The effect of stocking density and other parameters of the susceptibility of
Atlantic salmon (Salmo salar L.) to infectious pancreatic necrosis virus
Infectious Pancreatic necrosis virus
The virus has resulted in high levels of mortality in Atlantic salmon smolts following
seawater transfer. No reproducible challenge model existed for animals at this stage
in the life cycle. We have recently developed a challenge model in smolts and are
now investigating the aetiology of the disease (Bowden, Smail & Ellis 2002).
kidney
homogenate
1. the stocking density of fish?
2. the viral dose?
3. the isolate?
4. the time post transfer?
- aquabirnavirus
- non-enveloped
- double stranded RNA
- 7 kilobase genome
- 60nm diameter
- global distribution
100
Preliminary study on stocking density.
Low Density -20 fish / tank
High Density (100 fish/tank) can lead to cohabitation
mortalities.
60
Low density (30 fish/tank), mortalities only in injected
group.
40
20
Low Density
Preliminary study on related parr.
60
40
20
Not susceptible
80
60
Preliminary study on infective dose.
40
From 10 to 107 TCID50 per fish. Mortalities in all groups,
mortality levels lower and onset times longer in low dose
groups.
20
0
Percentage of group
Medium density - 80 fish/tank
100
Preliminary study on effect of time post
seawater transfer.
80
60
Infection from transfer point to 10 weeks post transfer.
Mortality rates higher in median groups.
40
In depth study (data shown here from 3 replicate tanks),
significant mortality and corresponding viral titres in all
groups.
20
0
1
3
5
7
9
11 13 15 17 19 21
Days post challenge
IP
Cohab
CHSE
supernatent
We have achieved mortalities in all challenges. By
checking viral titres and IPNV positive reaction we
ensure that death is due to viral infection. The
infection pattern shows mortalities occur after 4-7
days in the injected groups, this is followed after
another 4-7 days by mortalities within the
cohabitation groups. Viral titres in mortalities are
high, whilst in survivors it is low, strongly indicating
that the fish can clear the virus, at least to some
extent.
cohabitation
ratio 1:1
kidney
homogenate
CHSE
supernatent
CHSE
titration
IPN ELISA
23 25
2
3
4
5
6
7
8
9
10
5
6
7
8
9
10
Medium density
60
40
20
100
High Density - 160 fish/tank
1
80
0
Percentage of group
Cumulative percent mortality
100
Cumulative percent mortality
80
0
0
100
IPN outbreak
on farm
1
2
3
4
Stocking density high : Tank 3
80
60
40
20
0
1
2
3
4
5
6
7
8
Infectious units per g kidney (5 x log10)
Mortalities IP
Mortalities Cohab
Survivors IP
Survivors Cohab
9
10
11
Future Plans
Cumulative percent mortality
80
IP injection
(1x106 TCID50)
Figure 1. Caecal lumen of an infected salmon showing sloughed cells Figure 2. Pancreas tissue from infected salmon showing normal tissue
and exudate characteristic of infectious pancreatic necrosis.
at the top and necrotic acinar tissue at the bottom. Bar = 100µm
Bar = 100µm
100
Results
Is pathogenicity
effected by;
Methods
The Problem
Disscussion
The Agent
Percentage of group
Introduction
Tim Bowden*, Keith Lockhart, David Smail and Tony Ellis
1. Long term status of survivors – carrier status
2. Effect of viral isolate
3. In depth study of effect of time post transfer on
susceptibility
4. Interspecies infectivity
5. Anti-viral defences such as interferon, Mx and NF-kB.
This project has been supported by grants from the Crown Estates
Commission whose assistance is gratefully acknowledged.
*[email protected] | FRS Marine Laboratory, PO Box 101, 375 Victoria Road, Aberdeen, AB11 9DB, Scotland, UK