Is There Any Species Specificity in Infections with Aquatic Animal Herpesviruses?–The Koi Herpesvirus (KHV): An Alloherpesvirus Model
Herpesviruses constitute a diverse family of widespread pathogens inducing severe diseases of veterinary importance in all animals including animals used for human consumption. It is likely that they evolve within their host species over long periods of time and most of them induce very host-specific disease, although sometime infections occur in the absence of clinical disease signs. As a rule herpesviruses have large genomes and all of them induce a latent or a persistent phase of infection, sometimes in different hosts. For the family Alloherpesviridae it has been demonstrated that most of the virus species have developed a high level of host specificity. In the majority of animal herpesviral infections only mild symptoms (internally and externally) appear under natural conditions. Virulence associated with herpesviral infection is often initially displayed in immunologically weakened hosts or during primary infection of a naïve host. The characteristics of KHV, including the morphology investigated by electron microscopy [1,2], and phylogeny meets the taxonomical criteria for viruses of the family Alloherpesviridae , which have been shown to be non-host-specific, but sometimes causing disease in one species. Within this family are four genera grouped as the Batrachovirus, Salmonivirus, Ictalurivirus and Cyprinivirus. In Batrachovirus, the ranid herpesvirus 1 (RaHV-1) can be found in different leopard frog (Rana pipiens) (sub) species and has been identified as the causative agent of renal adenocarcinoma , and the ranid herpesvirus 2 (RaHV-2), which can cause infections in a lot of species of the family Ranidae (pets and wild) leading to skin lesions and sometimes inducing tumors . The members of the family ictaluriviridae, such as acipenserid herpesvirus 1 (AcHV-1), infect sturgeon spp.  and at least two sturgeon sp. can be infected with AcHV-2 . The most previously researched Alloherpesvirus, channel catfish herpesvirus (Ictalurid herpesvirus 1 (IcHV-1), CCV) infects channel catfish but also blue catfish  and possibly other catfish species or subspecies while IcHV-2 was detected in black bullhead (Ameiurus melas) and in channel catfish (Ictalurus punctatus) . Members of the salmonivirus group are known to often infect more than one species: salmonid herpesvirus 1 (SaHV-1) have been detected in rainbow trout (Ocorhynchus mykiss)  but also in Chum salmon (O. keta) and Chinook salmon (O. kisutch) . SaHV-2 was considered to infect all salmonid species  and SaHV-3 was detected in different fish and hybrids of the genus Salvelinus [3,11,12]. In addition to cyprinid herpesviruses 1,2 and 3 the eel herpesvirus (HVA, AngHV-1) is also included in the genus Cyprinivirus. HVA was detected during disease outbreaks in Japanese eel (Anguilla japonica) and European eel (A. anguilla) in Japan  and in American eel (A. rostrata) in Poland . A variant of the HVA in eels in Taiwan (eel herpesvirus Formosa, FEHV) has also been found to induce mortality in common carp (Cyprinus carpio) as well . From investigations conducted in our Lab it was confirmed that HVA may at least persist in artificially infected common carp (immersion) for up to six weeks, which was detectable directly by PCR and indirectly by serum neutralization assay (Bergmann, unpublished data). The cyprinid herpesvirus 1 (CyHV-1, carp pox virus) induced typical clinical signs in carp or koi (C. carpio), in golden ide (Leuciscus idus)  or in other cyprinids (carps and minnows) . CyHV-2 or goldfish herpesvirus was first detected in goldfish (Carassius auratus), inducing severe disease outbreaks in those affected populations [18,19]. Recently severe outbreaks of disease with high mortalities were observed in crucian carp (Carassius carassius)  and Prussian carp (C. c. gibelio)  where CyHV-2 was identified as the disease causing agent. In Germany severe mortality rates were also induced by CyHV-2 in a wild Prussian carp population (Bergmann, pers. obs.). For CyHV-3 or koi herpesvirus (KHV), although KHV disease (KHVD) is induced only in the species C. carpio (common carp and koi), it has been demonstrated that this virus can be present, and sometimes replicated, in a lot different fish species living in fresh and brackish water. KHV DNA has been identified by different PCR methods in a lot of members of the families Acipenseridae , Cyprinidae, including common carp or koi hybrids [23-25], and Percidae in many wild fish populations within close vicinity to, or in carp ponds [26,27]. Furthermore KHV has also been detected in fresh water mussels and crustaceans . In this study we have shown that a common salmonid species, rainbow trout, is able to transfer KHV to naive carp inducing KHVD under certain circumstances.