Top Quality Goldfish

For the purpose of this article I have gathered information from a variety of sources and from my own personal experience within the discipline of aquaculture/fish health and disease.

    SVC is a vast topic and constantly expanding due to ongoing research.

SVC accounts for extensive subject areas in:

·        fish pathology

·        the science behind the tests

·        the history and legislation behind controlling the disease.

This article should help the fishkeeper to:

·        be aware of SVC issues

·        recognize the disease symptoms

·        act accordingly in the event of an SVC outbreak

·        adopt appropriate measures for the prevention and the controlled spread of SVC within closed recirculation systems, aquaria and ponds.

    I have put forward some of my own opinions within the article, these opinions are purely those expressed by myself and not necessarily those of the majority.

    Within the text, there are some graphic images which some readers may find offensive.

Introduction to SVC in the UK

    SVC has occurred on the UK mainland for over the last few decades, with just a few cases in recent years.

    SVC continues to taint the reputation of the UK ornamental goldfish trade, with our most recent occurrence in 2008.

    The most recent case was traced to a shipment of imported fish from the Far

East ^(DEFRA) 

    Outbreaks in the UK are very rare in comparison to parts of Europe, such as

France,  Germany, Poland, Moldova and

Czech Republic ^(FDU)

The Virus

    Spring Viraemia of Carp virus (SVCV) is quite literally a 'bullet' shaped (bacilliform) virus belonging to the group of viruses known as Rhabdoviridae.

    Figure 1 shows a typical Rhabdovirus when viewed at magnification x 200,000

Fig. 1

    Despite, its size of less than 200 nanometres (nm) it is capable of causing widespread destruction in fish stocks.

    As the name suggests, it infects members of the carp family usually in the spring when water temperature is between 10-15 ^o C. Higher temperatures in the region of 17-22 ^o C have also been reported  ^(Fijan)

    There are many species of coldwater fish affected. However, in the UK the coldwater fish most likely to suffer are; goldfish, carp, rudd, roach, tench and pike.

    In experimental conditions tropical species such as guppies and zebra danios are susceptible to the disease ^(Post)

    At present there are known to be four main strains of the virus worldwide ^(FHI) Various molecular, immunological and culture methods are used to identify SVC The virus can be artificially grown in vitro using cell culture, whereby 15^o C is considered to be the optimum temperature for cultivation ^(OIE)

    There has been some development of a vaccine to SVC. Currently, there are no licensed vaccines commercially available. It is likely to be considerable time before a single vaccine will be effective against all four sub-types of the virus.

The Symptoms

    There are many disease symptoms associated with SVC in goldfish.

    The most common route of entry for the virus is via the gill epithelium.

    Within days the virus replicates and travels into the bloodstream leading to a systemic infection. Mortalities of fish can be observed within around 3 weeks after exposure.

    Some of the more common symptoms at the acute stages are:

·        dropsy

·        edema within the abdominal cavity

·        exophthalmia

·        pale gills

·        reddening around the mouth, eyes and vent

·        tiny haemorrhages on the skin, more noticeable on the underside of fish or on pale coloured specimens.

    Not all of the above symptoms are exclusive to SVC. Many bacterial infections will exhibit some of these characteristics.

    Particular attention should be given to any reddening on the skin and mouth. These signs are much more apparent when infection reaches a chronic stage.

    The most indicative external symptom for SVC at the chronic stage is haemorrhaging on the skin.

    In the laboratory, small biopsies of internal organs are sampled for further virus isolation and identification.

    Whenever a gross post-mortem is carried out, any internal symptoms of SVC at the chronic stage will be obvious. The haemorrhages are very distinctive on the swim bladder and to a lesser extent, the liver and intestine.

    Figure 2 below shows the internal appearance of a healthy goldfish.

Fig. 2

Fig. 3

    In contrast to this, Figure 4 shows the appearance of a swim bladder after chronic infection of SVC.

Fig. 4

    The haemorrhages can range in size from relatively small pinpoints to much larger areas. The virus replicating in the epithelial cells is the underlying cause for these haemorrhages.

    With the surface infection of the swim bladder, many chronically infected fish will have a tendency to lose balance.

    The kidney cells also undergo severe devestation. Infected fish lose the ability to control normal osmotic retention of fluids. The dysfunctional state of the kidney is considered to be the main cause of mortality. 

SVC slipped through the net

    You may ask.......... Are there no procedures set in place to prevent this? So howcome infected fish were able to reach the public domain?

    Within the UK, SVC is taken very seriously. It is a notifiable disease under European legislation, categorised as a 'List III' disease (EU Directive 91/67/EEC)

    For anyone who keeps coldwater susceptible species, such as, importers, retailers and fishkeepers, there is legal obligation to report an outbreak of SVC to local authorities.

In the UK, procedures are set in place

in order to help control outbreaks of SVC.

    These include, movement restrictions for suspect cases, 'test and slaughter' policies for confirmed cases, disinfection of premises and holding facilities after mass culling. Possibly a prolonged period of 'fallowing' tanks and ponds.

    There usually follows two years of testing for commercial premises that restock after an outbreak. This will determine if the virus is still present after the initial infection.

    It is also compulsory for imported fish to the UK to be covered by a valid Health Certificate.

Health certificates for commercial fish farms are based on internationally recognised diagnostic tests performed by competent authorities ^(OIE)

Generally, when screening for a disease on a commercial fish farm, a number of fish are randomly selected and sent to a laboratory

for testing. Sampling the appropriate number

is usually determined using a risk based calculation ^{(Ossiander, Wedemeyer)} 

    For very large farms and exporters of tens of thousands of fish, only a fraction of the stock will be sampled.

    The calculations give a certainty value of 95%. That means that there is a 5% chance of infected fish not being physically sampled.

    Therefore, it is possible that one or two SVC infected individuals remain in the population.

    It is also worth considering that whilst diagnostic tests are relatively accurate they are never completely fail-safe.

    Virus levels may well be present at a very low titer within the infected individual so that it remains undetected. Additionally, an individual fish may have overcome the infection (post-infection) so that there is no trace of the virus.

The virus may have undergone a latent stage of infection. Currently there is no evidence to

support complete immunity to SVC after

infection ^(Wildgoose)

    The outcome of the tests in both the above cases would result in a false negative.

Just because a virus was not isolated in the laboratory, does not necessarily mean that it was not there in the first place.

    Currently in the UK there are no compulsory requirements for wholesalers to quarantine imported ornamental fish.

    My own personal opinion is that any health certificate that allows infected fish to slip through the net, is not worth the paper it is written on. Quarantine of imported fish should go hand in hand with health certificates.

    For example; the SVC outbreak of 2008, a shipment of infected fish arrived at the consolidator and was distributed to around 20 retailers before the extent of the damage was realized ^(DEFRA)

    Despite authorities best attempts for controlling SVC, closing the stable door after the horse has bolted is pointless.

    If anything the current procedures can only be described as 'passive'. Quarantine of all imported fish at the importers should be imperative. This would be more of a 'pro-active' approach. 

    The Ornamental and Aquatic Trade Association (OATA) would argue that due to the sheer numbers of imported fish, large holding facilities would be required. Therefore making quarantine impractical. 

Future Outlook for the goldfishkeeper

    There is very little data published on the ability of SVCV to over-winter in deep ponds and mud.

Other agricultural viruses with a similar mode of action have shown to over-winter in organic matter at low temperatures.

    For example; Foot and Mouth Disease Virus (FMDV) is known to survive in slurry for up to 6 months during winter. In contrast, it will only survive in surface soil for 3 days in summer and 28 days in autumn ^(Douglas)

    Similarly, it may be possible for SVCV to over-winter in deep 'fishless' ponds containing nutrient rich organic sediment.

    For the forthcoming spring of 2009, extra vigilance should be given to all fish over-wintering outdoors as they emerge from hibernation.

At the lower end of the temperature range the virus will start to replicate within the fish, long before the host can mount an immune response.

    It has been shown for other fish viruses to be spread by vertical transmission.

For vertical transmission to occur, high titers of the virus are required in the broodstock ^{(Inst. of Aquaculture)}

    SVC has been known to survive in vitro within Carp ovarian cells (COC) at low temperatures for over a year ^(Rudikov)

    This may have future implications for imported broodstock.

Further Steps for control

    The safest way to obtain SVC-free stock is to purchase from suppliers that have a 'history of freedom' from the disease.

    Prevention is always better than the cure. This is the number one rule of disease management.

Sadly, there is no cure for SVC, that leaves prevention as the only option.

    In order for the goldfishkeeper to obtain SVC free fish, they should source locally bred fish from reputable breeders or suppliers within the UK. Local bred fish are much less likely to carry SVC than their imported counterparts. They will also be more hardy to regional water parameters, therefore less stressed by minor changes in environmental conditions. 

    As mentioned above, I can not place enough emphasis on the importance of quarantine facilities. The health status of newly acquired fish should never be taken forgranted.

    Even if fish have undergone 'optional' quarantine at the wholesaler or aquatic store, further quarantine (min 3 weeks) should follow after movement of fish to another location.

    If at all possible, quarantine at a cooler temperature range of 10-15 ^o C. This will enable the virus to replicate and manifest clinical signs of SVC in the host.

    Some other advantages of quarantine are:

i)  it helps any fish adapt to changes in physical or chemical water parameters.

ii) confines any disease in a 'closed' system, therefore preventing spread of infection to existing fish stocks or into the wider environment.

    Biosecurity is another issue that needs to

be addressed when controlling an outbreak of SVC.

    Disinfection of commercial premises after an outbreak of SVC is performed in accordance with the Fish Health Inspectorate (FHI), DEFRA and other Agricultural authorities. The disinfectants are approved by these authorities and used at specific concentrations (0.5-1%). VirkonS (hyperoxygen), Fam30 (iodophore based) and Trigene are types of commercially available disinfectants.

    2% washing soda is also used in agricultural practices.

    For the fishkeeper, standard household bleach will be just as effective as some of the above disinfectants.

    Strong household bleach usually contains <5% Sodium Hypochlorite (NaClO), sodium hydroxide (NaOH) and surfactants. They work by the same mechanism of the 'approved' disinfectants.

    The combined action of chlorination, extreme pH and surfactants disrupts the outer envelope integrity and subsequent destruction of the virus itself.

    The main agent of bleach (NaClO) is so effective as a disinfectant that it is used for chlorination in drinking water and certain waste water treatment works.

    When washing down tanks, equipment

and nets, allow the items to steep in 1-2% solution (10-20ml of neat bleach per litre of

water) for an hour or more. The bleach is simply washed away using copious amounts of

water.

    When performing a water change from a quarantine tank, I prefer to drain waste water into 30 litre carboys. This is treated with 1% bleach solution and left to stand overnight before pouring down the drain. This is purely an additional step in disease management and not a necessity. The additional step is to prevent any possible disease within the quarantine tank  from entering a public waterway.

    There are some alternative ways for controlling the virus. It is inactivated at temperatures of +60 ^o C. Washing equipment in hot water, followed by drying for dessication of the virus will also be successful.

    UV sterilizers found in pond filters will destroy any virus that passes over quartz tube. However, in reality, due to flow rate versus pond volume, UV's tend to only reduce virus numbers.

    It has been known for sometime that SVCV can be spread by other biological vectors such as fish lice and leeches ^(Ahne)

    It may well be possible for other parasites to act as vectors for the disease.

    Such vectors can be transported along

with wild plants and live foods taken from natural water courses. Under no circumstances should they be introduced into ornamental ponds or tanks.

    As for purchasing fish, it is good practice to source plants and live foods from reputable suppliers in order to prevent introduction of SVC or other diseases to ornamental stocks.

Author's final Note

    I have presented some of the major concepts surrounding SVC within this article.

    SVC is a very distressing disease for fish and for the fishkeeper unfortunate enough to witness it.

    The article is not intended to shock or cause unnecessary alarm to the goldfishkeeper.

Instead, I hope that the fishkeeper will be able to take an 'informed' and 'educated' approach for preventing the spread of the disease within the UK.

    Many of the concepts can be applied to both the novice and professional fishkeeper.

    By applying these concepts, this will help ensure SVC-free fish, possibly eradication of the disease in the UK.

    Thereby, preserving our heritage of the gold standard UK bred goldfish for future generations of fishkeepers to come.

    As my title suggests, the goldfishkeeper shouldn't have to 'bite the bullet! '

References

Ahne, W. (1985). Argulus foliaceus L. and Pisciola geometra L. as mechanical vectors of spring viraemia of carp virus (SVCV). J. Fish Dis. 8:241-242

Douglas A. (2000). Personal communication, Former lecturer of 'The Queen's University of Belfast', Department of Agriculture & Rural Development

DEFRA, Department for Environment Food & Rural Affairs

FDU, Fish Diseases Unit, Personal communication. Agri-food biosciences institute, formerly (DARD, Dept. of Agriculture & Rural Development)

Fijan N. (1972). Infectious dropsy in carp - a disease complex. In: Diseases of Fish, Mawdesley T., ed. Symposia of the Zoological Society of London. Acedemic Press, UK, 39-51

FHI - Fish Health Inspectorate, (CEFAS, Centre for Environment, Fisheries & Aquaculture Science)

Institute of Aquaculture, University of Stirling.

OIE, Office International des Epizooties, Paris. International Health Standards, Diagnostic Manual for Aquatic Animal Diseases (2000), 3rd Edition

Ossiander F.J. & Wedemeyer G. (1973). Computer program for sample size required to determine disease incidence in fish populatons. J. Fish. Res. Board Can., 30, 1383-1384

Post, G. (1987) Textbook of Fish Health, T.F.H. Publications, U.S.

Rudikov, N.I. et al, (1975). Vesennaja virus naja bolezj rhb, Bjull. Ves. ord Lenina inst. erksp. vet., 20:16-19

Wildgoose, W. (1997). Koi Therapeutics Pt 5, The treatment of virus & other organisms, Koi Health Quarterly.

Glossary

Acute - exposure to the virus or infection for a short period of time

Bacilliform - bullet shaped

Chronic - exposure to the virus or infection for a prolonged period of time

Edema - inflammation of fluid into the adominal cavity

Envelope (virus)  - outer coating of a virus composed of fatty acids/lipids

Exophthalmus - protuding or swollen eyes

Fallow - adopted from agricultural terms, to leave barren or deserted during the growing season or a substantial length of time

FMDV - Foot and Mouth Disease Virus, referring to the actual virus that causes the disease

in vitro - artificial culture of the virus outside the body

latent infection - period of dormancy of infection before becoming active again

nanometre - one millionth of a millimetre (0.000001mm)

SVC - Spring Viraemia of Carp, referring to the disease

SVCV - Spring Viraemia of Carp Virus, referring to the actual virus that causes the disease

Titer - unit of measurement for viruses, more specifically, the active virus particles present.

Vector - a living organism that is capable of carrying/transferring an infectious disease and passing it on to other susceptible species. The vector itself may or may not be affected by the disease