Diagnosis: Microscopic Examination (Part 2)

Once we've found the right microscope for our needs, the examination can begin. We'll start by taking a swab, and depending on the symptoms, we might also take a gill biopsy and examine the feces. In a future article, we'll discuss how to examine all of this with a microscope.

Make a smear

Once we've properly placed a fish in a bowl or measuring container, we'll scrape off some mucus from the skin with a microscope slide. Ground and rounded microscope slides offer less risk of damage but are more expensive to purchase. If you're hesitant to use a microscope slide, use a plastic coffee spoon or something similar. The downside of using a teaspoon is that you'll still have to manage the mucus on your microscope slide.

Taking a swab is easiest if you turn the fish on its back; it will generally remain much calmer this way. Do this by first pressing the fish against the wall on your side and then pushing the top of its back away from you. Then, clamp the fish back against the wall. The fish will now be upside down, clamped against the nearest wall of the tank. Hold the fish in place with your left hand/arm and take the swab with your right hand. The best place to take a swab is just behind the gills and near the cloaca (the opening through which the fish defecates and urinates). The gills are also prone to parasites, and the cloaca is a common area for parasites to reside. Use the long edge of the microscope slide and scrape a small amount of mucus from the skin (using the pressure you would use to write with a pen). Don't overdo it! The fish can then be released.

Now, use the coverslip to maneuver the mucus to the center of the slide. Add a drop of water, place the coverslip on the mucus plug, and gently press it down. If you've done it correctly, the surface under the coverslip should now be about one-third to one-half mucus and the rest water. These watery zones are important, because parasites are best detected in the transition zone between mucus and water. Dry the bottom of the slide with a paper towel to prevent it from sticking to the microscope stage. Your slide is now ready for microscopic examination.

If you suspect a parasite outbreak, stroke three fish per pond to get a good overview of the situation. Some fish will be weaker than others and carry more parasites.

Both microscopes are suitable for the koi hobby; the choice is yours. Don't confuse a binocular microscope with a stereo microscope; the latter isn't suitable for our hobby!

Taking a gill biopsy

Some parasites have a preference for the gills. To detect these, you'll need to examine a piece of gill under a microscope. If you're too shy, leave it to an expert! You'll need to anesthetize the fish, then turn it onto its back and hold the gill open with your left thumb while lifting the fish slightly above the water with your left hand. With your right hand, use bandage scissors to snip off about 1 to 2 mm² from the tip of a gill. Flatten this slightly on your microscope slide (add a few drops of water) while placing the coverslip. Then you can examine the result under the microscope.

stool

If you suspect intestinal parasites, you can also examine some feces under a microscope. Only use fresh feces for this! Feces from your filter or from what has been floating in the pond for a while are teeming with microscopic life and can lead you astray into thinking you recognize a parasite. You can obtain fresh feces by turning a fish over in the bowl and making a massaging motion from the head to the cloaca. With a bit of luck, you'll obtain fresh feces this way. Place a sample on your microscope slide, add a few drops of water, and gently flatten it with your coverslip. Your specimen is now ready for microscopic examination.