Sick fish: social distancing may help them too

By: Dana Sackett, PhD

Here at The Fisheries Blog our current situation begs the questions: can fish be infected with viruses too?  The answer is a resounding yes.  But before we get into the details, let’s first describe what a virus is and how it is different from other pathogens (things that cause disease), such as bacteria, fungus, or protozoa.

With the news constantly talking about the current coronavirus wreaking havoc in the world, many may have already learned some of the basics about viruses, but just in case you have been ignoring much of the news for sanity, here is a brief overview.  Unlike bacteria, which are a group of single-celled living organisms that can replicate themselves just about anywhere, and whose members are mostly harmless or even beneficial, viruses are generally orders of magnitude smaller and are not considered alive. Viruses are so small in fact that they can infect bacteria (these are called bacteriophages)! 

Pathogens | BioNinja
Pathogens cause disease and can be classified into the above general categories.  These groups increase in complexity from right to left with a prion being the simplest because it is just protein (most commonly known to cause mad-cow disease). Fungi (which include yeast, molds, and mushrooms) and protozoa also include members that can cause disease.  For fungi, these members most commonly harm plants rather than animals.  Source

Viruses are considered nonliving partly because they do not have the ability to replicate alone.  In fact, they can only duplicate themselves by invading a living cell (such as a human lung cell) and tricking that cell into making copies of the virus rather than the molecules the cell would normally make to survive; effectively hijacking the production machinery of the cell.  This causes the cell to fill with newly manufactured viruses, eventually bursting and releasing the viruses to infect more cells.  Also, unlike living organisms, viruses only have one type of genetic material (either DNA or RNA) rather than both. 

Difference Between Virus and Prion
Simplified structure of an enveloped virus, meaning it has a lipid or fat envelope around the protein capsid that surrounds the genetic information (DNA or RNA). If the envelope (or bubble of fat) was missing it would be called a nonenveloped virus. Source

One similarity among living organisms and viruses is that scientists group them based on physical and genetic similarities.  For instance, the same way an elephant, whale, and mouse are all animals classified as mammals, viruses can be classified based on particular characteristics that they share.  This in no way guarantees that two different virus species in the same group will have the same host or impact on its host (just as being a mammal doesn’t mean that that species lives in a particular place, or is a certain size). 

types of viruses
One of the many ways that scientists classify or group viruses is based on their shape. Above are several examples of common viral groupings. Source

For fish, the best understood viruses are those that have been found in aquaculture (otherwise known as farmed or domestic) fish species.  This is largely because one of the main factors limiting aquaculture production and profit is disease. However, only a select few of the more than 24,000 species of fish are reared in aquaculture. Consequently, there are likely many fish viruses that remain unknown in the wild.  One of the most well studied types of domestic fish viruses are Aquabirnaviruses (quite the mouthful). This category of viruses was first seen in farmed trout in the 1940’s and later found to cause damage to the pancreas of fish. Since that time these viruses have been identified in numerous freshwater and marine fish species, and invertebrates.  Farmed salmon have been plagued by these viruses because many salmon that survive the disease become asymptomatic carriers that continue to shed the virus in their feces infecting new populations, often killing entire generations of young salmon. 

The 3D structure of an Aquabirnavirus that causes disease in fish (left panel) and a virus from the same family that causes disease in poultry (right panel). The study notes that the spikes on the surface of the viruses cluster differently causing them to interact with different cell receptors for attachment and entry into a specific cell.  Source: Coulibaly et al. 2010.

Scientists continue to identify and expand our knowledge of wild fish viruses. A recent study examined wild caught fish sold at an Australian fish market.  One goal of this study was to test the well-established epidemiological theory that a population with higher contact rates has an increased likelihood of acquiring and transmitting viruses.  This theory is largely related to the fact that viruses cannot replicate unless inside a host cell and will only remain infectious for a limited amount of time outside of a host.  Therefore, contact or close proximity between an infected individual and an uninfected individual is needed for the virus to continue to spread.  Thus, to determine if this was the case for fish the scientists examined whether the shoaling behavior of the species tested (whether they were the type of fish to remain in close proximity to other fish or whether they were more solitary) influenced the amount of virus and the number of different viruses an individual carried.  Their results supported this theory because the most solitary fish were infected by the smallest viral loads while the fish species that had the most dense schooling behavior had the greatest viral loads.

The authors of this study also identified twelve potentially novel (brand-new) viral species, eight of which were associated with numerous other vertebrate species.  They concluded that fish species commonly sold as food may contain a wide range of mostly unknown fish viruses.  However, it is important to note that the major differences in fish virus biology and human virus biology means that they likely pose no risk to humans.  Even more, no fish virus to date has been known to cause a disease in a human.

vsv virion
The structure of the viral hemorrhagic septicemia virus, which causes a very serious often fatal disease in fish.  This virus is spread by water that has urine from infected fish. Source

Although, a novel virus does not have to cause human disease to affect the human population.  For example, a virus began wiping out a major source of global protein, tilapia, in Israel in 2005.  Alarming fishers, catch for wild tilapia initially dropped from 316 tons to only 8 tons in a matter of four years.  Following this massive die-off, farmed tilapia began dying in droves as well.  What was even more concerning was that this same disease began killing tilapia around the world.  However, with no known culprit causing the disease, scientists suspected a novel virus was to blame. They were right. This virus is still a major threat to the 7.5 billion dollar farmed tilapia industry and the millions of people that reply on tilapia as a source of protein today. 

Fish that perished from Tilapia Lake Virus at a fish farm in Isreal. Credit: Avi Eldar. Source

An emerging virus is one that has an increasing infection rate in the world and is therefore of concern.  However, an emerging novel (or new) virus may not necessarily be new, just new to us.  These types of viruses may have evolved some time ago but had not encountered the right conditions to spread. Unfortunately, the conditions conducive to viral spread are becoming more prevalent in aquaculture and farming industries.  For instance, many farmed animals and plants, including fish, are bred to have particular characteristics. For example, always choosing to breed the biggest fish in a population leads to an inbred population of big fish with little genetic diversity (these are called monocultures).  This means that if one fish is vulnerable to a disease than all the fish in the farmed population are equally vulnerable. Furthermore, trying to grow as many fish as possible to meet demand and maximize profit leads to crowding, increasing contact between fish and transmission of a disease through the population.  These crowded conditions and fish handling also increase stress, which can compromise the fish immune system, further allowing a virus to spread. 

A pen of farmed fish. Aquaculture demand is reported to likely rise in the coming years. Source

The common source of water for farmed and wild fish species (which can introduce previously unknown wild viruses into aquaculture where conditions would allow the virus to thrive), and long distance shipments of potentially infected fish, create an environment ripe for emerging fish viruses to spread globally.  But, we do have tools to fight the spread of fish viruses. Similar to humans, the primary preventative measure to stop the spread of a fish virus in aquaculture is a vaccine.  Indeed, fish immunization has been carried out for over 50 years and has been widely accepted as an effective method for preventing numerous viral diseases.  Thus, while an emerging novel fish virus could have a significant impact on the human food supply, scientists working in the field of virology are hard at work around the world studying, discovering, and developing solutions to prevent just such an event. 

References and other resources:

Coulibaly F, Chevalier C, Delmas B, Rey FA. 2010. Crystal structure of an Aquabirnavirus particle: insights into antigenic diversity and virulence determinism. Journal of Virology 84:1792-1799. doi:10.1128/JVI.01536-09

Crane M, Hyatt A. 2011. Viruses of fish: an overview of significant pathogens. Viruses 3:2025-2046. doi:10.3390/v3112025

Geoghegan JL, Di Giallonardo F, Cousins K, Shi M, Williamson JE, Holmes EC. 2018. Hidden diversity and evolution of viruses in market fish. Virus Evolution 4:vey031. doi: 10.1093/ve/vey031

Kurath G, Winton J. 2011. Complex dynamics at the interface between wild and domestic viruses of finfish. Current Opinion in Virology 1:73-80. DOI 10.1016/j.coviro.2011.05.010

Ma J, Bruce TJ, Jones EM, Cain KD. 2019. A review of fish vaccine development strategies: conventional methods and modern biotechnological approaches. Microorganisms 7: 569 doi:10.3390/microorganisms7110569

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