Histopathological investigation of complex gill disease in sea farmed Atlantic salmon.

Various agents including Ca. Piscichlamydia salmonis, Ca. Branchiomonas cysticola, Desmozoon lepeophtherii, Paramoeba perurans and salmon gill poxvirus may be associated with complex gill disease in Atlantic salmon. Co-infections involving two or more of these agents are common and histopathological interpretation of lesions is therefore challenging. In this study, we developed a semi-quantitative scoring system for examination of histopathological gill lesions in sea-farmed Atlantic salmon suffering from gill disease. Following qPCR analysis of gills sampled for Ca. P. salmonis, Ca. B. cysticola, D. lepeophtherii and P. perurans from 22 geographically spread outbreaks, five cases representing different infectious loads and combinations of agents were chosen for histopathological scoring. Twenty-eight histological features were evaluated and potential associations between individual pathological changes and the occurrence of individual agents studied. The inter-observer agreement in interpretation of histological parameters between the three pathologists involved, was calculated to validate robustness of the scoring scheme. Seventeen histological parameters met the criteria for inter-observer agreement analysis and were included in the calculation. The three most frequent findings were identification of subepithelial leukocytes, epithelial cell hyperplasia and mucus cell hyperplasia. While few findings could be specifically related to particular agents, necrosis in hyperplastic lesions, pustules and necrosis of subepithelial cells appeared to be associated with the presence of Ca. B. cysticola. Further, lesion profiles clearly support the previously identified association between P. perurans and pathological changes associated with AGD. Very few pathological changes were observed in the single case in which Ca. P. salmonis was the dominating agent. Some lesions were only very rarely observed e.g. chloride cell necrosis, epithelial cell apoptosis, lamellar deposition of melanin and haemophagocytosis. The scoring scheme developed and applied was robust and sensitive. A less extensive scheme for routine diagnostic use is proposed.

Piscine orthoreovirus-3 is prevalent in wild seatrout (Salmo trutta L.) in Norway.

In 2017, a PCR-based survey for Piscine orthoreovirus-3 (PRV-3) was conducted in wild anadromous and non-anadromous salmonids in Norway. In seatrout (anadromous Salmo trutta L.), the virus was present in 16.6% of the fish and in 15 of 21 investigated rivers. Four of 221 (1.8%) Atlantic salmon (Salmo salar L.) from three of 15 rivers were also PCR-positive, with Ct-values indicating low amounts of viral RNA. All anadromous Arctic char (Salvelinus alpinus L.) were PCR-negative. Neither non-anadromous trout (brown trout) nor landlocked salmon were PRV-3 positive. Altogether, these findings suggest that in Norway PRV-3 is more prevalent in the marine environment. In contrast, PRV-3 is present in areas with intensive inland farming in continental Europe. PRV-3 genome sequences from Norwegian seatrout grouped together with sequences from rainbow trout (Oncorhynchus mykiss Walbaum) in Norway and Coho salmon (Oncorhynchus kisutch Walbaum) in Chile. At present, the origin of the virus remains unknown. Nevertheless, the study highlights the value of safeguarding native fish by upholding natural and artificial barriers that hinder introduction and spread, on a local or national scale, of alien fish species and their pathogens. Accordingly, further investigations of freshwater reservoirs and interactions with farmed salmonids are warranted.

Effects of cnidarian biofouling on salmon gill health and development of amoebic gill disease.

This study examines the potential implications of biofouling management on the development of an infectious disease in Norwegian farmed salmon. The hydroid Ectopleura larynx frequently colonises cage nets at high densities (thousands of colonies per m2) and is released into the water during regular in-situ net cleaning. Contact with the hydroids' nematocysts has the potential to cause irritation and pathological damage to salmon gills. Amoebic gill disease (AGD), caused by the amoeba Paramoeba perurans, is an increasingly international health challenge in Atlantic salmon farming. AGD often occurs concomitantly with other agents of gill disease. This study used laboratory challenge trials to: (1) characterise the gill pathology resulting from the exposure of salmon to hydroids, and (2) investigate if such exposure can predispose the fish to secondary infections-using P. perurans as an example. Salmon in tanks were exposed either to freshly 'shredded' hydroids resembling waste material from net cleaning, or to authentic concentrations of free-living P. perurans, or first to 'shredded' hydroids and then to P. perurans. Gill health (AGD gill scores, non-specific gill scores, lamellar thrombi, epithelial hyperplasia) was monitored over 5 weeks and compared to an untreated control group. Nematocysts of E. larynx contained in cleaning waste remained active following high-pressure cleaning, resulting in higher non-specific gill scores in salmon up to 1 day after exposure to hydroids. Higher average numbers of gill lamellar thrombi occurred in fish up to 7 days after exposure to hydroids. However, gill lesions caused by hydroids did not affect the infection rates of P. perurans or the disease progression of AGD. This study discusses the negative impacts hydroids and current net cleaning practices can have on gill health and welfare of farmed salmon, highlights existing knowledge gaps and reiterates the need for alternative approaches to net cleaning.

Piscine orthoreovirus can infect and shed through the intestine in experimentally challenged Atlantic salmon (Salmo salar L.).

Piscine orthoreovirus (PRV) is a ubiquitous virus in Norwegian salmon farms associated with the disease heart and skeletal muscle inflammation (HSMI). Experimental challenge has shown that the virus replicates in circulating red blood cells of Atlantic salmon prior to infecting heart myocytes. The infection route from water to blood is however still unknown. The related mammalian orthoreovirus primarily infects the lungs and gastrointestinal (GI) tract and is proposed to spread mainly through the faecal-oral route. To investigate the role of the salmonid GI tract in PRV-infection, oral and anal administration of virus was compared to intraperitoneal (i.p.) injection. When administered anally, PRV was transferred to blood 4 days post challenge (dpc) and levels peaked at 42 dpc, similar to i.p. injected fish. PRV was detected in heart and faeces with corresponding kinetics, and inflammatory heart lesions consistent with HSMI were observed from 49 dpc. The orally intubated group showed slower virus kinetics in both blood and heart, and no signs of HSMI. Compared to the oral and i.p. administration routes, leakage of virus inoculate by anal intubation was minor and challenge was restricted to the mid- and distal intestine. These findings show that anal intubation is an efficacious method for PRV delivery to the GI tract and demonstrates that PRV can establish infection through the intestine with the potential for transmission via faeces.

Gizzard erosion and ulceration syndrome in chickens and turkeys: a review of causal or predisposing factors.

Gizzard erosion and ulceration syndrome (GEU) was described for the first time in the 1930s. The main focus of early studies was on nutritional deficiencies and peroxidation of highly polyunsaturated fatty acids as causative factors. During the 1970s and 1980s the focus was moved towards toxic substances in the feed. Scott's review in 1985 concluded that overproduction of gastric acid induced by gizzerosine was a major cause of GEU. During the last decades, serotype 1 of fowl adenovirus A and Clostridium perfringens have been implicated as important pathogenic agents in the development of GEU in chickens. Although GEU is globally distributed and its subclinical form appears to be common in commercial poultry flocks, the condition is rarely mentioned in standard textbooks on poultry health. This regrettable fact is probably due in part to the lack of one definitive cause of the syndrome.

A survey of the economic impact of subclinical Eimeria infections in broiler chickens in Norway.

The objective of this work was to examine the impact of subclinical coccidial infection on commercial performance, expressed as a modified European Production Index, in broilers. Performance data, and litter and faecal samples, were collected from two independent observational surveys of Norwegian broilers receiving in-feed narasin during 2000 to 2004. Numbers of oocysts per gram (OPG) of litter collected during rearing (Study 1) or faecal samples collected at slaughter (both studies), and relative frequencies of Eimeria species categories (both studies) were calculated. Polymerase chain reaction-based identification of Eimeria species was performed in Study 2. A definition of flocks at risk of impaired performance associated with coccidia ("risk flock"), using the predominant species and OPG level as criteria, was tested. Coccidia had a significant effect on performance in the first, but not the second study. In Study 1 the following coccidia variables were found to be associated with impaired performance in multivariate models: OPG at slaughter (ordinal), mean OPG during rearing (ordinal) and "risk flock" (binomial). The European Production Index was approximately 9% lower in flocks with infection levels >50 000 OPG at slaughter in Study 1. The composition of coccidial populations shifted between Study 1 and Study 2, from a dominance of medium and large oocysts to a dominance of small oocysts. There was a substantial increase in prevalence of coccidial infection from Study 1 to Study 2, but mean infection levels were similar in the two surveys. The "risk flock" definition was useful as an indicator of coccidia-associated performance loss in Study 1, where subclinical coccidiosis was an important factor. The results suggest that the economic importance of subclinical coccidiosis may vary substantially with time, and they emphasize the need for population studies on the importance and dynamics of specific coccidial infections under different field conditions.

Coccidial infections in commercial broilers: epidemiological aspects and comparison of Eimeria species identification by morphometric and polymerase chain reaction techniques.

The objective of this study was to add to existing knowledge of the epidemiology and the aetiology of coccidial infections in commercial broiler flocks. Polymerase chain reaction (PCR) and morphometric identification of the Eimeria species were compared as means of differentiation in the field samples of faeces and litter. For morphometry, the Eimeria species were categorized into three groups based on lengths of the oocysts. Two random samples of commercial broilers were studied, one during 2000/01 and the other during 2003/04. The prophylactic regime (in-feed narasin), husbandry and methods applied were broadly the same for both subpopulations. Coccidial infection prevalence increased from approximately 45% to approximately 75% during this period, but infection levels (oocysts per gram of faeces) did not significantly change. There were substantial geographical differences in both prevalence and infection levels. A change in Eimeria species profile occurred during the study period. Five Eimeria species were identified at slaughter, by PCR targeting the ITS-1 region of the genome; Eimeria acervulina (100%), Eimeria tenella (77%), Eimeria maxima (25%), Eimeria praecox (10%) and Eimeria necatrix (2%). PCR and morphometric tentative identification were in complete agreement in only 49% of the cases.