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Parasitology. What is a parasite?. Definition: An animal which lives in (endoparasite) or on (ectoparasite) another animal (the host). Is almost always a different species from the host Depends on the host for food an causes some degree of injury. General characteristics.

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what is a parasite
What is a parasite?
  • Definition:
    • An animal which lives in (endoparasite) or on (ectoparasite) another animal (the host).
    • Is almost always a different species from the host
    • Depends on the host for food an causes some degree of injury
general characteristics
General characteristics
  • Tremendous reproductive capabilities
  • Have physical adaptations that enhance attachment to the host (i.e. suckers, hooks or clamps)
  • Possess various mechanisms for avoiding the host’s immune response
  • Exhibit complex life cycles often with multiple hosts
typical indirect parasite life cycle
Typical Indirect parasite life cycle

Primary Host

Adult stage parasite infects host

Infective larvae

Able to infect primary host

Egg/Spore Stage

Transmission and spread into the environment

Intermediate host

Growth and developmental stage (may not cause damage to the host)

negative affects on the host
Negative affects on the host
  • Direct damage to host
    • inducing tissue and organ damage
  • Indirect effects
    • Stress causes an increase susceptibility to secondary infections
    • Direct damage can act as a portal for secondary infection
    • Parasite may serve as a carrier/vector for another viral/bacterial pathogen
signs of parasitic infections
Fish at surface gulping or piping

Suggests parasites on gills

Fish rolling/flashing

suggests protozoan or worm infestation (internal or external)

Lethargy or listlessness

Suggests gill parasite

Fish at bottom

Suggests gill parasite, especially “Ich”

Fin erosion/Lesions

Indicative of external parasite

Flared gills

Indicative of gill parasite

Excess mucus, fish “shimmies”/quivers, or is off feed

General indication of disease

Signs of Parasitic infections
major groups of fish parasites
Major Groups of Fish Parasites
  • Protozoa: single celled animals
  • Monogenetic Trematodes: Flukes (flatworms) with haptor (posterior attachement organ) an have simple life cycle (no intermediate host)
  • Digenetic Trematodes: Flukes (flatworms) with oral/ventral suckers and exhibit complex life cycles (involve intermediate hosts)
major groups of fish parasites8
Major Groups of Fish Parasites
  • Cestodes (Tapeworms): worms with flattened/segmented bodies, head usually has suckers/hooks/suctional grooves
  • Nematodes (Roundworms): Thin elongated worms with cylindrical bodies covered by a rigid cuticle
  • Acanthocephala (Spiny-headed worms): bodies cylindical or fattened with anterior end bearing elaborate hooked proboscis
major groups of fish parasites9
Major Groups of Fish Parasites
  • Copepods: crustaceans (sea lice) that may appear louse, worm, or grub like
  • Leeches: flattened or cylindrical, body segmented with anterior/posterior suckers
  • Glochidia: larval freshwater clams
  • Fungi: either as spores or as fungal hyphie
external protozoa

Ichthyobodo (Costia)


Ichthyophthirius multifilis (“Ich”)



External Protozoa
external protozoa11
External Protozoa
  • Common and usually occur in low numbers
  • Dense populations can cause serious epizootics (usually caused by some form of stress)
  • Symptoms include:

-Irritation (flashing) -Erosion of scales

-Erythema (reddening) -Hemorrhaging

-Excess mucus production -white spots on skin

  • Control by chemical treatment
myxobolus cerebralis
Infects cultured and wild salmonids

Specific tropism for cartilage

Infection can result in axial skeleton and neural damage

Myxobolus cerebralis

Found in marine and freshwater environments

Only infects salmonids

Susceptibility varies

Clinical signs vary among infected salmonid species

Identified by spore size, shape, and location

Ceratomyxa shasta


Ovoid, spherical, or lenticular spores

Usually cysts form around spores


general myxosporidean lifecycles
General Myxosporidean lifecycles
  • landmark discovery by Wolf & Markiw in 1984
  • a fish myxosporean alternates with an actinosporean from an oligochaete worm
  • both spore types represent alternate lifecycle stages of the one organism
  • morphologically distinct spores are genetically identical

General Diagnostic Procedure

  • Presumptive ID:
    • Wet preparation
    • Histology
      • Site of infection
      • Spore morphology
  • Confirmation of ID:
    • Molecular methods
      • Any level of infection, all stages, definitive

Gyrodactylus sp.


Bolbophorus damnificus

is often referred to as the “catfish trematode”

Misnomer because there are about 30 trematodes found in the channel catfish and because it is also found in the fathead minnow

digenetic trematodes
Digenetic Trematodes
  • Adult flukes reside in fish, birds, or mammals
  • Flukes lay eggs that pass through the definitive host, eggs hatch to a ciliated miracidia
  • The miracidia will develop to a cercariae if in contact with a snail or mussel
  • If the cercariae contacts invertabrate of fish hose it will encyst as a metacercaria
  • GI tract of fish, bird or Mammal
  • Eggs are laid to water and are eaten or hatch into a coracidium (C) and are then eaten by an invertabrate host
  • Larval development to a proceroid or a pleroceroid occurs invertabrate
  • Final host becomes infected by ingesting invertebrate
A.colex of Bothriocephalus acheilognathii from carp, Transvaal, South Africa (by courtesy of J.G. Van As).

B. B. acheilognathii, whole worm (living) from farmed carp, Israel.

C. Embryonated eggs of b.

D. Ligula sp. from Rastrineobola argenteus from L. Victoria. Infected fish are recognized by their inflated abdomen (top fish) and may accommodate even three worms (bottom group).


C. philippinensis egg

C. philippinensis adult

Unembryonated eggs are passed in the stool (1) and become embryonated in the environment (2); after ingestion by freshwater fish, larvae hatch, penetrate the intestine, and migrate to the tissues (3).  Ingestion of raw or undercooked fish results in infection (4).  The adults reside in the human small intestine mucosa (5).  The females deposit unembryonated eggs (can become embryonated) (6).  Also infects fish eating birds (7).

Capillaria philippinensis

  • A) GI tract of Fish, Acanthor larva released
  • B) Eaten by invertabrates and produces a cystacanth (C)
  • If eaten by suitable host, the cystacanth will develop into an adult

Neoechinorhynchus rutili

Adult female

Adult male

leeches hirudinea
Leeches (Hirudinea)

Leech with brood attached

Top view

leech characteristics
Leech Characteristics
  • Primarily occur in freshwater
  • Most are predators or scavengers which feed on fluids or soft tissues of live or dead invertebrates
  • Generally have 34 body segments and an anterior and posterior sucker
  • Parasitic leeches attach temporarily
  • Cause little noticeable harm
hirudinea problems








Weight loss

Some can be vectors of other parasistes

Hirudinea Problems

Lernaea sp.

Salmonicola sp.

copepods are a subclass of crustaceans
Copepods are a subclass of Crustaceans
  • Sexes are usually separate – with sexual dimorphism present
  • Heavy infections can cause severe damage to skin, muscle, and gill tissues
  • Can also lead to secondary infections, anemia, emaciation, and mortality
  • Mature copepods release eggs (B) that hatch to larvae (C)
  • D) After molting a copepod stage is formed and may attach to a host (E)
  • Larvae attach to gills or skin
  • Live as parasites then drop off and live independently
  • Some modify mantle tissue to help find host

Lampsilis reeveiana

Glochidia attaching to gill tissue



  • Saprolegnia “water molds”
    • Worldwide in freshwater
    • Appear as whitish cottony-like growths
    • Considered secondary invaders
    • Can attach to eggs and fish
    • Can be controlled with chemicals

Aphanomyces invadans






2° Zoospore

1° Zoospore

1º cyst