Three-legged frogs? There are plenty!

In the mid-1990s, a child at New Country School in Minnesota, USA, caught a three-legged frog. Since then, a drama with science fiction and horror temperament has begun, which has not yet ended.

Three-legged frogs? There are plenty!

Since the first frog, a large number of frogs with deformed hind legs have entered the American vision, some without hind legs, and some even with eight hind legs. People are worried that this is a mutation caused by environmental pollution, which has caused public panic. After the news spread to China, some people created horror stories about "mutated frogs are highly poisonous and attack people in groups", claiming that they were non-fiction literature. It was widely circulated in the era of paper media and scared many people.

But as the frog panic spread, Stanley Sessions of Hartwick College and Pieter Johnson of the University of Colorado pointed out that these deformed frogs were not the result of modern technology, but a product of nature.

Deformed frog | Brett A. Goodman, Pieter TJ Johnson / Wikimedia Commons

Long before the deformed frogs attracted attention, Sessions noticed that a parasite called Ribeiroia ondatrae, a trematode, would parasitize the hind legs of amphibians. Johnson and his colleagues then used the parasite to infect tadpoles in the lab. Strange things happened: the tadpoles grew into frogs with multiple hind legs (or no hind legs at all). After R. ondatrae entered the tadpoles, it concentrated in the position that would develop into legs in the future. There is a molecule called retinoic acid, which is similar to vitamin A and plays a key regulatory role in the development of frog limbs. The presence of trematodes would change the concentration of the molecule, causing developmental disorders and deformities.

Frogs infected with trematodes and deformed development | Pieter Johnson et al. / Science (1999)

Although it is not a human conspiracy, the reasons behind the multi-legged frog are complex and sinister. The life history of R. ondatrae is very complicated, and giving frogs extra legs is an important means for them to complete their life cycle.

The cruel fluke

The life of R. ondatrae trematodes begins as eggs, which are excreted into the water through the feces of water birds and hatch into the first stage of larvae, called miracidium. If they encounter aquatic snails of the genus Helisoma in the family Planctomycetes, the miracidium will drill into its body, where it will complete the three stages of development: sporocyst, mother rediae, and daughter rediae, and replicate itself in large numbers through asexual reproduction. Finally, the daughter rediae produce cercariae that can swim, leave the snail and enter the water to find tadpoles as the second host.

The life cycle of the trematode R. ondatrae | Dorina Szuroczki & Jean ML Richardson / Oecologia (2009)

The cercariae form metacercariae in the hind limbs of the tadpole, disrupting the development of the tadpole's hind legs and causing them to become deformed. The frog with extra legs has greatly reduced its jumping and swimming abilities, making it easy for birds to eat it. The metacercariae then migrate to the ileum of the bird, eventually becoming sexually mature, fertilized and laying eggs (by the way, it is hermaphroditic).

It's not uncommon for parasites to manipulate their hosts. We have fungi that manipulate ants to climb trees to spread spores, and tapeworms that make fish become bolder. But R. ondatrae is quite unique among parasites in that it causes such a radical change in the host's body.

Or is it humans' fault?

If R. ondatrae has always existed, why has it only recently attracted attention? Comparing past amphibian studies with frog specimens in museums shows that the spread and severity of the trematode have increased significantly in modern times. This is certainly not good news for frogs. The situation of some endangered amphibians has become even worse.

Although humans did not make frogs grow many legs, the sudden rise of trematodes is still inseparable from human activities. Since the arrival of the colonists, the United States has developed more than 100 million acres of natural wetlands. Humans have a great impact on the environment. Water storage activities, from dams to reservoirs, will breed a large number of freshwater snails, providing more hosts for trematodes (the prevention and control of schistosomiasis is an issue that must be considered in water conservancy projects). The fertilizers and animal manure produced by agriculture are discharged into the water, causing eutrophication of the water body and increasing the number of snails.

R. ondatrae Trematodes | Internet Archive Book Images

Human influence on the distribution of organisms is also an important factor. Aquariums and aquaculture, and even the ballast water of ships, have transported snails, frogs and other hosts of R. ondatrae to various parts of the United States, which may have facilitated the spread of the parasite. Humans have developed natural wetlands on a large scale, reducing the living space of waterfowl. The more birds that "crowd" into the few remaining wetlands, the higher the density, and the higher the possibility of R. ondatrae transmission. On the other hand, some birds with strong adaptability may adapt to living in artificial water bodies such as rice fields and fish ponds, and even the bird population will be more prosperous. As mentioned earlier, artificial water bodies are more likely to breed a large number of snails due to eutrophication. Coupled with a large amount of bird feces that may contain worm eggs, the result is a paradise for trematodes and a nightmare for frogs.

Variety is happiness

The disease of frogs reflects the sickness of the ecosystem. In this regard, Johnson proposed a meaningful hypothesis: use ecological "medicine" to cure ecological "disease". He and his colleagues examined hundreds of pools in San Francisco Bay and found that the pools with more species had a lower chance of frogs being infected with R. ondatrae.

Toads infected with parasites and deformed bodies | Pieter Johnson et al. / Canadian Journal of Zoology (2001)

In an ecosystem, as biodiversity increases, the chance of infection with a particular pathogen decreases, which is called the dilution effect. The more species there are, the more likely the pathogen will spread to organisms that it cannot survive, reaching a dead end. Therefore, the health of frogs truly reflects the health of the ecosystem, and the fate of frogs is also related to the fate of all organisms, including humans.

References

[1]Johnson, Pieter TJ, et al. "The effect of trematode infection on amphibian limb development and survivorship." Science 284.5415 (1999): 802-804.

[2]Lunde, Kevin B., Pieter TJ Johnson. "20. Parasite Infection And Limb Malformations: A Growing Problem In Amphibian Conservation." Amphibian Declines. University of California Press, 2005. 124-138.

[3]Venton, Danielle. "News Feature: Many species, one health." Proceedings of the National Academy of Sciences 112.6 (2015): 1647-1649.

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