Climate Change: What Impact Does It Have on Viral Diseases in Fish?

Already weakened by human activities and the overall degradation of their environment, wild marine fish populations are already bearing the full brunt of climate change’s impact on water quality and circulation. Several particularly aggressive fish viruses are finding conditions conducive to their replication and spread. These viruses have a significant impact on wildlife health and the health of farmed fish, even as the human population shows an ever-increasing interest in these animal resources.   Globally, 2024 is the hottest year on record for both surface waters and deep waters (deeper than 2,000 meters). Extreme phenomena are already affecting the seas and oceans and will intensify in the future, such as “marine heatwaves ” (periods of extreme heat affecting surface waters, lasting from a few days to a few weeks, over a large area), such as those experienced along the French coast in recent years . These marine heatwaves have direct harmful effects on many marine organisms that cannot tolerate such high temperatures—typically corals. But they also generate indirect effects that can be just as devastating: this is the case with infectious diseases affecting marine fish, particularly those caused by viruses. Although they receive very little media attention, viral diseases in fish nonetheless cause significant economic losses on a daily basis at most fish farms worldwide, across all species (salmon, sea bass, sea bream, tilapia, etc.). In the 2000s, outbreaks of salmon mortality caused by the ISAV virus cost the Chilean economy nearly $2 billion ; Chile had become the world’s second-largest producer of farmed salmon and took several years to recover from this serious health crisis. Wildlife is also not spared by highly pathogenic viruses, some of which are, in fact, spread in large quantities by fish farms located in marine environments. In the summer of 2024, the RGNNV virus decimated a large number of fish of wildlife in southern Italy and Greece, amid a marine heatwave with peak temperatures nearing 30 °C. Three species of grouper were particularly affected, including the brown grouper, a highly susceptible species considered a heritage species in France. Furthermore, for the first time, the virus has affected several fish species that had previously been spared, such as the flying gurnard , further expanding the already broad host range of this virus, which affects several dozen species. If heat waves become more frequent in the coming years, as scenarios predict, there is reason to fear that mortality due to this virus will continue to rise throughout the Mediterranean basin, once again affecting fish farms and wildlife along the southern coast of France.  

An Ocean of Viruses

To fully understand the threat, one must realize that viruses are the most abundant biological entities in the oceans, and that a single drop of seawater contains between 1 and 100 million viral particles, all invisible to the naked eye. While the vast majority of aquatic viruses are bacteriophages (viruses that infect bacteria), several viral families have been found to infect more complex hosts such as plankton, mollusks, crustaceans, fish, and even marine mammals. Viruses are ubiquitous and have colonized all of the world’s aquatic ecosystems, including surface waters and the deep ocean, forming what could be described as the “dark matter” of the oceans. The global genetic reservoir constituted by aquatic viruses is therefore virtually infinite, especially since it is constantly evolving due to the combined effects of mutations and adaptation to hosts and the environment (it should be noted that viral genomes often mutate much more rapidly than those of their hosts).  

Temperature: A Key Factor in Infectious Fish Diseases

One of the direct consequences of warming waters is a decrease in their oxygen concentration. Warming therefore particularly affects fish species that require high oxygen levels, notably salmon and trout.

Some of the viruses most frequently detected in salmonid farms worldwide—Piscine reovirus (PRV) and Piscine myocarditis virus (PMCV)—target heart tissue and red blood cells, as well as the gills. One can therefore imagine the combined effects of reduced respiratory capacity in fish caused by a virus and oxygen deprivation in water that is too warm. For example, during a heat wave affecting salmon cages in Newfoundland in the summer of 2019, the salmon exhibited increased metabolic activity and heart rates, indicating acute stress. However, it was the interaction between salmon infection by pathogenic organisms and water temperature that caused high mortality rates—up to 100% at some sites. Another factor to consider is the ability of certain viruses to replicate more rapidly in their hosts under high-temperature conditions. Viral proteins are believed to be more active within a specific temperature range, leading to accelerated viral production. In the laboratory, it has been demonstrated that a RGNNV virus experimentally inoculated into sole can see its virulence increase dramatically with temperature, causing a mortality rate limited to 8% when the water is maintained at 16 °C but reaching 100% at 22 °C.  

From the Tropics to Warming Regions

The RGNNV that decimated groupers last summer is not the only virus adapted to warm waters. Other viruses that are currently absent or only just emerging in the Mediterranean could be introduced there and cause uncontrollable “epizootics” (rapidly spreading animal epidemics), such as megalocytiviruses. In many respects, the Mediterranean is an ideal area for the introduction and adaptation of exotic viruses. It is a commercial waterway traversed by numerous cargo ships from other seas and oceans that may carry viruses in their ballast water. Since the opening and expansion of the Suez Canal, it has been colonized by numerous fish species from the Red Sea, such as the lionfish. These species are likely to infect endemic species—whether in aquaculture or in the wild—with exotic viruses. Another route of entry for exotic viruses is the international fish trade. For example, ornamental fish transported by air from South America have tested positive for a megalocytivirus in Europe . This is far from a minor issue: the intense and, at first glance, harmless international trade in exotic fish actually poses a major risk of introducing highly virulent viruses to native fish populations.  

What can be done to combat emerging viral diseases?

It is difficult to control epizootics in an environment as vast and open as a sea or ocean. Nevertheless, surveillance of fish farms and wildlife must be intensified to limit the introduction and spread of viruses. To this end, effective diagnostic tools must, of course, be available, and stakeholders in the aquaculture sector must be made aware of the issue. In addition to quarantine measures for imported farmed fish, efforts must focus on the early detection of viral populations—on the one hand, at the farm level, to attempt to eradicate any new pathogen by culling the fish population in the event of an outbreak at a farm, and second, in the water itself, by analyzing “environmental DNA” (DNA present in the environment) that contains genomics sequences of known and unknown viruses. Laurent Bigarré , Project Manager, French Agency for Food, Environmental and Occupational Health & Safety (ANSES) This article is republished from The Conversation under a Creative Commons license. Read the original article.