Lecture 23 Flashcards
Define parasite.
an organisms that obtains its nutrients from a host or very few hosts, normally causing harm to the host but not necessarily causing death.
Define vector.
an organism that carries and transmits an infectious pathogen into another organism.
Most parasites only have ____ host
ONE, or a limited number
called host-specificity
What are features of micro-parasites?
- small and often intra-cellular
- multiply directly within their host
- often extremely numerous (e.g. viruses, bacteria, protozoa)
What are features of macro-parasites?
- grow on / in but do not multiply their host
- produce infectious stages which they release into the environment to find new hosts
- often live on the body or in body cavities (e.g. gut / intestine) rather than being intracellular
e.g. ticks, fleas, lice, tapeworms
How are parasites transmitted?
- DIRECT – do not require a vector to reach their hosts- include parasites of vertebrates like lice, mites, viruses, bacteria
- TROPHICALLY TRANSMITTED – eaten by hosts, when eaten, they survive digestion and mature into their adult form. Can have complex life cycles w one or more intermediate hosts
many protozoa, round worms,
can alter intermediate host behaviour so have greater chance of being eaten
- VECTOR TRANSMISSION – carried by other organisms b/w their hosts; are microparasites, like bacteria, protozoa, viruses, mosquitos, fleas, etc.
Parasitic castrators
- parasitic castrators: reduce or remove their host’s reproductive ability & use the energy that would have gone into host production for parasite growth. The host survives and sustains the parasite.
Micropredators
- micropredators: attack several hosts, usually feeding on blood. Invertebrate examples include leeches, mosquitos, flies, fleas, ticks, etc. Vertebrate examples include lampreys, vampire bats, and false cleaner fish
Parasitoids
- parasitoids: insects which eventually kill their hosts. They either 1) sting their large prey, carry it to a nest & lay an egg on top which hatches and feeds on the prey or 2) lay their eggs directly into the host - the eggs hatch and grow in the living host, eventually emerging from the host
What kinds of strategies do parasites have?
Parasitic castrators
Micro-predators
Parasitoids
Brood parasites
Brood parasites
use other species to raise their young (e.g. insects, cuckoos)
To what extent are animal & plant populations affected by parasitism and disease?
The answer depends on:
1) pathogen virulence
2) whether the pathogen reduces host survival (death rate), reproduction (birth rate), or both
microparasites (small, intracellular, multiply really quickly) drive really abrupt and severe changes in populations, whereas macroparasites (on / in host, but don’t multiply) are a bit different
Epidemic diseases
characterized by rapid changes in the prevalence of infection
when outbreaks occur, these pathogens cause waves of infection that can cause rapid population declines
short / long periods of no infection after it disappears
Endemic infections
persist for long times in populations, showing relatively little fluctuation in prevalence
Epidemics can cause . . .
mass mortalities
What happened after the black death?
- increased resources available for those that survived
- population grew at a greater rate after the epidemic than before
Micro- and macro-parasites that don’t kill their prey can affect (EPIDEMIC parasites can suppress populations):
1) birth rates
2) death rates, via increased predation
3) movement (immigration, emigration)
Can diseases cause populations to go extinct?
- density dependent specialist pathogens, those that infect a single host and where transmission is more successful when population numbers are high, they very rarely cause hosts to go extinct, although host numbers could be reduced to quite low levels to the point that other processes could cause extinctions
- frequency dependent transmission is more likely to cause extinction (i.e. transmission rate is similar regardless of population size), have long lived infectious stages, or infect multiple diff host species.
Strategies for disease prevention & control of microparasites
- culling –> used when transmission is thought to be density dependent
reduction of a wild animal population by selective slaughter.
Behavioural modifications including quarantine & social distancing
Vaccination – herd immunity
What is the virulence-transmission trade-off hypothesis?
virulence is an unavoidable and increasing cost because the parasite uses host resources to replicate
transmission is highest when virulence is intermediate
What is host tolerance?
the ability of a host to tolerate infection with a pathogen by minimizing the damage done but without impeding replication or transmission of the pathogen
What is host resistance?
the ability of a host to reduce the probability that it is infected, reduce pathogen replication within the host ,and/pr increase the speed of pathogen clearance (Recovery)
What are the 2 ways that hosts evolve?
- host tolerance
- host resistance
