The Creation of Sea Monsters: Limb Malformation in Amphibians

By Christie Jackson

Introduction:

      Do you believe the Swamp Thing or the Loch Ness Monster is real? Probably not. But believe it or not, real aquatic monsters do exist. Neither Hollywood nor the media is responsible for the creation of these creatures -- the credit belongs to Ribeiroia ondatae, a species of parasitic flatworm (trematode) that, at 500-5,200 µm as an adult, is visible to the naked eye [1].

    R. ondatae is found mostly in North America where it has been observed in 46 states in the United States and in parts of Canada [2]. It can live practically anywhere containing freshwater including ponds, lakes, and drainage canals [7]. To complete its lifecycle, it must infect three hosts at different stages: planorbid snails, amphibians (specifically frogs, toads, and salamanders), and birds or mammals [1]. Although it causes particularly grotesque limb mutations in amphibians (particularly the addition of limbs, but also the deformation or absence of limbs) and infertility in the snails it inhabits, birds and mammals, the definitive hosts for R. ondatae, are not affected [1-4].

Amphibian malformations [3]

Lifecycle:

     R. ondatae eggs are released into the water by a definitive host, either a bird or a mammal. The eggs become a free-swimming, larval stage of the parasite, a miracidia. The miracidia then enters the first intermediate host, a planorbid snail. The snail may be from either the genus Planorbella or Biophalaria as both have shells resembling a ram’s horn [1,3]. The miracidia travel to the either the snail’s kidneys or the veins in the snail’s lungs to asexually reproduce becoming rediae. Often, R. ondatae will remove the snail’s reproductive organs during its own reproduction [3].

Ribeiroia lifecycle [2]

     Once expelled, the rediae become free-swimming cercariae. The cercariae locate the second intermediate host, a frog, a toad, or a salamander and attaches to the area that will become the host’s limbs. After attachment, the cercariae become metacercariae. As the host matures and goes through metamorphosis, the metacercariae signal the growth of additional, malformed limbs.

     Malformed individuals are often slower than normal individuals and are easily caught by predators. Once ingested by birds or mammals, R. ondatae mature into adults and sexually reproduce in the digestive tract of its host. The R. ondatae eggs are released into water with the definitive host’s feces, completing their lifecycle [1,3]
 

Ecology:

     The destruction of marine and freshwater ecosystems worldwide plays a major part in the parasite’s prevalence [4, 5]. Pollution often caused by agricultural run-off creates a build-up of nitrogen and phosphorus in aquatic habitat of the panorbid snails. This allows the snails to reproduce more rapidly creating more first intermediate hosts for R. ondatae to inhabit.

     The desire for the human population to expand creates a limited area for amphibians to reside. As they are being pushed into small geographical areas, the number of ponds or lakes decreases. Coupled with the rise in panorbid snails, the likelihood that an amphibian will become infected greatly increases. In some wetlands, greater than 50% of the amphibian population has been reported malformed [7].

     Prevention is key to help stop the decline of North American frogs, toads, and salamanders. Educating the public and encouraging activism to help clean up our water will not only positively impact amphibian population and health, but also the health of humans. Humans, wildlife, and the environment are all interconnected.

  

Host-parasite ecological continuum [5]

References:

[1] Johnson, P. T. J., D. R. Sutherland, J. M. Kinsella, and K. D. Lunde (2001). Review of the trematode genus Ribeiroia (Psilostomidae): ecology, life history and pathogenesis with special emphasis on the amphibian malformation problem.  Advances in Parasitology, 54: 192-253. 

[2] Johnson, P. T. J. and K. B. Lunde (2005). Parasite infection and limb malformations: A growing problem in amphibian conservation. Amphibian Declines: the Conservation Status of United States Species, 124-138.

[3] Johnson, P. T. J. and D. R. Sutherland (2003). Amphibian deformities and Ribeiroia infection: An emerging helminthiasis. Trends in Parasitology, 19: 332-335.

[4] Johnson, P. T. J. and J. M. Chase (2004). Parasites in the food web: linking amphibian malformations and aquatic eutrophication. Ecology Letters, 7: 521-526.

[5] Daszak, P., Cunningham, A. A., and A. D. Hyatt (2000). Emerging infectious diseases of wildlife – Threats to biodiversity and human health. Science, 287:443-449.

[6] Docampo R. (2002). Fifth Annual Conference on New and Re-Emerging Infectious Diseases. Emerging Infectious Diseases, 8(9): 1008-1010. 

[7] Johnson, P. T. J., K. B. Lunde, R. W. Haight, J. Bowerman, and A. R. Blaustein (2002). Parasite (Ribeiroia ondatrae) infection linked to amphibian malformations in the western US. Ecological Monographs 72(2): 151-168.