Until recently, covert mortality nodavirus was a problem mainly for shrimp farmers. First identified in Chinese aquaculture ponds in 2014, CMNV killed animals invisibly. Infected shrimp sank to the bottom of ponds and died out of sight, so farmers had no idea they were losing stock. The virus spread through the global seafood supply chain, infecting more than twenty species of fish, crustaceans, molluscs and other marine animals. Mortality in shrimp reached 80%. The World Organisation for Animal Health initially classified it as an emerging disease. But when WOAH last updated its official disease card in February 2024, it listed the virus’s zoonotic importance — its capacity to infect humans — as “None.” That was not a stale document left to gather dust. The 2024 card was itself an update of a 2023 version that said the same thing. Someone reviewed the evidence twice and saw no threat.
That assessment is now obsolete. A study published in Nature Microbiology in March reports that CMNV is the cause of a new human eye disease called persistent ocular hypertensive viral anterior uveitis, or POH-VAU. The evidence is not circumstantial. A team led by researchers at Qingdao’s Chinese Academy of Fishery Sciences and Shandong First Medical University found viral particles physically present in the iris tissue of affected patients, identified by electron microscopy and confirmed by proteomics. All 70 patients in their clinical study showed antibodies to the virus. Mouse experiments recapitulated the disease: animals injected with CMNV developed elevated eye pressure, inflammation and damage to the cornea, iris and retina.
What the disease looks like
POH-VAU is not subtle. Patients develop keratic precipitates, clumps of inflammatory debris on the inner surface of the cornea, along with iris atrophy and dangerously elevated intraocular pressure. The condition recurs. It resists standard treatment. Roughly a third of the patients in this study eventually required antiglaucoma surgery. One lost vision permanently.
Most patients were between 45 and 70 years old. The strongest risk factor was the number of severe exposures to aquatic animals within two years before disease onset. More than half the patients were people who handled seafood bare-handed at home. Another 17% ate raw or uncooked aquatic products. Direct contact with animals or contaminated environments accounts for the vast majority of cases. But about 16% of patients were close contacts of high-risk groups rather than seafood handlers themselves — suggestive, as a companion commentary in the same journal noted, of possible onward transmission between humans.
A virus that was already everywhere
That clinical picture raises an obvious question: how many people are exposed? The study surveyed CMNV prevalence in aquatic animals globally. In Asia, 35% of shrimp samples and 40% of fish samples tested positive. The virus turned up in specimens from the Americas, Africa and Antarctica. In Chinese seafood markets, testing of 351 products across six provinces found CMNV in 33% to 62% of samples of fish, shrimp, crabs, molluscs and cephalopods.
Looking at the molecular sequence, an isolate of the virus taken from a human groups with strains from fish rather than shrimp, though all CMNV strains are so closely related that the branches separating them are very short. The spillover route into people remains an open question.
Provincial POH-VAU rates in China now correlate significantly with aquaculture production. That pattern has strengthened in recent years. Global fisheries and aquaculture produced 223 million tonnes in 2022, supplying roughly 15% of the world’s animal protein and more than half in several countries in Asia and Africa. The industry is expanding, particularly in the Global South, and with it the surface area of human contact with animals carrying this virus.
CMNV is a single-stranded RNA virus that infects species spanning seven phyla — an extraordinarily broad host range. Closely related nodaviruses survive freezing but are inactivated by heat, which means cooking likely eliminates the risk from food. But for the millions of people who process aquatic animals for a living, heat treatment is not the relevant exposure. The relevant exposure is the work itself.
