Researchers are looking for more information about how fish feel pain. Photo by Patrick Robertson.

Researchers examine aquatic painkillers

Until recently, veterinarians removed tumours, installed prosthetic eyes and performed other painful medical procedures on beloved pets as well as on animals in zoos and aquariums without providing their patients with any painkilling drugs.

One thing sets this group of animals apart – they have scales and gills instead of fur and paws or hoofs.

Common knowledge has long held that fish can't consciously feel pain. But science has recently turned that thought on its head and brought about the moral realization that veterinarians must provide proper pain relief when performing procedures that would be considered painful in other warmer-blooded vertebrates.

However, little is known about how fish respond to different analgesic (painkilling) drugs, and some pain killers used in cats and dogs may not be effective in fish.

This presents a major problem for practitioners such as Dr. Justin Rosenberg, a former veterinary fellow at the Vancouver Aquarium and now a zoological medicine resident at the University of Florida.

Rosenberg often does as many as two surgical procedures on fish each week, and several hundred fish may come across his operating table when he's helping with research projects.

But when it comes to post-operative analgesia, his options are limited.

"The bottom line is that not much is known about fish analgesia. There are very few peer-reviewed articles that evaluate efficacy," he says.

Many small animal veterinarians would reach for meloxicam, a common nonsteroidal anti-inflammatory drug (NSAID). However recent evidence has shown that NSAIDs may not be effective for fish.

"NSAIDs have a relatively short half-life and would be difficult to maintain therapeutic levels at the dose used," says Rosenberg.

This problem is driving research at the University of Saskatchewan's Western College of Veterinary Medicine (WCVM) where scientists are trying to determine if dexmedetomidine, a common anesthetic and sedative drug, can provide effective pain relief to aquatic patients.

Dexmedetomidine is an alpha-2 adrenergic receptor agonist — one of the drug classes that hasn't been investigated yet as an optional analgesic drug for fish.

Testing pain in fish provides its own unique set of challenges as they don't display the same behavioural signs as mammals. Fish can't wince, cry out, whine or lick their wounds. Their pain responses tend to be species-specific; common responses include rubbing the painful site, a change in respiration rate, rocking back and forth on the bottom, and instability (a loss of buoyancy equilibrium).

To test for pain and the efficacy of an analgesic drug, the standard protocol is to inject a small volume of acetic acid (the acid found in vinegar) into the cheek or lip of the fish.

Without any pain medication, the fish will usually show a pain response and change of behaviour to the acetic acid. But if the fish ingest the analgesic drug and do not display a pain behaviour to the painful stimulus, the drug is likely providing a painkilling effect.

In the WCVM study, researchers recorded the baseline or "normal" behaviour of goldfish before anesthetizing and injecting them with one of three treatment doses of dexmedetomidine or a saline solution. The fish that were treated with saline solution served as the study's control group. All of the treatments were blind, so researchers didn't know which treatment each fish had been randomly assigned.

Once the fish recovered from anesthesia, researchers injected a small volume of acid into the cheeks of each fish and recorded their behaviour. At the end of each trial, the team members re-anesthetized the fish and drew blood samples so they could measure the levels of cortisol — a stress hormone. Once the fish were humanely euthanized, the researchers removed samples of brain tissue for a more specific investigation of chemical processes.

Researchers are still collecting data for the study, but ideally, long hours spent watching goldfish swimming in their tanks will result in a better understanding of whether dexmedotomidine can help veterinarians to prevent pain in their aquatic patients.

Patrick Robertson of Red Deer, Alta., is a third-year student who was part of the Western College of Veterinary Medicine's Undergraduate Summer Research and Leadership program in 2015. Patrick's story is part of a series of stories written by WCVM summer research students. 
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