James M. Forwood

Efficacy of bath and orally administered praziquantel and fenbendazole against Lepidotrema bidyana Murray, a monogenean parasite of silver perch, Bidyanus bidyanus (Mitchell)

We investigated the efficacy of praziquantel (PZQ) and fenbendazole (FBZ), each administered by bath and orally, against the monogenean Lepidotrema bidyana Murray, a gill parasite of the freshwater fish silver perch, Bidyanus bidyanus (Mitchell). PZQ and FBZ were each administered by bath at 10 mg L⁻¹ for 48 h and on surface-coated feed pellets at 75 mg kg⁻¹ per body weight (BW) per day for 6 days. Bath treatments of PZQ and FBZ had an efficacy of 99% and 91%, respectively, against adult L. bidyana. Oral treatments of PZQ and FBZ had an efficacy of 79% and 95%, respectively, against adult L. bidyana. Fish rejected feed pellets surface-coated with PZQ, suggesting that palatability of surface-coated PZQ-medicated feed is poor, which undermined efficacy. In all trials, some juvenile parasites were present on fish after treatment during efficacy assessment, indicating that efficacy may be lower against juvenile parasites or that recruitment occurred post-treatment, demonstrating that repeat treatments are necessary to effectively control L. bidyana in aquaculture.

Host impact of monogenean Lepidotrema bidyana infection and intensity estimates for onsite monitoring

We developed a rapid effective method for accurate estimation of intensity for the monogenean Lepidotrema bidyana, a gill parasite of silver perch Bidyanus bidyanus. This parasite requires monitoring because high-intensity infections reduce host growth and can lead to secondary bacterial and fungal infections. The most accurate method for counting L. bidyana was visual examination of fresh gills. There was a significant relationship between fish size and parasite intensity; however, there was no significant relationship between fish condition and parasite intensity. Parasite intensity estimates were generated by using the mean intensity of worms on the posterior hemibranch on the first left gill arch, compared to the total mean intensity of worms on all hemibranchs. Estimates were validated by predicting L. bidyana intensity from a random sample of silver perch obtained from aquaculture ponds. Parasite intensity estimates correlated strongly to real counts, and this method can be used to accurately predict parasite intensity on an individual host, and thus represents an improvement over previous methods.

Life cycle and settlement of an Australian isolate of Ichthyophthirius multifiliis Fouquet, 1876 from rainbow trout.

Ichthyophthirius multifiliis Fouquet, 1876, a ciliate parasite, is a cosmopolitan and problematic parasite of cultured freshwater fish. Each geographical isolate of I. multifiliis has variations in life cycle timing under different abiotic water conditions, such as temperature and salinity. We assessed the effects of salinity and temperature on the development and the preferred settlement site of a temperate Australian isolate of I. multifiliis. The time until theront release was significantly different between each temperature; development time was longest at 5 °C with a mean time of 189 h and decreased to a mean time of 11.7 h at 30 °C. At 5 °C our isolate produced a mean of 267 theronts per tomont, which increased to a mean of 493 theronts at 25 °C and reduced to a mean of 288 theronts at 30 °C. Theront length showed an inverse relationship to temperature; mean length was 62 μm at 5 °C and 41 μm at 30 °C. Our isolate reproduced faster at all temperatures and a greater sensitivity to salinity than all reported profiles for temperate isolates. Parasite abundance was highest on the dorsal region of the fish. An accurate understanding of temperature-life cycle information and optimal region to sample for surveillance will aid in the development of specific management plans for the Australian isolate of I. multifiliis, facilitating the strategic timing of treatments.

Validation of a rapid counting method for assessing treatment efficacy against Lepidotrema bidyana infecting silver perch Bidyanus bidyanus

We used a published sub-sampling method for estimating the abundance of the monogenean Lepidotrema bidyana, a gill parasite of silver perch Bidyanus bidyanus, and determined that it also accurately predicts parasite abundance post-treatment. Post-treatment parasite abundance estimates based on the number of parasites on the first left posterior hemibranch were compared to actual counts on fish after bath and oral treatment trials with praziquantel and fenbendazole. Post-treatment parasite abundance estimates were significantly correlated to real counts of all individual hemibranchs, accurately predicting the parasite abundance on an individual host. There was no significant difference in the post-treatment parasite abundance between individual hemibranchs, however, indicating that the treatment affected L. bidyana abundance on each hemibranch unequally. Use of this method is ineffective at predicting post-treatment abundance; however, it accurately predicts the remaining parasite abundance, aiding evaluation of treatment efficacy, while reducing post-treatment sampling time or facilitating larger sample sizes.

Histological evaluation of sodium percarbonate exposure on the gills of rainbow trout.

Ichthyophthirius multifiliis is a recurring problem in Australian rainbow trout Oncorhynchus mykiss farms and requires strategically timed, repeat treatments for effective management. Sodium percarbonate (SPC) is permitted for use in Australia, with host safety margins based on the toxicity of acute exposures to hydrogen peroxide (HP), the active product released when SPC is added to water. The effects of exposure to HP released by SPC, of repeated doses and of doses exceeding 100 mg l-1 on rainbow trout are unknown. We exposed juvenile rainbow trout (mean weight: 30.5 ± 9 g) to repeated doses of 50, 150 and 250 mg l-1 SPC for 1 h on Days 1, 2, 7 and 8 of a treatment regime. The effect of SPC was assessed by histological evaluation of structural changes in gill tissue. Survival was 100% in all groups, but some fish exposed to 250 mg l-1 SPC displayed impaired swimming performance, and on Day 9 after the final treatment, oedema was present in 9.8% of lamella, which was significantly higher than the mean occurrence of 1.7, 4.2 and 1.3% in fish treated with 0, 50 and 150 mg l-1 SPC, respectively. These changes resolved within 24 h of the cessation of treatment. We conclude that SPC is safe to use on rainbow trout in doses of ≤150 mg l-1 at 17°C, however caution is advised at doses approaching 250 mg l-1. Water temperature, fish age, fish size and maturity, intensity of parasite infection and stocking density could alter the sensitivity of rainbow trout to SPC treatments.