Ictavirus ictaluridallo2

Ictavirus ictaluridallo2 (IcHV-2) is a species of virus in the genus Ictalurivirus, family Alloherpesviridae, and order Herpesvirales.

Also known as the black bullhead virus, it is phylogenetically similar to IcHV-1. Distinction between the two species is reliant on the places of detection and the severity of clinical signs. IcHV-2 has been experimentally observed to result in more severe clinical signs in channel catfish than IcHV1.

Background
IcHV-2 was first observed in 1994 after two mass mortality events occurred in two different catfish farms in Italy, resulting in severe economic loss.

As observed in experimental trials, there is a higher mortality rate in channel catfish when exposed to IcHV-2 than in trials and previous background studies where channel catfish were exposed to IcHV-1. It is also shown that through syncytial cytopathic effect, both strains of Ictalurid herpesvirus are homologous to the syncytia produced.

Genome
Next-generation sequencing performed on IcHV2 revealed its complete genome to be 142,925 bp, along with 77 (predicted) protein-coding regions with 12 open reading frames (ORFs) where these ORFs are seen to be homologous with every other Alloherpesvirus sequenced. High synteny between IcHV1 and IcHV2 was also observed.

Diagnosis
Detection of IcHV-2 uses quantitative PCR (qPCR) for any potential carriers or for checking for isolates. While IcHV-2 was detected in all organs, the virus was seen the highest in the kidney and spleen; meanwhile, liver and gill concentrations were much lower.

Other uses of qPCR can help detect IcHV-2 in latently infected carrier fish. This helps reduce the risk of potentially relocating the virus and introducing it into new aquaculture and whether or not high morbidity rates are from IcHV-2 or another pathogen.

Pathology
Clinical signs of IcHV-2 include disoriented swimming or instances where they dwell at the surface of the pond bank. Other signs include scattered hemorrhages and kidney tissue death.

IcHV2 propagation control is a highly researched topic due to it showing much more intensive mortality rates in channel catfish than IcHV-1. The high mortality rate is due to the fact that IcHV-2 propagation and viral transmission has an optimal temperature of 24°C, which is relatively lower than IcHV-1. Furthermore, overcrowded pens contribute to the transmission of the virus.