The impact of co-infections on fish: a review

International audienceAbstractCo-infections are very common in nature and occur when hosts are infected by two or more different pathogens either by simultaneous or secondary infections so that two or more infectious agents are active together in the same host. Co-infections have a fundamental effect and can alter the course and the severity of different fish diseases. However, co-infection effect has still received limited scrutiny in aquatic animals like fish and available data on this subject is still scarce. The susceptibility of fish to different pathogens could be changed during mixed infections causing the appearance of sudden fish outbreaks. In this review, we focus on the synergistic and antagonistic interactions occurring during co-infections by homologous or heterologous pathogens. We present a concise summary about the present knowledge regarding co-infections in fish. More research is needed to better understand the immune response of fish during mixed infections as these could have an important impact on the development of new strategies for disease control programs and vaccination in fish

Disease resistance in Atlantic salmon (Salmo salar) Coinfection of the intracellular bacterial pathogen Piscirickettsia salmonis and the sea louse Caligus rogercresseyi

Background Naturally occurring coinfections of pathogens have been reported in salmonids, but their consequences on disease resistance are unclear. We hypothesized that 1) coinfection of Caligus rogercresseyi reduces the resistance of Atlantic salmon to Piscirickettsia salmonis; and 2) coinfection resistance is a heritable trait that does not correlate with resistance to a single infection. Methodology In total, 1,634 pedigreed Atlantic salmon were exposed to a single infection (SI) of P. salmonis (primary pathogen) or coinfection with C. rogercresseyi (secondary pathogen). Low and high level of coinfection were evaluated (LC = 44 copepodites per fish; HC = 88 copepodites per fish). Survival and quantitative genetic analyses were performed to determine the resistance to the single infection and coinfections. Main Findings C. rogercresseyi significantly increased the mortality in fish infected with P. salmonis (SI mortality = 251/545; LC mortality = 544/544 and HC mortality = 545/545). Heritability estimates for resistance to P. salmonis were similar and of medium magnitude in all treatments (h2 SI = 0.2360.07; h2 LC = 0.1760.08; h2 HC = 0.2460.07). A large and significant genetic correlation with regard to resistance was observed between coinfection treatments (rg LC-HC = 0.9960.01) but not between the single and coinfection treatments (rg SI-LC =20.1460.33; rg SI-HC = 0.3260.34). Conclusions/Significance C. rogercresseyi, as a secondary pathogen, reduces the resistance of Atlantic salmon to the pathogen P. salmonis. Resistance to coinfection of Piscirickettsia salmonis and Caligus rogercresseyi in Atlantic salmon is a heritable trait. The absence of a genetic correlation between resistance to a single infection and resistance to coinfection indicates that different genes control these processes. Coinfection of different pathogens and resistance to coinfection needs to be considered in future research on salmon farming, selective breeding and conservation. © 2014 Lhorente et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited

Quantitative genetic basis for resistance to Caligus rogercresseyi sea lice in a breeding population of Atlantic salmon (Salmo salar)

A total of 1511 Atlantic salmon smolts representing 75 full-sib and 40 half-sib families from the Antares S.A. breeding program were challenged with Caligus rogercresseyi in order to assess the quantitative genetic components of resistance to infection by this parasite. After three weeks of acclimatization in the experimental hatchery conditions, fish were distributed in three 6-m 3 tanks, with an equal number of fish per family per tank. An infection rate of 100 copepods per fish was used for the experiment. Resistance/susceptibility was recorded individually at approximately 5days (range=4-7days) after infestation as the number of sessile lice per fish on all fins (FSL), the estimated total number of sessile lice per fish (TSL), and the total number of sessile lice per fish per unit of body weight (TSL/BW s). Resistance/susceptibility was also recorded at approximately 25days (range=24-26days) after infestation as the total number of mobile lice per fish (TML) and the total number of mobile lice per fish per unit of body weight (TML/BW m). The level of infestation on days 5 and 25 post-infestation was 30.7 (SD=16.3) sessile parasites (TSL) and 13.2 (SD=6.0) mobile parasites (TML), respectively. A high level of phenotypic variation was observed for parasite load traits when considering fin and total counts as well as counts per unit of body weight (CV=46-56%). Significant differences between tanks (P<0.05) were observed in FSL and TSL. Weight was included as a covariate (P<0.05) when performing the genetic analysis on FSL, TSL and TML. Estimated heritabilities for parasite counts in the sessile stage were of low to medium magnitude (0.22-0.34), whereas in the mobile stage heritabilities were very low (0.03-0.06) and not significantly different from zero (P>0.05). The genetic correlations between parasite counts in the sessile (FSL, TSL) and the mobile (TML) stages were very high (0.99). Also, body weight shows a high genetic correlation with fish parasite count measured at both the sessile (0.61-0.65) and the mobile stages (0.95). These results show that there is enough additive genetic variation for selection to be applied for improving resistance to sea lice. Measurement of genetic resistance in the sessile stage is a better option than measurement in the mobile stage as a selection criterion in breeding programs of Atlantic salmon aimed at improving resistance to C. rogercresseyi. © 2011 Elsevier B.V