General considerations:

Defining parasites:

• There is no easy definition that will separate everything biologists consider parasites from herbivores or predators
• almost never kill the host directly (although some diseases do this, of course)
• usually live in intimate contact with their hosts (although some insects commonly considered parasites, like ticks and mosquitoes, spend much of their life span not in contact with a host)

Parasitoids are smaller than their hosts, like parasites, but kill their hosts, like predators

• almost always insects, often in the Hymenoptera (bees, ants, wasps)

Host is the organism from which the parasite or parasitoid derives its sustenance

• some parasites have only one species as a host
  • many diseases infect only one or a couple of host species
• some parasites can have many (usually related) species as hosts
  • ticks and mosquitoes will bite any warm-blooded animal they find
• Host Range is the number of host species one parasite species will attack (thought to be narrower for parasites that are totally dependent on the host for all feeding)
• some parasites require more than one host species before they can complete their life cycle
  • these parasites (many parasitic worms [Platyhelminthes and Nematoda]) have complex life cycles
  • often two different species as hosts (rarely three)
    • often the hosts are not closely related for parasites with complex life cycles
    • schistosomiasis nematode must infect both a freshwater mollusk and a vertebrate to complete the life cycle
  • host in which meiosis takes place is called the Definitive Host
  • other hosts are referred to as Intermediate Hosts
    • parasite reproduces asexually in intermediate host
• Vectors are organisms that are necessary to transmit the disease
  • some vectors are not affected by parasite and can't be considered hosts, just vehicles to transport the parasite
  • some vectors are also hosts
• Reservoirs are alternative hosts where the infection may remain if it is eliminated from another population (deer are reservoirs of eastern equine encephalitis for human populations)

Parasites can be divided into:

• Ectoparasites that remain outside of the host's body
• Endoparasites that enter the host's body
• Microparasites that reproduce in the host and are usually single-celled
• Macroparasites that release juvenile stages to the world outside of the host
• Holoparasites (used for plants only) plants that parasitize other plants and no longer photosynthesize but get all water and food from the host (ex: Dodder, Dutchman's Pipes)
• Hemiparasites (used for plants only) plants that parasitize other plants for water and minerals, but photosynthesize to make their own food (ex: Mistletoe)
  • Don't confuse hemiparasites with epiphytes, plants that grow on other plants but do not invade their tissues to steal water and nutrients (ex. many orchids)

Effect of Parasite on host

• Parasites may kill (as when a disease kills its host)
• Parasites may reduce host fitness through lost growth or lost reproduction due to stress from harboring parasite
• Parasites may sterilize the host
• Parasites may alter the hosts phenotype
  • some parasites change the sex of the host
  • some parasites alter behavior of the host so that the host acts to benefit the parasite (at its own expense)

Parasites

• Come from almost all taxonomic groups
  • parasitic bacteria. plants, fungi, protists, and animals
• Attack all kingdoms, including bacteria (which have viruses)

Parasites and Hosts are also coadapted

• Coadaptation often due to arms race type of coevolutionary changes in host and parasite
• Parasites differ with respect to their host specialization
  • Monophagous parasites attach a single species of host
  • Polyphagous parasites attack several species of hosts (usually they are related)
• Endoparasites are, in general, more often monophagous than are ectoparasites, although there are many exceptions to this observation.

Host defenses:

• Cellular Defense Reactions
  • Encapsulation of parasite's cells (often reproductive cells) by the host so that they are non-functional
  • Cell surface changes
    • Change the marker molecule and the parasite may not recognize the host
  • Immune response
• Defensive behaviors
  • Avoidance of parasites
  • Defensive Displays intended to deter parasites
• Grooming and preening to remove ectoparasites

Models of the spread of a parasite:

Epidemiology is the science that studies disease

• Infectious diseases are caused by parasites and are the most intensively studies parasite systems (due to their importance to our health)
• parasites are referred to as pathogens in epidemiology

Factors in the spread of disease

• Note that the parasite population is not usually studied, but the number of infected hosts is studied
  • If all parasites are in one host, and it dies, the pathogen population dies, no matter how large it is
  • If a smaller number of parasites are spread into many hosts, the death of a single host will not eliminate the pathogen
• The variables below are needed to model the spread of a disease
  1. S = the density of susceptible hosts (notice that density is usually used here, not total population size)
  2. B = transmission rate (depends on virulence of disease, mode of transmission, and host behavior)
  3. L = the average period during which a host will be infectious
  4. R_p = the replacement rate of infected hosts (note that it is not the R₀ of the parasite, as there may be lots of parasites and lots of parasite reproduction in each host). However, a bit like R₀, if R_p is less than 1, the rate of infection is such that there are fewer and fewer infected hosts as time goes on, if R_p is equal to 1 then the number of cases is stable, and if R_p is over 1, then the disease is increasing in incidence

  We can relate these factors with the following equation:

Rp = SBL

Some consequences:

• The longer the host is infective, the greater the replacement rate of parasitized hosts, so there is pressure on the parasites to keep the host alive (increase L, increase Rp)
• High transmission rates (large B) leads to greater replacement rate of parasitized hosts, so there is pressure on the parasites to evolve greater rates of transmission (increase L, increase Rp)
• Given the limited resources of the host, it may not be possible to do both of the above

An important consideration (for the parasite) is N_T

N_T = threshold population of susceptible hosts at which R_p = 1 and below which R_p <1

If R_p in the first equation is set to 1, then

Some consequences:

• If N_T is not constant, an increase in either transmission rate or infectious period will reduce the size of the host population needed to maintain the parasite
• If N_T is constant, then an increase in one parameter (either transmission rate or infectious period) will lead to a decrease in the other parameter (in other words, an increase in transmission rate will reduce the infectious period and vice versa)

Epidemiologists often try to define N_T so that they can predict the critical density of a susceptible population

Studies of the impact of Parasites:

Often can see the effect of an addition of the parasite to the host population as an epidemic (outbreak) of a disease

Difficult to remove the parasite and so it it difficult to do field experiments with parasitic systems

If this was not so, we would have performed many such removals in trying to cure us and our crops and livestock of disease

Impacts:

• Disease may show cycles similar to predator-prey cycles (in humans, whooping cough and measles show this cycling)
  • basis is the proportion of susceptible hosts
    • susceptible hosts become non-susceptibles after infection, as immunity's memory system makes a second infection unlikely
    • After an outbreak, enough hosts become immune to drop Rp below 1, so the disease declines in the population
    • As disease prevalence falls, new individuals entering the population (births and migration from populations without the parasite) boost the proportion of susceptibles
    • When this proportion is high enough to boost Rp over 1, another outbreak begins, starting the cycle over again
    • Can you see why this cycling is most apparent in diseased that affect children?
• Disease can set limits to the population size or the distribution of a host or hosts
  • Rinderpest in Southern Africa - virus with wide host range (large, grazing mammals)
    • Buildup of host (cattle) after establishment of European-style ranching
    • Outbreak of parasite after introduction of diseased cattle from Southeast Asia caused decline in cattle
      • also led to loss of natural populations of other hosts
        • Decline in wild populations of large grazing animals (antelopes, gnu, etc.) lead to:
          • change in vegetation over wide areas
          • reduction of tsetse fly population, which feed on large mammals
  • Decline in tsetse fly population lead to decline in cases of sleeping sickness caused by a trypanosome transmitted by the tsetse fly from human to human and from other large mammals to humans
• Competition can be mediated through parasite  - called Apparent Competition
  • White-tail deer and Parelaphostrongylus tenuis
    • White-tail deer are tolerant
    • Other cervids (moose, other deer like the mule deer, pronghorn) are harmed
  • Where white-tail act as a reservoir, other cervids do not occur

• Evolution may change the character of host and parasite
  • Evolution of resistance to antibiotics an example of the evolutionary potential of parasites
  • Virulence (transmission rate and infectious period) may vary through time
    • Rabbits and Myxomatosis
      • Less virulent strain of virus evolved
    • When both are present in a rabbit, virulent strain grows faster, overgrowing the less virulent strain, and wins by being the strain transmitted to the next host
      • When alone, less virulent strain meant that rabbits would live longer, infect more bloodsucking insects (vector)
    • More vector meant the less virulent strains had a higher rate of transmission as the rabbits lived longer to be bitten and the less virulent strains would win at the global level, although it loses at the individual rabbit level to more virulent strains
      • after time, Myxomatosis became a non-lethal disease and now a second virus, Calcivirus, is being used

Biological control through the use of parasites and parasitoids:

Attempt to reduce the population of a pest to an acceptable level through manipulation of the population ecology of that pest

• Note that it says reduction and not elimination of the pest
• elimination may sometimes occur
• usual outcome is the reduction of host to lower population levels than without the parasite

Not all biological control involves parasitism

• Herbivores and predators are also used
• sterile male release also used (screw worm)

Strategies can attack either death rates or birth rate (or both)

• Death rate strategies
  • Rabbits and Myxomatosis
  • Poses potential problems as the disease might jump to new hosts in the new environment and kill non-target species
• Birth rate programs
  • Sterile male programs
  • Med fly and Screw worm programs

Terms:

Parasitoids, Host, Host Range, Parasites, Ectoparasites, Endoparasites, Microparasites, Macroparasites, Holoparasites, Hemiparasites, epiphytes, Definitive host, Secondary host, Vectors , Reservoirs , Coadaptation, coevolution, Host defenses, Epidemiology, Infectious diseases, S, susceptible hosts, B = transmission rate , L, infectious, R_p , N_T = threshold population, Rinderpest, tsetse fly, sleeping sickness, Myxomatosis, virulence, Biological control, Sterile male programs, Med fly, and Screw worm, Defensive Display

Last updated October 11, 2006