CORVALLIS, Ore. - A study by microbiologists at Oregon State University has concluded that an unsuspected bacterial infection, rather than a viral disease, was associated with the stranding and death of seven harbor seals on the California coast in 2009.
The research, made with a powerful investigative method called "meta-transcriptomics," found a high incidence of infection in the seals with the bacterial pathogen Burkholderia, and provides the first report in the Americas of this bacteria in a wild harbor seal.
The bacteria probably did not directly cause the death of the seals, researchers say, but this provides further evidence of the increase in emerging marine pathogens, and the need for improved monitoring and study of zoonotic diseases that could affect both human and wildlife populations.
In light of these findings, OSU researchers also remind the public that they should not touch stranded or dead marine mammals.
The research was recently published in PLOS ONE, in work supported by the Oregon Sea Grant program and the National Science Foundation.
"We now have improved tools to better identify new diseases as they emerge from natural reservoirs, and can record and track these events," said Rebecca Vega-Thurber, an assistant professor of microbiology in the OSU College of Science. "It's becoming clear there are more pathogens than we knew of in the past, and that some of them can move into human populations.
"This is why it's increasingly important that we accurately pinpoint the cause of these diseases, and understand the full range of causes that may factor into these deaths."
Cases such as this, the researchers said, point out that it's not always a single pathogen that causes death, but a combination of pathogens, changing environmental influences, weakened hosts and other forces. In this seal-stranding event, the scientists also found evidence of Coxiella burnettii, another bacterial pathogen, at high levels in one animal.
Advances in this type of monitoring are being made with the comparatively new field of meta-transcriptomics, which has been referred to as a way to eavesdrop on the viral and microbial world, to catalogue and compare sequences from suspected pathogens. It's just now being applied to marine systems, which are often reservoirs for pathogens that can emerge into terrestrial populations.
This phenomenon seems to be picking up speed, the researchers noted in their study.
About 61 percent of emerging human diseases arise from zoonotic pathogens, and about 70 percent of these originate from wildlife. The recent Ebola outbreak in Africa was one example; the bacterial pathogen that causes tuberculosis was introduced to the Americas from pinnipeds; and influenza has been shown to be transmitted from seals to humans. In recent years, viral disease has been implicated in the deaths of tens of thousands of harbor seals.
Almost half of marine mammals die from unknown causes, the researchers said, but the use of new high-speed, analytic tools could offer ways to change that. The techniques don't require prior information about the viruses and bacterial infections that may be affecting wildlife.
In the case of the stranded harbor seals in this study, it was initially suspected that viruses were the cause. This study largely ruled that out, but identified bacterial infection in the animals' brains. The final cause of death is still unknown and research on that issue is continuing.
"These analytic tools should be increasingly useful in the future, and show us just what genes the pathogens may be using during an infection," said Stephanie Rosales, a doctoral student in the OSU College of Science, and lead author on this study. "A lot of new environmental changes and stresses are taking place that may lead to new emerging diseases, and we should be tracking them as they evolve."
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Rebecca Vega-Thurber, 541-737-1851