Availability of effective treatments for control of infectious diseases is a critical requirement of the
Scottish and wider UK rainbow trout industry. The purpose of this project was to: identify the key
diseases that affect freshwater aquaculture operations in Scotland, particularly the trout sector, and
determine their relative impact; to identify the main methods used to control these diseases; identify
the potential consequences if any of the main control methods were to be withdrawn; and finally, to
identify any new potential treatments that could be used instead, if any of the main treatments were
to be withdrawn.
Producers, vets and health professionals surveyed confirmed that production was constrained by a
limited group of common diseases that affected rainbow trout producers in England and Scotland.
These included rainbow trout fry syndrome (RTFS) caused by the bacterium Flavobacterium
psychrophilum, white spot disease caused by the endoparasite Ichthyophonus multifiliis, enteric
redmouth disease (ERM) caused by the bacterium Yersinia ruckeri, proliferative kidney disease caused
by the myxozoan parasite Tetracapsuoidesa bryosalmonae, red mark syndrome (RMS) and bacterial
gill disease (BGD).
The main treatments available to control these conditions were limited, with florfenicol reportedly
used by all producers to control RTFS, formalin used extensively to control white spot and a range of
parasites and chloramine T to treat bacterial gill disease. ERM was mainly controlled by vaccination,
particularly via dip vaccination of fry with the Relera dual antigen vaccine. Other licensed antibiotics
(oxytetracycline, amoxicillin and oxolinic acid) were used to treat sporadic outbreaks of ERM, in fish
where vaccine protection had waned, and furunculosis.
The major reliance of the industry on florfenicol and formalin was concerning. Firstly there were
limited identified alternatives to control RTFS in the event of RTFS-causing strains of F. psychrophilum
developing resistance to florfenicol. There is also pressure at an EU level to withdraw formalin from
sale as a biocide. Possible alternatives to the use of formalin products purchased for biocidal
applications were reviewed in the event of their withdrawal from sale. For control of white spot it may
be possible to use a licensed product marketed in Spain for the control of parasites of turbot under
the veterinary cascade. The bronopol containing medicine Pyceze is one identified alternative that
may be used. Where systems can be engineered to allow its use, Salt (sodium chloride), either via low
concentration continuous dosing for several days, or short duration high concentration flushes is also
a potential treatment. Practical issues with regards either maintaining low concentrations of salt, or
dealing with high concentration effluents, may limit the use of this treatment strategy though. Project
staff also consulted with Danish producers who are trialing the use of peracetic acid. For control of
some ectoparasites, particularly flukes (e.g. trichodina), praziquantel, either as a water-based or in
feed treatment, may also be an option to explore. Review of the literature suggested that caprylic
acid, green tea extract and epigallocatechin gallate (EGCG), Piscidin 2, quinine, Triclabendazole and
potassium ferrate may all have some promise as alternative treatments. Selection of any alternative
treatments should be guided by whether they are likely to be readily useable. In this regards, products
that already have approval for use in food animal production, either as biocides, feed additives or as
medicines should be preferred in the first instance.
Undertake further controlled studies (laboratory and field based) on the effectiveness of
peracetic acid for the control of white spot and other production diseases.
Obtain further information on the margin of safety of peracetic acid at different
temperatures via target animal safety studies, at both a farm and laboratory scale.
Continue to support efforts to develop alternatives vaccines for the control of RTFS.
Determine the effectiveness of alternative antibiotics to florfenicol to control RTFS infections
caused by Flavobacterium psychrophilum.
Explore practicalities of importing formalin-containing medicinal products licensed in other
Ms for control of fish diseases for controlling white spot and other diseases.
Investigate use of mechanical control measures to reduce the impact of white spot in
rainbow trout production systems.
In vivo trials are needed to follow up some of the potential alternative chemical treatments
identified (e.g. caprylic acid, green tea extract and epigallocatechin gallate (EGCG), Piscidin 2,
quinine, Triclabendazole and potassium ferrate).
The survey clearly demonstrates that the rainbow trout industry is heavily reliant on a very limited
range of treatment options to control major production diseases.
Discussions with fish medicine producers and veterinarians also suggest that the freshwater stage of
the Atlantic salmon industry is similarly reliant on a small range of similar treatments to those used in
the trout industry. In particular, there is also heavy reliance on formalin to control white spot disease
and Costia in some hatcheries, and similar reports that florfenicol is the only effective treatment for
the control of Flavobacterium psychrophilum. They also report that formalin is used quite extensively
to control saprolegniasis in vaccinated salmon smolts prior to seawater transfer.
These findings are collectively concerning as either the withdrawal of formalin from sale, or the
development of resistance to florfenicol in Flavobacterium psychrophilum, could affect the viability of