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Biological Hazard – Deadly Chytrid fungus: (pathogenesis) Experimental infection

2017/04/26

CHYTRID FUNGUS, ZEBRA FISH: EXPERIMENTAL INFECTION
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Published Date: 2017-04-24 11:40:36
Subject: PRO/AH/EDR> Chytrid fungus, zebra fish: experimental infection
Archive Number: 20170424.4990724

Date: Thu 20 Apr 2017
Source: Phys.org [edited]

The deadly chytrid fungus has for the 1st time been found to infect and kill species other than amphibians, giving clues on how it causes disease.

The fungus, _Batrachochytrium dendrobatidis_ (Bd), is a type of chytrid that has severely affected over 700 amphibian species worldwide, and has made more species extinct than any other infectious disease known to science — at least 200 so far. It causes chytridiomycosis, a disease that damages amphibian skin and rapidly kills its host.

Until now, chytrid was thought to only affect amphibians, a group that includes toads, newts, salamanders, and frogs. However, researchers from Imperial College London have now demonstrated in the laboratory that Bd can also infect zebrafish at the larval stage — the developmental phase just after they hatch from eggs.

Research into Bd currently relies on studying infected amphibians. However they are difficult to study and also need to be captured in nature, which is not sustainable in the longer term. Amphibians that are sourced from different natural populations also may respond very differently to the fungus.

Zebrafish are some of the most widely-used biological model species owing to their transparency at the larval stage, which allows scientists to use microscopy to easily track infections. Their immune systems also have many parallels with that of humans and other vertebrates such as frogs.

The team behind today’s discovery, which is published in Nature Communications [see reference below], say their work will lead to zebra fish as a new model for studying the disease. This could give scientists the opportunity to understand in more detail how the fungus harms its amphibian hosts.

Professor Mat Fisher, a co-author from Imperial’s School of Public Health, said: “The fact that chytrid is able to infect zebrafish larvae could mean that we now have a more effective animal model with which to study the fungus and continue our research in how to save these amphibians.”

The researchers found that Bd infection took hold in zebrafish larvae in a similar way to how it does in amphibians. Professor Fisher added: “The natural bacterial coating found in young zebrafish appeared to protect them from harm during infection, and meant they could fight off the chytrid. This is a far more humane way to study the fungus than our previous models, and means we now have a new laboratory model.”

Furthermore, because zebrafish breed quickly, the researchers can use many more than they can with frogs. This would help to make research go further and faster.

Co-author Dr Serge Mostowy from Imperial’s Department of Medicine said: “A zebrafish model represents a brand new opportunity to study the disease process of chytrids. Young zebrafish have fully developed innate immune systems, which means we can now easily study host-fungus interactions in real time using non-invasive techniques. We can also control their environment with antibiotics, allowing us to study the role of already-present bacteria in influencing chytrid infection.”

The findings may also offer clues into how the fungus spreads between hosts. The researchers suggest that zebrafish larvae and other fish species could act as environmental reservoirs in the wild, and may pass the infection onto amphibians.

Ms Nicole Liew, lead author of the paper from Imperial’s MRC Centre of Molecular Bacteriology and Infection said: “The more we know about how Bd can infect hosts and where it resides in the environment, the better we can prepare for it and prevent more deaths. Our findings today give us an exciting wealth of information to work with, opening a whole new avenue of research. From our experiments, we now know some of chytrid’s hiding places, and present a new lab model highly suited for fluorescent microscopy, enabling us to learn more about the disease process.”
The scientists also managed to infect another species of fish, the guppy, but these fish ended up clearing their infection eventually. The authors say that although their research shows that young zebrafish can be infected, further studies are needed to determine the extent that fish might act as reservoirs of infection in the environment.

Professor Fisher added: “Our knowledge of this devastating fungus is growing in leaps and bounds, and we are excited to see where this new information will take us in terms of saving our amphibian friends.”

[Byline: Caroline Brogan]

Communicated by:
ProMED-mail from HealthMap Alerts
<promed@promedmail.org>

[The reference for the scientific article describing the finding is
Liew N, Mazon Moya MJ, Wierzbicki CJ, et al: Chytrid fungus infection in zebrafish demonstrates that the pathogen can parasitize non-amphibian vertebrate hosts. Nat Commun. 2017 Apr 20;8:15048. doi: 10.1038/ncomms15048;
available at https://www.nature.com/articles/ncomms15048.

The abstract reads, “Aquatic chytrid fungi threaten amphibian biodiversity worldwide owing to their ability to rapidly expand their geographical distributions and to infect a wide range of hosts. Combating this risk requires an understanding of chytrid host range to identify potential reservoirs of infection and to safeguard uninfected regions through enhanced biosecurity. Here we extend our knowledge on the host range of the chytrid _Batrachochytrium dendrobatidis_ by demonstrating infection of a non-amphibian vertebrate host, the zebrafish. We observe dose-dependent mortality and show that chytrid can infect and proliferate on zebrafish tissue. We also show that infection phenotypes (fin erosion, cell apoptosis and muscle degeneration) are direct symptoms of infection. Successful infection is dependent on disrupting the zebrafish microbiome, highlighting that, as is widely found in amphibians, commensal bacteria confer protection against this pathogen. Collectively, our findings greatly expand the limited tool kit available to study pathogenesis and host response to chytrid infection.”

This discovery is significant in 2 ways: first, it allows the development of an interesting animal model for this disease, although inference should be made with caution given the taxonomic distance between fish and amphibians and the artificial settings. Second, it hints that the infection should be studied in fish, and perhaps in other animals too. In fact, a few years ago the chytrid fungus was found in crayfish.

_B. dendrobatidis_ is generally thought of as an amphibian specialist that consumes host keratin for sustenance, despite it commonly being maintained in the laboratory on nonkeratinized media, such as tryptone. Numerous vertebrate and invertebrate taxa possess keratin or keratin-like compounds in their gastrointestinal tracts. Hence, it is not surprising that previous researchers have hypothesized that there might be non amphibian hosts or vectors of _B. dendrobatidis_. – Mod.PMB]

See Also

2013
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Chytrid fungus, crayfish – USA: non-amphibian hosts 20130105.1483017
2011
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Chytrid fungus, frogs – worldwide: mechanism of spread
2010
—-
Chytrid fungus, frogs – Worldwide: possible recovery 20101212.4421
Chytrid fungus, frogs – worldwide: review article 20100130.0323
2009
—-
Chytrid fungus, frog – South Korea 20090920.3301
Chytrid fungus, frog – Philippines: (Luzon) 20090527.1976
2008
—-
Chytrid fungus, frogs – Panama 20081014.3246
Chytrid fungus, frogs – Spain (Majorca) 20080928.3065
2007
—-
Chytrid fungus, frogs – Japan (02): wild frogs 20070613.1924
Chytrid fungus, frogs – Japan 20070113.0176
2006
—-
Chytrid fungus, frogs – worldwide: possible source 20060524.1463
Chytrid fungus, frogs – South Africa 20060203.0344
2005
—-
Chytrid fungus, frogs – UK (England) 20050916.2741
2004
—-
Red leg disease, frogs, fatal – UK (02) 20040914.2560
Red leg disease, frogs, fatal – UK 20040912.2542
2002
—-
Frog deformities – USA (02) 20020425.4030
Frog deformities – USA 20020422.4012
Frog mortality, virus – UK 20020201.3458
2000
—-
Chytrid fungus, frogs: background 20001201.2096
Frog deformities – USA (Northeast) 20000420.0579
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