Parasitology

I. TermsA. Parasite: a living organism which lives upon or within another living organism

1. Ectoparasite – a parasite living on the outside of an animal

2. Endoparasite – a parasite living in the tissues or body cavities of an animal

3. Periodic parasite – a parasite that is on and off the host during part of its life cycle

4. Permanent parasite - a parasite that stays on the host all the time

5. Obligate parasite – an animal that must live as a parasite

6. Facultative parasite – an animal that can live as a parasite or as a free-living organism

7. Pseudoparasite – something that is called a parasite which is not a parasite.

B. Parasite/Host relations1. Symbiosis – 2 organisms living together

2. Mutualism – 2 organisms living together with both being benefited

3. Commensalism – 2 organisms living together where one organism is benefited but the other is not harmed

4. Parasitism – 1 organism living at the expense of another organism

C. Host – an animal where a parasite lives within or upon at any given time1. Final/Definitive host – an animal which harbors the adult stage of a parasite

2. Intermediate host – an animal which harbors some larval stage of a parasite which is necessary for completion of that parasite’s life cycle

3. Transport/Paratenic host – an unnecessary, but convenient animal that harbors some developmental stage of a parasite

D. Life Cycle – the reproduction and maturation series of a parasite1. Direct life cycle – the reproduction of a parasite without the use of an intermediate host

2. Indirect life cycle – the reproduction of a parasite through the use of an intermediate host

E. Reproductive1. Egg/Ova – 1st growth stage of a parasite

2. Cyst – a protozoan ova or egg

3. Larva – growth stage of the egg; baby worm

4. Hermaphroditic – containing both sex organs but neither being functional

5. Monecious - containing both sex organs which are functional

6. Dioecious – containing individual sex organs which are functional

7. Oviparous – production of ova within the parasite which pass out of the parasite, into the host and out of the host as ova

8. Ovoviviparous – production of ova within the parasite that passes into the host as ova, then hatch into larvae and pass out of the host as larvae

9. Viviparous – production of ova within the parasite which hatch into larvae within the parasite, pass into the host and then out of the host as larvae.

II. Ways of transmission1. Directly eating an infective larva or egg. Eating feces or contaminated grass.

2. Eating the intermediate host. The principal host may ingest the intermediate host harboring the infective stage.

3. The parasite actively penetrates the principal host.

4. Becoming infected by means of the intermediate host.

5. Eating the paratenic host.

6. The parasite may be maternally transmitted.

III. Fecal Examination TechniquesA. Direct Smears

B. Centrifugation

C. Passive Fecal Flotation (Fecalyzer)

D. Gross examination

IV. Internal ParasitesA. Nematodes (roundworms) – They have a complete digestive tract and a

tough, elastic, skin-like cuticle. The mouth may be specialized for attaching to or feeding on the host.

1. Ancylostoma (Hookworm) – found in the small intestinea. Host: man, dog, cat

b. Life Cycle: eggs develop in larvae which enter the host by being eaten, by skin penetration, or by penetrating a rodent transport host. Ingested larvae usually mature without migrating further. Larvae that penetrate the skin usually travel in the circulation through the heart to the lungs, break into the lung tissue, ascend the trachea and are swallowed before maturing. Some bypass the lungs and are dormant in the muscle. Adult worms may live for 4-24 months in the small intestine.

c. Symptoms: ground itch at penetration, anemia, enteritis, cutaneous larval migrans (“creeping eruptions”). All hookworms suck blood. They secrete enzymes and anticoagulants to digest intestinal mucosa and facilitate blood sucking. Humans usually get the hookworm

through skin penetration from infected pet. Common around the southeastern and Gulf Coast states.

2. Toxocara (T. cati, T.canis) – found in the small intestinea. Host: dogs, cats

b. Life cycle: can be transmitted by milk, tracheal migration, eating feces or dirt with contaminated feces, and paratemic host (rodents, birds). The adult worm lays eggs which pass out in the feces. The eggs are then eaten by the animal. If transmitted through the placenta or young animals, the larvae migrate through the liver and lungs, carried up the mucocillary apparatus, swallowed into the small intestine.

c. Symptoms: “pot belly” appearance, can vomit up adult worms if heavily infested, humans can have visceral larva migrans and ocular larva migrans from infection.

3. Dirofilaria imitis (Heartworm) – found in the tricuspid valve; greatest number of heartworm cases are seen in the southeastern US and the Mississippi River Valleya. Host: carnivores

b. Life cycle: Female mosquito (intermediate host) bites in infected animal and picks up the larval stage (microfilaria). After two molts (2 weeks), L3 the infective larval stage is found on the mouth parts of the mosquito. L3 migrates from the bite into the host. L4 then migrates through the tissue for several weeks. Heart worms live approximately 5-7 years in the dog.

c. Symptoms: cats – respiratory signs, pleural effusion, distended jugular veins; dogs – CHF

4. Trichuris (Whipworm) – found in the cecum.a. Hosts: dog, fox, coyote, cats (rare in North America)

b. Life cycle: transmitted through ingestion of larva eggs. Infective eggs can remain viable for many years. Larvae hatch from eggs in the small intestine and mature to adults. Adults begin producing eggs between 74 and 90 days after infection. Each female worm can produce more than 2000 eggs per day. Dogs tend to become re-infected.

c. Symptoms: rectal prolapse, inflammation, enteritis

5. Large Strongyles (Strongylus Vulgaris, S. Edentatus, S. Equinus) –found in cecum and colon

a. Hosts: Horses

b. Life Cycle: Most common horse parasite. Can migrate to tissues and other organs. Common cause of colic – S. vulgaris causes aneurysm in the arteries in the colon.

B. Cestodes (tapeworms) – Small intestineCestodes absorb nourishment directly through the skin from the gut contents of the host. a. Hosts: carnivores, man, ruminants, omnivores, herbivores

Life cycle: Cestodes have indirect life cycles that require specific intermediate hosts. Dogs and cats infected with adult tapeworms shed egg-laden proglottids in their feces.

a. Symptoms: weight loss, rough hair coat, or the extreme

C. Trematodes (Flukes) – hermaphroditic parasitesa. Hosts: everyone

a. Life Cycle: Indirect life cycle that requires one or two intermediate hosts to reach the infective stage. Eggs are passed through the feces where the larva is hatched. There the intermediate or paratenic host ingests the larva.

C. Protozoan – single cell organisms1. Giardia –found in the small intestines; water born parasite

a. Hosts: everyone

b. Life cycle: Transmission is by fecal oral route, with ingestion of cysts shed by animals or humans. Cysts are acquired from contaminated water, food, or fomites or through self-grooming.

c. Symptoms: fever, weight loss, lethargy; neurological disease, birth defects, stillbirths and ocular disease in humans

V. External ParasitesA. Fleas – blood sucking parasites; small brown, wingless insects with bodies that are flattened laterally. The thorax is divided into 3 sections each with a pair of legs. Females are larger than males.

They are vectors of several diseases including Bubonic plague, intermediate host for tapeworms, allergies, and typhus.

a. Hosts: They show little host specificity

b. Life Cycle: A female flea can lay as many as 40-50 eggs per day. 2-16 days the egg hatch, larvae (maggot) emerge and begin to feed on the skin. The life span of the adult flea ranges from 4-12 months.

c. Symptoms: pruritus, iron deficiency anemia (severe), FAD (flea allergy dermatitis)

B. Lice – wingless, flat parasites that are about the same size as fleas. Their body is divided into a head, thorax, and abdomen with 3 pairs of legs.

a. Life cycle: female lays several eggs (nits) daily throughout her life, which is about 30 days. Lice can only survive 3-7 days if separated from the host. They tend to be host specific. .

b. Symptoms: irritation, rubbing, scratching, biting at areas, or with severe cases alopecia, anemia, rough coat, pruritus.

C. Mites – infestations are more common in dogs than in cats. 1. Demodex

a. Hosts: human and domestic animals; dogs are commonly affected. Host specific

b. Life Cycle: Development from egg to adult takes approx. 20-35 days and is completed entirely on the host. The adult stage looks like a “cigar”.

c. Symptoms: mild, nonpruritic, patchy alopecia on the head or limbs, crusts around the face.

2. Sarcoptes scabei (mange)a. Hosts: relatively host specific

b. Life Cycle: The mites burrow into the epidermis layer of the skin.

c. Symptoms: pruritic, scaling, crusting, self-trauma.

d. Zoonotic

3. Otodectes (ear mites)