When talking about diseases which affect frogs, the parasite group is often overlooked. This group includes tapeworms, nematodes, flukes, trematodes (flatworms), filaria, protozoa and fly larvae.
In the past, it was thought that parasites weren't too detrimental to the frog's well being - just incidental freeloaders - and they weren't supposed to be capable of actually causing the frog's death. However, the pathology results from frogs that we received during the North Queensland winters of 1999 through 2002 revealed that the worms themselves were the cause of death in these frogs. The worms are actually the middle domino in a set.
We still believe that a disease pathogen (the primary pathogen) is first getting into some frogs and disabling their immune system. Once that has occured, the frog has lost its biological defense system and becomes prey to a variety of parasites which infest the frog in huge numbers. The parasites themselves can cause death through the damage they do to internal organs and essential body tissues (such as liquifying the thigh muscles so that the frog can't catch its food or escape from predators). Other times, the worms dissolve holes in the skin which leads to the frog being swamped by bacteria and fungi and these act as the cause of death.
One of the nastiest parasites being seen thus far is the tapeworm Spirometra erinacei (see photo at right) which becomes a breeding adult once it has been picked up by cats. This worm migrates through several internal organs in the frog, damaging them as it goes, until it finally burrows into the muscle tissues in the thighs. Once there, the muscle tissue is liquified, lymph sacs damaged and the protective skin over the bone breached which leads to breaks in the thigh bone. Holes appear in the skin which allows the frog to be flooded with bacteria which leads to scepticaemia (blood poisoning). Other times, the thigh muscle is compromised by cavities in the muscle and these fill with blood. Spirometra (the sub-adult stages of which are also referred to as spargana) lives in many hosts including insects, frogs, reptiles, cats and dogs but it has now become a serious predator of frogs in Far North Queensland.
There is a special worming regime needed to kill spirometra in cats and this is described on our Cat Alert page in the threats to frogs section.
Once the frogs are infected, the worms are extremely difficult to treat and different medications used thus far have only acheived limited success. The Spirometra worms in particular might have their own 'radar cloaking device' as they migrate through the frog's body, excreting both an immune suppressant and an anti-inflammatory agent which enables the worms to hide their presence from the frog's natural defenses. This lack of recognition by the immune system might also be because of a possible primary pathogen which is blinding the frog's defense system from recognising any invaders. By the time the frog starts exhibiting external symptoms (usually lesions and lumps but also fluid retention in the legs), the infestation is already quite severe and the frog's tissues have been damaged.
As more of our backlog of specimens is tested, more and more parasite problems are emerging. A blood parasite called a filaroid has been found at massive infestation levels in emaciated frogs which were turned in during our very dry winter and excessively hot spring in 2001. We've also found Capillaria (threadworms, bladder worms) in many frogs, Rhabdias (a lung parasite), nematodes and flatworms. Hydatid parasites also cause an unusual symptom of turning the frog's dorsal skin black, like soft rubber, which then smears off.
Some worms could not be identified and have been sent on to other researchers who might be able to identify them using DNA analysis. Some worms show up in the most unusual places such as these flatworms (right) which were found infesting the liver of a squamous cell cancer case. (the black colour of the liver shows that a systemic infection - which could have been bacterial, fungal or viral - has been in progress for some time)
While cleaning out a large backyard pond in Smithfield, Cairns, every tadpole we found had these snowflake-like growths all over the body. Some successfully metamorphed but died within days. The lab result identified the growths as a soil parasite called a ciliated protozoa. The particular pond involved had been allowed to silt up with soil carried into the pond during rainy seasons and this muddy bottom is where the protozoa were living until they attacked the tadpoles. (A good reason to keep your frog ponds from silting up - leaf litter on the bottom is good but not mud and soil!)
Another prominent parasite of the Far North is not a worm but a maggot which usually only occupies the frog on a temporary basis and we've given this temporary house guest (the Bot fly larvae, batrachomyia, its own page.