A study at Colorado University, Boulder, shows when toad tadpoles were raised in tanks with parasitic trematods, forty percent of the frogs were deformed. But when toad tadpoles shared the tank with grey tree frog tapoles, parasitic infections in the toads dropped almost in half. The tree tadpoles acted as ‘sponges’ for the trematods, which were killed by the stronger immune systems of frog tadpoles. Gray tree frogs and American toads are found throughout the Midwest and eastern United States, often in the same wetlands.
"This is one of the first experimental studies to definitively show that an increase in diversity of host species actually can reduce parasite transmission and disease," said Johnson of CU-Boulder’s ecology department."The study has implications for the declining global diversity of wildlife species that are susceptible to parasitic infections," said Johnson.
Without the parasites, toads and frogs are competitors. But when trematode parasitism is part of the ecosystem, toads are shielded from infections by the tree frogs.
Lifecycle of Trematod Parasites
The Trematode is a class of the phylum Platyhelminthes that has two groups of parasitic worms, also known as flukes. Nearly all trematodes are parasites of molluscs and vertebrates and can’t live without their host or a series of hosts. The relationship between the mollusk and worm is called a symbiotic relationship, a long term relationship between two species. Parasites benefit from the relationship with the host, which they exploit for food or habitat. The host is usually harmed in some way.
For such a small worm, the trematode has complex life cycle. Host snails release parasite trematode larvae into the water, which infects frogs and toads and causes deformed amphibians with extra limbs or strange limbs. Because they’re more susceptible to predators like white herons or other wading brids, deformed frogs and toads have a short life span. The birds eat them and later release their feces into ponds or wetlands, which releases trematodes back into the water to infect other snails and complete their life cycle.
Deformed frogs first made national news when they were spotted by school children in Minnesota in 1995. It quickly became apparent that the frogs were indicative of major enviromental issues. Ongoing research since then, shows that increases in ultraviolet radiation, contaminated water and a parasitic trematode are the leading culprits in the wave of deformed legs, eye damage and other ailments found in more than 60 species of frogs, toads and salamanders in 46 states and across four continents.
Although the results vary, most scientists agree the parasite appears to be the major cause of many of the deformities. Water pollutants or UVB radiation, while not directly causing the majority of deformities, may weaken an amphibian’s immune system and make it more vulnerable to infection.
Frogs with stronger immune systems can increase the diversity of frog populations, allowing them to adapt easier to environmental changes in habitat, water quality, pollution and parasitic diseases. And in the long run, frogs may also help our environment.