The region of Ostwestfalen-Lippe is characterized by only a limited number of small standing waters and a total absence of bigger lakes. At first sight those general conditions doesn´t benefit the occurence of trematodes because many of them need an obligate first limnic snail host and a common further development in water fowl. Therefore, for the first time in 1997 we started research on the occurence of trematodes in this region and most surprisingly found a vast number of different species in the small ponds located in the city and surroundings of Bielefeld. Up to now more than 25 different species of trematodes have been identified in snails of the genus Lymnaea, Planorbarius, Radix, Anisus and Physa, among others the pathogen of swimmer´s itch Trichobilharzia ocellata. The identification of the trematode species on the basis of the developmental stage of cercariae offen is exceptionally difficult and most frequently doesn´t result in a confident identification of the exact species (see below). Furthermore, the differentiation between germane species on the basis of cercariae is not reliable, supposing a much higher number of species. See the Cercariae Gallery for a further description of the encountered cercariae so far.

- please make a click here -

The characteristic feature in the life cycles of a vast number of trematodes is a host alternation between a vertebrate in which the adults live (final host) and molluscans (first intermediate host). The sexual reproduction occurs in the vertebrate hosts resulting in the production of eggs. The eggs are containing the first larval stage, called miracidium. Its task is to search and find the first intermediate host and cause infection. In case of an aquatic life cylce this next host most commonly is a snail. Within the snail the miracidium develops into a mother sporocyst shedding its ciliated epithelium and forming a new tegument (--> Neodermata). The mother sporocyst is a germinal sac in which the next generation develops. The next generation occurs in two morphological different forms, called (daughter) sporocyst or redia, depending on the trematode species. These sporocysts or rediae are leaving the mother sporocyst and migrate usually to the hepatopancreas of the snail where they start cercarial production. The cercaria is the following larval stage of trematodes which is released from the sporocyst or redia and leaves the snail (in most cases) following an adequate hatching stimulus to infect the next host. Their aim is to search and infect an appropriate final host. The next host could be a further (second) intermediate host or, in some species, the final host. Deviating this principle there are evolved many different life cycles of trematodes integrating various intermediate hosts or a quiescent stage (metacercaria).

The procedure for detecting trematode infections in snails during the patent period is easy to perform and possible also e.g. for school lessons. Collected limnic snails are placed in small vessels containing tap water or filtered pond water during the morning hours and are exposed to a source of light a few hours. The light and the rising temperature in the small vessels function as hatching stimulus for the cercariae inside the snail which initiate leaving the snail. Vessels containing hatched cercariae can be identified afterwards with assistance of a binocular. For further identification the cercariae are transferred to slides and stained with e.g. Neutral Red or Nileblue Sulphate and viewed with a microscope.

The identification of cercariae is recognizing various features of the cercariae. Roughly cercariae can be categorized in several types following their morphological variations. Furcocercariae are featured with a forked tail, Xiphidocercariae can be identified due to their stylet in the lumen of the oral sucker. Both of these types develop in sporocyst. The most conspicuous feature of echinostome cercariae is their spined collar embracing the oral sucker and their well developed collecting tubules of the excretory system containing in many cases a vast number of granulae. Monostome cercariae are characterized by only an oral sucker and the absence of a ventral sucker (acetabulum). Whereas an amphistome cercaria only shows a huge ventral sucker which is positioned in the rear part of the body. The development of the former three types take place in rediae. In some cases a rough systematic classification of cercariae following their main displayed features seem to be possible, but, as a rule, there have to be considered a bigger variety of features. E.g. Furcocercariae are the cercarial stage of various trematode orders. As mentioned above, a differentiation of trematode genus or species most frequently is exceptionally difficult. As well because of multiple descriptions of the same species by different authors always identifying a "new" species. The further identification of cercariae depends on the type of cercariae. Wherefore advanced features of e.g. echinostome cercariae are among others the number, localization and size of collar spines, while in xiphidocercariae size and form of the stylet is a unique feature. An additional criterion can be e.g. the structure of the excretory system with number and position of the flame cells (protonephridia). Further details can be obtained by chaetotaxy though this technique is difficult to utilize and needs experienced workers. Summarizing, the identification of trematode species using the larval stage of cercaria up to now resides difficult and unreliable and, as a rule, should be decided only after regarding the complete life cycle including all stages.

Further information concerning e.g. morphology, reproduction, development of trematodes is given e.g. in:
Schmidt, G.D., Roberts, L.S., Foundations of Parasitology, 6th ed., McGraw-Hill Comp., 2000

Download here the corresponding chapter about trematodes Trematoda.pdf 2,2 MB