Viruses and Prions

Chytrid fungus, as awful as it is, is not the only major disease of concern for those interested in healthy frog populations. There is another group of pathogens which are already present in Australia and causing havoc overseas called viruses. Some viruses have a wide target range such as iridoviruses (ranaviruses) which attack most cold blooded animals including fish, reptiles and amphibians while others are quite specialised and attack only one type of animal such as Chronic Wasting Disease of cervids (deer and elk).

Viruses have some nasty survivalist qualities that should be given their due consideration when it comes to frog conservation:

• they are easily adaptable and can mutate into new strains to survive better or find new hosts (such as Mad Cow Disease which attacks cattle but has a strain called Crutzfeld Jacob (new var.) which attacks people)
• they can combine with other viruses to form a new virus (this is the great fear around the world right now concerning Avian Influenza - it can't go from person to person in its present form but if it were to migrate to somewhere in the world where a susceptible human flu virus is present, the two could combine to form a new virus which would have the virulence (killing strength) of the original avian flu and the person to person transmission capability of the original human flu)
• some viruses can remain harmless in the body until a particular helper virus also turns up and then they activate to cause disease (for example, some research into Chronic Fatigue Syndrome suggests that an echo or polio virus is responsible for CFS but it does not activate until the host has also become ill with another virus such as Epstein Barr, Ross River, Glandular Fever (called mononucleosis if you are in the US), etc.)
• viruses can remain dormant for several years which means an animal can be carrying them for a long time but not become ill until something else happens which stresses the animal and causes the virus to activate (like a seed in dry soil which remains viable for a period of time but only germinates when it rains); Note: dormancy is not the same as incubation which is a period of growth and reproduction where the disease has become active and building its numbers up to reach the stage where its host becomes ill.
• viruses generally can't be cured by they sometimes trigger a healthy animal to create antibodies which fight the virus the first time it appears and then linger on to protect the animal for a period of time in case it comes into contact with the same virus again; this is how vaccines work - by introducing the target virus into a body but in a weakened form so that the immune system will do its thing and create the antibodies while reducing the possibility that the body will be overtaken by the vaccine. But how would you create a vaccine to protect all frogs from viruses?

Some viruses are so strong (virulent) that they kill the host faster than the host can possibly respond with antibodies. What if the frogs are already weakened by being sick with something else? If an amphibian's immune system is already trying to fight off another problem such as chytrid fungus, a bacterial infection or too many parasites for example, then it might not be able to produce antibodies against the weakest of viruses. So trying to prevent viruses from arriving in the areas where amphibians live is pretty important.

One of the ranaviruses that has already been found in wild caught Australian frogs is Bohle Iridovirus (BIV). There have been two incidences of frogs from Townsville dying from Bohle which is endemic to Australia (in other words, this virus only occurs here and nowhere else). An exotic ranavirus known by the acronym of ENHV (Epizootic Haematopoietic Necrosis Virus) occurs in New South Wales. A batch of tadpoles collected from that region and raised in captivity, died suddenly and was tested at James Cook University. They were positive for a ranavirus but it wasn't clear if ENHV was responsible.

As part of its genetic engineering project to create a virus which interferes with toad metamorphosis (see our GMO toad virus page), CSIRO found that antibodies against ranavirus were present in cane toads caught in Australia. This means that those individuals were exposed to ranaviruses here. There are likely to be many other Australian cases where frogs have died from ranavirus but this research has, sadly, not attracted much funding or official interest thus far (but we're trying to change that!).

Being aquatic, ranaviruses in particular can have serious impacts on frog populations by killing off large numbers of tadpoles. Over time, the overall population dwindles because fewer and fewer offspring are surviving to replace the adults. And once the virus is triggered in an adult frog, it can quickly spread to other adults who hadn't previously been exposed to the virus while they were tadpoles.

There are several unidentified virus problems being investigated/studied in Australia that are targeting specific types of animals other than frogs:

There are wildlife health organisations which are chasing funding to try to keep up with all the new wildlife disease problems popping up all over Australia. (see our links page for contacts) Viruses are obviously getting around so it stands to reason that some of the viruses arriving in Australia are going to target frogs. And we know that this is happening. Visit our Immuno-deficiency, Redlynch virus, and cancer pages for more info on these amphibian problems which we believe are caused by viruses.

Many other amphibian viruses are active in other parts of the world and can be very easily brought into Australia. In the United States, there must be at least half a dozen active iridovirus (ranavirus) problems around the country and some of these are involved in the endangered status given to rare toads there. These diseases are monitored by the Animal and Plant Health Inspection Service (APHIS) and some of the government's wildlife disease investigation facilities like Pautuxet and Michigan. (We hope to add more information on this subject soon and some links to follow for more information about the US situation but we didn't want to hold up the entire disease section in the meantime! Come back to this page again for updates.)

In the meantime, you can help reduce the spread of viruses. Just follow the guidelines in our Precautions page, especially if you rescue tadpoles or do any bushwalking or frog monitoring activities. The guidelines on the Tadpole distribution page and keeping chytrid out of ponds will help too.