activity increased but appeared to be normal 1–2 h posttreatment. No negative effects on feeding activity and mortality were observed. When the treated tanks were reconnected
to the recirculation system no negative effects were seen on the feeding or on the biofilters. A bath treatment of formalin at 75 ppm for 3 h reduced the mean infection
Ž .
category from 3.0 to 1.0 F1, Table 3 . Formalin also reduced the mean infection category from 2.6 to 1.2 when added to a concentration of 85 ppm in a recirculation
Ž .
system biofilters isolated 1 h . However, the mean infection category increased to 2.8 Ž
. Ž
. three days posttreatment F2 . Formalin 60 ppm added to a complete recirculation unit
Ž .
Ž .
biofilters included had no apparent effect on the parasites, fish nor biofilters F3 . All tested substances caused a slightly elevated mucus secretion on skin and gills.
4. Discussion
4.1. Aquarium trials Thirty different substances were screened as potential parasiticides for ectoparasitic
T. jadranica in eels. Eight substances had a high parasiticidal effect and out of these the anthelmintic bithionol and the disinfectants, Detarox AP
w
and Virkon PF
w
vet., appeared to be the best potential alternatives to formalin for treatment of trichodiniasis.
4.2. Full-scale trials in eel farms Highly infected eels in production units were treated with bithionol, Detarox AP
w
and Virkon PF
w
vet. Using these substances high infection levels in stocks with thousands of eels were reduced to levels where hardly any parasites were found. The
outlined procedures caused a temporary elevated swimming activity and mucus secretion during treatment. The treatments caused no observable effects on the biofilters and the
feeding activity was only reduced for a few hours.
Bithionol seemed to be attractive as a therapeutic because of its effectiveness and its ability to treat the entire recirculation system, although the drug has some serious
disadvantages. Its therapeutic index was relatively narrow, so a minor overdosing might be toxic. In addition, intensive data must be developed describing bithionol residues and
metabolites in eel flesh before introduction of this drug to aquaculture. Such work will probably not be implemented as long as it is possible to control trichodiniasis with easily
decomposable oxidizing disinfectants. The quick acting Detarox AP
w
is the most attractive substance as it can be added to the recirculation system with the biofilters
isolated for only 1 h. Virkon PF
w
vet. required a treatment period of 3 h in isolated tanks and was considered to be an alternative to Detarox AP
w
only. Formalin had parasiticidal effect when biofilters were separated from the system. The
less effective traditional treatment with formalin included the biofilters, suggesting that much of the formalin might be absorbed andror decomposed in the biofilters before its
parasiticidal effect was obtained. This observation was supported by our aquarium experiments where formalin was much more parasiticidal in tap water than in process
water. Therefore, some of the expected effect of formalin might be gained if treatment was conducted while isolating the biofilters.
None of the experimental treatments were capable of eradicating T. jadranica from production units. Although T. jadranica was undetectable after treatment with Detarox
w
Ž .
AP 55 ppm trichodinids became apparent in few days, possible due to the experimen-
Ž
w w
. tal procedures Detarox AP
and Virkon PF vet. that did not remove trichodinids
from the biofilters. A possible source of infection might be infected eels living in the biofilter and connecting pipe systems. Even if there were no parasites in the biofilters
reinfection may still occur because one or more parasites survived the treatments, e.g., Ž
. trichodinids may be found deep in the epithelia Frank, 1962 . Treating trichodiniasis
with current therapeutics should focus on controlling the abundance of the parasite, not on eradication. The interval between required treatments may depend on both the
condition of fish and water quality. Weakened eels are more susceptible to infection Ž
. Ž
Lom and Dykova, 1992 and high organic load favours trichodinids Schaperclaus,
´ ¨
. 1979; Stoskopf, 1993; unpublished data .
5. Conclusions
