Fish Disease and Human Health
ections or infestations of animals that can be transmitted to humans are called zoonoses. Although cases of human disease occurring from fish and shellfish are rare in Australia, there are a few diseases of fish that workers in the aquaculture and fishing industries need to be conscious of when handling or processing fish.
Mycobacteriosis/Nocardiosis
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| Rainbow trout with mycobacteria lesions |
Mycobacteriosis and nocardiosis are bacterial diseases that affect a wide range of freshwater and marine fish, but particularly aquarium fish. The bacteria cause chronic systemic infections that form lesions internally and externally.
Affected aquarium fish exhibit several symptoms such as anorexia, popeye, skin discolouration and external lesions such as ulcers, nodules and fin rot. Cold water salmonids may show no external signs of disease other than mortality, but upon post mortem, greyish-white lesions may be seen in the kidney, liver and spleen.
There is some debate as to whether infected fish can be successfully treated for these bacterial diseases and destruction of infected stock and comprehensive disinfection of premises is usually recommended.
Several species of these bacteria are capable of infecting humans. The bacteria enters the skin as a result of abrasions incurred in swimming pools, tropical fish aquaria or from handling guts of infected fish, and may produce skin granulomas of the elbow, knees, fingers and feet.
The condition may persist for quite some time and must be treated with antibiotics for an extended period.
Symptoms in Humans
- Three to four weeks after the bacteria enters the skin, a swelling develops over a bony prominence or the site of an abrasion. A cyst, or abscess develops, that may be filled with pus and may ulcerate and scar.
- Swelling of the lymph nodes may occur.
Treatment
- See your local doctor as soon as possible.
Prevention
- Ensure adequate chlorination of swimming pools. Wear protective gloves when cleaning fish aquaria or diving.
- If infected stock are suspected, wear protective gloves when cleaning and filleting fish and disinfect equipment afterwards.
Anisakid nematodes
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| Anisakis worms in the viscera of a blue mackeral |
Anisakis and anisakis-like parasites are common nematode worms, the larvae of which infect many species of locally caught fish. The parasite is transmitted by fish eating crustacean zooplankton that is the intermediate host of the parasite, or by preying on fish that are already infected.
It has a complicated lifecycle with many intermediate hosts and may be transmitted several times from fish to fish before the final host is reached. The final hosts are marine mammals such as dolphins, whales and seals, where the adult worms cause serious inflammation of the stomach wall.
Anisakis larvae are 10-50 mm in length, white and normally tightly curled in a cyst in the guts and muscle of the fish. Anisakiasis in humans can be acquired by eating raw or partially cooked marine fish, and the use of marine 'trash fish' as food in aquaculture can facilitate the transfer of the parasite to freshwater species.
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| A simple life cycle of Anisakis nematodes |
Symptoms in Humans
- Severe gastric and intestinal pain, vomiting, and diarrhoea. In some severe cases, fever and blood in the stools may occur.
- Note: Anisakiasis is often misdiagnosed a stomach ulcers or appendicitis.
Treatment
- See your local doctor as soon as possible.
Prevention
- Avoid eating raw fish dishes When cooking fish, cook for a minimum of five minutes at 60 degrees Celsius or a shorter time at higher temperatures. If fish is to be eaten raw, it should preferably be frozen at -30 degrees Celsius. If storage occurs at a temperature of -20 degrees Celsius, the fish should be frozen for at least five days. It is preferable to use farmed fish for raw dishes as these fish appear to be virtually free of infection.
- Marination and smoking will not necessarily kill Anisakis.
Crayfish Handlers Disease
The condition known as "crayfish handlers disease" or "seal finger", is well known in the fishing industry. It can be caused by various bacteria, but Erysipelothrix rhusiopathiae and various species of the Vibrio genus have been specifically incriminated.
The bacteria gain entry to the skin via abrasions, lacerations or fissures and cause a painful itching or burning sensation. Swelling then occurs and the adjacent joints become stiff and painful. If left untreated the injury will heal in three weeks, although a relapse at a later date is possible.
Symptoms in Humans
- An itching, burning sensation at the site of the injury, followed by swelling. Stiff painful joints.
- Lymph node enlargement and inflammation may occur, but is rare.
Treatment
- See your local doctor as soon as possible.
Prevention
- Thoroughly clean all marine cuts and injuries with antiseptic and keep dry and clean. If handling marine animals, wear thick protective gloves. Wear thick boots when wading or fishing.
- Use a knife to touch things when diving, not your hands.
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