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

• 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

Primary HostAdult stage parasite infects host

Egg/Spore StageTransmission and spread into

the environment

Intermediate hostGrowth and developmental stage

(may not cause damage to the host)

Infective larvaeAble to infect primary 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

Major Groups of Fish Parasites

• Protozoa: single celled animals

• Monogenetic Trematodes: Flukes (flatworms) with haptor (posterior attachment organ) and have a 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 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 cylindrical or fattened with anterior end bearing elaborate hooked proboscis

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 hyphae

External Protozoa

• Flagellates– Ichthyobodo

(Costia)

• Ciliates– Ichthyophthirius

multifilis (“Ich”)

• Trichodinids

• Epistylis

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

Internal Protozoa

• Myxobolus cerebralis (whirling disease)

• Ceratomyxa shasta

• Henneguya

Myxobolus cerebralis

• Infects cultured and wild salmonids

• Specific tropism for cartilage

• Infection can result in axial skeleton and neural damage

Typical Indirect parasite life cycle

Primary HostAdult stage parasite infects host

Egg/Spore StageTransmission and spread into

the environment

Intermediate hostGrowth and developmental stage

(may not cause damage to the host)

Infective larvaeAble to infect primary host

Ceratomyxa shasta

• Myxosporea• 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

Henneguya• Myxosporea• Ovoid, spherical, or

lenticular spores• Usually cysts form around

spores

• morphologically distinct spores are genetically identical

• 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

General Myxosporidean lifecycles

• Presumptive ID:• Wet preparation• Histology• Site of infection• Spore morphology

• Confirmation of ID:• Molecular methods

• Any level of infection, all stages, definitive

General Diagnostic Procedure

Trematodes

• Monogenetic– Gyrodactylus sp.

• Digenetic– 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 TrematodesA) Adult flukes reside in fish,

birds, or mammals

B) Flukes lay eggs that pass through the definitive host, eggs hatch to a ciliated miracidia

C) The miracidia will develop to a cercariae if in contact with a snail or mussel

D) If the cercariae contacts invertabrate of fish host it will encyst as a metacercaria

CestodesA) GI tract of fish, bird or

Mammal

B) Eggs are laid to water and are eaten or hatch into a coracidium (C) and are then eaten by an invertebrate host

C) Larval development to a proceroid or a pleroceroid occurs invertebrate

D) 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 adult

C. philippinensis egg

Nematodes

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 invertebrates and produces a cystacanth (C)

D) If eaten by suitable host, the cystacanth will develop into an adult

Acanthocephala

Adult female

Neoechinorhynchus rutili

Adult male

Leeches (Hirudinea)

Leech with brood attached Top view

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

• Hemorrhaging• Inflammation• Edema• Ulceration• Fibrosis• Hyperplasia

• Necrosis• Irritation• Weight loss• Some can be

vectors of other parasites

Copepods

Salmonicola sp. Lernaea sp.

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

Copepoda

A) 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)

Glochidia

• Larvae attach to gills or skin• Live as parasites then drop off and live

independently• Some modify mantle tissue to help find

host

Glochidia

Lampsilis reeveiana Glochidia attaching to gill tissue

Fungi• 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

Aphanomyces invadans

Hyphae

Sporangia

1° Zoospore2° Zoospore

Cyst

Germination

Sporulation

1º cyst