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Research Looks at How Cumulative Pathogens May Create More Hazards for Migrating Salmon by Staff Columbia Basin Bulletin, March 4, 2016

The combination of a parasite found in juvenile salmon along with a bacterial infection, both common in Northwest waters, can make the bacterial infection more lethal, according to a recent study.

The study found that a high intensity of the parasite Nanophyetus salmincola (N. salmincola), which is found in 60 percent to 80 percent of juvenile chinook and coho salmon, but alone seldom causes death, when coupled with the common bacterium Flavobacterium columnare (F. columnare) causes a rise in mortality that would not otherwise occur at the same rate by itself.

On the other hand, the combination of N. salmincola with another bacterium, Aeromonas salmonicida (A. salmonicida), did not cause a mortality difference with either N. salmincola-parasitized fish or unparasitized fish.

The bottom line is that the encysted parasite has an impact on the outcome of some bacterial infections in juvenile salmon, but not others and that could have implications for disease in wild fish, the report says.

"We know quite a bit about how fish are affected by single pathogens, but, in the wild, fish typically encounter a variety of types of pathogens and we don't know as much about how they handle these co-infections," said Jerri Bartholomew, professor and head of the Department of Microbiology at Oregon State University.

She said that infections by trematode parasites, such as N. salmincola, are common and fish may become infected as juveniles in tributaries and carry these parasites for life.

"These infections can result in a range of effects, from severe to unapparent," Bartholomew said. "The bacterial pathogens we used in this study are also ubiquitous, and outmigrating juvenile salmon often encounter them as water temperatures increase during summer, particularly in the mainstem rivers."

The study, "Effect of Nanophyetus salmincola and Bacterial Co-Infection on Mortality of Juvenile Chinook Salmon," was published online December 15, 2015, in the Journal of Aquatic Animal Health.

Bartholomew's co-authors are Kym Jacobson, zoologist, NOAA Fisheries, Northwest Fisheries Science Center; Sean Roon, microbiologist, U.S. Fish and Wildlife Service; and Julie Alexander, post-doctoral research associate, Department of Microbiology, OSU.

"The experimental infections demonstrated that prior infection with the trematode could significantly increase mortality from a subsequent bacterial infection, supporting an earlier study by one of our co-authors," Bartholomew said (that co-author is Jacobson). "However, we didn't see that effect with both pathogens, which of course means we need to be careful about making generalizations. There are a lot of things that might be important, from virulence of the parasite strain to timing of the infections."

Salmon become infected in freshwater with N. salmincola. The parasite becomes encysted as metacercariae, mostly towards the posterior kidney and muscle. When quickly exposed to high concentration of the parasite, physical damage can occur, but if exposed over a period of time, the parasite will encyst as metacercariae without lethal effects, according to the study.

F. columnare and A. salmonicida are also found everywhere in Northwest streams. The pathogens have been linked to "mortality in hatchery and wild salmonid populations in the PNW and therefore pose a potential threat to out-migrating juvenile salmonids," the study says.

A real life example, according to the study, is the exposure of subyearling and yearling chinook to the parasite while they rear in the Willamette River and its tributaries. They may accumulate relatively high N. salmincola intensities before entering the ocean, given that the parasite is present everywhere in the Willamette River system. As the juveniles migrate into the mainstem river, they will encounter "increasing densities of bacterial pathogens, particularly when fish densities and river temperatures are high," the report says.

To get a better idea of what the parasite-bacteria co-infection might have on the ecology of disease in wild salmon populations, the authors simulated in the laboratory co-infection situations that would typically be experienced by salmon in streams.

Using juvenile salmon with and without N. salmincola, they infected the salmon with either F. columnare or A. salmonicida and found that mortality in fish infected with F. columnare was higher in parasitized fish than in fish without the parasite. There was no difference in mortality when the fish were infected with A. salmonicida, regardless of the parasite's presence.

The authors also discovered that the intensity of the encysted N. salmincola made a difference, when an intensity greater than 200 of the encysted parasite paired with the F. columnare having the most impact.

"Understanding all of the factors that affect the survival of juvenile salmon is complicated and we tend to look at the effects of one at a time, or when considering disease, at the effects of a pathogen and physical parameter like temperature," Bartholomew said. "Studies like this are helping us to better predict disease risks for juvenile salmon as they migrate through a river system, encountering different pathogens and environmental conditions."

The impact of the combined pathogens may not end in freshwater, according to Jacobson.

"These juvenile salmon that survive exposure to freshwater pathogens may then be exposed to additional potential pathogens and stressors in estuarine and coastal ocean habitats," she said.

"This is a really hard topic to study because we know that with laboratory studies we can't really replicate exact conditions that juvenile salmon experience in nature (which may be much harsher or more stressful than what they experience in a controlled study) and when we sample fish from their natural habitats we are sampling the survivors," Jacobson said.

Describing the process of studying the impact of pathogens on fish, she said that they start with basic questions or simple scenarios, building from there to more complicated scenarios that fish are likely experiencing in the wild. That includes "exposure to multiple pathogens, exposure to varying (often extremely high) temperatures, predator avoidance, food availability, smartification, etc. All of these factors can affect how well a fish deals with infections."