Lecture 1- Principles of parasitism

veterinary medicine importance of parasitism

• Treatment and control of parasitic health issues in daily practice

• Over $11 billion dollars spent annually worldwide (veterinary only)

• $$ amount higher w/ human parasite control & research is accounted for

• Cost of parasitism is significant

o Lost agricultural & human productivity

o Human and animal suffering & premature mortality

o All rational efforts at parasite control depend on understanding of the

phenomena of parasitism

commensalism

one organism benefits and the other is unaffected

parasitism

One organism benefits and the other is harmed

parasite fitness is characterized by a parasite's ability to-

- Colonize/become established in host

- Ability to resist environmental stresses

• Drug resistance

• Seasonal climatic extremes

- Survive host defenses

• Immunomodulation

• Immune avoidance

- Reproduce and disseminate its progeny

• May incur "Fitness Cost"

do parasites have the ability to change based on their environment?

- Parasite populations are plastic and dynamic

• capacity to respond to a variety of conditions and stimuli (selection pressures) by permanent alterations of their genetic composition

- Hypobiosis (arrested development)

- Vertical transmission of developmental stages

- Shorter or abbreviated lifecycle development

• drug resistance (survive drug selection pressures)

• ability to colonize new hosts/sites within hosts

how parasites cause disease

- suck blood, lymph, or exudates

- feed on solid tissues directly or after liquefying them

- compete with the host for ingested food

- traumatic injury by mechanical obstructions

- destroy host cells by growing in them

- production of various toxic substances that aid in their ability to enter host tissues, feed, or reproduce

- cause various host reactions such as allergic, inflammatory, hypertrophy, hyperplasia, and nodule formation

-stimulate the development of cancerous cell growth

- carry (vector) additional diseases (parasites, viruses, bacteria)

- reduce host resistance to other disease and parasites

obligatory parasite

- Cannot complete its lifecycle without spending part or all its time on or within a host.

- May have free-living stages

- May use a successive series of different hosts

- May spend entire lifecycle within or upon a host

facultative parasite

- not normally parasitic, but become so when eaten or opportunisitically enter a host through and orifice or wound,

- Naegleria fowleri causative agent of Primary Amoebic Meningoencephalitis (PAM) "Brain eating amoebae"

- Halicephalobus a causative agent of encephalitis inhorses

endoparasite

organisms living within a host environment

ectoparasite

organisms living on host (external environment)

aberrant parasite

- Organisms found in locations where they do not normally occur

• Generally, do not mature or facilitate transmission

• Often result in disease state- "Ectopic infections"

• Toxocara larvae in the eye of its host,

• Dirofilaria immitis in the brain of a tiger

• Paragonimus in the liver

accidental or incidental parasite

- Parasitic organisms that enter or attach to host species that are different from the one(s) with which they are normally associated.

- May not be able to survive in the host or often elicits a major pathological response from the host.

- Repeated successful colonization of an accidental host can become the basis of "host switching" and development of a new host association.

pseudoparasite

- Diagnostic parasite stages or other artifacts (plant pollen, fungi, arthropods) that superficially resemble protozoan cysts or helminth eggs in the feces, blood, or urine of an examined animal.

- Misnomer because "pseudo-parasite products" are not associated with a true host-parasite association.

- Eimeria oocysts passed in dog feces, various pollen mistakenly attributed to parasitic species infecting a host.

hyperparasite

- When a parasitic organism is host to another parasite species.

- Mosquito that is host to Plasmodium sp. or Dirofilaria immitis;

- Flea that is host to Dipylidium caninum

definitive host

- Where a parasite attains reproductive maturity

- evidenced by the circulation or passage (in feces or other host tissues) of progeny capable of colonizing additional host species

intermediate host

- in which a larval or intermediate stage parasite must undergo development to become infective for the "definitive or final host.

- Often an obligate or required developmental stage for completion of the life cycle.

paratenic host

- A facultative host used by a larval or intermediate stage parasite to facilitate transmission to the "definitive or final host.

- Parasites that infect a paratenic host do not undergo any development.

- This is an important distinguishing difference between paratenic and intermediate hosts.

- This facultative relationship (convenience and opportunity) often bridges an ecological gap between passage of infective stages and successful transmission and establishment in a new host for completion of a lifecycle and perpetuation of the parasite population.

reservoir host(s)

- Maintain a parasite in the population at sufficient levels to facilitate its transmission between susceptible hosts.

• Population parameter rather than Individual parameter

- For zoonotic parasites and infectious diseases, the reservoir hosts are animals

• Lyme disease: deer, white-footed mice

- Coyotes, and stray and unprotected canines are reservoir hosts for potential infection with Dirofilaria immitis

• Stray/wild canine vs. Companion (Pet) Canine

- Reservoir host(s) may be Definitive (adult parasites),Intermediate (obligate developmental stages), or Paratentic (facultative developmental stages)

- Emphasis on Population rather than Individual

vectors

organisms that facilitate transmission of infective parasite stages between different individuals of a host population

what do biological vectors assume?

assume the existence of an obligate relationship for successful transmission and maintenance of the parasite species in the host population

direct lifecyle

• Direct Lifecycle

- Parasites infect host directly

- Stages passed from host in feces/other tissues

- Infective to another "Definitive host" (dog to dog)

- May require period of development to infective stage

• Larval development w/in eggs

• Free living larval stages in environment

indirect lifecycle: obligate

• Require intervention of an intermediate host

• Stages passed from host in feces/other tissues

• Infective to the "intermediate host"

• Obligate requirement for development to infective stage

• Indirect transmission to "Definitive Host"

• May involve paratenic host

- "bridge the ecologic gap"

indirect lifecycle: facultative

• Indirect transmission to "Definitive Host"

• Optional intervention of an intermediate or paratenic host

• Stages passed from host in feces/other tissues

• Infective to definitive host or the "optional host"

• "bridge the ecologic gap"

horizontal transmission

- Parasites colonize susceptible individuals by usual routes

- Infection across different generations of host population not required

vertical transmission

- Transmission of parasites from mother to offspring by transplacental or lactogenic routes

- Trans-generational requirement

- Prenatal or perinatal

host specificity (momospecific): high specificity

• limited number of hosts capable of completing lifecycle

• Pinworms in primate hosts

host specificity (heterospecific): low specificity

• wide variety of species capable of hosting parasite

• Typical of intermediate and paratentic hosts

• Toxoplasma gondii, Baylisascaris procyonis

zoonotic

animal to human transmission

anthroponotic

human to animal transmission

Endemic (Enzootic)

The normal or usual distribution of parasites in a host population or geographic area

• Eimeria sp. is endemic in Tennessee cattle

Epidemic (Epizootic)

• An excess number of parasite cases, far exceeding the normal or usual distribution in a host population

• Temporal and Spatial characteristics

• " 50% of last years kid crop were aborted in an epizootic of toxoplasmosis "

prevalence

the occurrence of a parasite in a population defined by time and space

incidence

the rate at which new cases are added to a host population defined by time and space