An organism that lives in or on another organism of a different species (host).
Usually harmful
What is parasitism?
A kind of symbiotic relationship between two different species of organisms in which the parasite is metabolically dependent on the host.
What is a facultative parasite?
An organism that is capable of living either a free or parasitic existence.
The threadworm Strongyloides stercoralis in dogs or humans.
What are helminths?
Roundworms or nematodes (e.g., ascarids) and flatworms (e.g., tapeworms or cestodes) and flukes or trematodes.
What is a pseudoparasite?
An object (e.g., pollen) or nonparasitic organism (e.g., grain mite) that is mistaken for a "true" parasite.
Definitive host
An obligatory host in which a parasite develops to sexual maturity.
Intermediate host
An obligatory host in the life cycle of a parasite in which immature or asexual stages undergo essential development and/or proliferation before transmission to the definitive host (e.g., mosquitos).
Mechanical vector
A nonessential host which passively carries infective parasite stages on its body; source of parasite contamination (e.g., boot, syringe).
Biological vector
Usually an arthropod (e.g., face fly); may be an intermediate host or mechanical vector.
Paratenic host
AKA Transport host - a nonessential host (e.g., rodents, roach), in the life cycle of a parasite capable of harboring and maintaining immature stages
Parasites undergo NO development or proliferation within these hosts but are merely carried within the body until consumed by its proper definitive host.
Reservoir host
A population of infected hosts which serve as a potential source of infection for other species of susceptible domestic animals or humans (e.g., wild [feral] animals)
Infective stage
Developmental stage of a parasite capable of initiating a new infection within another host (e.g., eggs, oocysts, larvae).
Direct life cycle
Transmission of parasitic infection from one host to another without the requirement of an intermediate host; one-host life cycle.
Indirect life cycle
Transmission of parasitic infection from one host animal to another through one or more essential intermediate hosts; multi-host life cycle.
Pre-patent period
The time between ingestion and invasions of infective stages of a helminth parasite in its definitive host until eggs or larvae are first produced (e.g., the time between ingestion of infective L3 nematode larvae until the appearance of eggs in the feces).
Patent period
The life span of the adult parasite in the host.
Infection
Establishment of a parasite within a host, with or without clinical signs; genrally endoparasites infect their hosts (e.g., nematodes, cestodes and flukes).
Infestation
Establishment of a parasite on the surface of a host with or without development of clinical signs. Ectoparasites infest their hosts (e.g., insects and arachnids).
Acute disease
Rapid onset
Large parasite numbers
Usually dramatic clinical signs (e.g., anemia, death)
Chronic disease
Slow debilitation
Small to moderate parasite numbers
Clinical signs usually slow in developing
Subclinical infection
No clinical signs are manifested
May be detected by decreased growth, loss of production, etc.
85% subclinical
Ways parasites injure their hosts
They feed on blood, lymph or exudate (e.g. mosquitoes, flies)
They feed on solid tissues
They compete with the host for nutrition by ingestion or absorption through the body wall
They may cause mechanical obstruction of intestines, etc.
They may produce pressure atrophy
They may destroy host cells by growing in them
They may produce toxic substances
They may produce allergic reactions, e.g. flea allergy
They may produce host reactions
They may directly/indirectly promote the neoplastic transformation of cells
They may carry diseases and parasites, e.g. malaria, heartworm
They may reduce their host’s resistance to other diseases and parasites
Characteristics of insects
Antennae: single pair
Body segments: 3
Legs: 6
Wings: often found
Characteristics of arachnids
Antennae: none
Body segments: 2
Legs: 8
Wings: wingless
Simple metamorphosis
Egg > Nymph > Adult
Nymphal stage resembles the adult
Nymphs are smaller and do not possess wings
Nymphs are not sexually mature, they cannot reproduce
Ex: the cockroach
Complex metamorphosis
Egg > Larva > Pupa > Adult
Each of these developmental stages bears no resemblance to the adult
The larva is worm-like (maggot)
The pupal stage is a resting stage (may have a cocoon)
The adult emerges from the pupa
Ex: housefly or butterfly
"Modified" Complex metamorphosis
Female fly retains larvae in her body and they are laid on the host instead of eggs. The larvae quickly pupate after being laid (e.g., Sarcophaga, Oestrus ovis, Melophagus & other hippoboscid flies)
Characteristics of all flies
One pair of wings (halteres are vestigial wings; organ of balance)
