Viruses are everywhere…they can make us sick, they can be highly contagious, and some of them are downright dangerous. Fish get sick from viruses, too!
What is a virus? Viruses are very specialized biological entities and for years scientists have argued whether or not they are even organisms at all. They do contain genetic material and evolve through natural selection and at the very least interact with living organisms and have probably been present on Earth since the origin of life. They are basically bits of genetic material (DNA or RNA) coated in protein and/or lipids, are so small that they are able to get inside of the cells of a host organism and replicate. The large majority of virus exposure prompts an immune response from the host which destroys them. Common viruses are rabies, hepatitis, influenza, and even the common cold. Some are more serious than others. It is the viruses that persist that can cause some problems – they basically take control of cellular functions and all sorts of chaos ensues – usually ending poorly for the host.
Let’s get to the lampreys. Why study lampreys and virus? There have been plenty of fish viruses identified with some wonderfully creative names. Some examples are largemouth bass virus, channel catfish virus, and carp herpes virus. In the Pacific Northwest, infectious hematopoietic necrosis virus (IHNV) is a virus that attacks salmon and trout. It causes hemorrhaging, necrosis (tissue death), and organ failure and is particularly harmful to young fish. It has the potential to kill large numbers of fish if it spreads into a hatchery facility. Viral hemorrhagic septicemia virus (VHSV) is another nasty that does some of the same things and has been attributed to mass die-offs of fish in the Great Lakes region.
So we’ve blogged about lampreys on here plenty of times and why they’re important. We wanted to know if larval Pacific lampreys (the ammocoetes) are susceptible to some viruses that infect salmon of the Pacific Northwest. And we wanted to know not only if they were susceptible but also if they could carry the virus, replicate it, and therefore potentially transmit it to other fish. We began talking with Susan Gutenberger, one of our fish health expert colleagues at the Lower Columbia River Fish Health Center about it. We eventually connected with Gael Kurath and Jim Winton of the USGS Western Fisheries Research Center in Seattle – more fish health experts. We hatched a plan…we decided to pull our resources and expertise and see if we could figure out if larval lamprey could be infected with viruses.
We saw low levels of mortality in the immersion trials (less than 15%) and those that died tested negative for virus, suggesting a different cause of death. There was higher mortality for those that were injected (30-80%) but again they were negative for virus and we also had mortality in the control fish (30%). In fact, virus was only ever detected in fish that were injected. IHNV was detected in three fish and VHSV was detected in two fish – six days after injection (these were sacrificed fish) and in levels lower than they were originally injected. Basically, there was no evidence of replication of the virus and the virus was rapidly cleared from the fish.
Gael gets the viruses from the -80oC storage.
We conducted direct virus challenges using three viruses, two strains of IHNV and one strain of VHSV. Native Pacific lamprey larvae were collected from the North Fork Eagle Creek (Clackamas River drainage, OR), a basin that does not contain these viruses, and transported to Western Fisheries Science Center. Larvae were either immersed in a solution containing a high dose of virus or actually had the virus injected directly into their body cavity (the direct challenge)! Even though larvae would never be exposed to a virus in this manner in the wild we wanted to be certain that they had as much exposure – and therefore probability of contracting it – as possible. We also used control lamprey that experienced all the same conditions except the virus; they were handled similarly and injected with saline solution. Larvae were monitored for mortality and disease signs for 45 days. Subsamples of larvae were sacrificed at regular time intervals to test for presence of virus.
I'm injecting a larval lamprey with IHN virus.
Gael and Tarin add virus for an immersion trial. We really are nice people, though!