Ranavirus in wrasse: investigate beyond the skin

An acute mortality event does not need ulcers, haemorrhage or dramatic skin changes to justify a full diagnostic investigation. In 2025, 30 Choat’s leopard wrasse, Macropharyngodon choati, were collected from Australia’s Great Barrier Reef and transferred to a multispecies holding facility. Nine days later they developed abnormal swimming and impaired respiration. All died within one to two days after signs appeared. A study published in May 2026 associated the episode with a new member of the viral species Ranavirus micropterus1, provisionally called Macropharyngodon choati ranavirus, or McRV.

For public aquariums, the practical message is not that one assay can explain every sudden loss. It is that non-specific signs and nearly normal skin can conceal important internal disease. When mortality follows an introduction, teams need to preserve the timeline, suitable carcasses and complementary samples for a coordinated investigation using necropsy, histopathology, molecular testing and virology.

A severe event after transfer

The fish were collected in March 2025 and held in a 170-litre recirculating system at approximately 25°C and 31 ppt salinity. Clinical signs began nine days after transfer. No other species at the facility showed the same signs at the time of sampling, although the report was not a controlled challenge comparing species, groups or husbandry conditions.

Spleen, liver and brain samples were collected from 10 affected fish. Only one whole fish, less than 8 cm long, was fixed for histology. It had acute multifocal tubulointerstitial necrosis in the kidney, with some locally extensive areas. Skin and scale pockets were largely unremarkable. The limited gill changes were considered incidental. Encapsulated parasites were also found, but their distribution and associated tissue response did not support them as the principal explanation.

That contrast matters at the tank side. External examination remains essential, but “no skin lesions” is not equivalent to “no disease”. Abnormal swimming and impaired respiration are equally non-specific. They may occur with water-quality failure, transport injury, gill disease, toxic exposure or several other infections. The initial case definition should therefore describe what staff can observe without assigning a cause prematurely.

Independent findings converge on McRV

The investigators produced 18 metatranscriptomic libraries from spleen, liver and brain tissues from six fish. McRV was present in every sampled tissue type and accounted for an exceptionally large proportion of non-host reads in several spleens. No other vertebrate-associated virus was detected, while opportunistic bacteria and parasites occurred at much lower abundance or inconsistently.

The conclusion did not rest on sequencing alone. McRV was isolated in BF-2 fish cells, where a cytopathic effect developed and recurred after passage. Conventional PCR targeting the major capsid protein was positive in every tested tissue and homogenate, while negative controls remained negative. Sequencing from cell culture produced a complete 98,903-nucleotide genome, deposited in GenBank as PX883867. Raw reads are available under BioProject PRJNA1403177.

High viral abundance, active virus isolation, PCR detection, renal pathology and host immune-gene expression form a coherent body of evidence. The authors nevertheless identify important limitations. The signs were non-specific, histology did not cover every organ or multiple whole fish, there was no comparable healthy control group and experimental infection did not reproduce disease. McRV was therefore the likely cause of this event, but the study did not complete every step needed to demonstrate causality beyond doubt.

Phylogeny is not a transmission map

The McRV genome shared roughly 98% nucleotide identity with Ranavirus micropterus1 members reported from China and around 97% with North American members. At the major capsid protein, it was identical to a virus detected in 2021 in an apparently healthy tail-spot wrasse, Halichoeres melanurus, at another Great Barrier Reef location.

This relationship supports the possibility that related viruses circulate in wild wrasse and that subclinical infection preceded captivity. It does not identify where the affected group acquired infection, prove a transmission direction between wildlife and the ornamental trade, or define the host range within a public aquarium. The absence of contemporaneous signs in other fish at the facility cannot establish that those animals were uninfected or resistant.

World Organisation for Animal Health standards include a chapter on infection with Ranavirus species, but its current scope applies to amphibians. Finding McRV in a marine fish does not by itself create a new international notification rule for aquarium fish collections. Facilities should check any reporting, import or movement requirements with the competent authority for their country, species and circumstances rather than infer them from the virus name.

Protect the evidence during the first response

When grouped mortality follows an introduction, staff can pause movements from the affected unit and reduce shared water, nets, siphons and equipment. These are proportionate biosecurity measures, not a declaration that a specific pathogen is present. Water results, alarms, feeds, treatments, handling events, transfers and deaths should be recorded with real times so that exposure windows can be reconstructed.

Sampling should be planned with the veterinarian and diagnostic laboratory before every carcass deteriorates or is placed in one preservative. The investigation may need chilled fresh fish, frozen tissues for molecular or virological work, and separately fixed tissues for histology. Tissue immersed in fixative is no longer available for virus culture. Each animal and container needs a traceable identity linked to the tank, group, date and sampling order.

PCR also needs context. Detecting a genetic target confirms its presence in the submitted material; it does not automatically establish the cause of every death or show how widely the agent is distributed. A negative result can follow unsuitable tissue selection, decomposition, inhibition, late sampling or an assay that does not cover a divergent target. Pathology, sequencing, culture and epidemiological observations help determine what the molecular result means.

Do not invent a treatment or disinfection protocol

The study tested no treatment, vaccine, quarantine duration or disinfection programme. It cannot support a universal chemical, contact time or isolation period. Decisions depend on the working diagnosis, species tolerance, construction materials, hydraulic design and products authorised in the relevant jurisdiction. Poorly selected disinfection can fail while damaging systems or exposing animals and staff.

A more defensible response turns the event into structured evidence: define cases, plot mortality over time, map hydraulic and equipment links, retain appropriate controls and document each movement. Quarantine is not merely a number of days. It is an arrangement that makes observation, separation, sampling and traceable release decisions possible.

Communication matters as well. A provisional result should be labelled as such, and internal updates should distinguish a detected virus from confirmed disease, and confirmed disease from a demonstrated route of transmission. This prevents a scientifically strong laboratory finding from becoming an unsupported claim about all wrasse, suppliers or connected tanks.

Look beyond the external examination

This episode does not show that McRV threatens every reef-fish species or every aquarium. It demonstrates how a fast-moving mortality can require complementary tools even when obvious external lesions are absent. The case is persuasive because field history, pathology, PCR, cell culture and genomics converge. It remains bounded by the small number of fish examined comprehensively and the lack of an experimental challenge.

Vetofish can support public aquariums with mortality-investigation plans, sample selection and preservation, coordinated interpretation of laboratory findings, and reviews of quarantine and equipment flows. The goal is to secure useful evidence early without diagnosing a virus from one sign, image or isolated result.