Parasitology Notes

Definitions:

  • Facultative parasite: Doesn’t require a host to be parasitic.

  • Opportunistic infection: Caused by pathogens that take advantage of an opportunity not normally available, such as a weak immune system.

  • Parasitism: Organism that spends its life eating from a host, harming the host in the process (e.g., flea living on a dog).

  • Ectoparasite: Outside the host.

  • Endoparasite: Inside the host.

  • Phoresies ("being carried"): Two organisms traveling together.

  • Commensalism ("sharing"): One partner benefits, the other is not affected.

  • Mutualism: Both organisms benefit.

  • Hypobiosis: Arrested development.

Parasites in Veterinary Medicine: Helminth [parasitic worms]

  • Phylum Nematoda: Roundworms.

  • Phylum Platyhelminthes:

    • Class Cestodes: Flatworms.

    • Class Trematoda: Endoparasite fluke.

    • Class Monogenea: Ectoparasite fluke.

  • Phylum Acanthocephala: Thorny-headed worms.

  • Kingdom Protozoa:

    • Flagella [Mastigophora]

    • Ciliates

    • Apicomplexa

  • Kingdom Animalia:

    • Myxozoa

Key Concepts To Know:

  • Parasite usually smaller than the host.

  • Reproduce at a faster rate.

  • Typically don't kill the host.

  • Reduce host fitness.

Mode of Infection:

  • Oral

  • Skin penetration

  • Injection

  • Transplacental

Host types:

  • Intermediate [IH]:

    • Required

    • Parasite continues asexual development

  • Definitive [DH]:

    • Parasite continues sexual development

  • Paratenic [PH]:

    • Non-required

    • Transfer host

  • Aberrant:

    • Host not typically used

    • Development slow or incomplete

  • Dead-end:

    • Doesn’t allow transmission to the definitive host

    • Prevents development completion

Life Cycles:

  • Direct Cycle:

    • DH only

    • Reproduce without an intermediate host

  • Indirect Cycle:

    • Two or more hosts required

    • Intermediate host required

  • Sexual:

    • Genetic diversity

    • Longer life cycle

    • At least 2 parasites

  • Asexual:

    • No genetic diversity

    • Short life cycle

    • Only 1 parasite needed

Fitness/Survival:

  • Hypobiosis: Arrested development

  • Predilection site: Preferred site in or on the host

  • Aberrant site: Abnormal location

Strategies:

  • Trophic transmission: Linked by food chain; loses predator avoidance.

    • Ex: Fish swimming close to water surface.

  • "Zombie Snail": Leucochloridium paradoxum

    • Parasitic flatworm [trematode]

    • Gastropod intermediate attracts definitive bird host

  • Toxoplasma gondii

    • Only reproduce sexually in cats

    • Rodents lose fear of cats

Host Balance:

  • Parasite x host x environment

  • -iasis = presence of parasite (subclinical)

  • -osis = disease caused by parasite

    • Ex: Coccidiasis vs. Coccidiosis

  • Parasites CAN benefit host:

    • Healthy skin

    • Immune boost

Intro to Parasitology: Trematoda ENDOPARASITE Worm

  • Key Characteristics:

    • Oral and Ventral suckers

    • Broad host spectrum

    • Do NOT have a direct life cycle

    • Hermaphroditic but can cross reproduce and large eggs

    • Diagnosis -> feces sedimentation

  • Life Cycles:

    • Indirect life cycle

    • Host-specific for certain snail

    • Indirect COMPLEX life cycle

    • Second IH, PH, or environment

  • Location:

    • Species-dependent

    • Rumen, liver, lungs, blood

Fasciola hepatica:

  • Sheep LIVER fluke

  • Infects the bile duct

  • Zoonotic

  • Final Host: Ruminant

  • Pathology: Pipestem liver with chronic infection

  • Signs: Bottle Jaw & Anemia

  • Intermediate host: Snail

Monogenea ECTOPARASITE Worm

  • Flatworms:

    • DIRECT life cycle

    • Attachments by HOOKS (Gyrodactylus)

    • Udonella lost their hooks; Use copepods as ectocommensals

  • Demodex

    • Not usually contagious

    • Usually in old animals

  • Oncomiracidium stage

Protozoa Definitions:

  • Zoite: "little creature"

    • Sporozoite: Infective form

    • Trophozoite: Feeds and grows

    • Tachyzoite: Divide rapidly

    • Bradyzoite: Divide slowly

    • Merozoite: Many

Key Characteristics:

  • Unicellular & Eukaryotic

  • Anerobic or Aerobic

  • Feed on bacteria via osmotrophy or phagocytosis

  • Many do not cause disease

  • Grouped by locomotion

  • Asexual reproduction: Large exposure leads to significant damage

  • Host:

    • Host-specific or broad spectrum

    • Zoonotic

    • Chronic infections

Asexual Reproduction:

  • Binary fission -> budding, Schizogony

  • Exponential increase in numbers

  • Multiple divisions of the nucleus and cell

  • Serious cellular damage

  • Very pathogenic

Sexual Reproduction:

  • Merozoites:

    • Macrogametocyte to Macrogamete

    • Microgametocyte to microgametes

  • Microgamete fertilizes macrogamete forming a zygote into an oocyst

  • Not pathogenic

Flagellates: [Mastigophora]

  • Giardia: Travelers Diarrhea, chronic infection

  • Histomonas: Important in poultry

    • Clinical signs: Blackheads

    • Infection sight: Liver and cecum

    • Usually in nematode egg or earthworm containing the eggs

  • Trypanosomes: Sleeping Sickness and Chagas disease

Ciliates:

  • Balantidium: Zoonotic

    • Direct life cycle

    • Transmission contained to water

Apicomplexa:

  • Mostly intracellular

  • Sexual reproduction happens in the epithelial cells

  • Order Eucoccidiorida: Zoonotic. Asexual and sexual stages.

    • Asexual stage causes damage

  • Eimeria -> 4 sporocysts with 2 sporozoites

  • Goussia -> 4 sporocysts with 2 sporozoites, longitudinal suture

  • Sacrocystis -> Causes Equine Protozoal Myeloencephalitis

    • DH usually an opossum not a horse

    • Horse is an aberrant host

    • Form cysts

  • Toxoplasma -> Alters IH behavior to facilitate its way to the DH

    • Obligate intracellular

    • IH -> mouse

Order Piroplasmorida:

  • Blood-cell parasites

  • Use ticks -> DH

  • Babesia:

    • Texas cattle fever

    • Anemia

    • DH = TICK!

  • Theileria:

    • East coast fever

Acanthocephala Thorny-headed worms

  • Not usually pathogenic

  • Related to rotifers

  • No mouth or digestive system

  • Subclinical

  • Sexually dioecious (either male or female)

  • Intermediate host

  • Diagnosis:

    • Egg in the feces

    • Adult always in the small intestines

    • St Kitts mongoose/monkey dead-end host

Cnidaria Myxozoa

  • Parasitic jellyfish

  • Obligate parasites

  • Indirect life cycle

  • Polar Capsules

  • Highly adapted

  • Vermiform stage

  • 2 Hosts:

    • Fish

    • Annelid worm

  • Spores:

    • Myxospore

    • Actinospore

  • Whirling disease

  • Multivalvulida:

    • Histozoic -> pathogenic

    • In muscles

    • Coelozoic -> not pathogenic

    • Gallbladder

  • Kudoa -> Human GI problems

    • Food poisoning

Nematoda Round Worms

  • Key Characteristic: Males and Females are separate

  • Oviparous -> egg-laying

  • Viviparous -> Live young

  • Offspring must leave the host

  • Free-living or parasitic

  • Elongate/cylindrical

  • Alimentary canal present

  • Do not multiply w/in the host

  • Mouth has teeth -> draws blood -> causes anemia

  • Female nematode: "finger-like appendage" = vulva

  • Male nematode: Bursa

Life Cycle:

  • Egg-L1-L2-L3-L4-L5-Adult (1st-5th stage)

  • Pre-L1 -> microfilaria (mff)

    • Not L1 (after egg)

    • Can be considered L1 of its own stage

  • L5 -> immature adult

  • Insert host:

    • Final or Definitive

    • Intermediate

    • Vectors, paratenic, etc.

Insert mode of infection:

  • Per os (consuming)

  • Percutaneous

  • Transmammary

  • Transplacental

  • Direct:

    • Infective larva -> L3

    • Infective larva inside egg -> NOT L1; In dried-up old larva not fresh

  • Indirect:

    • L1 infective to intermediate host

    • L3 to the final host

  • Primary diagnostic stage -> egg or L1; In fresh feces

  • In a dead animal, what stage will you need to identify? Adult -> full grown and visible

Insert means of survival:

  • How they ensure offspring get into a host

    • Survival of eggs w/ larva -> Thick-shelled egg

    • Paratenic hosts

    • Transmammary, transplacental -> naïve host (puppies)

    • Periparturient -> egg increase in/around birth

  • Avoid immune system/crowding/bad-weather

    • Arrested development (hypobiosis)

Life Cycle: Periparturient Rise

  • Producing eggs in/around lambing

  • Immune exclusion -> L3's gone in 48 hr

    • Strong immunity

    • Previous exposure

  • Delayed rejection -> L3's enter mucosa and eventually die

    • Naïve sheep

  • Crowding -> arrests

    • Adult excretion

    • Older L3 -> slower development

  • Inhibited "take a nap"

  • Immune projection -> arrests and facilitates feeding

    • Small L4 or adults

    • Fewer eggs

  • SPRING TIME!

    • L4 resume development

    • Rise in egg count

    • New L3 causes "self-cure" -> premonition

  • Epidemiology: If you know how the species perpetuates, you can know how and WHEN to intervene

Ancylostoma caninum:

  • Transmammary

  • Hookworms

  • Direct life cycle

  • Puppy gets L3

  • Bursa

  • Predilection site: Adult in small intestines, larva in lungs

  • Pre-patent period PPP -> 2-3 wks

    • Period between growth of L3 to adult

  • Puppy clinical signs

    • Acute or chronic hemorrhagic anemia, bloody diarrhea, respiratory disease [larval migration]

  • Treat immediately

  • Zoonotic -> cutaneous larval migrants

Pastures:

  • Animals on pasture in spring, in barn in winter

  • Grazing animals eat L3 -> more grazing, more chance of exposure

  • Maximize nutrition -> pasture rotations

    • Often 30 days -> depends on weather, time of year, type

  • Accumulation of larvae

Periparturient rise:

  • Lambing has an increase of eggs in the feces [solid line]

    • Immune system is decreased, so adult eggs increase in amount when consumed

    • Starts in early parturition and ends in lactation

  • Accumulation of L3 in the environment

  • Maturation of egg depends on temperature and perfect environment -> eggs would rather grow to L3 in spring than winter

  • Lambs consume large amounts of eggs from the environment [dotted line]

    • Immune system also decreased

    • Adult worms reach their max numbers (peak of dotted line) and lambs start to become sick

    • Peak goes down because lambs are beginning to get some immunity

    • Adult eggs already around tell others not to grow

Prevention/Intervention:

  • Rotate pasture half way through lambing/late spring early summer [April - June]

  • Deworm ewe's halfway through lambing /late spring early summer [April -June]

Cestodes Tapeworms

  • Key Characteristics:

    • Hermaphroditic

    • INDIRECT life cycle

    • Larval stage causes damage

  • Scolex -> head

    • 4 suckers, Hooks

    • Larvae main goal is to form a scolex

  • Strobila -> body

    • Proglottids [segments]

    • Mature ones further away from the head

    • Hermaphroditic

    • No alimentary canal

  • Pseudophyllidea -> Bothria

    • 2 IH, Cilia

  • Cyclophyllidea -> Scolex and suckers

    • 1 IH, Adult in FH, Eggs in IH develops into larvae

  • Segments

    • Oncosphere, Hexacanth embryo

  • Do not usually cause problems in the final host

  • No clinical signs

  • Why do we treat?

    • Aesthetic purposes and zoonotic potential

Taenia Solium:

  • From eating undercooked pork

  • Final host -> humans

    • Adult in intestines

    • Normally NOT pathogenic in FH

    • No clinical signs

    • Final host produces eggs; Proglottids/oncosphere in feces

  • Intermediate host -> pigs

    • Eats human feces w/ hexacanth embryo

    • Forms a larva in muscle/tongue

    • MAY have clinical signs

  • Neurocysticercosis -> human eating human feces; In the brain

  • Human is aberrant and dead-end host in larval stage