Download Chytrid Fungus Discovered in Western Toads in Klondike Gold Rush

Chytrid Fungus Discovered in Western Toads in Klondike Gold Rush National
Historical Park, Skagway, Alaska
Last summer, National Park Service and U.S. Geological Survey researchers tested nine
western toads for the chytrid (kit-rid) fungus in the Dyea area of Klondike Gold Rush
National Historical Park and five individuals tested positive for the potentially lethal
fungus. Detection of the chytrid fungus in western toads in Klondike Gold Rush is the
first diagnosis in a toad in Alaska. A dead wood frog infected with the chytrid fungus
was discovered on the Kenai Peninsula, Alaska in 2002.
Once considered widespread and abundant in Southeast Alaska, western toads are
considerably less common today than they were twenty years ago according to long-time
area residents. Retired NPS trail crew leader Jerry Watson recalls that toads were once so
thick along the Chilkoot Trail in Klondike Gold Rush National Historical Park, that he
and his crew could not avoid stepping on some of the thousands of metamorphing toads
that blanketed portions of the trail each summer. For unknown reasons, western toads
(the park’s only amphibian) appear to have undergone sharp population declines not only
in Klondike Gold Rush but throughout Southeast Alaska’s coastal rainforests. Well
documented amphibian die-offs, malformations, and apparent extinctions worldwide
coupled with local knowledge of declining populations prompted Klondike Gold Rush to
begin studying western toads in 2003. Extensive surveys of potential breeding sites over
two summers by park researchers yielded observations of tadpoles at only six of the more
than 39 sites visited. In 2005 the park began working with the USGS to link park-level
efforts with a major national initiative to monitor trends in amphibian populations and
investigate causes of declines.
Amphibian population declines have been attributed to multiple factors working in
consort including habitat loss, exotic species, UV-B radiation, contaminants, disease and
climate change. Chytridiomycosis, an infectious amphibian disease caused by a chytrid
fungus, was first identified in 1998 by an international team of scientists from Australia,
the United States and Great Britain. The disease has been implicated in mass amphibian
die-offs and species extinctions in pristine areas of Central America and Australia, and is
considered a probable cause of precipitous boreal toad (a sub-species of the western toad)
declines in Colorado. The chytrid fungus has been responsible for sporadic deaths in
some species and 100% mortality in others. Recent studies in tropical areas of Australia
suggest that the survivors of severe chytrid-driven population crashes may be resistant to
the lethal effects of the disease and function as carriers.
Chytrid fungus damages the keratin layer of the skin, impairing the toad’s ability to
breathe and absorb water. Live toads show few clinical signs of the disease, but some
may appear weak or lethargic, exhibit excessive shedding of skin and may be reluctant to
flee at the approach of humans. Toads, as opposed to more aquatic amphibians such as
salamanders, may be more susceptible to the disease as they have a limited area of skin
over which to uptake water (i.e., the abdomen). Chytrid fungus is likely transferred by
direct contact between infected animals or through exposure to infected water. The
origin of the fungus is uncertain but evidence suggests it originated in Africa and was
transported worldwide through the international trade in African clawed frogs beginning
in the 1930s. Retrospective studies of archived specimens have enabled scientists to
establish a timeline for the introduction and spread of chytrid in some areas.
A non-destructive Polymerase Chain Reaction (PCR) technique was used to test for
chytrid fungus in western toads in Klondike Gold Rush. After the skin of the abdomen
and/or foot webbing of each toad was swabbed 25 times with a sterile cotton swab, the
toad was released unharmed. The samples were then sealed and sent to the USGS lab
and analyzed for the presence of chytrid DNA. To prevent the spread of chytrid and
other potential disease risks for western toads, the NPS and USGS follow strict disease
prevention protocols in the field which include disinfecting all footwear and equipment
before and between pond visits. To help stop the spread of the chytrid fungus, these same
precautions should be taken by anyone visiting amphibian breeding ponds in the wild and
the handling of toads should be avoided whenever possible.
We are using a variety of techniques to monitor western toad populations in Klondike
Gold Rush, including annual breeding habitat assessments, capture-recapture techniques
and chytrid fungus tests. Over time, we will estimate the size of the adult breeding
population, document timing and success of reproduction, and investigate toad
movements and habitat use within the park. Information about western toad movements
between seasonally important habitats can be important to park managers in their efforts
to maintain suitable habitat for healthy populations. This long-term study will improve
understanding of the importance of environmental factors such as water levels,
temperature, vegetation and water chemistry on western toads. Through a partnership
with the Canadian Government, the park is hoping to learn more about trans-pacific
transport of air pollutants such as mercury that have been implicated in the disappearance
of amphibians in other protected areas including Acadia National Park. This summer, the
NPS and USGS will continue testing western toads for the chytrid fungus in order to
better understand the distribution of the pathogen, the infection rate within populations,
and the role of the disease in toad population dynamics.
Contact: Meg Hahr, [email protected], (907) 224-7542