Parasitology: Definitions and Veterinary Importance

• Parasites are covered within general concepts and ectoparasites.

Definitions

• A facultative parasite can become parasitic but doesn't need a host to complete its life cycle.
• An opportunistic infection occurs when pathogens exploit an opportunity not normally available, such as a weakened immune system.
• Parasitism occurs when an organism derives nourishment from another living organism for all or part of its life, causing harm.
• Epibiosis and Phoresis is the act of being carried; organisms are nutritionally independent, with the larger basibiont carrying the smaller epibiont for transportation.
• Commensalism is when a smaller symbiont feeds on food available on the host's surface without harming or benefiting the host i.e flagellates on fish; Entamoeba in human intestines.
• Parasitism = Ht - X, Commensalism = Ht ± 0(-x), Mutualism = Ht + Y
• Ht is the host table complex (food available).
• Latin: com = together; mensa = table

Parasites of Veterinary Importance

• Helminths are large, multicellular parasitic worms with similar shapes.
• Phylum Nematoda consists of roundworms.
• Phylum Platyhelminthes consists of flatworms.
• Class Cestoda consists of tapeworms.
• Class Trematoda consists of flukes which are endoparasites.
• Class Monogenea consists of skin flukes that are ectoparasites.
• Class Turbellaria consists of free-living flatworms, some parasitic.
• Phylum Acanthocephala consists of thorny-headed worms.
• Phylum Annelida consists of segmented worms that are not parasitic such as leeches.
• Kingdom Protozoa (Protists) includes: Phylum Mastigophora (flagellates), Phylum Apicomplexa, and Phylum Ciliophora (ciliates).
• Kingdom Fungi is also a type of parasite
• Kingdom Animalia includes: Phylum Arthropoda (insects/arachnids, copepods), and Phylum Cnidaria (Myxozoa).

Key Concepts

• Parasitism is a non-mutual, non-symbiotic relationship where the parasite benefits at the host's expense
• Parasites live in or on their host, either ectoparasite or endoparasite.
• Parasites reduces the host's fitness and reproduces faster than the host.
• Parasites use hosts for food, water, heat, habitat, and transmission.

Mode of Infection

• Oral, Spore, Oocyst/eggs, Larvae, Cyst
• Via skin penetration.
• Via injection.
• Via transplacental (vertical) transmission.

Hosts and Life Cycle

• An intermediate host is required for parasite development.
• A paratenic host is a transport host where no parasite development occurs.
• An aberrant or abnormal host isn't usually used by the parasite, resulting in slow/incomplete development.
• A dead-end or incidental host prevents parasite transmission to the definitive host, stopping development.
• Direct life cycles involve only one host; parasites without an intermediate host have a direct life cycle.
• The definitive host is where sexual reproduction takes place.
• Indirect life cycles involve two or more hosts.
• The Final host (FH) is the host where sexual reproduction (DH) occurs.

Direct Life Cycle; Host Specific

• General nematode life cycle
• Ingested L3 develops through L4 and L5 to adult egg laying worms
• Adults worms lay eggs leading to the formation of Eggs in faeces, and then eggs emryonate in 5 days +
• Next is the formation of First stage larvae in faeces (L1), then Second stage larvae in faeces (L2)
• Lastly there is the Third stage (infective) larvae that migrate onto grass in approximately 3 weeks

Direct Life Cycle; Paratenic Host

• Toxocara canis uses vertebrates and some invertebrates as a paratenic host.

Host, Life Cycle, Mode of Infection

• Sexual reproduction involves: Genetic diversity, a Longer life cycle, requires at least 2 parasites (M & F).
• Asexual reproduction involves: a Lack of genetic diversity, a Short life cycle, only one is needed.

Parasite Fitness-Survival

• Parasites benefit from the host for survival, heat, food, water, habitat, also transmission or maturation/reproduction
• Arrested development/hypobiotic stage is often seasonal in nematodes; is also caused by host immune responses or overcrowding, where high adult numbers may prevent larvae
• Parasites rely on: a Predilection site for the preferred site on the host, a food source, and avoidance of the host's immune system.
• The aberrant site is not a normal location in or on the host.
• Complex life cycles involving multiple hosts create Transmission opportunities.
• Trophically transmitted parasites tricks enhance the chances their hosts are eaten, if infected, hosts lose predator avoidance and attract predators.

Parasite-Host Balance

• Consists of: the parasite, host & host response, and environment

Host Response

• Previous exposure.
• Type of host response (inflammatory, immune).
• Nutritional and immune status

Parasite x Host x Environment

• Environmental factors that effect it are : parasites, hypobiosis, immunity
• It is directly correlated with treatment.
• Geographical location matters.
• Temperature / weather is a factor.
• Physical parasite presence, activity, and toxic products affect the balance.
• Size, numbers, and parasite behavior affects the balance.
• Consider how much the parasite feeds.
• Remember that -iasis is the presence of parasite (sub-clinical)
• -osis is disease caused by parasite
• Example: Coccidiasis versus Coccidiosis

Can parasites benefit the host?

• Can boost the immune system.
• Provide allergy relief
• Improves reproduction and healthy skin.
• Many taxa have co-evolved with their parasites

Ectoparasites

• Ectoparasites are on the outside of a host.
• Demodex is an Ectoparasitic mite (Arachnida), diagnosed microscopically from a deep skin scrape of infected animal usually present in low numbers in healthy dogs, and is not usually contagious: suppressed immune system
• Biting ectoparasites carry serious pathogens.
• Ehrlichia is a genus of rickettsial bacteria transmitted to vertebrates by ticks.
• Anaplasma phagocytophilum is a bacterial infection transmitted by deer ticks.
• Heartworm, a parasitic roundworm, is transmitted by mosquitoes.
• Class Monogenea is made up of ectoparasitic flatworms, found on the skin, fins, and gills of fish: Direct life cycle causes cage culture problems for fish.
• Gyrodactylus consists of viviparous monogeneans and some extremely pathogenic species.