Showing posts with label Bd. Show all posts
Showing posts with label Bd. Show all posts

Tuesday, September 27, 2011

MHC Genes Convey Resistance to Bd in Some Frogs

The BBC Science and Nature News is carrying this story by Richard Black. It comments on Bd research done at Cornell University by Anna E. Savage and Kelly R. Zamudio in an attempt to discover how amphibian populations might be best able to fight off the Bd fungus.

'Scientists have taken a big step toward understanding why some frogs survive the fungal disease chytridiomycosis while others quickly die.

A group from Cornell University, US, identified genetic factors that seem to make some individual frogs immune.

This could improve captive breeding schemes, the team writes in Proceedings of the National Academy of Sciences.

Chytridiomycosis is slowly spreading across the world, and has already sent a number of species extinct.

Just two years ago, researchers discovered how it kills - by damaging the skin so that key nutrients cannot pass through, resulting eventually in cardiac arrest.

Frogs and other amphibians that have no resistance succumb quickly, with some populations being extirpated in just a few weeks.

Yet some populations and some entire species survive intact, which has long perplexed researchers; and salamanders and caecilians (limbless amphibians bearing a superficial resemblance to earthworms or snakes) appear more resistant as groups than frogs - another mystery.

The Cornell group collected lowland leopard frogs (Lithobates yavapaiensis) from five places in Arizona.

"The study goes a long way toward understanding the genetic basis of resistance to the amphibian chytrid”

Reid Harris, James Madison University

In the lab, they infected the animals with the chytrid fungus (Batratochytrium dendrobatidis, or Bd).

All the frogs collected from three of the locations died; but from the other two locations, some survived, and fought off the infection completely within two weeks, remaining healthy.

The researchers traced the difference back to regions of DNA that form part of the immune system called the major histocompatibility complex (MHC).

Its role - in humans and other animals, just as in amphibians - is to "present" bits of invaders such as fungi or bacteria to other immune cells, identifying them as things that must be tackled.

"All amphibians (and for that matter, all vertebrate animals) have MHC genes that play the same 'gatekeeper' role in initiating immune responses," said lead researcher Anna Savage.

"So our finding that MHC genotypes contribute to chytridiomycosis outcomes has potential ramifications for all amphibian species currently threatened by Bd."

It appears likely that the two populations whose members survived infection in the lab are the ones that had been most strongly exposed to the fungus in the wild since it was detected in Arizona in the 1970s.

In this case, they probably carry the "resistant genes" because there has already been quite a lot of selection - only the strong have survived.

"It means frogs may have the evolutionary potential to adapt [to Bd]," Ms Savage told BBC News.

"Natural selection can only result in disease adaptation if genetic variation for that trait exists, and we have shown that it does."

The next step along this avenue of research is to find out whether the same thing provides immunity in other species - and whether it can explain why entire species such as the infamous cane toad survive Bd.

Then, conservationists will have to work out how the information can be used.

Ms Savage suggests a role in captive breeding programmes - the last resort for species that cannot remain in their native habitat because it is infested with chytrid.

Chytridiomycosis can kill amphibians in less than a week - depending on their genes
The idea would be to screen amphibians' MHC genes before breeding, to increase the odds of producing Bd-resistant tadpoles.

Prof Reid Harris, an amphibian specialist at James Madison University in Virginia who is trying to develop new treatments for chytrid, described the latest news as "very exciting".

"The study goes a long way toward understanding the genetic basis of resistance to the amphibian chytrid, and it opens up the possibility of selecting for resistance to the disease," he told BBC News.

"However, amphibian defences are multidimensional and include innate immune components and microbial defences.

"It is likely that a successful mitigation strategy will be multidimensional as well."

However, chytrid is only one of the many threats that amphibians face today.

The most profound is loss of habitat, as marshes are drained, forests cleared, and wild areas tamed for human use.

"Although our study provides a new hope that amphibians may bounce back from chytridiomycosis, it does not eliminate the need for human conservation efforts," Ms Savage stressed.

"Habitat loss, invasive species and habitat degradation are other major factors leading to amphibian declines; and if we can work to provide good habitats so that amphibian population sizes and genetic diversity can increase, they will be much more likely to have the genetic capacity to adapt to Bd."

Anna E. Savage and Kelly R. Zamudio 2011. MHC genotypes associate with resistance to a frog-killing fungus.PNAS Published online before print September 26, 2011, doi: 10.1073/pnas.1106893108

Thursday, September 22, 2011

Diverse Ecosystems May Reduce Threat From Bd

The Western Toad, Anaxyrus boreas.

Photo Credit: Ivan Phillipsen, OSU

The following is a press release from Oregon State University.

CORVALLIS, Ore. – Researchers at Oregon State University have shown for the first time that loss of biodiversity may be contributing to a fungal infection that is killing amphibians around the world – a finding that provides more evidence for why biodiversity is important to many ecosystems.

The research, being published this week in Proceedings of the National Academy of Sciences, used laboratory studies of amphibians to show that increased species richness decreased both the prevalence and severity of infection caused by the deadly chytrid fungus, Batrachochytrium dendrobatidis.

“With greater diversity of species, you get a dilution effect that can reduce the severity of disease,” said Catherine Searle, an OSU zoologist and lead author on the study. “Some species are poor hosts, some may not get infected at all, and this tends to slow disease transmission.

“This has been shown in other systems like Lyme disease which infects humans, mice and deer,” she said. “No one has really considered the dilution effect much in amphibians, which are experiencing population declines throughout the world. It’s an underappreciated value of biodiversity.”

It’s generally accepted, the researchers said, that a high diversity of species can protect ecosystem function, help to recycle nutrients, filter air and water, and also protect the storehouse of plant or animal species that may form the basis of medicines, compounds or natural products of value to humans.

Protection against the spread of disease should more often be added to that list, they said.

“Emerging infectious diseases are on the rise in many ecosystems,” said Andrew Blaustein, a co-author on this study, professor of zoology at OSU and leading researcher on the causes of amphibian declines.

“Protection of biodiversity may help reduce diseases,” he said. “It’s another strong argument for why diverse ecosystems are so important in general. And it’s very clear that biodiversity is much easier to protect than it is to restore, once it’s lost.”

The fungus, B. dendrobatidis, can lead to death from cardiac arrest when it reaches high levels in its amphibian hosts. It is not always fatal at lower levels of infection, but is now causing problems around the world. One research team has called the impact of the chytrid fungus on amphibians “the most spectacular loss of vertebrate biodiversity due to disease in recorded history.”

Amphibians face threats from multiple causes, including habitat destruction, pollution, increases in ultraviolet light due to ozone depletion, invasive species, and infectious disease.

The dilution effect can occur in plants and animals, but also in human diseases. In a different report published last year in Nature, researchers noted an increased risk of West Nile encephalitis in the U.S. in areas with low bird diversity. And in more diverse communities, the infection of humans by schistosomiasis – which infects 200 million people worldwide – can be reduced by 25-99 percent.

Catherine L. Searle, Lindsay M. Biga, Joseph W. Spatafora, Andrew R. Blaustein. A dilution effect in the emerging amphibian pathogen Batrachochytrium dendrobatidis. Proceedings of the National Academy of Sciences, 2011; DOI: 10.1073/pnas.1108490108

Monday, August 29, 2011

Daphnia Feed on Bd: Can this Save Frog Species From Extinction?

Researchers have confirmed that this
zooplankton, Daphni magna, will eat
a deadly fungus that is devastating
amphibian populations around the world.
 It may provide a new biocontrol agent
to help address this crisis. (Photo
Credit:Oregon State University)
Zoologists at Oregon State University have discovered that a freshwater species of zooplankton will eat a fungal pathogen which is devastating amphibian populations around the world.

This tiny zooplankton, called Daphnia magna, could provide a desperately needed tool for biological control of this deadly fungus, the scientists said, if field studies confirm its efficacy in a natural setting.

The fungus, B. dendrobatidis, is referred to as a "chytrid" fungus, and when it reaches high levels can disrupt electrolyte balance and lead to death from cardiac arrest in its amphibian hosts. One researcher has called its impact on amphibians "the most spectacular loss of vertebrate biodiversity due to disease in recorded history."

The research, reported August 26 in the journal Biodiversity and Conservation, was supported by the National Science Foundation.

"There was evidence that zooplankton would eat some other types of fungi, so we wanted to find out if Daphnia would consume the chytrid fungus," said Julia Buck, an OSU doctoral student in zoology and lead author on the study. "Our laboratory experiments and DNA analysis confirmed that it would eat the zoospore, the free-swimming stage of the fungus."

"We feel that biological control offers the best chance to control this fungal disease, and now we have a good candidate for that," she said. "Efforts to eradicate this disease have been unsuccessful, but so far no one has attempted biocontrol of the chytrid fungus. That may be the way to go."

The chytrid fungus, which was only identified in 1998, is not always deadly at low levels of infestation, Buck said. It may not be necessary to completely eliminate it, but rather just reduce its density in order to prevent mortality. Biological controls can work well in that type of situation.

Amphibians have been one of the great survival stories in Earth's history, evolving about 400 million years ago and surviving to the present while many other life forms came and went, including the dinosaurs. But in recent decades the global decline of amphibians has reached crisis proportions, almost certainly from multiple causes that include habitat destruction, pollution, increases in ultraviolet light due to ozone depletion, invasive species and other issues.

High on the list, however, is the chytrid fungus that has been documented to be destroying amphibians around the world, through a disease called chytridiomycosis.

Its impact has been severe and defied various attempts to control it, even including use of fungicides on individual amphibians. Chytridiomycosis has been responsible for "unprecedented population declines and extinctions globally," the researchers said in their report.

"About one third of the amphibians in the world are now threatened and many have gone extinct," said Andrew Blaustein, a professor of zoology, co-author on this study and an international leader in the study of amphibian decline.

"It's clear there are multiple threats to amphibians, but disease seems to be a dominant cause," he said.

Although they have survived for hundreds of millions of years, amphibians may be especially vulnerable to rapid environmental changes and new challenges that are both natural and human-caused. They have a permeable skin, and exposure to both terrestrial and aquatic environments.

Because of this, OSU researchers said, other animals such as mammals, birds and fish have so far not experienced such dramatic population declines

Original Citation
Julia C. Buck, Lisa Truong, Andrew R. Blaustein. Predation by zooplankton on Batrachochytrium dendrobatidis: biological control of the deadly amphibian chytrid fungus? Biodiversity and Conservation, 2011; DOI: 10.1007/s10531-011-0147-4

Friday, August 19, 2011

Researchers Complete First Major Survey of Amphibian Fungus in Asia

Rana similis, a frog species found in the Philippines.
The survey conducted by Vance Vredenburg and
colleagues found that this species has one of the
highest infection levels of the chytrid fungus in Asia
 and is potentially threatened by the disease. Photo
Credit: R. Brown, University of Kansas.

An international team of researchers has completed the first major survey in Asia of a deadly fungus that has wiped out more than 200 species of amphibians worldwide. The massive survey could help scientists zero in on why the fungus has been unusually devastating in many parts of the globe -- and why Asian amphibians have so far been spared the same dramatic declines.

The disease chytridiomycosis, caused by the fungus Batrachochytrium dendrobatidis or Bd, is the culprit behind amphibian extinctions in Central, South and North America, Australia and Europe. The new Asian survey of the fungus, which was published Aug. 16 in the journal PLoS One, shows that Bd is prevalent at very low levels in the region.

Asia is home to a highly diverse set of amphibian species, and potentially could be vulnerable to Bd. But Vance Vredenburg, assistant professor of biology, said very little is known about the fungus and its impact on the health of amphibians in Asia.

"That's why we're excited about this first really big survey," said Vredenburg, who led the research team. "If you look at chytrid worldwide, Asia's been the black hole in our data."

From 2001 to 2009, Vredenburg and his colleagues surveyed more than 3,000 amphibians -- mostly frogs -- from 15 Asian countries, swabbing the toe webbing, thigh and abdomen of the animals to pick up any signs of Bd, which infects the skin of amphibians.

They found that the prevalence of Bd was very low throughout the region, appearing in only 2.35 percent of the frogs. The Philippines, Kyrgyzstan, Laos, Indonesia, Malaysia and South Korea were the only countries with any Bd infection.

The survey suggests that Bd is either emerging in Asia, or may have been in Asia at low levels for a long time or that some other factor is preventing Bd "from fully invading Asian amphibians," the researchers write.

Each site in the study was only surveyed once, Vredenburg explained, so it's difficult to determine whether Bd infections in the countries are newly expanding. It will be critical, he said, "to see how Bd prevalence is changing through time, because this is key to understanding the ultimate outcome of the disease."

If Bd has been in Asia for a long time, researchers would like to know why amphibians there have managed to co-exist with a fungus that has proved so destructive elsewhere. It is possible, for instance, that Asian amphibians might bear some sort of bacterial protection against Bd in their skins.

Other scientists are analyzing the genes of the Bd fungus collected globally, Vredenburg said, "to find out whether strains from different parts of the world also differ in their virulence."

Vredenburg said the possibility of another wave of extinctions highlights the need to follow the Asian survey with further research to answer all of these questions.

And if Asia is on the brink of a chytrid epidemic, Vredenburg and colleagues think it might start in the Philippines. "The prevalence and intensity of Bd infection is much higher here than anywhere else in Asia," he said. "Bd in the Philippines today looks similar to Bd in early outbreaks in California and South and Central America."

"This study is the first important step to understanding Bd in Asia," Vredenburg said. "It provides a solid foundation that future studies can build upon."

Link to Original Article: Swei A,  et al. (2011) Is Chytridiomycosis an Emerging Infectious Disease in Asia? PLoS ONE 6(8): e23179. doi:10.1371/journal.pone.0023179

Friday, May 6, 2011

The Spread of an Amphibian Epidemic, Bd in Mesoamerica

Male (top) and female (bottom)
Golden Toads, Incilius periglenes
(Savage, 1967). Monteverde, Costa
Rica. Described by science in 1967
it is likely now extinct, the victim of
 Bd. This was a lower montane
rainforest species. JCM
Evidence suggests that we are seeing the start of the 6th mass extinction event in Earth's history, and some of the evidence comes from amphibians, 40% of which are considered endangered. Hypotheses have implicated habitat destruction, overexploitation, pollution, and climate change in the loss of amphibians, but the infectious fungal disease, chytridiomycosis, has been the main suspect in the disappearance of many different species in many different families in geographically distant locations. The chytridiomycete fungus Batrachochytrium dendrobatidis (Bd) has a flagellated infective life stage called the zoospore that penetrates the skin of amphibians causing hyperkeratosis, this results in a loss of skin function, osmoregulatory failure, and death. First reported in 1999, Bd has been shown to be closely associated with the collapse of amphibian populations in Australia, Panama, California, and Peru and has been linked to amphibian declines that occurred decades ago. Bd is unusual because multiple host species in at least one locality have been extripated before density-dependent factors could slow the spread of disease. Now, Tina Cheng and colleagues (2011) present evidience on how Bd has spread using museum specimens from vanished populations. Using two well-studied cases of amphibian decline in Mesoamerica: the decline and disappearance of anurans from Costa Rica’s Monteverde Reserve in the late 1980s, and the decline and disappearance of plethodontid salamanders from the mountains of southern Mexico and western Guatemala in the 1970s and 1980s, the authors test retroactively whether Bd emergence was linked to earlier declines and extinctions. They use a noninvasive PCR sampling technique that detects Bd in formalin-preserved museum specimens and found Bd using the PCR technique in 83–90% (n = 38) of samples that were identified as positive by histology. They examined specimens collected before, during, and after major amphibian decline events at established study sites in southern Mexico, Guatemala, and Costa Rica and found a pattern of Bd emergence coincident with decline at these localities. The absence of Bd over multiple years at all localities followed by the concurrent emergence of Bd in various species at each locality during a period of population decline. The geographical and chronological emergence of Bd at these localities suggests a southward spread from southern Mexico in the early 1970s to western Guatemala (1980s/1990s), and to Monteverde, Costa Rica (1987); thus the authors found evidence of a historical “Bd epidemic wave” that began in Mexico and subsequently spread to Central America. The full article is available on-line, follow the link below.