Pacific Lamprey Larvae Are Resistant to Rhabdoviral Pathogens of the Pacific Northwest

Pacific lamprey Entosphenus tridentatus populations have declined across their entire range.  Many aspects of their biology and ecology are unknown, including the response to fish pathogens.  We conducted direct virus challenges with three viruses present in the Pacific Northwest that are known to infect bony fishes.  Our goal was to determine if Pacific lamprey larvae can be biological hosts or carriers of these viruses.  We used two strains of infectious hematopoietic necrosis virus (IHNV, types U and M) and one strain of viral hemorrhagic septicemia (VHSV, genotype IVa).  Naïve 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 the USGS Western Fisheries Science Center (Seattle, WA).  Larvae were challenged via immersion at moderate and high doses of virus as well as by direct injection of virus.  Controls were treated by immersion and injection procedures but without virus.  Larvae were monitored for mortality and disease signs over a 45 d period.  Subsamples of larvae were sacrificed at regular time intervals to test for presence of virus by plaque assay titers.  Low levels of mortality were observed (<15%) in the immersion trials but the fish that died were negative for virus, suggesting virus was not the cause of death.  Higher levels of mortality were observed (30-80%) in the injection trials but again all were negative for virus, and in this case mortality also occurred in the control groups (30%).  Among fish sampled early after exposure IHNV and VHSV were detected only in three and two fish, respectively, sampled 6 days after injection of virus.  The virus quantities detected were lower than the amount injected, so there was no evidence of virus replication, and it is likely the virus was being cleared.  In addition, virus was not detected in any of the surviving larvae at the end of the 45 day trial. The results indicate that Pacific lampreys are largely resistant to the viruses examined, which represent the most commonly found viral pathogens in Pacific salmon and trout of the Pacific Northwest. Lamprey may be susceptible to other pathogens and further study is warranted.  There is a need for captive populations of Pacific lamprey for basic research questions.  An understanding of lamprey pathology will assist in using the best protocols and utmost care to minimize any risk to other fish at the facility (e.g., hatchery facilities).