Aquatic species susceptible to diseases listed in Directive 2006/88/EC

Species which may act as vectors or reservoirs of diseases covered by the Animal Health Law: Listed pathogens of fish

Vector or reservoir species of five fish diseases listed in the Animal Health Law were identified, based on evidence generated through an extensive literature review (ELR), to support a possible updating of Regulation (EU) 2018/1882. Fish species on or in which highly polymorphic region-deleted infectious salmon anaemia virus (HPR∆ ISAV), Koi herpes virus (KHV), epizootic haematopoietic necrosis virus (EHNV), infectious haematopoietic necrosis virus (IHNV) or viral haemorrhagic septicaemia virus (VHSV) were detected, in the field or during experiments, were classified as reservoir species with different levels of certainty depending on the diagnostic tests used. Where experimental evidence indicated transmission of the pathogen from a studied species to another known susceptible species, the studied species was classified as a vector species. Although the quantification of the risk of spread of the pathogens by the vectors or reservoir species was not part of the terms or reference, such risks do exist for the vector species, since transmission from infected vector species to susceptible species was proven. Where evidence for transmission from infected fish was not found, these were defined as reservoirs. Nonetheless, the risk of the spread of the pathogens from infected reservoir species cannot be excluded. Evidence identifying conditions that may prevent transmission by vectors or reservoir fish species during transport was collected from scientific literature. For VHSV, IHNV or HPR∆ ISAV, it was concluded that under transport conditions at temperatures below 25°C, it is likely (66-90%) they will remain infective. Therefore, vector or reservoir species that may have been exposed to these pathogens in an affected area in the wild, aquaculture establishments or through water supply can possibly transmit VHSV, IHNV or HPR∆ ISAV into a non-affected area when transported at a temperature below 25°C. The conclusion was the same for EHN and KHV; however, they are likely to remain infective under all transport temperatures.

Susceptibility of pike Esox lucius to VHSV and IHNV and potential transmission to rainbow trout Oncorhynchus mykiss

Determining the origin of recurrent outbreaks of fish diseases occurring on fish farms is essential for disease prevention and control measures. In this study, we investigated the potential reservoir role of wild fish species living near salmonid farms which were regularly found to be positive for viral hemorrhagic septicemia virus (VHSV). In addition to VHSV, infectious hematopoietic necrosis virus (IHNV) was also isolated from several pike Esox lucius samples collected from a pond near the salmonid farms of interest. All isolates of VHSV and IHNV analyzed had 100% identical partial glycoprotein gene sequences. VHSV pike strain OO128-25 belonged to the Ia genotype and shared 99.1 to 99.5% nucleotide identity with strains recently isolated from the farms. IHNV pike strain OO121-8, European genotype, appeared to be different from strains from France characterized since the first isolation in 1987. Isolates representative of both viral species were highly virulent in rainbow trout Oncorhynchus mykiss. OO128-25 induced 65% mortality in pike fingerlings, whereas only weak mortality was observed with OO121-8, despite characteristic symptoms in infected fish. High levels of specific antibodies to VHSV and IHNV were detected in adult pike in the absence of clinical signs. Infection of rainbow trout in contact with experimentally VHSV- or IHNV-infected pike fingerlings indicates possible horizontal transmission. These results suggest that pike could act as a reservoir for VHSV and IHNV in the wild, providing additional evidence to explain viral persistence and resurgence in certain areas.

Potential Role of Different Fish Species as Vectors of Koi Herpesvirus (CyHV-3) Infection

Introduction

Koi herpesvirus (KHV) has infected farmed common carp in Poland clinically and asymptomatically since 2004. The role of non-carp species as vectors of virus transmission is well known except for in the case of KHV. The aim was to better understand this virus' infection and transmission pathways in common carp, looking at the potential vector role of fishes kept with them.

Material and Methods

Eight species were experimentally infected with KHV by immersion in a suspension at 20°C ±1 and transferred to a tank after 45 minutes. Specimens were euthanised at intervals up to 56 days post infection (dpi) and tissue was examined for KHV DNA. Surviving infected fishes were introduced at intervals, each time into a separate tank, to naïve common carp for experimental infection. These were observed daily for symptoms, sacrificed along with controls after three months, and dissected to provide tissue samples. Also fish from 14 species collected from a farm with a history of KHV were sampled from 3 to 22 months after disease was confirmed. Organ sections from single fish were collected in a single tube.

Results

Viral DNA was detected in tench and roach samples up to 49 dpi, but in three-spined stickleback and stone maroko samples only up to 14 dpi. Transmission of KHV to naïve carp occurred after cohabitation. KHV DNA was detected in three fish species three months after the farm outbreak.

Conclusion

We confirmed that grass and Prussian carp, tench, roach, and brown bullhead can transfer the virus to naïve common carp.