Investigating frog disease

New Zealand has 4 species of native frogs that are all endemic. All 4 are threatened with extinction. A major focus of the amphibian research at the University of Otago is how to save these unique animals.

Professor Phil Bishop, along with colleagues and postgraduate students, is working on techniques to successfully keep and breed frogs in captivity. An important aspect of this work is the research into frog disease.

Frog diseases

Phil’s frog disease research has 2 main strands – metabolic bone disease and chytrid fungus.

Metabolic bone disease is affecting reptiles and amphibians throughout the world and is unusual because it only affects animals in captivity.

Chytrid fungus is an ancient type of fungus. Scientists think that it originated in Africa and was spread when people shipped a species of frog used for pregnancy testing around the world. The chytrid fungus causes chytridiomycosis, an infectious disease that has now affected almost every frog population in the world. Scientists are not sure about how the fungus works. They know that it attacks the skin, and this may affect the ability of frogs to stay hydrated. Some scientists believe that the fungus secretes a toxin that eventually poisons the frog.

Chytridiomycosis

Chytridiomycosis was first discovered in New Zealand frogs in 1999. It has affected the two known populations of Archey’s frogs. Fortunately, no other native frogs have tested positive at this stage. Scientists are not sure whether this is because they are resistant to the disease or whether they haven’t yet been exposed to it.

Chytridiomycosis is a difficult disease to diagnose. The fungus often causes some slight redness on the stomach and legs of infected frogs, but there are no obvious ulcers or lesions. The biggest impact is on the behaviour of the frogs, but even this is hard to spot unless you are an expert in frog behaviour. Scientists use swabs and DNA analysis in the lab to test for the disease.

Why worry about the chytrid fungus?

Chytridiomycosis kills frogs. Research into the treatment and transmission of the fungus is important internationally as chytridiomycosis is blamed for the decline and extinction of numerous species of frog around the world. It is also very important in New Zealand. Archey’s frog is critically endangered. It is already vulnerable to introduced predators and loss of habitat. There is also concern that the disease could spread to our other native frogs.

Finding a cure for chytrid

The frog research team at the University of Otago discovered that a common antibiotic (chloramphenicol) used for humans was able to kill samples of chytrid fungus in the lab. They decided to test this antibiotic on some Archey’s frogs that were infected with the disease. They found that they were able to cure the diseased frogs and keep them alive in the lab – an important breakthrough for this critically endangered species.

This antibiotic treatment is very useful in New Zealand. Phil has also been collaborating with scientists overseas to test the treatment of infected frogs with this same antibiotic. However, there are some problems. For example, even if the frog has a negative test for chytridiomycosis, the scientists are reluctant to put the animal back into the wild in case the result is a false negative. Additionally, this treatment only works in captivity, as it’s not possible to spray antibiotics in the wild.

However, one very positive aspect of this research is that, if a vulnerable population is infected in the wild, the scientists can at least cure them in captivity. The hope is that, in the near future, it may be possible to re-release these frogs as well as to develop alternative treatment options for wild populations.