Literature Review
ENV 250
Daniel Sylvester
Dr. Collins
Introduction
Chytrid mycosis is a fungal disease that infects the skin of amphibians and is caused by
the pathogens B. salamandrivorans and B. dendrobatidis (https://www-mdpicom.ezproxy.snhu.edu/2309-608X/6/4/234/htm). Chytrid is responsible for population declines
and extinction of amphibian species around the world. The unprecedented loss of biodiversity
around the planet due to this fungal pathogen has sparked a flurry of research and investigation
into the fungus. Around 501 different amphibious species have been discovered to host the
fungus and 90 of those amphibian species have gone extinct due to the pathogen, making Chytrid
mycosis and its destruction of amphibian life on Earth one of the most pressing extinction events
in today’s time. It is approximated that over forty percent of amphibian species could face
extinction (Lambert, 2020). The fungus works by infiltrating the skin of an amphibian.
Amphibious adaptations to water and evolutionary adaptations that make frogs ectotherms make
their skin an extremely favorable environment for an invasive fungus like chytrid. The fungus
has been observed, exhibiting immunosuppressive properties that allow it to prevent the innate
immune response of amphibians from properly defending against epidermal fungal infection.
More research is needed in understanding how chytrid is disrupting the immune response, but
many researchers and biologists conclude that prevention will be the best cure (Grogan, 2020).
Other research suggests it is stress that suppresses the immune response (Kelly, 2020). It seems
all possibilities could be correct or wrong and the path to prevention begins in a better
understanding. The origins of chytrid are still up to dispute but one study suggests that Chytrid
mycosis is native to Southern Africa where it once was an endemic infection in Xenopus laveis
frogs. It is thought that people spread the fungus out of its natural host population when humans
first began trading the amphibian species in the early 1900s. The frogs were used by the
scientific community in the early 50s to study immunity, embryology, and molecular level
biology. It is proposed that specimens that escaped during transit spread the fungus to local
native frog populations in countries that received frog exports from Aftrica. Xenopus is not
thought to be the primary vector for the fungus, the American Bullfrog’s trade as a food item, is
thought to be a secondary vector that contacted infected Xenopus laveis and then spread chytrid
to species around the world by way of international food trade. (Weldon, 2004). Keep in mind
that there is still much controversy and debate on the origin of chytrid. A new theory uses a
phylogenic tree displaying the global genetic diversity of chytrid to determine its origin. This
study outlines “hyper diversity” in Asia suggesting it originated from the Korean Peninsula
(O'Hanlon, 2018). No solid conclusions can be made about its true origins, just highly educated
predictions. One thing that does seem to be consistent within available literature is that human
beings removed amphibians from their native habitat and spread the fungus. Just about every
aspect of chytrid is poorly understood. It is alarming how little we actually know about an
infection that has been so widespread for so long. Once again, humanity’s industrious ambitions
have blindly stepped on the toes of the natural world. Amphibians, a natural indicator of the state
of the environment, are in peril due to the actions of humans. The fight against chytrid is in dire
need of standardization and a solid procedure that will help bridge together the large amount of
research being conducted. Identifying the commonalities and differences in available literature is
crucial to making the next step.
Clinical Identification
Identifying the fungus can be divided into two categories one of identification in larvae
and the other in developed amphibians. In larvae, the mouth area will appear to be a lighter color
and have loss of pigment, under development due to lethargy and inability to find food. In adult
amphibians, excessive shedding, red discoloration, poor reflexes bad posture and lethargy are all
symptoms of a chytrid infection (Van Rooij, 2015). Identification methodology is one of the
most apparent hurdles the conservation community needs to face. There are many studies that
have used oral deformation in tadpoles to indicate the presence of the fungus while some
research argues against this. It is arguable that before more studies are conducted researchers
must create a standard practice and identification procedure for chytrid. Research was conducted
in the Atlantic Forest in Brazil to determine if oral deformation can be a reliable way to detect
chytrid and the study concluded that in some cases which differ from species to species, there is
a strong connection between the two but there were false positive and false negative rates of up
to 100% where there could be no reliable correlation gleamed from the statistics. (NavarroLozano, 2018). If there is research out there that has been relied upon for our current
understanding of chytrid that utilized oral deformity as a method to identify and quantify
prevalence, then our understanding is wrong, and a step must be taken back before we can take
one forward to make sure identification practice is proper and consistent. This same research
recommended the use of more reliable testing such as histology, PCR and histochemistry.
Research conducted on the viability of PCR as a detection technique claims it is twice as
successful in detecting pathogens than histology and it is less invasive and can be used on
endangered species of frogs without putting a vulnerable population at further risk (Kriger,
2006). There are still issues with the PCR method of detection. In some cases, in this same study,
PCR failed to detect any chytrid while histology was able to detect the fungus on the same
organism. A large takeaway from examining diagnosis techniques is that different things can
work better for different people or in different circumstances. If an organization were to
standardize chytrid detection I would infer that they would prefer a genetic PCR analysis but not
stamp out other forms of testing. Oral studies can be very informative but should not be included
in prevalence testing.
Pathology
Both of the pathogens that cause chytridiomycosis are known to be ubiquitous aerobic
saprotrophs which means they create food from dead and decaying matter and typically thrive in
water and soil-based areas, requiring oxygen to live (Grogan, 2020). Infection begins when a
zoospore inserts its flagellum into the skin of its host. These zoospores are highly mobile and are
oft spread in large numbers by the excessive skin shedding of infected frogs. The spore becomes
encysted within the first eight hours after contact and then forms a germ tube which grows
deeper into the epidermal cells of the host which inserts infectious contents into the lower cells
of the epidermis all within the first 24 hours. Within two days a developed sporangium will
begin cellular division and create zoospores which are released into the environment to seek out
new hosts. Fungal growth and keratinization of the epidermis compromises the skin which
allows bacteria and other infectious materials to enter the organism. Understanding the pathology
of chytrid is a very important first step because it allows researchers and conservationists to
devise a strategy to combat the disease. In this case, the extremely high mortality rate, which can
be observed at up to 100% in certain species, has urged scientists to focus on understanding how
chytrid is introduced into a population so it may be prevented as the logistics to finding a cure or
medical approach is poor (Kilpatrick). A prevention-based methodology becomes even more
favorable when one begins to understand the massive variety in immune response and how the
fungus affects species differently. Toxin secretion, immune suppression, electrolyte depletion
and co -infection have all been mentioned in assays examining how this fungus is so good at
killing off amphibians.
Epidemiology
Chytrid mycosis is caused by two separate but evolutionarily similar pathogens. B. dendrobatids
is the pathogen that has spread far and wide across the globe. The fungus is known to be endemic
in some populations and novel in others. By testing museum specimens researchers have been
able to identify chytrid in Asian frogs over 100 years old, thought to be populations that the
fungus could have originated in. (Van Rooj, 2015). Since then, the fungus has been identified on
every continent except the
Arctic. There are two primary hypothesis that can explain how emerging infectious diseases
originate. The Endemic Pathogen Hypothesis explains that a pathogen that once coexisted in a
population has become increasingly pathogenic due to higher susceptibility in a host population,
or due to changes in environment. The second is Novel Pathogen Hypothesis which states that a
pathogen has been introduced to a new geographic location and new populations of susceptible
organisms that have had no prior exposure to the pathogen, making them highly susceptible (Van
Rooj, 2015). There is still much debate on which hypothesis or if both could apply to the spread
of chytrid. One of the most recent and well-known developments in chytrid epidemiology is the
outbreak of B. Salamandrivorans in alpine newts in Southern Germany. Chytrid was first
identified in Bavaria in 2020 and a high death rate was reported by a village called Memmingen
where in garden ponds and fountains, alpine newts were found deceased (Schmeller, 2020). The
previously nearest outbreak was over 170 km away and plants being purchased by people to put
in their garden ponds are thought to be a possible vector for the disease. The newts in this study
are not endangered by the fungus and have shown some resistance but the main concern is that
chytrid will continue to spread to other more at-risk species such as the alpine salamander which
is endangered. Much more testing will need to be done throughout Germany in the coming years
to keep track of the spread and vectors for the fungus to prevent further loss of biodiversity. If
left unchecked, chytrid will continue to extinct important species on all continents. It is not too
late, but conservationists must begin to fill in grey areas where very little has been done to
understand the spread and reach of chytrid.
One such place that has had very little research conducted on the prevalence of chytrid is
the North East United States. As of 2007 conservationists only had one study to gleam an
understanding of prevalence in New England. It was a prevalence test involving 751 specimens
collected from five states including New York, New Hampshire, Vermont, Maine,
Massachusetts. The study highly favored collection in Maine as out of 86 collection sites, 74
were located in Maine (Richards-Hrdlicka, 2013). The Connecticut study took an approach
where they focused only on their own state to create a more localized understanding of chytrid
before surveying beyond their borders. The Longcore et al. study published in 2007 was still an
excellent study and opened a gateway for further study in the North East and did focus regionally
on Maine. Spreading resources too thin and across large areas helps us understand that chytrid is
present over a large area but we already know that due to its prevalence across the globe.
Choosing a smaller area like CT and testing opportunistically over the course of several years
helps us understand the epidemiology on a regional basis. Both of these studies found similar
prevalence of around 0.28 across all species of amphibians that were tested (Richards-Hrdlicka,
2013). Chytrid has been hypothesized to have began spreading around the globe starting in
American Bullfrogs. It would make sense for future research in the area to do genetic testing of
strains of chytrid in the area to help aid the search for the origin of chytrid. Ecologists of New
England should follow in the footsteps of the CT study. It was vigorous, ambitious, and
successful in quadrupling our available information on the fungus in that state and other states
wildlife agencies should be taking note of the methodology used in this study.
Conclusion
People have only begun to scratch the surface of chytrid mycosis and the implications it
has on the environment and the state of amphibian life on Earth. Chytrid has already aided an
ongoing extinction of amphibians, threatening 40% of amphibian life on Earth. We know that
humans have most likely spread this once endemic and co-existent fungus across hundreds of
species. We are responsible and it is our duty to study and aim to understand this amphibian
pandemic to hopefully stop its spread and learn to never cause such a seemingly irreversible and
detrimental issue. As mentioned above, prevention is the best cure. Ethical practices in the
animal trade must be enforced and people will need to get their fog legs from their own countries
because these luxuries have major repercussions. This is very much an ethical issue and
hopefully by examining chytrid, people will be able to learn and realize how far reaching our
abuse of the planet is. After reading about how little information is available on local New
Hampshire and New England chytrid occurrences, members of the Southern New Hampshire
scientific community should begin gathering attention and aid in researching the chytrid
epidemic in our own backyards. Imagine a New England spring without the sound of peepers,
the rare coastal cranberry bog toad of Plumb Island would cease to exist if its small population
were wiped out by chytrid. It is reaffirming to see how much overall data is available on chytrid
mycosis, but it is hard to find a report that claims any part of the epidemic is well understood or
documented. Research practice commonly involves PCR analysis of the fungus as a primary
diagnosis technique but other less reliable forms of diagnosis like physical assumption and
histology should be phased out so a standardized testing procedure can emerge. This would
allow people interested in fighting chytrid all around the world to be using a similar language
and build a stronger overall knowledge of this issue. A united front dedicated to compiling good
data and practice, eliminating poor data and practice, and then making said practices accessible
to conservation organizations around the globe would turn this ecological pursuit from a blob of
separate research into a community of shared knowledge that us much more able to confront
chytrid mycosis and protect biodiversity on this planet.
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