Molecular Biology and Genetics
Statistics of barcoding coverage
Specimens with Sequences:421
Specimens with Barcodes:346
Species With Barcodes:50
Schistosomatidae is a family of digenetic trematodes with complex parasitic life cycles. Immature developmental stages of schistosomes are found in molluscs and adults occur in vertebrates. The best studied group, the blood flukes of the genus Schistosoma, infect and cause disease in humans. Other genera which are infective to non-human vertebrates can cause mild rashes in humans.
Schistosomatids are dioecious (individuals are of separate sexes) which is exceptional with regards to their phylum, Platyhelminthes, in which most species are hermaphrodidic (individuals possess both male and female reproductive systems).
The eggs of these parasites were first seen by Theodor Bilharz, a German pathologist working in Egypt in 1851 who found the eggs during the course of a post mortum. He wrote two letters to his former teacher von Siebold in May and August 1851 describing his findings. von Siebold wrote a paper (published in 1852) summarizing Bilharz's findings. Bilhart's wrote a paper in 1856 describing the worms more fully and he named them Distoma haematobium. Their unusual morphology meant that they could not be comfortably included in Distoma so in 1856 Meckel von Helmsback created the genus Bilharzia for them. In 1858 Weinland proposed the name Schistosoma (Greek: 'split body') after the male worms' morphology. Despite Bilharzia having precedence the genus name Schistosoma was officially adopted by the International Commission on Zoological Nomenclature.
In 1898 all the then known species were placed in a subfamily by Stiles and Hassel. This was then elevated to family status by Looss in 1899. Poche in 1907 corrected a grammatical error in the family name. The life cycle was determined by da Silva in 1908.
There are a number of different families of blood fluke including the Schistosomatidae. The others include the spirorchids (turtle parasites) and the sanguinicolids (fish parasites).
The Schistosomatidae are considered venous system specialists and their sister group are vascular system generalists - the Spirorchidae.
The Schistosomatidae differ from the other blood flukes in having separate sexes and with one exceptional having homeothermic hosts. They have compensated for the reduction in potential reproductive partners by
- an increased overdispersion in the vertebrate host
- the reduced egg hatching time in the external environment
- the formation of permanent pairs mimicking the hermaphroditic condition
- the increased longevity in the definitive host
- increased fecundity.
Colonization of the venous system was necessitated by
- the evolutionary radiation into terrestrial vertebrates
- the increased immunopathology associated with the high, constant body temperature of homeothermic vertebrates.
The arterial dwelling spirorchids release eggs in the direction of blood flow, resulting in a wide dissemination of eggs within the host. The lower body temperature of poikilotherms is accompanied by a seasonal nature of the immune response in these hosts resulting in a quantitatively reduced pathogenesis. Hosts that did succumb to the infection would most likely die in water where eggs could be released by predation, scavengers, or decomposition and develop successfully.
Colonization of the venous system by schistosomes required precise egg placement because their eggs are released against the blood flow. Eggs are then sequestered within the portal system (or perivesicular plexus in some species) of homeotherms which restricts egg dispersal but limits the resulting pathology to less sensitive organs. A significant number of eggs may escape into the external environment before a heavily infected host is incapacitated by, or dies from, the infection.
The genera are:
- Allobilharzia - birds
- Austrobilharzia - birds
- Bilharziella - birds
- Bivitellobilharzia - mammals (elephants)
- Dendritobilharzia - birds
- Gigantobilharzia - birds
- Griphobilharzia - reptiles (crocodiles)
- Heterobilharzia - mammals
- Microbilharzia - birds
- Orientobilharzia - mammals
- Ornithobilharzia - birds
- Schistomatium - mammals
- Schistosoma - mammals including humans
- Trichobilharzia - birds
The genera Bivitellobilharzia, Orientobilharzia and Schistosoma form a clade in this family. Austrobilharzia and Ornithobilharzia are the closest relations of this clade.
Heterobilharzia and Schistomatium form a separate clade indicating that adaption to mammalian hosts has occurred at least twice.
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