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A connected world gives viruses the edge
May 28, 2009That's one conclusion from a new study that looked at how virulence evolves in parasites. The research examined whether parasites evolve to be more or less aggressive depending on whether they are closely connected to their hosts or scattered among more isolated clusters of hosts.
The research was led by Geoff Wild, an NSERC-funded mathematician at the University of Western Ontario, with colleagues from the University of Edinburgh. Their paper will be published on Nature's Web site on May 27.
"Our study follows up on some recent findings that suggest that reduced dispersal of parasites across scattered host clusters favours the evolution of parasites with lower virulence - in the case of influenza, for example, a milder, possibly less deadly, case of flu," said Dr. Wild.
"Some researchers had contended from this that the parasites were evolving to support the overall fitness of the group," he added. "The argument for adaptation at the group level is that the parasites become more prudent to prevent overexploitation and hence to avoid causing the extinction of the local host population."
However, Dr. Wild and his colleagues were not convinced that Darwinian theory - so successful in providing explanations based on the notion that adaptation maximizes individual fitness - was ready for such a major makeover.
The researchers decided to move the arguments from words to harder science. Together they developed a formal mathematical model that incorporated variable patch sizes and the host parasite population dynamics. It was then run to determine the underlying evolutionary mechanisms, the results of which were published in the Nature paper.
"The model revealed solid reasons why lowered virulence enhanced individual fitness," said Dr. Wild.
The researchers used an "inclusive" notion of individual fitness that has been used by biologists in other situations since the 1960's. This "inclusive" approach recognizes that an individual has a vested interest not only in its own success, but also the success of its relatives (not the group as a whole, per se).
"Basically, we replace the notion of self-interest - an idea that underlies much early evolutionary theory - with the notion of self-and-family interest," he said. "The difference between self-and-family interest versus group interests is subtle, but important."
"There are several reasons why lowered virulence enhances the success of genetic lineages of parasites," he said. For one thing, he explained, it means lower host-to-host disease transmission.
"While the more virulent strain of parasite can move among hosts readily, it does so to the detriment not of the group, but rather certain members of the group (namely individuals of the same strain - its relatives)," said Dr. Wild.
"Besides settling an argument over adaptation, we now understand better the importance of dispersal to the evolution of parasites."
"The findings also suggest that as human activity makes the world more connected, natural selection will favour more virulent and dangerous parasites."
Dr. Wild said the modeling approach the group took makes it possible to expand virulence theory to examine a range of potentially important biological factors.
Natural Sciences and Engineering Research Council
"Some researchers had contended from this that the parasites were evolving to support the overall fitness of the group," he added. "The argument for adaptation at the group level is that the parasites become more prudent to prevent overexploitation and hence to avoid causing the extinction of the local host population."
However, Dr. Wild and his colleagues were not convinced that Darwinian theory - so successful in providing explanations based on the notion that adaptation maximizes individual fitness - was ready for such a major makeover.
The researchers decided to move the arguments from words to harder science. Together they developed a formal mathematical model that incorporated variable patch sizes and the host parasite population dynamics. It was then run to determine the underlying evolutionary mechanisms, the results of which were published in the Nature paper.
"The model revealed solid reasons why lowered virulence enhanced individual fitness," said Dr. Wild.
The researchers used an "inclusive" notion of individual fitness that has been used by biologists in other situations since the 1960's. This "inclusive" approach recognizes that an individual has a vested interest not only in its own success, but also the success of its relatives (not the group as a whole, per se).
"Basically, we replace the notion of self-interest - an idea that underlies much early evolutionary theory - with the notion of self-and-family interest," he said. "The difference between self-and-family interest versus group interests is subtle, but important."
"There are several reasons why lowered virulence enhances the success of genetic lineages of parasites," he said. For one thing, he explained, it means lower host-to-host disease transmission.
"While the more virulent strain of parasite can move among hosts readily, it does so to the detriment not of the group, but rather certain members of the group (namely individuals of the same strain - its relatives)," said Dr. Wild.
"Besides settling an argument over adaptation, we now understand better the importance of dispersal to the evolution of parasites."
"The findings also suggest that as human activity makes the world more connected, natural selection will favour more virulent and dangerous parasites."
Dr. Wild said the modeling approach the group took makes it possible to expand virulence theory to examine a range of potentially important biological factors.
Natural Sciences and Engineering Research Council
Parasites Current Events and Parasites News RSSPATH Malaria Vaccine Initiative shares strategy for developing 'next-generation' malaria vaccines
Marking its tenth anniversary year, the PATH Malaria Vaccine Initiative (MVI) today unveiled a new strategy that sets the stage for an aggressive push targeting the long-term goal of eliminating and eradicating malaria. Malaria is one of the world's deadliest infectious diseases, killing nearly 900,000 people a year, most of them children in sub-Saharan Africa.
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Responding to the HIV/AIDS pandemic and tackling so-called neglected tropical diseases are the focus of the November/December 2009 edition of Health Affairs.
Media availability: The role of biomedical research in malaria eradication
Although malaria has been controlled in many local and regional populations, the permanent elimination of malaria parasites throughout the world remains an elusive goal, and the disease continues to claim nearly one million lives each year.
New insight in the fight against the Leishmania parasite
Professor Albert Descoteaux's team at Centre INRS - Institut Armand-Frappier has gained a better understanding of how the Leishmania donovani parasite manages to outsmart the human immune system and proliferate with impunity, causing visceral leishmaniasis, a chronic infection that is potentially fatal if left untreated.
UM School of Medicine researchers find extreme genetic variability in malaria parasite
Researchers at the University of Maryland School of Medicine Center for Vaccine Development (CVD) have charted the extreme genetic differences that occur over time in the most dangerous malaria parasite in the world.
Major discovery opens door to leishmania treatment
Leishmania is a deadly parasitic disease that affects over 12 million people worldwide, with more than 2 million new cases reported every year.
African cattle to be protected from killer disease
Millions of African families could be saved from destitution thanks to a much-needed vaccine that is being mass-produced in a drive to protect cattle against a deadly parasite.
Gut worms may protect against house-dust mite allergy
A study conducted in Vietnam has added further weight to the view that parasitic gut worms, such as hookworm, could help in the prevention and treatment of asthma and other allergies.
New research confirms potential deadly nature of emerging new monkey malaria species in humans
Researchers in Malaysia have identified key laboratory and clinical features of an emerging new form of malaria infection.
New data from 10-year global flea monitoring program confirms imidacloprid remains highly effective
Bayer Animal Heath announced today at the 22nd World Association for the Advancement of Veterinary Parasitology (WAAVP), Calgary, Canada the latest results of a large-scale, international, independent Flea Susceptibility Monitoring (FSM) program, demonstrating that imidacloprid remains highly effective for the control of fleas in companion animals.
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