Popular Science: Astonishing diversity of trypanosomes in frogs
Even though the genus Trypanosoma comprises over 500 species of flagellates, only two causing severe human diseases have become the predominant subject of human research – T. cruzi in the case of Chagas disease in Latin America and T. brucei in the case of sleeping sickness in Africa. Some other species that are important parasites of farm animals got much less attention and those that do not cause any obvious health problems remained completely neglected. This neglect was unjustified, however, since the very first described trypanosomes belonged to the latter group. Furthermore, they were discovered in animals in which nobody would expect it – in frogs. The reason why these were described first is mainly their substantially larger size, which made them easier to observe even with the rather less technically developed microscopes used in the first half of the 19th century. At the same time, this group differs from other trypanosomes due to its relatively large shape variability, which is one of the reasons why they now belong to the least known groups. Indeed, within one life cycle one individual of such a trypanosome may go through several shape stages: besides the classical fusiform it may also appear as an oval, fan or spiral. At first, they were not even described as flagellates thanks to their weird appearance. For instance, T. rotatorium was considered to be an amoeba, T. loricatum and T. ranarum to be infusorians. Apart from the variability, research in this group is also burdened by the fact that we usually find several species in one host individual. Indeed, these trypanosomes are not host specific. That means that when we get a sample with rounded trypanosomes, for example, they may belong to several species and we cannot distinguish them with a light microscope. Therefore, we cannot be surprised that, until molecular methods became routine, not many parasitologists were confident to look at this group properly.
The authors of this study used this gap in human knowledge and tried to map trypanosomes occurring in two frog species. They collected samples from Edible Frogs and Lake Frogs in two locations in southern Bohemia and in two locations in Ukraine, the homeland of most of the authors. Using molecular methods, they tried to separate the obtained parasites into their particular species and, using a light microscope, they tried to assign particular shapes to these species. In addition to finding three already-known species, they also found two completely new ones. As they originally expected, they confirmed that each particular species can have several shapes and most of the frogs are infected with more than one species. The authors then described particular shape form characteristics for each species (see figure). They still did not describe the two newly discovered species, but assigned them nicknames derived from the most common shape forms instead. Trypanosoma sp. “nautilus” is most frequently notably similar to shell of a Chambered Nautilus (Nautilus pompilius) and the nickname of Trypanosoma sp. “ring” is derived from its apparent ring-like shape. The latter is also the only species found only in Czech locations, the others were found in both Czechia and Ukraine. Though particular species can be faithfully separated using molecular methods, the details of their mutual relationships remain unclear and further research is needed for their clarification.
This study clearly proved that, in the case of trypanosomes in frogs, one can get very interesting results even when rather small-scale research is done. In total, only 16 frogs were examined (11 in Ukraine, 5 in southern Bohemia), whereas each of them was infected with at least two trypanosome species. Furthermore, the infestation rates were by far not low. Some details discovered about these parasites are very surprising, e.g. the closest relative of Trypanosoma sp. “nautilus” was so far only known to have come from Japan. The study of their shapes also proved that the morphological characters used until now are not reliable. Though Czech and Ukrainian trypanosomes show some of the same features as species from North America, they are genetically distant. This raises the question as to where a continuation of this research could lead. If, in the future, scientists manage to gather samples from more frogs from larger areas, we might reasonably expect discoveries of an entirely new world of trypanosomes.
One of the authors, Jan Votýpka, told us that the research is still continuing. New samples from southern Europe can show an even greater diversity of trypanosomes!
Spodareva VV, Grybchuk-Ieremenko A, Losev A, Votýpka J, Lukeš J, Yurchenko V, Kostygov AY. Diversity and evolution of anuran trypanosomes: insights from the study of European species; 2018; 11:447. https://doi.org/10.1186/s13071-018-3023-1