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What Bulldog and Trout Eyes Have in Common

Ever been poked in the eye? Eek! Turns out we need to be extra careful about eyes (especially the cornea) when handling brachycephalic dogs (the smushed-nose breeds like bulldogs and pugs) and trout.

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We're all more alike than we think.

Did you know that human and dog eyes evolved from fish eyes. Say what? So flipping cool, right? I see a lot of brachycephalic dogs at our hospitals (Bulldogs, French bull dogs, Shih Tzu, Pug, etc...). This patient population is prone to various disorders involving the eye called Brachycephalic Ocular Syndrome. If you'd like to nerd out more, check out this awesome open access article in the Journal of Veterinary Ophthamology, "The pandemic of ocular surface disease in brachycephalic dogs: The brachycephalic ocular syndrome" by Sebbag et al.

Ever looked at a pug? See how this guy's eyeballs stick out? Well, so do trout eyes. Part of our anesthesia plan specific to brachycephalics is to be extra careful about lubricating and protecting the eyes during surgery and transit, when they can't protect their eyes themselves.

So try not to drop that trout in your boat!

Shit happens, oh do I know. You may practice catch and release, but when you grab the fish it drops in your boat. We all make mistakes. We all fail, right? But one of the reasons you should be extra careful (and brief) when you hold a fish, is to protect their eyes. Dropping them can easily cause a corneal ulcer. And fish feed by sight. Damaging the cornea of a fish can mean no meals for awhile!

Fluroscein stain works in fish too...

When we look for ulcers in dogs, and cats (and people), we use a fluroscein stain that highlights damaged areas of the cornea neon green. Turns out, this is the same way you look for ulcers in fish. On the left is a photo of a Cavalier King Charles eye with a corneal ulcer highlighted in neon green. On the right is a tilapia fish who also has a bad ulcer on his eye. Ouch!

Fig. 1 Corneal ulcer in a Cavalier King Charles Dog. Fig. 2 A Tilapia with a corneal ulcer*.

Why it's important to buffer the water during fish anesthesia

MS-222 or Tricaine (also known as TMS, tricaine methanesulfonate) is an agent we put in the water to anesthetize fish. Oh yes, you can anesthetize fish just like you can people and dogs! This particular agent can cause corneal ulcerations if it is not buffered in the water you put the fish in.

Below are some cool images of (C) a fish cornea that was anesthetized with buffered Tricaine. And (D) shows the cornea of a fish that was anesthetized with non-buffered Tricaine. You can see all the cellular damage that has occurred in D. The poor fish with unbuffered anesthetic water.

Be careful with those nets. You can poke an eye out!

If you use a net, that's another potential source of corneal ulceration. Although net technology has advanced in the past decades, there is still opportunity to injure the fish's eye. So be gentle when putting fish in the net, and when taking them out of the net.

Fish in freshwater take longer to heal their corneal ulcers compared to fish in salt water.

One cool paper looked at the wounded eye of the rainbow trout. The found that the low osmolality of the environment delayed wound healing. Severe corneal edema and cataracts develop following epithelial wounding, and the cornea heals at 0.64 mm2/hr. Although the healing rates in teleost fish differ from those in mammals, histology shows that the corneal healing mechanism is essentially the same in fish and mammals.

Fish and Bulldogs have decreased corneal sensitivity

What does this mean? Compared to other non-brachycephalic breeds, brachycephalics have decreased corneal sensitivity. Their eyes just aren't as good as other dogs in recognizing when there is something poking their eyeball! Which partly explains why these breeds of dogs get more ocular disease.

Fish also have decreased corneal sensitivity. We know that rainbow trout (Oncoryhnchus mykiss) have polymodal, mechanothermal and mechanochemical receptors in their cornea. The cool thing is they have no cold receptors other cornea. The paper concludes that, "the lack of cold sensitive neurons may provide evidence for the evolution of cold nociceptors in vertebrates that is related to the transition from poikilothermy to homeothermy."

How do you anesthetize a fish?

If you'd like to learn more about fish anesthesia, check out this AWESOME blog post from the Exotic Pet Vet Blog. Dr. McCormack is so amazing!!! He talks about how to anesthetize a koi for a surgical mass removal, as well as how it's important to give them pain medication too.

Do fish have eyelids?

No! Get it? Got it? Good. Another reason they aren't meant to be out of water! And we need to be extra loving to their amazing eyeballs.

The End! If you made it to the end, thank you! You are a fish nerd like me. Yay!

Keep your eyes on the stars, and your feet on the ground. - Teddy Roosevelt

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