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New fish imaging mode could reduce harm

Researchers are trying a new imaging technique on rainbow trout. Photo by Paige Borrett.

Because fish are a valuable tool for determining whether environmental contamination has occurred, scientists commonly use them for environmental assessments. Unfortunately, gathering this information requires lethal sampling – the fish have to be removed from the population and euthanized so they can be dissected and analysed.

That’s a concern to Dr. Lynn Weber, a toxicologist at the Western College of Veterinary Medicine (WCVM) who’s heading a team of researchers, with a goal to develop and validate an effective method for using ultrasound technology to collect important data about fish so that information about site contamination can be collected without lethal sampling.

Using ultrasonography to examine fish is not a novel concept; ultrasound technology is commonly used to evaluate the reproductive status, reproductive quality and filet quality of commercial species. However, a successful method has not yet been developed for assessing the liver size of fish.

In current environmental monitoring programs, fish are euthanized and dissected to assess liver weight. This information is used to determine the Hepatasomatic Index (HSI), the weight of the liver relative to the whole body weight.

In an environment filled with toxins, the HSI increases as the liver enlarges from an accumulation of toxins as well as proteins to neutralize the toxins – that makes the HSI an important tool for determining whether or not there are toxins in the environment.

Although there are other factors that can cause a fish’s liver to enlarge, researchers in most environmental monitoring programs will follow up with additional environmental testing once they find changes in the HSI.

“Finding a way to determine HSI non-lethally will alleviate pressure on fish populations while allowing the same environmental assessment to continue,” says Weber.

Any textbook on fish anatomy would confirm that the liver size and shape vary among species, individuals and seasons. For many species, the liver shape is described by stating that it “occupies the space that is not occupied by other organs”. In addition, many species of fish have a diffuse liver, meaning they have many small livers attached along the length of the intestine.

A sample 3-D reconstruction of a rainbow trout liver from the front. Image by Paige Borrett.

The classical approach to using ultrasound for calculating an organ’s volume is to assume a specific shape or apply a method called Simpson’s rule of disks to attain an approximation. This system works well for organs of a consistent or symmetrical shape, but it’s not easily applied to an irregular shape like the liver.

Instead of applying this traditional approach, Weber and her team members are working with a computer imaging program known as Imaris as well as other programs that allow for the three-dimensional reconstruction and visualization of ultrasound scans. Using these visualization programs, they can obtain an accurate volume calculation of the irregularly shaped livers.

Once Weber and her team have developed an accurate methodology for calculating the volume of the liver using ultrasonography, they’ll work to develop a standard mass per unit volume to calculate the liver weight and the HSI. They hope to tackle the question of density by observing variance in the echogenicity, or the “greyness” of the ultrasound image. With rainbow trout as their research subjects, the scientists will then compare the estimated HSI they’ve attained through ultrasound images with the HSI that’s been attained using traditional lethal methods.

This methodology will be used in the Experimental Lakes Area – one of the world’s most influential fresh water research facilities where many toxicology studies are performed. In this setting, the duration of the study has often been limited by the number of fish that can be lethally sampled. Having a non-lethal sampling method will allow scientists to perform important long-term toxicology studies at the facility.

Veterinary clinicians and researchers now have greater access to state-of-the art technology that can be used to benefit environmental monitoring programs, and their application becomes increasingly important as fish populations are depleted and we learn more about the impact of long-term exposure to toxins.

Having a verified method for measuring the HSI using ultrasound rather than the conventional lethal sampling method ensures that monitoring programs are better able to collect environmental information by examining fish without any detriment to their population. This is especially important in aquatic systems with endangered species.

Written by Paige Borrett. 

Borrett of Duncan, B.C. is a second-year veterinary student who was part of the WCVM’s Undergraduate Summer Research and Leadership program in 2016. Paige’s story is part of a series of articles written by WCVM summer research students. 

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