammation on the cornea (corneal ulcer)

The various conditions can usually be treated by surgery and/or medications, but it is not possible to reach a result where eyes and adnexa (the area around the eyes) become like those of a normal and healthy dog. Even after surgery, the Pug may need lifelong treatment with medicine.

Goal: No Pugs shall be born with BOS.

Strategy: Since the problem arises from the brachycephalic skull, the skull of the Pug must become less brachycephalic. A normal and healthy dog ​​has almond-shaped eyes and therefore breeders should aim at having the eyes of the Pug almond-shaped. Because the brachycephaly is so strongly rooted in the breed, we need to bring in genetics from a breed with a non-brachycephalic skull to moderate the Pug’s skull. (See further under “Genetic diversity”.) This is also recommended by ufaw. In addition, only those Pugs that are least brachycephalic and which only have a low degree of BOS should be used in breeding.

The Swedish kennel club has a very good document called “Hundars ögonhälsa” (“Dogs’ eye health”) on its website (Wallin Håkansson et al. 2014). There, the authors introduce the concept of “critical control points” which are boundaries that should not be exceeded in breeding. The critical control points that are applicable on the Pug to be considered in breeding are the following:

  • protruding eyes
  • the hairless “rigid edge” between outer skin and eye cannot be seen (= entropion)
  • large nose fold
  • tear flow in the face

If we were to completely avoid breeding with Pugs that have these characteristics, all Pug breeding would end, and that is not realistic. Instead, each breeder should consider these critical control points and only let Pugs go into breeding that have these characteristics to a low degree.

To the left, the round form of the eyes because of protruding eyes, to the right, more almond-shaped eyes:

Sources and further reading:

Genetic committee of the American College of Veterinary Ophthalmologists (ACVO). 2015. Ocular Disorders presumed to be inherited in purebred dogs. www.ofa.org/wp-content/uploads/2016/09/Blue-Book-2015-8th-Edition.pdf Accessed 250319.

Tierklinik Hofheim. “Augenprobleme bei kurznasigen Hunden (Mops, Französische Bulldogge und Co)”. Paper version received 20-03-19.

ufaw.org.uk “Pug: Brachycephalic Ocular Syndrome”. Genetic welfare Problems of Companion Animals.
https://www.ufaw.org.uk/dogs/pug-brachycephalic-ocular-syndrome Accessed 250319.

Wallin Håkansson, Berit, Sheila Crispin and Nils Wallin Håkansson. 2014. “Hundars ögonhälsa”. www.skk.se/globalassets/dokument/uppfodning/externa-artiklar/ogon-berit-w/hundars-ogonhalsa.pdf Accessed 250319.

Chapters in Strategies for the breeding of Healthy Pugs