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Outbreaks of infectious disease in natural communities have increased in frequency in several major groups of marine life during the past 30 years, suggests a new review of scientific literature by Cornell University and the U.S. Geological Survey (USGS) published in the April issue of the Public Library of Science (PloS) Biology. This study lends support for the need to understand disease dynamics in the oceans and marks the first quantitative use of normalized trends in the scientific literature to test an ecological hypothesis.
The study found an increased number of reports in the scientific literature of disease outbreaks among turtles, corals (noninfectious bleaching), mollusks, mammals, and urchins, said Kevin Lafferty, a USGS marine ecologist in Santa Barbara, CA, and coauthor with Jessica Ward, a graduate student at Cornell University. The scientists detected no significant trends for seagrasses, decapods, corals (infectious disease), and sharks/rays (close relatives grouped together for the study). Most surprisingly, the scientists found evidence of a decreased number of reports of disease outbreaks in fishes.
Although stress may make individuals more susceptible to disease, disease outbreaks are also associated with population density, which aids in the transmission of disease between individuals of a species, noted Lafferty.
"Infectious disease should increase in thriving populations as much as or more than in stressed populations. Probably the most alarming result of our study is the suggestion of fewer diseases in fishes. As we fish stocks down, the remaining individuals may be too sparse to transmit infectious diseases," said Lafferty.
The USGS scientist noted that this research would not have been possible until a couple of years ago because the databases on scientific research only recently became extensive enough to allow scientists to use the data from previous research efforts to test scientific hypotheses, including trends in population status possibly attributable to specific factors.
"Ideally, we would have complete records of disease outbreaks for all marine organisms," commented Ward. "However, that is an unrealistic expectation, even today, given that extensive surveys are difficult in terms of both time and money. So, we developed and tested a method to circumvent this problem."
In the absence of baseline data on changes in disease occurrence over time, this new study used publication effort by marine scientists as an indirect way to detect important trends in disease occurrence. Ward and Lafferty searched the Science Citation Index Expanded online database, which references almost 6,000 journals, for titles from 1970 to 2001 that reported on disease in organisms from naturally occurring populations of nine marine groups: turtles, corals, mammals, urchins, mollusks, seagrasses, decapods, sharks/rays and fishes.
The idea is that scientists, through their publication efforts, can collectively document changes in natural phenomena over time, assuming that scientists actually pursue problems in proportion to their frequency in nature. Simply counting reports, however, is misleading because of the general increase in the number of scientific publications and other potential biases. Ward and Lafferty quantified the reports of disease, and normalized the data by taking an annual proportion of disease reports and eliminating various biases, such as multiple reporting of the same disease outbreak. Their approach worked well for a test case, raccoon rabies. Publication effort followed the actual spread of this disease over time. The disparity in the results among different types of marine life suggests that the trends in disease reports were not simply a result of disease becoming a fashionable topic for study in recent years.
With rising human stresses on the environment, understanding disease dynamics is vital to conserving marine ecosystems, warn the authors. Environmental factors such as global climate change could have complex effects on disease, and they recommend that future research look more closely at disease impacts within each marine group and consider the wide range of environmental factors that can affect disease, including warming, pollution, exotic species, and fishing. Although marine organisms are hosts for a diversity of infectious diseases, mortalities in hostsfor example, a loss of seagrass bedscan cascade throughout ecosystems.
"Although we are concerned that human impacts to the ocean may favor diseases, our study does not simply give the impression that global warming and pollution are making everything sick," Lafferty said. "Disease responds in complex ways to the environment, and it will be a challenge to better understand and manage disease in the ocean."
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