Fish Parasites – Comprehensive Study Notes

Water Quality & Aquatic Disease Context

• Aquatic animal medicine uniquely integrates toxicology + water-quality management.
• Major pathogen groups encountered in fish:
  • Bacteria, viruses, true fungi, oomycetes (water-molds).
  • Parasites
  • Protozoa (flagellates, ciliates, apicomplexans, amoebae).
  • Helminths (flatworms, roundworms, thorny-headed worms).
  • Arthropods (copepods, branchiurans, insects, arachnids).
  • Cnidarians – Myxozoa / Myxosporea.

Epidemiology – Land vs Sea

• Physical environment shapes parasite biology:
  • Motile larval stages (e.g. monogenean oncomiracidia, planktonic copepodids) thrive in water.
  • \uparrow population connectivity → rapid geographic spread.
  • Fewer need for vectors; many direct life cycles.
  • Vertical transmission uncommon in marine settings.

Protozoan Parasites (Kingdom Protozoa / Protista)

Taxonomic Snapshot

• Excavates: flagellates – Kinetoplastida (Trypanosoma, Cryptobia), Diplomonads (Hexamita, Spironucleus).
• Apicomplexa – Eimeria, Goussia.
• Ciliophora – free-living + parasitic ciliates (Ich, Cryptocaryon, Trichodina, stalked forms).
• Amoebozoa – Neoparamoeba (agent of AGD).

Ciliates

• Trichodina spp.
  • Free-living/commensal; can become facultative parasites (epibionts on skin/gills).
  • Ring-shaped, denticulate disc visible in wet mounts.
• White-spot disease (Ichthyophthirius multifiliis – “Ich”)
  • Obligatory protozoan of FW fish.
  • Life-cycle
  1. Trophont penetrates epidermis/gill epithelium; feeds, enlarges.
  2. Leaves fish → tomont; encysts in substrate and divides asexually.
  3. Cyst releases \approx thousands of tomites/theronts → infect new hosts.
  • Control: repeated chemical baths (formalin, malachite green, salt) timed to theront window.
• Marine Ich (Cryptocaryon irritans)
  • Salt-water analogue; large macronucleus aids diagnosis on skin scrape.
  • Management: \text{low}{\,}/\text{high salinity} shifts, formalin, copper, fallow tanks, quarantine new stock.
• Scuticociliates (Philaster, Pseudocohnilembus, Philasterides dicentrarchi)
  • Normally free-living; opportunistic systemic infections ("scuticociliatosis").
  • Implicated in Caribbean mass die-offs; first isolated from European sea bass.
• Velvet disease (Amyloodinium – marine; Oodinium – FW)
  • Dinoflagellate with photosynthetic relatives.
  • Fomite spread (nets, probes, live feed).
  • Clinical: fine “dust” on skin, severe hypoxia, gill necrosis.
  • Therapy: copper \rightarrow methylene blue, acriflavine, formalin baths; some agents banned.

Apicomplexans (Eimeria / Goussia)

• Heavy pyloric caeca infections cause malabsorption, wasting.
• Evolutionary note: All terrestrial coccidia evolved from aquatic ancestors.

Diplomonads – Hexamita / Spironucleus

• Intestinal flagellates related to Giardia.
• “Hole-in-the-head” in cichlids and salmonids.
• Treatment: metronidazole (targets anaerobic metabolism).

Kinetoplastida – Cryptobia (Trypanoplasma)

• Haemoflagellates; leech vector essential for transmission.
• Spectrum: free-living ↔ endocommensal ↔ blood parasite.

Amoebozoa – Neoparamoeba perurans (AGD)

• Free-living marine amoeba → attaches to gills → epithelial hyperplasia, mucus overproduction.
• Fulfilled Koch’s postulates molecularly.
• Losses: > 10^6 salmon annually.
• Control: \text{FW} (~2–3\% salinity) or \text{H}2\text{O}2 baths, nutraceutical feeds.

Cnidarian Parasites – Myxozoa (Class Myxosporea & Malacosporea)

Generalities

• Highly reduced cnidarians; indirect life cycle: annelid ↔ fish (or bryozoan for Malacosporea).
• Two spore forms
  • Actinospores in invertebrate host.
  • Myxospores in fish (diagnostic).
• Pathotypes: Coelozoic (body cavities) vs Histozoic (tissues).

Key Species & Diseases

• Myxobolus cerebralis – Whirling Disease
  • Juvenile salmonids; cartilage destruction → skeletal deformity, blackened caudal region, tail-chasing.
  • Mortality up to 90\% in \textit{Oncorhynchus mykiss}.
  • First described 1898 (Hofer); spread via live/frozen fish trade; no treatment → hatchery mitigation.
• Tetracapsuloides bryosalmonae – Proliferative Kidney Disease (PKD)
  • Salmonids in Europe/N. America; Arctic char collapse in Iceland as water warms +13^{\circ}\text{C}.
• Kudoa spp. (e.g., K.\;islandica,\;K.\;thyrsites,\;K.\;septempunctata)
  • Histozoic in skeletal muscle; white cysts, post-mortem myoliquefaction (“soft flesh”).
  • Food safety: gastroenteritis (diarrhea, vomiting) minutes-hours after ingestion of raw fish (tuna, flounder, amberjack).
  • Morphology: \ge 3 shell valves, multiple polar capsules (e.g., 7 in K.\;septempunctata).

Arthropod Parasites (Kingdom Animalia – Phylum Arthropoda)

Caligid Copepods – Sea Lice (Lepeophtheirus salmonis et al.)

• Major salmonid pest; adults \approx 5\,\text{mm}.
• Economic burden: \$800\,\text{million} (USD\/yr) globally.
• Control: SLICE® – emamectin benzoate in-feed; resistance documented.
• Integrated solutions: warm-water or freshwater thermal delousing, cleaner wrasse, laser systems, vaccines (in R&D).

Branchiurans – Argulus spp. (Fish Lice)

• Leave host to mate/oviposit; attach behind operculum.
• Feed on mucus, epidermis, blood; heavy infestations → argulosis outbreaks in FW aquaculture & aquaria.
• Eggs on hard surfaces; manage via tank sanitation, organophosphates, EM bath treatments.

Lernaeid Copepods – Lernaea spp. (Anchor Worms)

• Mating at final free-swim stage; female burrows in flesh, posterior anchors protrude.
• Typical on koi and goldfish; manual removal + organophosphate bath; manage waterfowl vectors.

Helminths (Parasitic Worms)

At-a-Glance Taxonomy

• Nematoda – roundworms (e.g., Anisakis).
• Platyhelminthes
  • Cestoda – tapeworms (fish & broad tapeworm Diphyllobothrium).
  • Trematoda – flukes (blood flukes Cardicola; general multi-host cycles).
  • Monogenea – ectoparasitic flukes (direct life cycles).
• Acanthocephala – thorny-headed worms.

Monogenea – Skin/Gill Flukes

• Key traits
  • Direct life cycle; oncomiracidium infects host directly.
  • Haptor hooks for attachment; lack suckers.
  • Hermaphroditic; most oviparous; some (e.g., Gyrodactylus) viviparous.
• Economic importance
  • Cage culture: rapid amplification; eggs adhere to netting (Benedenia seriolae in amberjack).
  • Treatments: praziquantel, \text{H}2\text{O}2 baths, copper alloy nets (anti-fouling).
  • Gyrodactylus salaris: microscopic yet lethal to Atlantic salmon smolts.

Trematodes

• Typical 3-host cycle: bird (definitive) → mollusk (1° int.) → fish (2° int.).
• Example: Scaphanocephalus sp. (presumed cycle illustrated).
• Blood flukes (Cardicola spp.)
  • Eggs lodge in gills of Pacific bluefin tuna → acute branchitis, lamellar distortion.

Cestodes – Diphyllobothrium latum (Broad/Fish Tapeworm)

• Zoonosis via raw/undercooked freshwater or anadromous fish.
• Definitive hosts: humans, bears, dogs, cats, pinnipeds.
• Clinical: \downarrow \text{vit B}_{12} → megaloblastic anemia, abdominal pain.
• Cultural practice: Hokkaido “ruibe” (frozen sashimi) originated as anti-cestode measure.

Nematodes – Anisakis spp.

• Marine mammal definitive hosts; fish/shrimp paratenic hosts.
• Humans accidental hosts → anisakiasis:
  • Larva penetrates gastric/intestinal wall → acute ulceration.
  • Allergic reactions can follow exposure to dead larvae (heat/freeze tolerant antigens).

Comparative & Clinical Insights

• Aquatic systems favour direct cycles, complicating control in recirculating aquaculture systems (RAS), ornamental tanks.
• Temperature crucial: e.g., PKD pathogenicity above 13^{\circ}\text{C}; parasite development rates determine treatment interval (Ich theronts).
• Host stress / immunosuppression (crowding, poor water quality) often precipitates clinical disease from endemic parasites (Velvet, Trichodina).
• Chemical control limitations: toxicity to invertebrates (copper), regulatory bans (malachite green), resistance (emamectin).
• Biosecurity pillars:
  1. Quarantine & health screening.
  2. Equipment disinfection (fomites).
  3. Stocking density management.
  4. Integrated pest management – biological (cleaner fish), physical (filtration, UV), medicinal.
• Zoonotic risk is limited but notable in parasites with mammalian definitive hosts or broad host spectra:
  • Diphyllobothrium, Anisakis, certain Kudoa spp.

Quick Reference – Treatment & Prevention Matrix

Consolidated Take-Home Messages

• Several parasite groups (Monogenea, parasitic ciliates, copepods) exist only in aquatic systems and have direct life cycles, making them troublesome in high-density fish culture.
• Others parallel terrestrial relatives with complex cycles (trematodes, cestodes, nematodes) and may pose zoonotic threats.
• Effective parasite management hinges on coupling environmental control and integrated therapeutics, cognisant of drug resistance and ecosystem impacts.