ORIGIN OF SCHISTOSOMES ; STATUS OF BIVITELLOBILHARZIA
In the evolutionary scale proboscids (elephants) have originated during late ecocene epoch while ungulates evolved much later. For this reason, schistosomes of proboscids may be considered Parental stock from which schistosomes of ungulates evolved. Though there are numerous schistosome species affecting ungulates , there is only one genus i.e. Bivitellobilharzia with two species which is known to occur in elephants. B.loxodonte occurs in African elephants and B.nairi in Indian elephants. The snail host for B.loxodonte is a Planorbid while that of B.nairi is still not confirmed.
As elephants arrived much earlier than the ungulates, it is possible that Bivitellobilharizia has evolved much earlier in elephants and existing till date.The other possibility is of its originating in ungulate era and than transmitting to elephants. But this does not appear probable as Bivitellobilharizia is not reported from any ungulate. Therefore, B.nairi and B.loxodonte have neither switched their host specificity nor enlarged their host range as they are still reported only from the elephants. It is interesting to investigate how the schistosomes was able to survive millions of years only on the basis of single host specificity – both on intermediate and final host level.
If we consider that Bivitellobilharzia has not affected ungulates during evolution, than what is the origin of Schistosoma or Orientobilharzia-both occurring in ungulates or prodontocotyles but not in proboscids. On morphological grounds, Orientobilharzia is more close to Bivitellobilharzia as both have large number of testes (30-70) and a terminal spined egg. But neither Orientobilharzia is occurring in elephants nor any species of Bivitellobilharzia is seen in ungulates making it doubtful if they evolved from each other. The case of Schistosoma is more difficult to explain.
Does it mean Schistosoma and Orientobilharzia have not evolved from Bivitellobilharzia ? If so , what are the reasons ? Who is the parental stock of Schistosoma or Orientobilharzia ?
On evolutionaly scale, birds have come much earlier than mammals.It is also an accepted fact that mammalian schistosomes have evolved from avians.The best example was genus Ornithobilharzia that contained schistosomes normally infecting birds but occasionally observed in mammals too. This genus was dismantled by Dutt and Srivastava (1955) who instead created two genus. Orientobilharzia who infects exclusively mammals and Sinobilharzia who infects birds. This suggests Orientobilharzia may be the parental stock for Schistosoma species .However, it fails to answer position of Bivitellobilharzia in this evolutionary hypothesis which is important to understand schistosome evolution.
I will highly appreciate to receive a response from the Parasitologists/Taxonomists on my our contention. If any institution is interested to search in above line, he is welcome to contact the undersigned.