Veterinary Parasitology: Helminths and Monogenea

• Study notes related to parasites of veterinary importance are outlined, focusing on helminths, acanthocephalans, and myxozoa.

Helminths

• Helminths are parasitic worms, large and multicellular with vermiform or worm-like shapes.
• They include various phyla with superficial similarities but unrelated phylogenetically.
• Phylum Nematoda consists of roundworms.
• Phylum Platyhelminthes consists of flatworms.
• Class Cestoda includes tapeworms.
• Class Trematoda includes flukes that are endoparasites.
• Class Monogenea includes skin flukes that are ectoparasites.
• Class Turbellaria contains free-living flatworms and some parasitic forms.
• Phylum Acanthocephala are thorny-headed worms.
• Phylum Annelida contains segmented worms, such as non-parasitic leeches.

Class Monogenea

• Monogenea are ectoparasitic flatworms within the Phylum Platyhelminthes.
• They are ectoparasites found on the skin, fins, and gills of fish, and also in exotic animals like frogs, reptiles, and hippos (in the eye).
• Monogenea lack a true body cavity or coelom.
• These can range in size from 2-3 cm to microscopic, e.g. Gyrodactylus.
• They have a simple digestive system consisting of a mouth, pharynx, and intestine, lacking an anus. Flame cells are specialized excretory cells.
• Attachment occurs via hooks.
• They are hermaphroditic, and are mostly egg-producing, though some are viviparous.
• There is a direct life cycle involving one host
• Monogenea are ancestrally related to Turbellaria and are closest to modern cestodes.
• A direct life cycle leads to significant problems in cage culture of fish.
• The life cycle is a direct cycle involving one host.
• Adult monogeneans live on fish gills or in the urinary bladder of frogs or turtles.
• They attach to the host and lay eggs that are shed into the water and hatch.
• Free-swimming larvae possess cilia, a distinct opisthaptor, two eyespots, and a digestive tract with protonephridia.
• Gyrodactylus are viviparous monogeneans, with some species extremely pathogenic.
• Benedenia seriolae parasitize amberjacks and feed on epithelial cells of the skin, growing up to 1 cm long.
• Pseudorhabdosynochus sp. infests groupers, feeding on epithelial cells and producing eggs with long filaments.

Phylum Acanthocephala

• Acanthocephala are thorny-headed worms.
• They are highly adapted parasites.
• These have lost many organs and structures through evolution, making taxonomy difficult.
• No mouth or digestive system is present.
• Nutrients are absorbed from the host's digested gut contents via the integument.
• Can be quite large, over 10 cm.
• The earliest description of Acanthocephala, a worm with a proboscis armed with hooks, was made by Francesco Redi in 1684.
• Recent genomic data suggests Acanthocephala are closely related to rotifers (freshwater zooplankton).
• The intermediate host is an invertebrate.
• The adult stage is usually in the digestive tract and not usually pathogenic.
• Acanthocephala are sexually dioecious (separate male and female individuals).
• Life cycles are complex and many are not host-specific.
• Acanthocephala infect invertebrates, birds, amphibians, fish, and mammals.
• Diagnosis is made by observing eggs in feces.
• Acanthocephaliasis refers to the presence of the parasite, which may be sub-clinical.
• In a case study at RUSVM, unusual cysts were found in the skeletal muscle of a mongoose used in a leptospirosis trial. These morphology was not readily identifiable. Later, similar cysts were seen in monkeys
• The parasite's cystacanth stage is encysted. No further parasite development happens its intermediate host, which is typically an invertabrate.
• A vertebrate definitive host must ingest the cystacanth.
• ITS sequencing provided the information identifying the same Oncicola venezuelensis species in both mongoose and monkey, with almost 100% DNA identity.
• Some questions arise from this case, such as what the definitive host is in St Kitts, and whether mongooses and monkeys serve as intermediate, paratenic, or dead-end hosts.
• In Brazil, the intermediate host of Oncicola venezuelensis is unknown.
• Cystacanths have been recovered in Caribbean termites, subcutaneous nodules of lizards, Indian mongooses, and pearly-eyed thrashers.
• Adult parasites in St Kitts probably require a cat to eat the cystacanths to reach the adult stage. Thus, other hosts are likely dead ends.
• Thorny-headed worms are worm-like but unrelated to nematodes or Platyhelminthes and are related to rotifers.
• Individuals are either male or female.
• Adult parasites are almost always in the intestine.
• Many are not host-specific, especially in intermediate hosts.
• They are highly adapted parasites with no mouth, absorb via cuticle.
• Mongoose and monkey are not the final hosts given that the cystacanth stage is found encysted in the muscle and not developing into an adult in the intestine.

Kingdom Protozoa, Fungi, and Animalia

• Study notes related to parasites of veterinary importance are outlined, including Kingdom Protazoa (Protists), Kingdom Fungi, and Kingdom Animalia.
• Kingdom Protozoa (Protists) consists of; Phylum Mastigophora (flagellates), Phylum Apicomplexa, and Phylum Ciliophora (ciliates).
• Kingdom Animalia consists of; Phylum Arthropoda (insects/arachnids, copepods), and Phylum Cnidaria (Myxozoa).

Class Myxozoa/Myxosporea

• Myxozoa or Myxosporea are in the Phylum Cnidaria.
• They are parasitic cnidarians with indirect life cycles.
• Polar capsules/filaments are used in sporoplasm release; an evolved/derived nematocyst.
• Myxobolus cerebralis causes whirling disease, first described in Germany in 1898,
• This disease destroys cartilage (especially spinal column and cranium) in juvenile salmonids, leading to death or skeletal deformities.
• Characteristic erratic tail-chasing behavior and blackened caudal area are from damage to the central nervous system.
• Rainbow trout are the most susceptible of 11 salmonid hosts, with losses reaching 90%.
• The parasite spread from Eurasia to many countries via live or frozen fish.
• The parasite's life cycle has been elucidated, and programs are active to minimize infection in hatchery-reared rainbow trout.
• Host type and tissue tropism (predilection site) groups relate to DNA data for this species.
• A hepatic biliary group of myxosporeans reveals infection with myxospores in other groups.
• These infect reptiles (freshwater turtles), mammals (shrew/mole), birds (ducks/waterfowl), and amphibians (frogs).
• Representations of genera major myxospore morphotypes are used.
• The presence of myxospores continues to be the important method of diagnosing for myxosporean infections.
• The finding of a myxospore is diagnostic, but not useful for species identification.
• A Salmonid nervous tissue contains; 100/1.0 M. fryeri EU346372
• The protoacanthopterygii contains; Protacanthopterygii, Myxidium clade and Sphaerospora oncorhynchi AF201373.

Malacosporea

• Tetracapsuloides bryosalmonae is a myxozoan parasite of salmonid fish, causing Proliferative Kidney Disease (PKD).
• Population collapse of arctic char in Iceland is related to this parasite, where parasitic infection can occur in warmer waters with a increased tempature of about 13C.

Order Multivalvulida

• The Multivalvulida is an order of histozoic marine myxosporeans (Kudoidae & Trilosporidae).
• This order is typically found in skeletal muscles of marine fish, appearing in brain, heart, gills, kidney, gall bladder, ovary and intestines, and is often associated with muscle tissues.
• There are 3 or more shell valves with apical polar capsules.
• They are mostly pathogenic, developing inside tissues (Histozoic) unlike many myxozoans that are 'Coelozoic', developing in coelom-like spaces, e.g. the gallbladder.
• They cause significant losses to commercial catches of wild fish due to white cysts in the muscle and are related to post-mortem myoliqufaction of fish fillets.
• Genera under Multivalvulida are: Kudoa iwatai in red sea bream, Japan (Kudoa amamiensis in yellowtail, Japan), Kudoa thyrsites in olive flounder, Japan, and Kudoa thyrsites in Atlantic mackerel, UK
• Kudoa-infected is a food safety concern.
• Affected products can cause food poisoning, with diarrhea and vomiting shortly after ingestment.
• This parasite is known in flatfish (Paralichthys olivaceus), tuna (Thunnus spp.), and amberjack (Seriola dumerili) cases happened nationwide in Japan and Korea. More reports recently in Pacific bluefin tuna.
• Kudoa hexapunctata lives in the somatic muscle of wild Pacific bluefin tuna (Thunnus orientalis).
• Kudoa septempunctata lives in the trunk muscle of cultured olive flounder (Paralichthys olivaceus).

Myxozoa Facts

• After years of being described as protozoans, we know now they are parasitic jellyfish (Cnidaria).
• They are obligate parasites with a two-host life cycle; two spore forms are produced.
• These parasites all have polar capsules to infect a new host. These are related to nematocysts in non-parasitic Cnidaria.
• They can cause very serious diseases in commercial fish species, both farmed and wild.
• Problematic species are typically histozoic, being inside the host tissues.
• Coelozoic forms are generally not pathogenic, but interesting!
• They effect birds, amphibians, reptiles and some mammals.
• Some Kudoa species can cause human gastrointestinal problems following the consumption of undercooked fish