As to “catching” something from your Koi, let me stress that you should always wash your hands thoroughly before and after handling fish. For Koi hobbyists, the potential for human illness can come from the pond water, fish parasites or actual diseases. Infections or infestations of animals that can be transmitted to humans are called zoonosis or zoonotic. Remember the old school nurse who always told you to wash your hands to prevent infections? It is still true, especially when working on or in your pond.

Our pond water is normally not a problem, however, if there is a run off into the pond, the pond is a drinking source for local animals, or if water (in the form of wild water plants) is brought in the form unknown sources, Giardia can become a problem. Giardia (Giardia lamblia) makes its home in the small intestine of mammals (not fish) and the symptoms can range from none in light infections to severe, chronic diarrhea in heavy infections. Don’t drink the pond water.

Many of our fish have parasites, such as crustaceans, flukes, protozoans, roundworms, and tape-worms. Other than the worms, most of these parasites don’t want much to do with us humans. Worms are usually only transmitted by ingestion, so don’t eat your Koi! Most Koi professionals don’t wear gloves when handling fish as they are concerned with dropping the fish, but they all wash their hands frequently after handling a fish and so should you.

Catching a disease from Koi is extremely rare, but possible. One of the most troublesome is the bacteria belonging to genus Mycobacterium. It’s the same type of bacteria that can cause tuberculosis in humans, can be found in fish and in the pond. Affected fish may exhibit several external symptoms such as anorexia, Popeye, skin discoloration and external lesions such as ulcers, nodules and fin rot. Some may exhibit no external symptoms, but upon post mortem, grayish-white lesions may be seen in the kidney, liver and spleen. The usual suspect is a fish that is thin, losing weight and probably older. The bacteria usually prefer the cooler temperatures of fish, but can infect humans through cuts or abrasions or handling infected fish. Such an infection in humans usually shows in the form of an infected nodule in the skin (granulosa), although there is a chance of a more serious internal infection. The other common types of pond or fish bacteria, Aeromonas or Pseudomonas, can also infect humans. Avoid these bacterial diseases by wearing gloves and washing hands thoroughly with antibacterial soap before and after handling the pond water or fish (the before is to protect the fish, the after is to protect the people.)

Do not use this information to avoid touching your fish. There is probably more potential for disease transmission in petting your dog after a romp in the woods or cleaning your cat’s litter box. In disease prevention, like pond keeping, cleanliness is our best tool. Wash your hands after playing in the pond or cleaning filters and wear gloves if working on an obviously infected fish.

Please note that people who have a compromised immune system (from HIV, chemotherapy, organ transplantation, heavy doses of Prednisone or other steroids) should check with their doctor before handling fish, especially sick fish.   Thank you to Sandy Yosha, DVM, for her contributions.

Sources: http://www4.nationalacademies.org/cls/ijhomes.nsf/   http://www.biosci.ohio-state.deu/~parasite/giardia.html