Parasitism and Disease Flashcards
Provide examples of microparasites and macroparasites and give two ways in which each can be transmitted.
- Microparasites—bacteria and viruses, plasmodium (malaria)
o Transmitted directly (one host to another)
o Vector required (mosquito-malaria, aphids-plant diseases) - Macroparasites— helminth worms such as tapeworms, flukes, nematodes
o Transmitted directly (intestinal nematodes of humans, lice, fleas, gall-forming insects, fungi and mildews that affect plants)
o Transmitted indirectly—trematodes carried by snails, microfilariae carried by mosquitoes, tapeworms carried by fleas and lice
Distinguish between a definitive host and an intermediate host.
- Definitive host—all parasites have, one in which the parasite reaches maturity and becomes the adult
- Intermediate host—some parasites, allow parasite to complete certain developmental phases
List some possible responses of plant and animal hosts to their parasites.
- The plants and animals function as a living habitat for the parasites.
- Changing its nature developing an immune reaction or digesting it
- Evolve in response to the presence of the parasite which carries important implications for the long term relationships between the two
List some of the possible effects of parasites on their host populations.
- Directly increase death rates of host populations (human pops in underdeveloped countries)
- Decrease birth and growth rates of their hosts (energy going to parasite so less for growth and reproduction)
- Increase the susceptibility of their hosts to predation (loss of energy leaves host weak/vulnerable)
- Affect the outcome of competition (two species, dominant always excludes weaker except in areas where stronger species afflicted with malarial parasite which commonly affects stronger but not weaker species)
- Effects of parasitism on host pop may be outcome of combined effects of infestations by a number of different species of parasites (diffuse parasitism)
What is meant by diffuse parasitism?
- The combined effects of infestations by a number of different species of parasites
How are the reproductive rates (Rp) of microparasites and macroparasites defined?
- Microparasites—the average number of new cases of the disease that would arise from a single infectious host introduced into a population of susceptible hosts
- Macroparasites—the average number of established, reproductively-mature offspring
What will happen to an infection if Rp1?
- If Rp>1—the infection will spread
- If Rp<1—the infection will die out, each present infection or parasite leads to less than one infection or parasite in the future
What three factors dictate Rp for directly transmitted microparasites?
- The average period of time over which an infected host remains infectious
- The density of the susceptible individuals in the host population (S)
- The transmission rate of the disease (β)
Give two factors that determine the transmission rate (β) of a disease.
- The infectiousness of the disease, the probability that contact leads to transmission
- The patter of host behavior as it affects the likelihood of coming in contact with infected individuals
In what way is Rp host-density dependent?
- The greater the density of the susceptible hosts, the higher the basic reproductive rate of the parasite
What is the relationship between the critical threshold density (ST) and rate of transmission (β) of a disease?
- The critical threshold density for a parasite decreases as its rate of transmission (β) and the length of infectiousness (L) increase
- If diseases are highly infectious (large β’s) or give rise to long periods of infectiousness (large L’s), they will have relatively large Rp values even in a small pop
- If diseases are of low infectivity or have short periods of infectiousness, they will have relatively small Rp values and will only be able to persist in large pops
Give two reasons that the number of susceptible hosts (ST) might decline when a disease spreads through a population.
- They will die
- They will become immune
What is the relationship between the epidemic and endemic phases of a disease and Rp?
- Epidemic—rapid increase in the incidence of a disease because Rp exceeds 1
- Endemic—infection in which Rp is close to 1
Explain why diseases usually show cyclic phases within a population.
- Immunity induced by a number of bacterial and viral infections reduces number of susceptible (S) and therefore reduces Rp which eventually leads to a decline in the incidence of the disease. Over time, influx of new susceptibles into the pop by birth or immigration allowing the disease to increase again. Incidence of disease increases and decreases
What is herd immunity and how is it achieved?
- Achieved by holding the number of susceptibles below ST so that Rp is below 1
- The critical proportion of the population (pc) needs to be immunized to provide herd immunity
