Date: 3 July 1998
From: Veter Quimica bjauregu@reuna.cl
To: MARINE_PATHOL@VS.vims.EDU
QUESTION:
Due to the increase of Flavobacterium psychrophilum in salmon rearing farms in Chile, my interest is to create an open discussion about the necessary strategies to control the diseases caused by this bacteria. According to the information gathered so far it would be advisable to control the following putative bacterial sources:
Broodstock
Eggs
Alevins
Water
I would really appreciate information about how to control this disease either from literature or from personal experience.
Tomislav Jakovljevic
Marine Biologist
Veterquimica
Camino Melipilla 5641 Santiago, Chile
e-mail: bjauregu@reuna.cl
***************
COMMENTS 1:
Flavobacterium spp. are among several resident pathogens inclusive of Flexibacter spp. and Cytophaga spp. which can cause Bacterial Gill Disease in salmonids reared under suboptimal water conditions.
The predisposing factors to disease outbreak are:
High organic load from uneaten rotting feed, suspended solids (following heavy rains), fish feces.
Low oxygen levels associated with excessive stocking densities, increased water temperatures and insufficient water flow/turnover/aeration.
Overcrowding of fish which increases the host-pathogen contact, stress levels are high which impairs the fishes' immunity & gives gill parasites a foot hold.
Control should be aimed at solving the above, viz:
Reducing stock density below 15kg fish/cubic metre water
Reduce feeding rates or cease feeding when water temperatures are high or during mortality outbreaks.
Increase water exchange rates (monitor NH3, suspended solids, O2 levels) so as to reduce the BOD (biological oxygen demand) of the system. Optimum water parameters for salmonids are:
O2 greater than 5mg/L
NH3 less than 0.005 mg/L
Water temp 12-17 C
Suspended solids less than 80 mg/L
Daily removal of all dead, moribund fish. Clean up the sediments built up in pond bottoms. Emergency harvest if up to 50% of fish in an enclosure are already affected or dead. Tow healthy cages of fish to new clean sites. Drain, dry and lime ponds following destocking.
If the disease is just starting ie 2% mortality daily, then steps 1-3 must be attended to and treatment may be attempted. Do not simply rely on drug treatment without attention to improving the culture environment otherwise it's just a waste of time and money.
Treatment:
Benzalkonium chloride by bath
10mg/L for 5-10 mins,5mg/L for 30 mins,2mg/L for 60 mins, 1 mg/L for few hours. The toxicity of this drug is greater in soft water, so half dosages. Treat a few fish first to assess tolerance to the regime. Supplemental aeration essential.
b) Potassium permanganate bath
1-5 ppm for 1 hour repeat as necessary for 2-3 days. Note: in muddy water (high organic load), the compound is rendered ineffective and may be toxic due to deposition of manganese oxide on already damaged gills. Toxicity also greater at warmer water temperatures. Always have supplemental aeration.
Despite treatment, heavy losses still occur. The key is prevention.
Dr Roger Chong
e-mail: rswchong@HK.Super.NET
***************
COMMENTS 2:
There is only one publication about F. psychrophilum in Chile:
Bustos P.A. et al, (1995) First isolation of Flexibacter psychrophilus, as causative agent of Rainbow Trout Syndrome (RTFS), producing Rainbow trout mortality in Chile. Bull. Eur. Fish Pathol. 15(5): 162-164
Although the number of publications is low, the outbreaks of the disease have been numerous. The problems start at the hatchery in piscines that are water-fed from a river or spring water with temperatures below 10 C. It seems that at this level there is a portation condition in the fishes with a great number of bacteria. We do not know the portation tissue, because we have sampled small fishes as a whole. There has been some outbreaks too at this stage.
Next, the fishes are transported to the lake cages, there they suffer outbreaks of the disease (we think the temperature raise up to 15 C is the detonating agent) and are treated with antibiotics.
Tomislav Jakovljevic
Marine Biologist
Veterquimica
Camino Melipilla 5641 Santiago, Chile
e-mail: bjauregu@reuna.cl