An Assessment of Three Major Impact Species
© K Gordon 2003
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Bacterial Fish Diseases In Australian Aquafarming
It is also well documented that many of the pathological organisms have been found to co-exist on or in the animal and only become opportunistically pathogenic (increase in virulence (epizootic)) with environmental change. Commercially, bacterial fish diseases represent a significant problem in Aquafarming? The epizootic pathway is known but the environmental control necessary for prevention is beyond effective application in the larger majority of commercial aquafarming and aquaculture.
Currently the use of antibiotics, environmental-management culture systems, (tank systems) early disease diagnosis, farm control, disease research and constant vigilance are the tools available in the 'quest to sustain the unsustainable'. Bacterial Fish Diseases in Aquaculture can be thought of as a battle where the unnatural is attempted and fortified against the enemy; natural evolutionary balance. Ever waiting, ever vigilant and always testing for weakness.
Most bacteria have a defined temperature range where pathogesis is most prevalent. Such information is helpful in identification of a particular species. This information can also assist you in disease control if temperature management is available. Lighter & Redman (Lighter & Redman 1998) define disease as “ any alteration from the normal state of health” which is a useful definition in terms of aquaculture, where the “normal state” can be tested considerably. So successful treatment is as much, or more, about environmental stability as it is about defining the epizootic and treating the pathogen.
“Bacteria are prokaryotes (cells with no distinct nucleus) ranging in size from 1um to 10um”. They are identified by how they react colour wise to the process of Gram Staining ( positive = blue; negative = red), by their shape, motility and the ability to grow in various cultures and the subsequent appearance, shape and smell of the culture colonies.
Some significant pathogens, such as Vibrio spp. ( CRC Handbook of Mariculture pp295) are facultative anaerobes (the organism can grow in either the presence or absence of oxygen, but they grow better if oxygen is available (The Anaerobic Jar, Web Ref). Vibriosis is of major significance to worldwide marine aquaculture.
Typically the nutrient loading in culture waters will reflect the concentrations of bacteria. Low concentrations would give counts of 102-103 cells per/ml. However as nutrient loading increases from faeces, uneaten feed and other metabolites this concentration increases. Concentrations of bacteria are usually distributed in bottom layers and anywhere nutrient build-up can occur. Many pathogenic bacteria can be isolated from healthy specimens and appear to co-exist harmoniously.
Bacteria are grouped according to their optimal incubation temperature as follows;
Psychrophiles…………………temp. range 0 - 20oC
Mesophiles…………………….temp. range 20 - 45oC
Thermophiles………………….temp. range 45 – 80oC
Halophiles are further grouped according to their culture levels of salt, which can be quite high (1%-25% where seawater is 3.5%). Because of their importance to humans many techniques and tests have been established for identifying specific species of harmful bacteria. These culture tests reflect the way the organism grows naturally and examples of these tests include oxidase, Dnase, salt%, indole, and examples of biological enzymes, sugars and metabolites added to culture media, for example, urease and sucrose.
The process of fish diseases and identification is, at present, tedious requiring specific laboratory sampling and testing which takes considerable time when bacterial diseases require quick application of treatment. However experienced farm persons do make accurate, presumptive diagnosis’s based on post mortem examination and assessment of the clinical signs. And then apply affirmative control measures. Interestingly, specific testing equipment such as “DNA Probes” utilising DNA amplification using polymerase chain reaction (PCR) methods are becoming available (Lighter & Redman 1998). This type of modern genomic (chromosome) probe is being made for use with bacteria and viruses in kit form for quick on location diagnosis (Anderson 1995). Such technological advances will improve the treatment process as specific identification is crucial to applying the correct treatment.
However the situation at present is one where observation of clinical signs is quickly assessed and acted upon. But the treatment may or may not be correct and reaction time for treatment from fish disease diagnosis is crucial as bacteria multiply rapidly. DNA probes should allow much earlier warning of increasing bacterial levels and treatment, be that increased turnover or less feed, but their greatest significance may be in that they allow the manager to build up a profile into what events precursor epizootics and within what time. To date, there are significant numbers of disease related bacteria that affect Australian Aquaculture and these are listed below.
Disease Chart - Click To Magnify
Bacterial Fish Diseases In Australian Aquaculture are signified by their commercial impact on the culture biomass. They can cause massive mortality but can also linger and kill off perhaps one or two fish per day over many months. Three Genus have worldwide implications for Aquaculture and are listed below. Bacterial disease organisms such as the one known to cause Furunculosis have been documented since the thirties (Blake et al. 1930) and it’s interesting to note the evolution in the scientific classification of this particular organism. As DNA testing becomes more common identification and classification will become a significant management tool for immediate farm diagnosis and treatment.
In assessing the three most important bacteria, for Australian Aquaculture, considerations have been given to the organisms that cause the most commercial damage and the organisms that are the most difficult to treat or are the most persistent. Piper (1982) talks of the prophylactic nature of good husbandry and, as mentioned earlier, this concept has to be the real commercial goal. He describes the nature of the fish environment where, for an epizootic to occur the fish population must interact with a pathogen in an unfavourable environment. Piper is saying, the environment is always present and an epizootic is the result of an out of balance of the environmental ingredients that function in aquaculture.
The Genus Vibrionacae consists of ubiquitous halophilic facultative anaerobes. They are Gram-negative (mostly), motile rods. They represent the major genus of pathogenic bacteria to saltwater aquaculture (McVey pp295). The disease associated with this Genus is called Vibriosis. It affects many marine (and some freshwater) species. (Bullock, G. l., 1987) Vibrio is an extremely virulent organism with one species known to be a pathogen of cows while another 11 species causes diseases in man. Vulnificus illness (V. vulnificus) kills a number of people each year in North America from eating infected oysters (Florida Dept of Health and Rehabilitative Services Web Page).
Vibriosis is a systemic bacterial infection know world wide through many fish species that inhabit both warm and cold waters. This genus has led to catastrophic epizootics in the Penaeid Prawn Industry and prompted the application of modern biotechnology to improve diagnostics, which led to the development of DNA Probes. (Lighter & Redman 1998).
From the above table, the infection produces either skin ulcers or a septicemia characterised by erythema (patchy redness of the skin), haemorrhaging and anaemia. For example, Vibrio anguillarium produces red necrotic boil like lesions in the musculature and erythema of the fin bases and mouth in salmonids and has been reported in more than 42 species in widely distributed regions (Bullock 1987).
Shrimp Vibriosis is a major problem in Latin America and is typified by prawns swimming on the surface (Fox, J. Web Page 2002) and the numerous pathogenic agents are listed in that article. 'The presumptive diagnosis signs are, black spots on cuticle, large amounts of bacteria in hemolymph, slow clotting and melanosis of the shell. The confirmative culture diagnosis is isolation/purification with appropriate media such as TCBS'. (Thiosulphate Citrate Bile Sucrose Agar) (Oxide TCBS Cholera Medium. Vibrio spp. grow small yellowish colonies).
Vibrio species are indicated as disease agents in most saltwater hatchery and growout operations involving commercial aquaculture of prawns, molluscs and fish. In each case the treatments appear to be similar. ”Improved husbandry and sanitation, avoiding crowding and handling stress, improved feed quality and water purity, use of probiotics / chemotherapeutants, vaccination and antibiotics”, (Fox 2002) (Bullock 1987).
In shrimp culture, in Latin America, antibiotics are used extensively in medicated feeds. Oxytet, (oxytetracycline), nitrofurizolidone and sarafloxathin are added at 4g/kg. (Fox 2002).
In Australia disease control can be a matter of choice where different antibiotics are used in differing climates for control in hatchery situations. In fish growout culture antibiotics such as oxytetracycline acid and potentiated suphonamides are administered in feeds at the rate of 50-75mg/kg fish/ day for 5–15 days and 30-80mg/kg fish/day for 5-7 days. Here the treatment situation appears to be, that if a particular method works well then that becomes the standard for the specific location. Antibiotics used in Australia include Furazolidone and oxolinic acid. For antibiotics to be effective they need to be administered quickly as fish will stop feeding as an early symptom of the disease. As well antibiotics appear to generate pathogenic resistance and should not be used as routine prophylactics.
The long-term answer appears to be in the further development and refinement of vaccine administration, which has been researched since the early 80’s. Bray (Bray et al.1985) discusses where a killed Vibrio species was inoculated into Penaeus setiferus males to reduce spermatophore infection from vibrio spp.
Bullock ( 1987) talks of formalin-killed bacterial cells being trialed as vaccines administered as intraperitoneal injection (injection into the gut cavity) or as an oral immunogen and having positive results in a number of different fish species including salmonids and stripped bass.
Robertson et al. 1982 ( from Bullock 1987) discusses positive results using spray or shower immunization, where Pacific salmon and ayu fish were immunised by high pressure spray of Oantigen extracted from viable cultures of Vibrio. He also discusses the positive results where striped bass were injected with viable anguillarium and survivors were found to have enhanced antibody levels. However Munday 1996 suggests that while effective, the vaccinated host can still be overwhelmed by severe challenge i.e. stress. The continued development of vaccination techniques for the ever-increasing number of aquaculture species will be of significance to increasing production particularly when dealing with such a potentially virulent Genus such as Vibrionacae.
“The genus Aeromonas consists of ubiquitous Gram negative, oxidase-positive rods and with one exception, are motile (Hayes 2000). They are non spore-forming facultative anaerobes. Aeromonas spp. cause significant mortality in aquaculture worldwide and have been identified in Australia since the early 70’s. They are found in both fresh and saltwater and cause cellutitis and gastroenteritis disease in humans. Aeromonas spp. have been isolated as pathogens in the freshwater crayfish industry (necrotic eye) and from prawn farms culturing Penaeus monodon (Lavilla-Pitogo 1995).
Aeromonas salmonicida was introduced to Australia on contaminated goldfish and has caused considerable problems in the goldfish industry as aquarium shops and wholesalers tend not to reorder stock from contaminated hatcheries. So the disease has been “hidden” and allowed to spread throughout many aquarium shops. From there it has found its way onto silver perch farms. The pathogen has been isolated from silver perch culture, (Humphrey & Ashburner 1993) which will, potentially, be highly significant for this industry as silver perch culture ponds age and contamination potential increases through, potentially, uninformed hatchery and farm procedures.
Aeromonas salmonicida is known to cause furunculosis, which is a major problem for commercial salmonids. It has been implicated in goldfish ulcer disease and carp erythrodermatitis but that pathogen differs in that it is Gram-positive and non-motile. However it is considered to be an Aeromonas salmonicida variant.
Furunculosis is a septicemic (blood) disease principally of salmonids and has been known since 1894 (Piper 1985). Furunculosis is enzootic to many hatcheries but is checked with good sanitation and drug treatment. The chronic signs are red, raised fluid filled lesions, “furuncules” and necrosis of the internal organs (Fox 2002). From the chart above, the fish can be disorientated and go off feeding which is a good indicator of problems. There can be intestinal swelling and inflammation with reddening around the bases of the fins (erythema). The disease is transmitted horizontally but not vertically (Hayes 2000) through a contaminated environment and severity increases with changes in environment and associated stress.
The epizootic is controlled by improving the culture environment and with antibiotics administered in the feed or with intraperitoneal injection particularly with Broodstock. Oxytet at 50-75mg/kg of fish for 10 days has been successful and sulfamerazine and sulphonamide have also been used effectively. Commercial vaccines are available and appear to be successful. Selective breeding may also be significant in epizootic reduction (Fox 2002). In prawn hatcheries the pathogen has been treated with; “Furacin, Furanace (1mg/l), Chloramphenicol (1-10mg/l) and Oxytetracycline (60-250mg/l) (CRC Handbook).
The pathogenic agents are Flexibacter columnaris (freshwater) and Flexibacter maritimus (saltwater). Both pathogens are Gram-negative long slender rods identifiable, microscopically as they form stacks (columns) and have a gliding/creeping motion. The disease is reported world wide but the host maybe opportunistic and of a secondary infectious nature. The pathogen is associated with lesions but can appear as white spots on the body. It is mostly an external infection but can occur as an internal systemic infection with no visible signs. (Methods Microbiology 1985) The disease is fast acting and can cause severe fin erosion and is highly communicable (DBWS Bacterial Diseases).
The disease is well known in silver perch Bidyanus bidyanus hatcheries in NSW and Southern Queensland and can cause major mortality from handling stress and crowding. The disease is more noticeable with small fingerlings (15 – 25mm) being harvested and graded in the warmer months (Personal Observation, Hallidays Point Fish Farm & Hatchery). It has been the probable cause of mass losses of fingerlings transported to farm operations. The farmer needs to be vigilant and observant when new stocks arrive as severe losses can occur 2-3 days later. Any fin erosion on newly transported stock should be discussed prior to purchase. In some cases silver perch fingerlings recover within a few days while in others the caudal fin is lost altogether and the tail flesh becomes severely necrotic prior to death. The epizootic is usually greater than 80% of the total number. The use of clove oil as a sedative during transport has reduced the incidence of columnaris in silver perch significantly.The reduced stress factor while grading and shipping has been almost revolutionary to the industry (Personal Observation).
Flexibacter appears as a long thin “dotty”, almost segmented, Gram-negative rod when cultured on Ordals Medium. It appears as a slightly yellow/orange pigment surrounding the culture colonies. The colonies have a dry, rhizoid shape (hair like, filament) and are cultured at 20oC for 3 days. Ordals medium has low levels of nutrients and retards the growth of A. hydrophila (which co-exists with F. columnaris) and favours Flexibacter columnaris. This method was used to positively identify this pathogen in silver perch fingerlings at Hallidays Point Fish Farm in 1991.
The disease can be significantly virulent if established in the gills and if unnoticed can quickly lead to high mortality. Identification needs to be assessed quickly and appropriate chemotherapy applied.
Columnaris disease is treated usually by first lowering the water temperature where possible and treating with Terramycin or Furanace / Malichite Green flushes. Increases in flow and aeration assist as does the addition of clove oil at 0.5ml/1000l in bath treatments. While Terramycin or Furanace has been effective against the disease in Tasmanian Atlantic salmon it has not been documented in warmwater fish culture.
Quarantine is an under-managed aspect of Australian Aquaculture and of concern. As a hatchery operator for 12 years the mistakes made, innocently, influenced the profitability and potential of both the farm and the hatchery.
It is probably a wise idea to assess every facet of aquaculture from a quarantine aspect. Australia’s geographic isolation has helped keep the number of aquatic pathogens low but the industry growth and species expansion is bound to increase the potential for a new epizootic. Quarantine concepts need adequate examination and education on the farm, during local transport, export and import and even with hatchery staff and equipment used. The whole process of aquaculture needs a positive quarantine spin.
The introduction of Aeromonas spp. in the 70’s is still rippling through the new industries and its full impact is as yet unclear as Aeromonas is just starting to have significance in the silver perch industry. However strict adherence to regulations did not prevent the introduction of disease onto my own farm and disease management became a significant limitation in the commercial operation. The development of DNA probes will assist AQUIS staff and associated import quarantine facilities and that will benefit aquaculture and reduce the risk. It would seem that the 2 week quarantine time for imported goldfish is not sufficient and DNA probes may help.
The significance of the translocation policies in NSW Australia, appears to be misunderstood by sections of industry and anecdotal examples exist where contaminated water has been dumped into rivers by fish transporters some many hundreds of kilometres from the point of origin. Samples of silver perch fingerlings bought in Qld and delivered to NSW include many different species of native fish caught in the harvest method. This is simply bad practice carried out by dangerously uninformed operators. (early 90's)
The introduction of regulations where fish movements are recorded may have come to late to prevent substantial epizootic in the future and as such the new fish culture industries needs to be consistently vigilant. There is an example of one operator who found he could grow barramundi new Newcastle in NSW in a flow-to-waste recirculating/holding system. He was unaware that this practice was illegal however he could buy stock at the local pet shop (personal Com R. Callinen). Potential problems occur when property owners buy exotic species and place them in farm dams where fish can escape into rivers. Good intension, perhaps, but it will only take one fish to contaminate a river. Such abnormalities appear to potentially exist in all states by way of the aquarium industry regulations.
The banning of goldfish into Tasmania was a very appropriate move to protect the young salmon industry (Personal Com John Purser). At the present time there is no highly virulent Aeromonas salmonicida and consequently no GUD or Furunculosis, although, a-typical endemic strains have been recorded. The Tasmanian Salmon Industry has the advantage that fish movement is “one way” from fresh to salt to chilled which is likening to a natural quarantining procedure. Broodstock movements, back to freshwater, are carefully monitored and broodstock are kept in isolation away from culture areas in the sea. All fish transportation is well documented. (John Purser).
As aquaculture grows in all countries the lessons don’t have to be repeated. Careful education of why regulations are necessary and how important a role every industry person can play should be a high priority in planning for industry awareness for the future. Matter of fact there should be a paper on that subject.
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