"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

February 17, 2013

Bivitellobilharzia loxodontae

Blood flukes from the genus Schistosoma are found in over 77 countries, infecting at least 230 million people, and second only to malaria as the most socioeconomically crippling parasitic disease in the world. But the majority of flukes from the family Schistosomatidae do not infect humans; they parasitise other species of mammals, as well as birds. There are about 100 known species of schistosome flukes around the world. Understandably, those species from the genus Schistosoma are the most extensively studied due to their public health importance. However, there are many other blood flukes for which very little is known on even the most basic aspect of their ecology.

Photo by Thomas Breuer from here
Meet Bivitellobilharzia loxodontae; a schistosome that parasitises African forest elephants (Loxodonta cyclotis). It holds the distinction of probably being the most poorly known of all the schistosomes. The first and only adult specimens of this fluke were retrieved from an elephant that had died in an animal park in Hagenbeck, Germany. To this day, almost everything known about this parasite had come from those samples which were described in 1940. The elephant that was hosting those blood flukes was likely captured from the region now known as the Democratic Republic of Congo.

Because B. loxodontae is an endoparasite (internal parasite) of elephants, adult specimens are hard to come by as they can only be retrieved via "destructive sampling" (dissecting the circulatory system of a dead elephant). And despite extensive sampling in the area where the forest elephant resides, the snail host (where the asexual larval stages of this parasite reside) has not yet been identified. Documenting the life-cycle of these parasites is a labour-intensive and time-consuming task as it requires finding all the different larval stages and demonstrating that all those different stage do indeed belong to the same species by performing experimental infections. Performing experimental infection on an animal like an elephant is out of the question due to its large size and rarity.

Photo of B. loxodontae egg
from the paper
With the advent of molecular techniques, it is now possible to confirm the identity of parasites at different stages of their life cycle without experimental infection (even though experimental infections are still useful for working out other aspects of a parasite's ecology). This can be done by sequencing specific sections of DNA which can serve as markers that identify the species and differentiate it from other parasites which might look similar. In the paper we are featuring today, the researchers extracted DNA from B. loxodontae eggs that were retrieved from samples of elephant dung in order to work out how this blood fluke fits into the schistosome family tree.

Their analyses showed that out of all the schistosome blood flukes, it is most closely related to Bivitellobilharzia nairi -  a species known from the Asian elephant (Elephas maximus). Taxonomically, the genus Bivitellobilharzia sits near the base of a branch within the schistosome family that contains mammal-infecting species (including those species from the Schistosoma genus). The pattern of branches in the schistosome family indicates that at some point in the past, the mammal-infecting group evolved in a divergent direction (in terms of host use) to the rest of the family, which is composed of species that infect birds. This raises intriguing questions about the deep evolutionary history of this group of parasites.

Brant SV, Pomajbíková K, Modry D, Petrželková KJ, Todd A, Loker ES. (2013) Molecular phylogenetics of the elephant schistosome Bivitellobilharzia loxodontae (Trematoda: Schistosomatidae) from the Central African Republic. Journal of Helminthology 87: 102-107.


  1. °°Surprised to learn that about elephants.
    Thank you.

  2. Thanks for sharing! I wonder has anyone considered looking for a 1st host species that is not a snail? I know that snails are the most common 1st intermediate host for trematodes, but not always.

  3. Sara, that is a very good question.

    You are right, there are indeed trematodes that do not use snails as a 1st intermediate host. There are a handful of families including the Gymphallidae, Fellodistomatidae, Bucephalidae (and others) that utilise bivalves as first intermediate hosts. And then there are a few oddball like the Sanguinicolidae which use *polychaete worms* as a first intermediate host (we featured one species in Sept 2011 - Cardicola forsteri).

    However, it seems that trematodes have a great degree of consistency *within families* in terms of their first intermediate host usage. For example while the fellodistomatids are different from other trematodes in using a bivalve instead of a snail as a first intermediate host, *all* the species in the family uses bivalve. The same apply to the schistosomes (which usually uses snails).

    Furthermore, there was a bit more that I didn't get to write about in the blog post because it would have dragged it out too long - back when the original specimens of the elephant schistosomes were found, the parasitologists who found them were able to obtain some live eggs from those worms and did some experimental infections. They exposed the hatched miracidia to different species of African and German freshwater snails. They were able to successfully infect some snails, but not the species they were expecting - the parasite successfully infected Stagnicola palustris, an *European* snail - but none of the African snails they were testing.

    Obviously, S. palustris cannot be the usual 1st intermediate host for this parasite, however, it seems to be physiologically similar enough that the parasite could jump over. Therefore, while it is still not clear what the first intermediate host might be, there are two clues which indicate the 1st intermediate host of this parasite is a freshwater snail:

    1) Schistosomes as a family utilise snails as a first intermediate host.
    2) The miracidia of this parasite is able to successfully infect a freshwater snail of European origin, which means its natural host would not be too different (it'd at least also be a freshwater snail of some sort).

  4. Cannot be less well known than the schistos yet to be discovered ;)

  5. Well Tim, if we know the existence of a species, then at least that's ONE fact we know about it - even if that is very little. And we can compare that with another species, for which we know a dozen, a hundred, a thousand little facts about it.

    But if we don't know about the existence of a species, then we cannot compare how little we know about it comparing with species that we know do exist.