Ribeiroia ondatrae

Ribeiroia ondatrae is a parasitic trematode, whose larvae can infect amphibians and can result in infected animals to birth defects such as deformed, missing or extra limbs.

In 2002 published Pieter Johnson of the University of Wisconsin and colleagues from other institutions in the journal Ecological Monographs ( Vol. 72, p 151 ) Research results on studies of 12,000 amphibians with deformities. In the trematode Ribeiroia ondatrae " infected animals are found encapsulated resting stages, so-called cysts under the skin at the insertion points of the limbs and tail. It turned out to be related to the deformations could be made only with the extent of parasitism " and not, as previously assumed a direct relationship with the concentration of nitrate, phosphate, or pesticides. " The stronger the amphibian population was infected with Ribeiroia, the more frequent and severe were the deformities. "

Life cycle

The main hosts of the adult Saugwurms are water birds, get worm eggs into the water with their droppings. These develop after about two weeks to miracidia, which penetrate into the host snail (first intermediate host ). The larvae develop in horn snail of the genus Helisoma ( syn. Planorbella ), especially the frequent type Helisoma trivolvis, family Planorbidae. There they walk in the heart of the host into sporocysts, release the rod larvae ( redia ), which migrate into the kidneys, gonads and the anterior part of the midgut gland. Can alternatively daughter redia redia or cercaria - release. The worm loses its fertility by the parasite ( castration). The cercariae leave the snail, then penetrate into tadpoles and form metacercariae, which are encapsulated by the host in cysts (second intermediate host ). Metacercariae cysts of the species are about 300 to 350 micrometers long. Characteristic of the genus are lateral diverticulum of the esophagus of the metacercariae. The cysts interfere with the normal development of limbs, either mechanically or possibly also by output signal molecules. The life cycle is completed when infected amphibians are eaten by aquatic birds ( 40 species known as host, very rare a development also occurs in mammals ). The liberated from the thin-walled cyst metacercariae evolve into the adult trematode, which reaches about 1.7 to 2.8 millimeters in length. The worm lives mainly in the glandular stomach ( proventriculus ) of the host. The studies, conducted by Pieter Johnson ( University of Wisconsin ) showed that the number of Helisoma snails represents a direct measure of the parasites of amphibians at a site.

Influences

The exact mechanism of malformation has not been determined, but it has been suggested that the deformations of the mechanical destruction of the cells by the encapsulated resting stages ( cysts ) result within the amphibian larval stage.

Next was observed that the parasite appears particularly abundant in eutrophic ( nutrient-rich ) waters. It is believed by researchers a connection with various fertilizers and phosphate input. The herbicide atrazine could weaken the immune system and make the frogs more susceptible to infection in amphibians. The influence of pesticides is examined.

Studies show that the most commonly affected in frogs or toads points for malformations are the hind legs. However, there seems to be a lot of dependency that affects where a deformity may occur: For example, a moderate amount of Ribeiroia ondatrae affect the front legs of amphibians, while a heavy infestation, however, not to affect the front legs, but only on malformations of the hind legs seems.

A high level of biodiversity, the researchers postulated to Pieter Johnson ( University of Colorado Boulder) in 2013, protect amphibians from disease caused by parasites. The study of 345 ponds have shown that the animal world is, the healthier it is ever mixed, ie the more species live in a pond. " In ponds with six different amphibian species, the number of malformed animals was more than half lower than in waters with only one kind, the transmission rate of the parasite fell by almost 80 percent."

Further studies showed that the water usually only a relatively small proportion of the worm population is infected ( 1 to 5 percent ), even if the infection of tadpoles already very high ( 50 to 100 percent). The parasite is therefore best detected in the amphibian host, this is done by dissection and subsequent microscopic search. The zystierten metacercariae usually sit just beneath the skin, above the skeletal muscle.

Dissemination

The species is so far proven safe only from North America, where it is widespread. In the Caribbean, lives a related species, Ribeiroia marini ( Faust & Hoffman, 1934). The very similar nature was subjected to molecular genetic distinction, their intermediate hosts are snails of the genus Biomphalaria. One, possibly two more species have been described from the Congo (Africa). Recently, a further, as yet undescribed species of the genus in Brazil was discovered. Deformities of amphibians by the other species of the genus are not known.

Swell

• Johnson, PTJ, Sutherland DR, JM Kinsella and KB Lunde ( 2004). Review of the trematode genus Ribeiroia ( Psilostomidae ): Ecology, life history and pathogenesis with special emphasis on the amphibian malformation problem-. Advances in Parasitology 57: 191-253.