Download Important diseases of salmonid fish and the risk they pose to New

Surveillance Vol.17 No.2 1990
Important diseases of salmonid fish and the risk they pose
to New Zealand
Thc economic well-bciiig and future developi n r i i t of Nezu Zraland's rt~crentioizal aiid
co~iimrrcialfishcric.5and aqzinciilt U re depeiici
011 l i d t h y f i s h stocks. Interiiatioiial trade iii
fish stocks nizdfish ~ ~ r o d iposes
~ t s a potential
risk ill so far a s snch trade could introdiiix~
s r ~ i o i i sfish dismsrs iiito N e w Zmland's
ri1i7tc~zc~ays.Once established, siich diseases
conld he inipossihl~to control or eradicatr. 171
this article Colin Atiderson, vctcririarinii niid
fish ptliuloSist, examiiics the iniportant dis~'asi'sujsalinoiiid fish and the risks thy pose.
The major diseases of salmonid fish
are listed in Table 1. New Zealand is in
the fortunate position of having only two
of these agents, MyXCJbCJlL4s c-iwhrnlis and a
Birna virus of the I I " group.
Table 1: Important diseases of salmonid
fish:
Viral diseases
Infectious haematopoietic necrosis (IHN)
Viral haemorrhagic septicaemia (VHS)
Infectious pancreatic necrosis (IPN)
Piscine erythrocytic necrosis (PEN)
Herpesvirus salmonis
Bacterial diseases
Bacterial kidney disease (BKD - Rmehacrerium salmoninarumj
Aeromonus salmonicida (Furunculosis)
Vibriosis (Vibrio ordalii, V . an,quillarum)
Protozoal diseases
Mxoholus cerebralis (Myxosonia cerehrulis - whirling disease)
Proliferative kidney disease
Ceraiomy.ra shasta
Henneguya sulmoniida
Parvicapsula disease
Fungal diseases
Rosette disease
M!yxoboliis c-cwhralis (whirlingdiseasc
was first reported in New Zealand from
a rainbow trout sport fish hatchery
(Acclimatisation Society hatchery) near
Dunedin in the South Island of New
Zealand in 1971.' Clinically, disease has
been observed once since then, again in
rainbow trout reared at a government
hatchery near Christchurch in the South
Island in 1980. The parasite has also been
identified in routine examination of
'healthy' wild salmonids in a number of
locations in the Canterbury and Otago
provinces of the South Island.
The Birna virus, the AB serotype, was
first isolated from healthy adult Quinnat
salmon (Chinook salmon, Oncorhynclzus
tsl~arcytsclia)in 1985.' Two further isolations were made i n 1989 from the same
class of stock. N o clinical disease has
been seen, nor has the virus been isolated
fromany other classofstockon the twenty
salmonid farms which have been regularly inspected and tested for export certification purposes over the last five years.
The economic significance of
salmon disease
The most important disease for salmon
farmers in New Zealand is vibriosis. This
disease has a world-wide distribution
and affects other marine species including shellfish and prawns. On New Zealand salmon farms, vibriosis appears in
the warm water temperatures of midsummer and is not usually seen on an
individual property every year. The
disease is frequently restricted to salmon
in their first year in seawater and these
outbreaks are adequately controlled with
vaccination and short treatments with
antibiotic medicated feed.
Vibriosis and whirling disease have
little economic impact on New Zealand
salmon farmers. Losses due to vibriosis
involve usually only 5-10% of the least
valuable year class on the property. The
presence of Myxobolus cerebralis has resulted in mild constraints to the sales of
live fish within New Zealand and the
provision of well water supplies to the
hatchery building on the three farms
involved.
In contrast, salmonid farmers and
sport fish hatcheries overseas have to
contend with a large number of serious
diseases. In the USA, the IHN virus has
devastating effects in both hatcheries and
wild populations of salmon.'f4
The bacterial diseases, furunculosis
and BKD, cause significant mortalities in
fresh water salmon culture, but when
affected fish are exposed to the stress of
transfer into sea water, the smolts often
suffer heavy mortalities which are difficult to control. The costs are considerable
in terms of replacement smolts, wasted
labour, delayed marketing and medicated
feeds (Canadian $70,000 to treat a 200
tonne farm for the usual 10 days).s Reduced marine survival of BKD infected
wild smoltshreducessalmon returns and
effects sport fishermen and businesses
involved in the angling industry.
In the USA and other Northern Hemisphere countries, singificant national
funding is being spent on researching
means of controlling salmonid diseases
(especially IHN and BKD in the USA).
VHS is a salmonid viral disease of
particular economic significance as it
often causes high mortalities in valuable
market-size fish.
The desire of national salmon fishing
industries to avoid the debilitating effects of these salmon diseases on their
native fishand their commercial and sport
fish industries has resulted in regulations to control internal and international
movements of fish. These have their
associated bureaucratic and technical
costs for the respective governments and
farming i n d ~ s t r i e s .Governments
~.~
have
also imposed restrictions on the types of
products they will import. New Zealand
and Australia only allow the entry of
canned or 'hot smoked' salmon, because
the required heat exposures are believed
to kill fish pathogens in the salmonid
flesh. The stipulated temperature-time
requirements may not meet the
marketer's ideal for a high quality product and could effect sales.
It is essential for the well-being of the
sport fish, native fish, and aquaculture
industries of New Zealand that the wide
ranging detrimental effects of these exotic fish diseases are avoided by preventing their introduction.
The possible means by which fish
disease could be introduced are many
and varied. Live salmonids and their
eggs pose the greatest risk and these
would be required to undergo secure
quarantine and rigorous disease testing
in New Zealand before release. There is
growing evidence that other fish species
can carry, and be affected by, 'salmonid'
diseases and vice versa. Cold water
species are especially important. Ornamental gold fish have introduced a strain
of Aerornonas salrnonicida which is highly
virulent to Atlantic salmon (Salrno solar)
into A ~ s t r a l i a . ~
Imported ornamental
fish should be subjected to quarantine
and disease examination appropriate to
the risk they pose.
Many fish diseases are preserved in
fish tissue by chilling and freezing'","
and the disease agents could remain
viable in table fish imported into New
Zealand. Discarding uncooked imported
salmon near waterways, or places where
feral animals could mechanically transfer diseased material to waterways, feeding imported salmon flesh to aquarium
gold fish or salmonids which are subsequently liberated into waterways (asoften
occurs when they get 'sick') are some of
the possible means of transferring exotic
fish pathogens to New Zealand fish.
It is important to note that fish diseases commonly thought of as salmonid
diseases are not necessarily restricted to
salmonids. VHS has been experimentally transmitted via a water borne route
to turbot, a marine flatfish (Scophthalrnus
and has caused disease outbreaks in a number of fish species including pike (Esox lucius).'3 PEN, a viral
disease of the erythrocyte, causes mortalities in a number of fish species including salmon, herring and cod.'4
Visitors and imigrants also pose some
risk. MAFs Border Protection Officers
examine tourists and goods moving
through our ports to prevent fish diseases gaining entry on equipment or in
products.
Ongoing surveillance, with the aim of
early detection of introduced fish diseases, is also important. MAF officers
routinely visit fish farms to look for clinical evidence of disease, and laboratory
testing is performed for salmonid export
certification and on diseased fish submitted from fish farms and by the public.
Surveillance I7(2) 17
Surveillance Vol.17 No.2 1990
There would probably be a long lag
time from the introduction of an exotic
disease until it reached a high prevalence
in the population. BKD was introduced
by transfer of infected Pacific salmon to
the Great Lakes, and it has taken 20 years
for its effects to become evident.15 This,
time lag together with the remote nature
of our waterways, the extreme difficulties involved in completely depopulating an infected waterway by fish poisoning, and the impossibility of disinfection
of even a small stream, means that preventive measures to exclude exotic fish
diseases need to be stringently applied.
The establishment of an exotic disease of
salmonids could be devastating for New
Zealand's freshwater fisheries
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Fresh Water Research 6 (1&2) : 1-10.
Tisdall, D, Phipps, J, 1987: Isolation and characterisation of a marine bimavirus from retuming Quinnat salmon (Oncorhync,hustshowtsc.huj
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Colru Anderson
Central Animal Health Laboratory