Demonstrating freedom from Gyrodactylus salaris (Monogenea: Gyrodactylidae) in farmed rainbow trout Oncorhynchus mykiss

Abstract

This paper describes an approach to demonstrate freedom of individual rainbow trout farms from Gyrodactylus salaris Malmberg, 1957. The infection status of individual farms is relevant should G. salaris be introduced into a country or zone previously known to be free of the parasite. Trade from farms where G. salaris may have been introduced would be restricted until freedom had been demonstrated. Cage, fish and parasite sample sizes were calculated based on the minimum detectable prevalence (P*), test characteristics, population size, and Type I and II errors. Between 5 and 23 cages per farm would need to be sampled to demonstrate freedom at a cage level P* of 10%. The number of fish sampled per cage depended mainly on the test sensitivity (probability of correctly identifying an infected fish). Assuming a test sensitivity of 99% at the fish level, 59 fish per cage are needed (P* = 5%). Since G. salaris may exist in mixed infection with G. derjavini, testing a sample of gyrodactylid parasites may not result in the parasite being detected when present. Test sensitivity at the fish level depends on the number of gyrodactylids on the fish, the proportion of which are G. salaris and the number examined. Assuming a P* of 5% (i.e. G. salaris are at least 5% of the gyrodactylid population), between 20 and 73 parasites per fish would need to be sampled (depending on abundance) to maintain the Type I error at 0.01 (thus a fish level test sensitivity of 99%). This work identifies the critical information, and further research, needed to assess freedom from G. salaris with a known level of confidence; this is essential to provide a sound scientific basis for decision-making about disease control measures. KEY WORDS: Gyrodactylus salaris · Rainbow trout · Surveys · Disease freedom · Contingency planning Resale or republication not permitted without written consent of the publisherDis Aquat Org 79: [47][48][49][50][51][52][53][54][55][56] 2008 1990, Bakke et al. 1990b, 1992a,b, Jansen & Bakke 1995, Soleng & Bakke 2001 and indefinitely in rainbow trout Oncorhynchus mykiss (RBT) populations (albeit at a low prevalence, low abundance and causing no signs of infection) . G. salaris has been found on farmed RBT in Denmark, Sweden and Finland (see Bakke et al. 2007 for details). A RBTadapted strain of G. salaris which is not pathogenic for salmon and can achieve infection rates of several hundred parasites per fish in RBT has been found in Denmark (Jørgensen et al. 2007). Recently, a strain of G. salaris has been found on Arctic charr, which has not affected Atlantic salmon in the same river. Under experimental conditions, salmon are found to be either resistant or only slightly susceptible (Olstad et al. 2007); this is the first published report of a variation in the pathogenicity of Norwegian populations of G. salaris.A limited amount of experimental work has demonstrated that Atlantic salmon in the British Isles are susceptible to Gyrodactylus salaris (Bakke & Mackenzie 1993). Other populations in Europe are also...

Reservoir hosts for Gyrodactylus salaris may play a more significant role in epidemics than previously thought

“…[ 11 ], where the examination of formalin-preserved material in farm archives indicated that G. salaris had been in Italy on O. mykiss for at least nine years prior to discovery. Such asymptomatic hosts may represent a serious problem in that they can serve as reservoirs playing an important role in the epidemiology and dispersal of G. salaris across Europe [ 26 - 28 ].…”

Development of a non‐lethal hydrogen peroxide treatment for surveillance of Gyrodactylus salaris on trout farms and its application to testing wild salmon populations

“…Although the majority of these are highly virulent against Atlantic salmon, some strains that exhibit limited reproduction and are non‐pathogenic to Atlantic salmon have been isolated from other host species, including rainbow trout (Lindenstrøm et al, ) and Arctic charr (Olstad et al, ), which are more susceptible to infection. Establishing the infection status and, more specifically, demonstrating freedom from a strain of G. salaris pathogenic to Atlantic salmon in rainbow and brown trout, with low levels of prevalence and abundance, presents serious challenges (Peeler & Oidtmann, ). This study aimed to identify a compound that could be used in a non‐lethal procedure to remove gyrodactylid parasites from fish hosts and develop a robust method to rapidly recover detached parasites that can be used to screen the large numbers of fish that would be required for testing non‐primary hosts.…”

“…Peeler and Oidtmann () detail a 3‐stage approach to structuring a survey on a rainbow trout farm which determines required sample sizes for rearing units (tanks or cages), fish within units and then parasites on individual fish to demonstrate freedom from G. salaris which may be present in mixed infections with other, non‐pathogenic, gyrodactylid species. When using the chemical approach, we simplify the survey structure by effectively sampling the tank‐level parasite population directly (assuming fish are selected at random for treatment).…”

“…A study conducted by Cefas demonstrated the difficulty in isolating non‐pathogenic gyrodactylid species in a farm survey (no gyrodactylids were recovered from 16 of 27 trout farms sampled) and highlighted the unacceptable time effort required to remove parasites from fish for identification (Anon, ). Current estimates of sample sizes required to demonstrate the absence of pathogenic parasites from a rainbow trout farm with an acceptable level of confidence (95%) stand at 295 fish (for whole‐body examination) or 595 fish (using fin samples; Peeler & Oidtmann, ), which clearly has very high ethical and resource implications should it be necessary to investigate more than a small number of sites (all fish would currently need to be killed for testing).…”

Checklists of Gyrodactylus Species (Monogenea) from Fishes of Iraq

“…Secondary infections of the epidermal lesions with bacteria or fungi may also play a significant role in the pathogenicity of Gyrodactylus (58). The pathogenicity of G. salaris on some strains of Atlantic salmon Salmo salar became clear in the early 1970s, when this parasite was introduced into Norway with juvenile Atlantic salmon imported from Sweden for aquaculture (94).…”