A Guide to Acceptable Procedures Practices for Aquaculture Fisheries Research - Part 3

1. Animal husbandry (next)

1.2. Diseases and health management 

Infectious diseases are common in intensive aquaculture and can cause significant mortalities. Regular monitoring and appropriate management are essential for the maintenance of good fish health. Information is available on aquatic animal diseases, particularly regarding diseases of native freshwater fish, and is described in Rowland and Ingram (1991), Callinan and Rowland (1995), Ingram et al. (2005) and Read et al. (2007) (this latter publication provides additional detail relating to the diagnosis, treatment and prevention of silver perch diseases). However in comparison with terrestrial animal diseases, much still remains unknown about many aquatic animal health issues. Therefore, while some information is provided below in relation to some of the more commonly encountered fish health issues this is provided for information only and must not be considered as an authoritative guide to the diagnosis and treatment of fish diseases. It is therefore very important that disease events in fish be appropriately investigated, particularly where the cause of mortality or disease event is not clear, and appropriate veterinary advice should be sought and/or material submitted to a diagnostic laboratory for further investigation.

1.2.1. Notifiable aquatic diseases 

Under the Fisheries Management Act 1994, a number of aquatic animal diseases are listed as Declared Diseases, and there are obligations relating to the notification of these diseases that researchers need to be aware of. Specifically, whenever a researcher knows or has reason to suspect the presence of any Declared Disease, they “…. must notify a Fisheries Officer as soon as practicable of the infection or suspected infection.” Notifications of the presence of suspected or confirmed Declared Diseases should be made to the Strategy Leader, Aquatic Biosecurity, NSW DPI who may be contacted through the office directory for the Port Stephens Fisheries Institute. Biosecurity staff can advise regarding appropriate submission of samples to a diagnostic laboratory to either confirm or rule out involvement of a suspected Declared Disease.

These obligations to report the suspected or confirmed presence of Declared Diseases not only relate to fish that are cultured or housed in experimental facilities, but also extend to fish populations in the wild that may be encountered by a researcher in the course of their work.

The NSW list of Declared Diseases under the Fisheries Management Act, 1994 in relation to finfish (additional diseases are listed for molluscs and crustaceans) can be downloaded from www.legislation.nsw.gov.au. As at February 2015 the Declared Diseases list is as follows:

• Epizootic haematopoietic necrosis—EHN virus 

• European catfish virus, European sheatfish virus (Ex)

• Infectious haematopoietic necrosis (Ex)

• Oncorhynchus masou virus disease (Ex) 

• Spring viraemia of carp (Ex) 

• Viral haemorrhagic septicaemia (Ex)

• Channel catfish virus disease (Ex)

• Viral encephalopathy and retinopathy

• Infectious pancreatic necrosis (Ex)

• Infection with HPR-deleted or HPRO infectious salmon anaemia virus (Ex)

• Bacterial kidney disease (Renibacterium salmoninarum) (Ex)

• Enteric septicaemia of catfish (Edwardsiella ictaluri) (A)

• Piscirickettsiosis (Piscirickettsia salmonis) (Ex)

• Gyrodactylosis (Gyrodactylus salaris) (Ex)

• Red sea bream iridoviral disease (Ex)

• Furunculosis (Aeromonas salmonicida subsp. salmonicida) (Ex)

• Aeromonas salmonicida—atypical strains 

• Whirling disease (Myxobolus cerebralis) (Ex)

• Enteric redmouth disease (Yersinia ruckeri—Hagerman strain) (Ex)

• Koi herpesvirus disease (Ex)

• Grouper iridoviral disease (Ex)

• Infectious spleen and kidney necrosis virus- ISKNV like viruses (Ex)

• Infection with salmonid alphavirus (Ex)

- Diseases marked (Ex) are considered to be exotic to Australia

- Diseases marked (A) are considered to be present in Australia, but absent from NSW

- Diseases not marked are known to exist in some areas of NSW, but remain notifiable to

NSW DPI under the Fisheries Management Act, as Declared Diseases

1.2.2. Ectoparasitic infestations

Ectoparasites are pathogens that commonly cause diseases in both freshwater and marine fishes. If left untreated, ectoparasitic diseases can cause very high mortalities. Parasitic diseases account for around 80% of all disease records for silver perch (Rowland et al. 2007). 

The parasites and their etiology, diagnosis and treatment in Australian native freshwater fishes are described in detail by Rowland & Ingram (1991), Callinan and Rowland (1995) and Read et al. (2007). The most common parasites of native freshwater fish are the ciliate protozoans Ichthyophthirius multifiliis (which causes the disease ichthyophthiriosis or white spot), Chilodonella hexasticha (chilodonellosis) and Trichodina sp. (Trichodinosis), the flagellate Ichthyobodo necator (ichthyobodosis), and monogenean trematodes or gill flukes. The species of gill fluke infesting silver perch is Lepidotrema bidyana. All these parasites infest gill and skin tissues, and can be readily diagnosed using a microscope. Outbreaks can usually be controlled by using formalin in ponds, or salt or formalin in tanks (for details see Read et al. 2007).

1.2.3. Fungal infections

Water moulds (Class Oomycetes) are ubiquitous and cause fungal infections of fish. There are several common fungal diseases of freshwater and marine fishes; two examples are fungus (cotton wool-like growths on skin or gills) caused by Saprolegnia parasitica, and Epizootic Ulcerative Syndrome (EUS or red spot disease) caused by Aphanomyces invadans. Fungi are frequently opportunists that normally feed on dead tissue and are able to infect fish when the epidermis is damaged and/or the immune system is suppressed. They are generally considered secondary pathogens. Fungal infections can usually be avoided by good husbandry, careful handling and the maintenance of good water quality. The prophylactic use of 2-5 g/L NaCl in tanks will prevent many fungal infections, and the application of formalin may reduce the severity of an outbreak once an infection has commenced in ponds or tanks. Prevention of fungal diseases is generally much easier than control.

Listed below are treatments for bacterial and fungal infections and for ectoparasites that are well established and commonly used by NSW DPI (Fisheries) scientists.

1.2.4. Bacterial infections 

The incidence of bacterial diseases in native freshwater fish is low (< 3%) and they cause few problems at facilities where good fish husbandry and health management are used. Bacterial infections usually follow periods of severe stress caused by rough handling and poor water quality, particularly low dissolved oxygen and high ammonia; a combination of these factors will predispose fish to infection. In fish larvae, bacteria can also be ingested with food and may proliferate in the gut. 

Bacterial diseases can be treated with antibiotics. As the action of antibiotics is often pathogenspecific, it is important to accurately diagnose a bacterial disease to ensure that the most effective antibiotic and dosage is administered (Rowland & Ingram, 1991). Antibiotics should only be used to treat a diagnosed bacterial disease, and should not be used prophylactically because prolonged or repeated application, at dosages insufficient to kill bacteria, may lead to the development of resistant strains that will subsequently prove difficult to treat (Rowland & Ingram, 1991). For most antibiotics no detectable residue is allowed in food fishes in Australia. Since ‘research’ fish should not be entering the food chain this should not be an issue.

Signs and Diagnosis

External bacterial infections are easily recognised by skin inflammation (redness and swelling), fin erosion, lesions and ulcers and/or gross colonies of bacteria (eg. Flexibacter) of the epidermis on the torso or fins. Other signs may be exophthalmia (pop-eye), swelling of the abdomen, fluid in the body cavity and lesions in tissues such as spleen, liver and kidney. The bacteria may be identified by sending fish to a histology/pathology laboratory for analysis. The best samples are moribund (dying) fish which should be sent live to the laboratory if possible. 

Otherwise recently-dead or freshly euthanased fish can be placed, in a plastic bag, on ice for preservation and may be suitable for post-mortem analysis if live moribund fish cannot be sent live to the laboratory. It is important that such samples are kept chilled but are not frozen

In NSW, the Elizabeth Macarthur Agricultural Institute (EMAI) Menangle, NSW is a diagnostic laboratory that accepts submissions of fish samples for disease diagnosis. 

For further information relating to descriptions of symptoms and treatment for some common specific fish bacterial diseases refer Read et al. (2007) .