Download Distribution of salmonid diseases in New Zealand

Distribution of salmonid diseases in New Zealand
Trout and salmon (salmonids) are Northern
Hemisphere species. Introductions to this
country began in 1867, when brozun trout
eggs were imported by a sportfish group, the
Acclimatisation Society,from Scottish stock
being reared in Tasmania. Further introductions of various species of salmonids from
Europeanand North American countriescontinued until the 1960s"'.
Commercial farming of salmon be-
gan in New Zealand in fresh water in the
mid 1970s and in sea-cages some 6 years
later. Intensive screening of salmonids
for disease followed this commercial development.
There are three sport fish hatcheries
in the North Island, at Tongariro,Ngongotaha and Napier (Figure l),which specialise in rearing rainbow trout for restocking angling waters. The South Island has one sport fish hatchery at Wanaka and 23 salmon farms rearing Quinnat
(Oncorhynchus tshauiytscha) and sockeye
(0.nerka) salmon. The salmon farms are
spread throughout the South Island and
in Stewart Island.
Information on salmonid diseases in
New Zealand has been collected during
investigations of disease episodes on fish
farms and on-site inspection, and laboratory testing for export certification. Export testing has been directed at the detection of Myxobolus cerebralis, the protozoan agent of whirling disease, and viral
haemorrhagic septicaemia (VHS).However, the tissue culture cell lines used are
susceptible to a number of fish viruses
and a birnavirus, infectious pancreatic
necrosis virus (IPNV),has been isolated
from healthy sea-run adult Quinnat salmon during export te~ting@)(~)(~).
In 1977 MAF established a specialist
fish disease diagnostic section at the Central Animal Health Laboratory (CAHL),
Continued next page
Figure 1: Locations offish hatcheries and isolations of disease organisrrzs
Surveillance 23(4) 1996 23
Wallaceville, Upper Hutt. Export testing
of salmon farms for the certification of
salmon meat commenced there in 1985.
All 'lots' (year, classes and species) on
about 20 South Island salmon properties
have been tested annually for whirling
disease and viruses. On average, 60-120
salmon from each property were tested
for Myxobolus cerebralis and 120-180salmon were tested for viruses. During 9 years
of export testing, diagnostic examinations and surveys, 12,920salmonids have
been tested at CAHL for M . cerebralis and
23,183 salmonids have been sampled for
viral tissue culture examination. For the
same period, 1,809 salmonid were examined for bacterial diseases.
Pathogenic organisms identified by
CAHL in salmonids over the period 1977
to 1995 include the following:
Virus:
infectious pancreatic necrosis
virus (IPNV)
Bacteria: Flexibacfer sp.
Hafnia alvei
Strepfococciis sp.
Vibrio ordalii
Yersinia ruckeri
Protozoa: Chilodonella sp.
Ichthyophtkirus multifilis
Myxobolus cerebralis
Paramoeba sp.
The locations in which disease agents
were isolated are shown in Figure 1.
Four disease agents in the above list
are of particular interest. M. cerebralis
was first detected in New Zealand at a
trout hatchery near Dunedin in 1972'". A
survey carried out at that time, and following detection of whirling disease in
Canterb~ry'~"'',identified the organism
in 1980 in rainbow trout farmed near
Christchurch. Since thenM. cerebralis has
been found at a further six locations, in
fish that were sampled for research or
export certification, but that showed no
signs of whirling disease. M. cerebralis
has never been detected in the North
Island.
V . ordalii was first identified in 1984
during disease investigation on a salmon
24 Surveillance 23(4) 1996
farm near Owaka in Otago(xl.In the early
years of sea-cage rearing of salmon in
New Zealand, vibriosis outbreaks were
common in the warm summer months.
Fish in their first year in seawater were
most often affected, and oral antibiotic
medication and vaccination were used to
limit mortalities. However, since 19S9
vibriosis outbreaks have been rare arid
antibiotic medication and vaccination are
no longer necessary.
Y. ruckeri was first isolated in 1989
from dying fingerling salmon on a Canterbury salmon farm. A survey was carried out to determine the distribution of
this agent and to test for the exotic fish
pathogen Aeronzonas salmonicida. Salmonids and native fish were sampled at
aquaculture and wild locations in both
Islands. Y . ruckeri was detected at two
sites already known to be positive. A.
saltnonicida was not i~olated'~'.
IPN virus was first isolated in 1985
duringroutine testing for export purposes of healthy sea-run Quinnat salmod2,.
It has since been isolated seven times
from this class of stock, but there have
been no isolations from other age classes
on these or other salmonproperties. Clinical signs of the disease, which are seen
with other strains of this virus group,
have never been observed in New Zealand.
The short list of diseases illustrates
New Zealand's relative absence of serious fish diseases compared with almost
all other countries that farm salmonids"0'.
Three factors have played major parts in
this favourable health status. Firstly, salmonids were introduced as eggs, which
are the safest life stage at which to translocate fish from the point of view of disease transmission'"'. These introductions
occurred when fish farming was in its
infancy and when the movement of salmonids, within and between countries,
had been minimal. Secondly, New Zealand is an island nation comparatively
isolated from the rest of world. Thirdly,
for the last 20 years New Zealand has had
in place border controls to prevent the
introduction of exotic fish diseases with
imports of salmonids and their unsterilised products.
References
( I ) Scott D. The migratory trout (Sulmo trurtu L.)
in New Zealand. 1. The introduction of stocks.
Transactions of the Royal Society of New
Zealand, Zoology 4(17), 209-227, 1964.
(2) Tisdall DJ, Phipps JC. Isolation and characterisation of a marine birnavirus from returning Quinnat salmon (Oncorhynchus
r.shawyrschu) in the South Island of New
Zealand. New Zealand Veterinary Journal 35,
217-218, 1987.
(3) ChiefVeterinary0fficer.AnnualReport 1992.
Surveillance 20(3), 7, 1993.
(4) Chiefveterinary Officer. AnnualReport 1993.
Surveillance 21(3), 8, 1994.
(5) Hewitt GC, Little RW. Whirling disease in
New Zealand trout caused by Myxosornu cerehrulis (Hofer, 1903) (Protozoa: Myxosporida). New Zealand Journal of Marine and
Freshwater Research 6(1 & 2), 1-10, 1972.
(6) Hewitt GC. Survey of New Zealand trout
hatcheries for whirling disease caused by
Mvxosomci cerebralis. New Zealand Journal
of Marine and Freshwater Research 6(4),
463-468, 1972.
(7) Boustead N. Whirling disease survey results.
Freshwater Catch, Summer, 9-1 1, 1982.
(8) Wards BJ, Patel HH, Anderson CD, de Lisle
G. Characterisation by restriction endonuclease analysih and plasmid profiling of Vibrio
ordalii strains from salmon (Oncorhynchus
tshawy/scha and Oncorhynchus nerku) with
vibriosis in New Zealand. New Zealand Journal of Marine and Freshwater Research 25,
345-350, I99 I .
(9) AndersonC,KnowlesG,deLisleG.Asurvey
for Yersiniu ruckeri and A e r o m o n u s
salmonicidu in farmed and wild fish. Surveillance 2 I (3), 39-40, 1994.
( I 0) Anderson C. Important diseases of salmonid
fish and the risk they pose to New Zealand.
Surveillance 17(2), 17-18, 1990.
( 1 1 ) Turner GE. Codes of practice and manual of
procedures for consideration of introductions
and transfers of marine and freshwater organisms. European Inland Fisheries Advisory
Commission, Food and Agriculture Organisation of the United Nations, Occasional paper 23, 1-44, 1988.
Colin Anderson
MAF Quality Management
Central Animal Health Laboratory
Email: andersoncm wallacevil1e.mqm.govt.nz