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Cyprinid herpesvirus 3 (CyHV-3) is a highly contagious virus that infects the common carp and koi. Initially, the virus caused mortality in koi which was observed by ornamental carp breeders and hobbyists in various European countries, Israel and the USA. In a short time, CyHV-3 became the causative agent of koi herpesvirus disease (KHVD) leading to the mass mortality of common carp in aquaculture and wild fish populations. The skin and digestive tract have been proved to be the primary portal of entry for the virus. Because of its latency ability, the virus can persist for many years in carp populations. CyHV-3 can infect carp at all developmental stages and can be responsible for up to 100% of the mortality in fish stocks, therefore, the virus is a global economic issue. Several solutions have been proposed to solve this issue. These include silencing with siRNA and the use of antiviral drugs which were primarily examined via in vitro experiments. The in vivo studies encompassed numerous methods of immunisation, immunostimulation, broodstock selection and testing of antiviral drugs. All the above-mentioned methods are briefly described in this review. Moreover, the major findings, efficiency and potential applicability are delineated. Further prospective studies may focus on the development of safe, easily applicable vaccines, natural medications as well as advancement in selecting the broodstock.

The common carp ( Cyprinus carpio L.) has a long-standing history in Polish pond aquaculture, which has given rise to a great number of local breeds, strains and hatchery populations. They are well adapted to their local conditions, play an important role for fisherman's livelihood s and embody valuable genetics for future breeding programs. Proper maintenance of animal genetic resources (AnGR) requires an appropriate strategy based upon the precise knowledge of their genetic structure. This study genetically characterizes 20 common carp strains breed in Poland alongside with wild carp from Onuma Lake in Japan. We study three different types of genetic markers including the 963 AFLP markers, a set of 11 microsatellite loci (STR) and DNA sequences of the mitochondrial control region (D-loop). Bayesian clustering analysis of microsatellite loci divided the studied strains into 17 distinct clusters. Using population assignment test 92.4% of carp individuals were correctly assigned to their strain of origin. Microsatellite and AFLP marker analysis have shown the moderate but significant level of genetic variation between strains (AMOVA 23% and 18.2% respectively). AFLP and STR analysis of genetic distances gave equivalent values and were positively correlated with high values of the Mantel test ( r = 0.553, n = 20, p = 0.001). Only two mtDNA haplotypes (H5 and H2) were detected in all studied carp strains with H2 being the most prevalent. This is the first comprehensive genetic research of common carp strains breed in Poland which provided crucial data for the national conservation strategy. This study means that this resource can now be placed within the context of similar studies worldwide allowing the further improvement and breeding work.

A controlled fish propagation is nowadays the fundamental prerequisite for well-managed aquaculture production. However, despite practices aimed at creating equal conditions during the spawning, the share of males giving offspring is highly diversified. In traditional pond aquaculture, the fish are exposed to various environmental factors that may affect their fertility through immune activation, among which parasites are one of the most influential. It is highly feasible that in conditions of sperm overload and short sperm-egg distance, the differences in fertilization success between males are related to sperm characteristics determined by male fitness. The present study aimed to investigate if mounting an immune response through parasitic infection affects common carp (Cyrpinus carpio L) sperm quality and reproductive success. Mature carp males were experimentally infected with a dose of 1 × 109 blood parasites Trypanoplasma borreli by intraperitoneal inoculation. Control fish were PBS-injected. Parasite number was determined at weekly intervals using a Burker counting chamber. The level of parasitaemia developed within four weeks post-infection served as the criterion to classify each individual either as parasite susceptible or resistant. On day 24 post-infection, carp males were engaged in controlled spawning. Equal volumes of quantified semen, obtained by gentle abdominal massage, were used to fertilize the egg portions obtained from four females of different genetic origin. The calculated hatchability rate was used as the indicator of the males' reproductive success. The parasite-infected males demonstrated average hatchability rates of 14.18 ± 4.83%, while the control males demonstrated 11.29 ± 3.93% rates. Parasites induced a robust adaptive antibody response, but individual antibody levels were not associated with hatchability rate. A significantly higher number of spermatozoids with damaged DNA was detected in parasite-susceptible males than in the resistant ones. The 11-ketotestosterone and most CASA parameters did not differ between parasite-infected and control fish except beat cross frequency (BCF), which was higher in infected fish (57.45 ± 18 Hz in infected vs 43.64 ± 9 Hz in controls). This CASA parameter, alongside the percentage of motile spermatozoa (MOT), was positively associated with the hatchability rate. Our study shows that in artificial spawning, a mild level of parasitaemia does not negatively influence common carp males' reproductive success. The only sperm characteristic altered by immune activation is BCF, which was correlated with hatchability in infected fish. However, the mechanism by which the activated immune system can alter BCF values and how this sperm parameter affects hatchability remains unknown and requires further investigation.