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

Occurrence of herpesvirus in fish

Introduction

Herpesviruses are common agents in animals of the aquatic environment. They infect many species of fish but only lead to disease in one or two species. Nevertheless, infected fish without clinical symptoms can actively transfer infectious agents to disease-susceptible species. The aim of the study was to identify and prove the natural presence of different herpesviruses.

Material and Methods

Koi, Nile tilapia, grass carp, goldfish and crucian carp were infected with a herpesvirus isolate 99% identical to goldfish herpesvirus (GHV) or cyprinid herpesvirus 2 (CyHV-2) obtained from crucian carp. Before and after infection, samples were collected non-lethally at different time points from all five fish species to identify and evaluate the replication of viruses naturally infecting the fish as well as the CyHV-2 experimentally infecting them. Gill swabs and separated leukocytes were subjected to PCR and the results compared.

Results

These samples yielded DNA of koi herpesvirus (KHV, also referred to as CyHV-3), GHV and a new herpesvirus. While Asian-lineage CyHV-3 DNA was detected in samples from crucian carp and goldfish, CyHV-2 DNA was found in samples from koi and tilapia. A new, hitherto unknown herpesvirus was identified in samples from grass carp, and was confirmed by nested PCR and sequence analysis. The survival rates were 5% for grass carp, 30% for tilapia, 55% for crucian carp, 70% for koi and 100% for goldfish at 20 days post infection. Evolutionary analyses were conducted and five clusters were visible: CyHV-1 (carp pox virus), CyHV-2 with sequences from koi and tilapia, CyHV-3 with sequences from crucian carp and goldfish, probable CyHV-4 from sichel and a newly discovered herpesvirus - CyHV-5 - from grass carp.

Conclusion

The results obtained with the molecular tools as well as from the animal experiment demonstrated the pluripotency of aquatic herpesviruses to infect different fish species with and without visible clinical signs or mortality.

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.

Serological responses to koi herpesvirus (KHV) in a non-cyprinid reservoir host

Koi herpesvirus (KHV) is a highly contagious virus that causes KHV disease (KHVD) inducing high mortality in carp and koi (Cyprinus carpio L.). In the late stage, latency occurs with very low, often non-detectable virus concentrations, which represents a challenge for virus detection. After validation according to OIE recommendations, an antibody ELISA was established to recognize antibodies of C. carpio against KHV infection. In this study, the ELISA was modified to detect anti-KHV antibodies from a non-cyprinid fish. Experimentally infected rainbow trout (Oncorhynchus mykiss) were able to transmit KHV to naïve carp at two different temperatures, demonstrating their potential as a reservoir host. At 20°C, KHVD was induced in carp but not at 15°C. Unexpectedly, rainbow trout developed humoral response against KHV at both temperatures. In contrast to carp, at 15°C trout produced neutralizing antibodies but not at 20°C. While antibodies obtained from infected carp sera reacted in a similar way against all KHV, antibodies from rainbow trout sera reacted differently to the same isolates by ELISA. The data show that even when non-cyprinid fish species are infected with KHV, they can produce antibodies that differ from those observed in carp.

Koi herpesvirus disease outbreak: Input for the implementation of a surveillance program in South Tyrol - Italy

A severe episode of high and abnormal mortality was observed in the population of Cyprinus carpio of Lake Caldaro (South Tyrol, Italy) in summer 2016. The diagnostic investigation carried out led to the identification of Koi Herpesvirus (KHV) as the etiologic agent. Following this disease outbreak and its socio-economic consequences, the local authorities, in agreement with the local Fishing Association, decided to implement a surveillance program for the achievement of KHV-free health status (Category I) in the Province, in accordance to the Implementing Decision (EU) 2015/1554. The selected area was a defined geographical compartment (the Monticolo lakes compartment, South Tyrol, Italy), which is located near Lake Caldaro, where the Koi Herpesvirus disease (KHVD) outbreak had occurred. This area is of particular interest because it supplies other water bodies with juvenile C. carpio individuals; with the achievement of a KHV-free health status, South Tyrol could possibly become independent in the breeding of this fish species. Suitable samples were collected and processed during a two-year period in order to detect the presence/absence of KHV. The same samples were tested for other viruses that can affect carp, namely spring viraemia of carp (SVCV) and carp edema virus (CEV). According to the results, the authors conclude that the Monticolo lakes area should be classified as KHV-free, as no sample has tested positively for the presence of this specific virus (KHV).

Cyprinid herpesvirus 3 (CyHV-3) transmission and outbreaks in Iran: Detection and characterization in farmed common carp

This study shows the presence of Cyprinid Herpesvirus-3 (CyHV-3) in common carp (Cyprinus carpio) from Iranian carp farms with cumulative mortality up to 80% during 2015-2016. Pathological signs of disease such as gill necrosis, sunken eyes, and increased slime secretion on the skin and fins were observed in affected fish. The extensive fusion of secondary lamellae with necrotic cells, margination of chromatin, and formation of intranuclear inclusion bodies in gill tissues were also observed by histopathological examination. Most tubular epithelial cells and some hematopoietic cells showed intranuclear inclusion bodies in the kidney. The Iranian CyHV-3 isolates showed identity with Asian strains, and displayed the I⁺⁺ II⁺ allele of the Asian lineage, as revealed by sequence analysis of the TK gene, Marker I, and Marker II. The detected isolates were also similar to those detected from koi in the same region of Iran, suggesting the probable transmission of CyHV-3 from ornamental to farmed cyprinids. This represents the first report of CyHV-3 from Iranian farmed common carp to the best of our knowledge.

Koi herpesvirus (KHV) and KHV disease (KHVD) - a recently updated overview

Over the last years, there has been an enormous increase in the knowledge on koi herpesvirus (KHV), koi herpesvirus disease (KHVD), pathogenesis and virus variants. Different KHV lineages have clearly been identified, possible genomic changes during replication in different cell cultures at different temperatures but also in several hosts have been identified, a persistent stage of infection has been specified and it has been shown that infection with KHV is not host specific at all, but KHVD is. Additionally, it has been shown that it is possible to combat KHVD by immunization with inactivated and attenuated live vaccines using different delivery systems but also to benefit from alternative treatments with e.g. exopolysaccharids obtained from Arthrospira platensis.