Schistosomatidae
Encyclopedia
Schistosomatidae is a family
of digenetic trematodes with complex parasitic life cycle
s. Immature developmental stages of schistosomes are found in molluscs and adults occur in vertebrate
s. 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).
in 1851 who found the eggs during the course of a post mortem. 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.
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
Colonization of the venous system was necessitated by
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 poikilotherm
s 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.
s: the others infect crocodile
s and mammal
s including human
s.
The genera are:
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.
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
of digenetic trematodes with complex parasitic life cycle
Parasitic life cycles
Parasite life cycles can take a variety of forms, all involving the exploitation of one or more hosts. Those that must infect more than one host species to complete their life cycles are said to have complex or indirect life cycles, while those that infect a single species have direct life...
s. Immature developmental stages of schistosomes are found in molluscs and adults occur in vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s. The best studied group, the blood flukes of the genus Schistosoma
Schistosoma
A genus of trematodes, Schistosoma, commonly known as blood-flukes and bilharzia, includes flatworms which are responsible for a highly significant parasitic infection of humans by causing the disease schistosomiasis, and are considered by the World Health Organization as the second most...
, infect and cause disease in humans. Other genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
which are infective to non-human vertebrates can cause mild rashes
Swimmer's itch
Swimmer’s itch, also known as lake itch, duck itch, cercarial dermatitis, and Schistosome cercarial dermatitis, is a short-term, immune reaction occurring in the skin of humans that have been infected by water-borne schistosomatidae...
in humans.
Schistosomatids are dioecious (individuals are of separate sexes) which is exceptional with regards to their phylum
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
, Platyhelminthes
Flatworm
The flatworms, known in scientific literature as Platyhelminthes or Plathelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...
, in which most species are hermaphrodidic (individuals possess both male and female reproductive systems).
History
The eggs of these parasites were first seen by Theodor Bilharz, a German pathologist working in EgyptEgypt
Egypt , officially the Arab Republic of Egypt, Arabic: , is a country mainly in North Africa, with the Sinai Peninsula forming a land bridge in Southwest Asia. Egypt is thus a transcontinental country, and a major power in Africa, the Mediterranean Basin, the Middle East and the Muslim world...
in 1851 who found the eggs during the course of a post mortem. 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
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
: '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.
Evolution
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 poikilotherm
Poikilotherm
A poikilotherm is an organism whose internal temperature varies considerably. It is the opposite of a homeotherm, an organism which maintains thermal homeostasis. Usually the variation is a consequence of variation in the ambient environmental temperature...
s 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.
Taxonomy
There are 14 genera in this family. Of these 9 infect birdBird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s: the others infect crocodile
Crocodile
A crocodile is any species belonging to the family Crocodylidae . The term can also be used more loosely to include all extant members of the order Crocodilia: i.e...
s and mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s including human
Human
Humans are the only living species in the Homo genus...
s.
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
- SchistosomaSchistosomaA genus of trematodes, Schistosoma, commonly known as blood-flukes and bilharzia, includes flatworms which are responsible for a highly significant parasitic infection of humans by causing the disease schistosomiasis, and are considered by the World Health Organization as the second most...
- 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.
External Links
Further reading
- Beltran S., Desdevises Y., Portela J. & Boissier J. (2010). "Mating system drives negative associations between morphological features in Schistosomatidae". BMC Evolutionary Biology 10: 245. doi:10.1186/1471-2148-10-245.