Spotlight on Vet Medicine
September 29, 2006
It's a Fluke
National Zoo veterinarians and pathologists diagnose and treat an outbreak of parasitic flukes in discus fish.
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| A discus fish in Amazonia. |
Native to the Amazon Basin of South America, discus (Symphysodon spp.) are freshwater cichlid fish related to angelfish and tilapia. Their common name comes from their round, flattened bodies, which resemble a dish or disc and are eight to ten inches both long and tall. Their genus name, Symphysodon, means “jaw grinder.” Large-lipped carnivores, they take in food, such as worms, insects, and crustaceans, grind it up, spit it out, and pick it up again for regrinding before swallowing.
In the wild, discus live in a unique freshwater environment. At certain times of the year, the water is very warm—up to 30°C (86°F) —and highly acidic. They also prefer “soft” water, which has low mineral content.
Discus and some other cichlids have an interesting adaptation for feeding their young (or fry), called micronipping. Parental fish produce large amounts of protein-rich mucus on their skin and their fry “graze” on this nutritious mucus.
Discus were recently added to the fish collection at the Zoo’s Amazonia exhibit, where they live in a pool with freshwater stingrays and arowanas. During the fishes’ initial acclimation period, keepers noticed erratic behavior and abnormal gilling (breathing). Zoo veterinarians and pathologists teamed up with Amazonia keepers and curators to investigate and monitor the health status of the discus. All of the discus were moved into holding tanks. Amazonia keeper Ed Smith briefly removed one discus from the water so veterinarian Suzan Murray could collect samples of its gill and fin.
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| Ed Smith holds a discus so Suzan Murray can take samples. |
A Fluke
Pathologist Tim Walsh reviewed the samples and saw living gill flukes (Dactylogyrus sp.) displaying their characteristic flex-and-recoil movement. These flukes have four eyespots on a wormlike body less than a centimeter long (1 cm equals just over one-third of an inch), making it easy to identify them.
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| Gill flukes are easy to identify. |
Gill flukes bear an array of hooks on one end of their bodies that enable them to latch onto a fish’s gills. The hooks irritate the skin as the flukes feed on the fish’s skin and mucus. The flukes also lay eggs, and the resultant larvae, which have hooks as well, also feed on fish gills.
A Salty Bath Does the Trick
Veterinarians prescribed a medicated bath to treat the discus, placing them in a mild salt solution for a short time. The salt removes the flukes and improves mucus production; the mucus layer of a fish’s skin is part of the immune system, so improving mucus production helps the fish fight infections that may result from fluke infestation. A salt bath also minimizes the stress of osmoregulation (maintaining the water and electrolyte balance in the body). Freshwater fish expend energy to osmoregulate and this is stressful for a sick fish. Salt makes this process easier, decreasing energy expenditure.
This treatment was successful and the discus are currently doing well. Come visit these unique fish at Amazonia!
