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InFocus

Treating a melted corneal ulcer in a tiger

A hood graft, routinely performed in domestic cats and dogs, worked just as well in 17-year-old Sumatran tiger, Ratna

It was Andy Warhol back in 1968 who coined the phrase “15 minutes of fame”, but it was not until the turn of the millennium when the term “viral” came into play with regard to social media feeds. But I seem to have had a good deal more than 15 minutes – maybe a couple of weeks – of “fame”, if you can call it that, with the viral spread of a story about an ophthalmic surgery on a tiger that, truth be told, did not last a lot longer than a quarter of an hour. A surgery that, really, I would recommend for melting corneal ulcers in domestic cats and dogs quite as much as tigers.

I received a phone call from Shepreth Wildlife Park as one of their Sumatran tigers, Ratna, seemed to have injured her eye. It took me only 15 minutes to get there which surprised them, I think, but I had been in the midst of paperwork and anything that can take me away from that is welcome! The tiger seemed to have damaged the eye a day or two earlier but had not shown much evidence of pain. Perhaps she had injured it on some spiky bamboo in her enclosure.

FIGURE (1) The cornea of Ratna’s eye was quite severely ulcerated and was starting to show signs of melting

Upon observation, the cornea was quite severely ulcerated and beginning to show signs of melting (Figure 1). The way I like to explain this is that bacteria are trying to get into the dense proteinaceous cornea and to do this, they produce enzymes. We used to simply call these enzymes gelatinases and collagenases, but so many different enzymes have been detected that they are now just termed matrix metalloproteinases – technically “calcium-dependent zinc-containing endopeptidases”, or so Wikipedia tells me – and given numbers up to 25. The trouble is that it is not just the bacteria that produce these: neutrophils entering the cornea to kill the bacteria also produce similar enzymes.

Note that these enzymes depend on calcium and zinc, so chemicals that chelate those anions – such as EDTA and acetylcysteine – can be useful in controlling the melting. But, more useful than that, the body has its own inhibitors of these enzymes and so serum in the cornea is the most useful plan of attack. Now, serum or plasma on an hourly basis has a beneficial effect, but unless you hospitalise a dog or cat with a melting ulcer this will be difficult to administer in a domestic pet and certainly not practical in a tiger, despite Ratna having been trained to accept eye drops having had cataract surgery in that eye a few years previously.

A better plan of attack, and one I would recommend for any animal with a melting ulcer, is a hood graft. A normal conjunctival pedicle flap requires fine suture – 8/0 Vicryl ideally – suturing the conjunctiva onto the cornea. This requires good magnification, microsurgical instruments and, most importantly, a reasonably healthy cornea around the ulcer. I could provide the former two, even in a tiger cage, but the whole of the cornea was not looking pretty.

A conjunctival hood graft involves incising the cornea around the dorsal 180 degrees of the globe, undermining that and then stretching it to be sutured to the ventral conjunctiva with 4/0 or 5/0 Vicryl. The cut surface of the conjunctiva then oozes serum onto the underlying cornea 24/7 and also adheres to the damaged stroma, protecting the cornea. It also means that any medication you give orally – antibiotics and non-steroidals – get straight onto the damaged cornea.

FIGURE (2) A conjunctival hood graft was performed to surgically treat the ulcerated cornea

As it heals, the epithelium gradually covers the denuded stroma, and the stratified epithelial cells gradually desquamate and lift off the hood graft. As the Vicryl degenerates the whole graft lifts off, apart from any areas where continued melting is still precluding full epithelial coverage. And so, in a little over a week, the graft lifted off, showing not a completely normal eye by any manner of means, but one with a smooth glistening surface: the ulcer had healed (Figure 2).

At this point, it was important to keep administering topical antibiotics but the job was pretty much done. Keeping an eye on the eye, if you will excuse the pun, was important, but essentially, if Ratna was holding it open and was not bothered by the eye, then I was not bothered either (Figure 3).

FIGURE (3) In a little over the week the graft lifted off, revealing not a completely normal eye, but one with a smooth glistening surface which, most importantly, no longer seemed to bother Ratna

Now in all that, the one person I have not acknowledged is Steve Philp from the International Zoo Vet Group who induced and maintained the anaesthetic. Honestly, the hood graft is one that any vet should be able to do – I would recommend it for any melting ulcer you see this summer – but ensuring that Ratna, then 17 years old and weighing 250kg, stayed asleep during the surgery and woke up at the end of it without harming anybody or herself was crucial, so thank you Steve for a tremendous job well done!

And we might ask if it is reasonable to keep a tiger like this in a zoological collection. I can honestly say that Ratna and her daughter seem to have a lovely life in Shepreth, and their willingness to be examined frequently shows, to my mind, that they are content. Their five freedoms are well catered for – fed and watered, comfortable and with any injuries much better treated than if such an ocular injury had happened in the wild. You might say that their natural behaviours – to chase and kill to eat – cannot be catered for, but if you are provided with your nutritional needs and have the opportunity to exercise in your sizeable enclosure then perhaps that is not essential. I really enjoy taking my students to see Ratna and, of course, her welfare is a key talking point, but just seeing her relaxing in front of the observing visitors, now that COVID restrictions have been lifted, suggests she is not in the least bothered by them.

David Williams

FELLOW AND DIRECTOR OF STUDIES at UNIVERSITY OF CAMBRIDGE

David Williams, MA, VetMB, PhD, CertVOphthal, CertWEL, FHEA, FRCVS, graduated from Cambridge in 1988 and has worked in veterinary ophthalmology at the Animal Health Trust. He gained his Certificate in Veterinary Ophthalmology before undertaking a PhD at the RVC. David now teaches at the vet school in Cambridge.


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