Gyrodactylus salaris, an important pathogen of Atlantic salmon Salmo salar, has been shown to be highly sensitive to factors in host serum and mucus, being killed rapidly (50% within 1 h) by serum at a dilution of 1:200. The time needed for killing was inversely proportional to serum concentration. Similar effects were noted using host mucus, which contained approximately 1/20th of the anti-Gyrodactylus activity of serum. Serum activity was abolished completely by heating at 45 degrees C for 30 min, and by addition of EDTA, but not by EGTA + 1 mM magnesium ions. Activity was not dependent on whether the serum was from infected or naive fishes, nor was it species specific. Attempts to pre-coat parasites in salmon anti-Gyrodactylus antibodies also failed to enhance the activity of fresh serum. These observations suggest that killing is due to the complement system of the host, acting via the alternate pathway. G. salaris appears to be exceptionally sensitive to complement, being killed at concentrations which could be experienced in vivo. The role of complement in the protection of fishes against gyrodactylid infection therefore deserves further investigation.

Salmo salar and Salmo trutta co-exist in coastal river systems in Europe and produce hybrids with little loss of viability or growth. This report describes the susceptibility of pure full-sibs of S. salar and S. trutta and their reciprocal half-sib hybrids to their respective gyrodactylids, Gyrodactylus salaris and Gyrodactylus derjavini. The pure-bred salmon and trout, and half-sib hybrids, were produced using eggs and sperm from wild anadromous S. salar (River Alta stock, North Norway) and wild anadromous S. trutta (River Fossbekk stock, Southwest Norway). Infections were initiated by exposing experimental fishes (0+) to S. salar naturally infected with G. salaris (River Lierelva strain) or S. trutta naturally infected with G. derjavini (River Sandvikselva strain). Fishes were then kept individually isolated under standardized conditions at 12 degrees C. Pure-bred S. salar were susceptible but frequently mounted a response to G. salaris without eliminating the infection, whereas pure-bred S. trutta were innately resistant to this species. Pure-bred S. trutta ranged from innately resistant to susceptible to G. derjavini but later most of the susceptible trout mounted a host response to G. derjavini. Pure-bred S. salar were also susceptible to this species, although parasite population growth rates were reduced and a host response frequently appeared eliminating G. derjavini. The abundance of both gyrodactylids was lower on the hybrids than on their respective pure-bred natural hosts, and a parental sire- and dam-influence on the resistance of hybrids was observed. When the sire was S. salar, the susceptibility of hybrids to G. salaris was similar to that of pure S. trutta; when the dam was S. salar both innately resistant, intermediately susceptible and responding individuals were present. In the case of G. derjavini, when the sire was S. trutta, infections on hybrids were similar to those on pure S. salar; when the dam was S. trutta, an increased level of susceptibility was observed. The present results provide evidence that: (1) Norwegian salmon stocks are variable in their susceptibility/resistance, with some fish able to control S. salaris infections; (2) trout stocks are innately resistant to G. salaris; (3) individual trout show a spectrum in susceptibility/resistance to G. derjavini, ranging from innate resistance through slightly susceptible to highly susceptible but with acquired resistance controlling infection; (4) although G. derjavini infections grow poorly on salmon, this host stock is susceptible to the parasite, but can limit infection by a host reaction; (5) susceptibility/resistance traits to gyrodactylids are genetically controlled and resistance can be transferred as a dominant trait through interspecific crosses between different salmonids; (6) interspecific hybrids between susceptible and resistant salmonids have a pattern of susceptibility to gyrodactylids intermediate to that of the parents; (7) resistance to gyrodactylids may be controlled by relatively few genes in salmonids; (8) epidem...

SUMMARYHead lice prevalence varies greatly between and within countries, and more knowledge is needed to approach causes of this variation. In the present study, we investigated head lice prevalence among elementary school students and their households in relation to individual and household characteristics as well as spatial variables. The investigation included households from 5 geographically separated municipalities. Present infestations among household members as well as previous infestations in the household were reported in a questionnaire. In elementary school students prevalence was low (1·63%), but more than one-third of the households (36·43%) had previously experienced pediculosis. Prevalence was higher in elementary school students than in other household members, and highest in third-grade children. Prevalence was also influenced by the school attended, which suggested that interactions between children in the same school are important for head lice transmission. Previous occurrence of head lice in homes also increased the risk of present infestation. Prevalence of previous infestations was higher in households with more children and in more densely populated municipalities, indicating that the density of hosts or groups of hosts influences transmission rates. These results demonstrate that information of hosts’ spatial distribution as well as household and individual characteristics is needed to better understand head lice population dynamics.

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The salinity tolerance of the freshwater monogenean Gyrodactylus salaris, infecting Atlantic salmon (Salmo salar) parr, was studied experimentally. Following direct transfer of infected fish from fresh water to 5.0omicron salinity, parasite population growth increased at the same rate as in fresh water and was positively correlated with temperature (1.4, 6.0, and 12.0°C). In 7.5omicron salinity the populations declined and became extinct after a maximum of 56 days, without any significant difference between 6.0 and 12.0°C. However, some infrapopulations demonstrated short periods of growth. At higher salinities (10.0, 15.0, 20.0, and 33.0omicron) the survival time decreased, and there was a negative correlation between survival time and temperature (1.4, 6.0, and 12.0°C). When transferred directly to sea water (33.0omicron) the parasites became opaque and ceased moving after a few minutes. There was no difference in parasite survival time between direct and gradual transfer from fresh water to 7.5 and 10.0omicron, except for one infrapopulation which demonstrated population growth from day 22 after some fluctuations following gradual transfer to 7.5omicron. The present findings support the hypothesis of brackish water dispersal of G. salaris with infected fish migrating between rivers in fjord systems.

Gyrodactylus salaris infects numerous salmonid species, ranging from the fully susceptible (Norwegian strains of Salmo salar), through species which, though initially susceptible, eventually eliminate their infections (Salvelinus alpinus and S. fontinalis) to entirely resistant (Salmo trutta) species. Here we describe experiments in which Salvelinus alpinus, S. fontinalis and Salmo trutta, implanted with hydrocortisone acetate to simulate stress-induced immunosuppression, were challenged with G. salaris. With previously uninfected Salvelinus fontinalis, G. salaris infections on fish treated with hydrocortisone acetate grew larger, and for longer, than on sham-treated controls. A similar result was obtained with S. trutta. Patterns of infection on Arctic charr, Salvelinus alpinus, were more complex, because individual fish varied from susceptible to highly resistant. Fish were therefore initially infected with G. salaris, and the most highly resistant group of individuals identified and disinfected. After 6 months recovery from this primary infection, hydrocortisone acetate was administered to half the fish, and all were challenged with G. salaris. Parasite populations on the hydrocortisone-treated individuals were consistently larger than those on the sham-treated controls, exceeding 30 parasites per fish after 5 weeks, in comparison with less than 10 parasites per fish on controls. These results indicate that hydrocortisone administration can lead to enhanced gyrodactylid populations on a range of salmonids. This suggests that the response to G. salaris is mediated by the immune system, and that the spectrum of responses observed in different species are, at least in part, due to the same mechanism. At a practical level, stress-induced immunosuppression during handling and transport of cultured salmonids may prove an important factor in the dissemination of G. salaris between watersheds.

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The attraction of the blowfly Calliphora vicina Robineau-Desvoidy, 1830 (Diptera: Calliphoridae) to single synthetic compounds, blends and authentic odours was investigated in a wind tunnel. A total of 1850 C. vicina (1750 females and 100 males) were tested. A comparison of male and female responses showed significant differences in attraction between the sexes. Females were more attracted than males to liver odour. The attraction of females lay in the ranges of 0-22% for single compounds, 26-64% for synthetic blends and 58-88% for authentic odours. Dimethyl trisulphide was the most attractive single compound. Significant improvement in attraction was achieved with blends and a three-component lure, consisting of dimethyl trisulphide, mercaptoethanol and o-cresol, was found to be the best solution for field trapping of C. vicina. Authentic odours from dead fish and mice were significantly more attractive than liver and the three-component blend, and the blend and liver were similarly effective as attractants. Field tests support the results of the wind tunnel study and a high number of C. vicina were caught in funnel traps. Overall, 99.1% of the specimens caught were females.