Aquatic Animal Diseases Diagnostic Testing

The importance of aquatic animals to New Zealand

Aquatic animals are important to New Zealand for many reasons:

• commercial fisheries target more than 130 species and are worth in excess of $1.1 billion to our economy
• aquatic animals grown in farms (aquaculture) add another $250 million of value to the economy. This industry plans to be worth $1 billion to New Zealand by 2021.
• more than 1.2 million individual aquatic animals are imported every year to satisfy the demand for aquarium fish. The species permitted for import can be found in this import health standard: fisornic.all.pdf (1190 KB)
• there are important customary fisheries for Maori
• our aquatic animals perform a vital role in the healthy aquatic ecosystem of our fresh and marine waters
• New Zealand is world renowned for the quality and size of its brown and rainbow trout, and the variety and quality of our natural freshwater fisheries. The dollar value of New Zealand’s freshwater fisheries has not been researched, but DoC advises that the Taupo fishery alone generates approximately $70 - $80 million for the local and national economy. (Source: Fish & Game NZ )

Protecting the industries and the environment

With aquatic animals being of such importance to the people of New Zealand, MAFBNZ have aquatic animal disease diagnostic capacity in fish, molluscs and crustaceans. This capability is enhanced continuously as new diagnostic tests are developed and introduced.

Aquatic animal health diagnostic tests are carried out in the Animal Health Laboratory at Wallaceville.

Aquatic animal disease diagnostic work

The animal health laboratory carries out diagnostic work in the following areas:

• diagnostic tests for the incursion investigators carrying out potential exotic disease incursions.
• routine diagnostic work for clients, where it is appropriate that the tests are carried out by a government diagnostic laboratory. This might be for export purposes, where no other expertise exists in New Zealand or the required tests are not commercially available.
• salmon farming approved establishments scheme – to permit export of salmonid products from New Zealand.
• ruling out of exotic pathogens in sick fish submitted from transitional, or quarantine, import facilities.
• investigation and characterisation of novel pathogenic agents found in aquatic animals here.
• development of new or improved diagnostic methods for the identification of pathogens of significance to New Zealand’s aquatic animals. This work aims at developing tests where none currently exist, improving the turnaround times for certain tests as well as improving the specificity and sensitivity of tests.

Our range of diagnostic tests

Aquatic animals are no different to terrestrial animals in that we have a range of diagnostic test disciplines available for us to use in any disease investigation. These include bacteriology, virology, pathology and parasitology. For more information on each discipline please see below:

Aquatic pathology:

When whole animals are received the first step is a gross post-mortem, also called a necropsy. Here the pathologist looks for visible changes in the animal(s) that could be caused by disease. At this time, where appropriate, wet preparations of the gills and skin scrapings will be prepared to check for external parasites.

During the necropsy, samples are taken for bacteriology, virology and histopathology. The lab also receives previously dissected samples for testing. In these situations the aquatic diseases lab will have sent out a sampling kit to the submitter with instructions as to which samples to take. This is usually done only after discussion between the diagnostic lab and the submitter.

Histopathology involves the preparation of very thin sections (5 μm thick) through important organs and tissues. These sections are stained, usually with a stain called H&E, and examined under a light microscope. The pathologist can examine each organ system for tell-tale changes due to pathogenic agents. Histopathology is a mainstay of aquatic disease investigation as it can provide so much valuable information.

Following the reading of an H&E stained slide the pathologist can ask for special stains to highlight specific structures, or carry out more specific investigations using immunohistochemistry and in-situ hybridisation.

Aquatic bacteriology:

Classical bacteriology involves the inoculation of nutrient media in Petri dishes with swabs or parts of the internal organs of the aquatic animals. These Petri dishes, or plates, are incubated at a variety of temperatures an in a variety of environments, such as in the presence or absence of oxygen (aerobic and anaerobic respectively) and in the presence of increased carbon dioxide. The laboratory uses both general and selective media. Selective media is chosen to enhance the growth of certain important bacteria.

Once any colonies that have grown have been examined, the bacteria in question are identified via a series of biochemical tests, gram staining and determination of motility.

The microbiology laboratory also uses some modern molecular methods to assist in the identification of aquatic bacteria. This involves classical polymerase chain reaction (PCR) tests for Brucella spp., Reninbacterium salmoninarum, Yersinia ruckeri and Aeromonas salmonicida. In addition 16s rDNA sequencing can be used to identify a bacterium.

Aquatic virology:

There are many viruses that can cause disease in aquatic animals and IDC Wallaceville has classical virus isolation capability on a number of fish cell lines: virus isolation – cell lines

Virus replication in the cell lines causes the development of characteristic cytopathic effects (CPE). The virus can then be further identified using:

• virus neutralisation tests
• ELISA (enzyme linked immunosorbent assays)
• PCR
• electron microscopy

Aquatic parasitology:

Parasites may be identified grossly at necropsy, by visualisation in skin and gill wet preparations and in histological sections. Where visualised whole in fresh preparations the identification of species can be attempted from the gross morphology, or appearance, of the parasite or its spores. It is harder to identify to species level parasites seen in histopathological sections as so little of the parasite is visible, but it can quite often be identified to genus level.

It is also possible to compare parasite DNA sequences to help the determination of genus and species. Where parasites are microscopic, it can be quite difficult to obtain pure parasite DNA.

Moving forward with diagnostic testing

A number of research projects are underway in the aquatic diseases section to develop more rapid, sensitive and specific diagnostic tests for certain pathogens of significance to our aquatic animals. Real-time PCR methods are being developed for a number of bacterial species, as well as some important viruses such as mollusc herpes-like viruses, aquatic birnaviruses and iridoviruses.

In addition classical virus isolation on cell culture is being researched for the mollusc herpes-like viruses and the iridoviruses.

A 3-year project is due to commence shortly to examine the diagnostic utility of examining fouling organisms (on boats, marinas and wharves) as early warning systems of pathogen introduction from shipping coming to New Zealand.

Page last updated: 23 June 2008