Parasitism and Disease

Parasitoids

In between true predators and parasites

As adults, they deposit eggs into hosts, larvae consume hosts from within

Mostly insects (mostly hymenoptera and diptera

Parasitoids as Natural Pest Control

Host species-specific

Shipped as adults or as mummified hosts

○ Advised to release regularly

Parasites

Live on or in a host

Do not necessarily kill the host

○ Can consume part of the host (fluids)

○ Hijack nutrients (tapeworms)

Can rely on multiple hosts or be specific

Kleptoparasitism

Parasitism by theft

hijacking food

Kleptoparasitism examples

interspecific: gull steals food from a penguin

intraspecific: small spiders steal from big spider

Brood parasites

kind of Kleptoparasitism

Manipulate/use host individuals to raise their young so that they can spend more time on other activities (and spread out eggs)

One percent of all bird species are brood parasites (50% of all cuckoos, two genera of finches, five cowbirds and a duck)

Host defenses against brood parasites

Avoid parasitism, After parasitism

Avoid parasitism

○ Select sites that are difficult to parasitize

○ Start incubation early

○ Display aggressive territory defense

○ Nest in aggregations

After parasitism

○ Eject the parasitic egg (if you can recognize it)

○ Grasp and eject or puncture

Mafia Hypothesis

Proposes that cuckoos or cowbirds repeatedly check their host’s nests

○ If the parasitic egg has been removed, they destroy the host’s nest and kill or injure the nestlings.

○ 56% of warbler nests were destroyed when cowbird eggs were rejected vs. 6% of the nests where the warbler did not reject the cowbird (Hoover & Robinson 2007)

○ Hosts will lay new clutches in destroyed nests, giving the cowbird another shot at parasitism

Endoparasites

inside of the body of the host

The greater the intimacy, the more likely a parasite will be restricted to a specific host

Ectoparasites

on the skin/fur/feathers

Microparasites

Small, often intracellular

○ Bacteria, viruses, protists

Reproduce within host

○ Typically measure the number of infected

hosts

Acquire nutrients at the expense of the host

○ Can cause symptoms of disease (pathogenic)

Infection can result in death of host but it may

be advantageous to allow host to live.

Transmission of microparasites

Can be instantaneous or via short lived agents

May remain dormant in food or water

May depend on a vector (agent of dispersal)

Why more advantageous to allow host to live?

○ A less deadly strain that kills a smaller proportion of hosts may succeed in infecting a larger number of new hosts!

Macroparasites

Live on body or in body cavities of animals, are intracellular in plants

Produce specialized infective stages which infect new hosts when released

Can be counted or estimated, viewed with naked eye

lice

Macroparasite example

Human Hookworm,

Transmission of macroparasites

direct or indirect

direct transmission

Lice, fleas, etc. are transmitted through contact

Common in fungal parasites of plants (wind dispersed spores contact the leaf surface, penetrate host cells

indirect transmission

Require a vector or intermediate host

Trematodes that cause human Schistosomiasis reproduce in mollusks and infect humans

diversity lowers parasitism

What do parasites do the host

Can affect behavior

Can decrease population size through reduced survival or reproduction

Can have community-level effects and can influence diversity

Programmed change in hosts

Fungal and nematode parasites of plants can result in nodules even if parasites removed

○ Actually inducing a morphogenetic response and a genetic transformation of the host cells

Some flies and wasps lay eggs in host tissue, which results in increased growth (galls

Change in behavior

Increasing transmission

Maximize chance of moving parasite

Can result in increased mortality

Gordian worm

parasitizes terrestrial insects but needs aqueous medium for reproduction; hosts develops hydrophilia

Killifish

exhibit behavior that makes them more vulnerable to predation by birds(herons) (the definitive host of the parasite!

trematode

Invasion of the body snatchers

parasite complex life cycle and alteration of prey behavior

pillbugs pick up ancanthocephalan from starlings feces(where eggs are laid) → change behavior making them more vulnerable to birds → birds pick up parasite and poop it out

adult flukes come out of cow poop → eaten by snails → offspring passed in snails slime balls → ants eat slime balls → behavior changes so they go to top of grass where they get eaten by cows

Ophiocordyceps unilateralis

the zombie fungus

lures males into having sex with dead females

Effects of parasites on hosts

Vulnerability to predation

○ Red grouse with nematodes are killed more frequently

Weakening competitive strength

○ When infected with a parasite, Anolis lizards that typically exclude other species allow for co-existence

Decreased reproductive success

○ Decreased expression of secondary sex characteristics (plumage, etc)

Parasite Removal Study

Determining the effects of parasitic blowflies on bird chick

w/out parasite larger chicks and less nest failure

Pathogens

disease-causing parasites (bacteria, viruses, fungi, or protists)

Non-native pathogens can have dramatic impacts

Dispersal and dispersion of pathogens

Transmitted directly or require vector

Rate of production of new infection depends on the per capita transmission rate and the number of susceptible hosts (S)

Per capita transmission rate depends on

○ Contact rate

○ Probability of infection

How does disease move through a population

Hosts fall into three categories:

○ S = susceptible

○ I = infected

○ R = recovered (or removed)

The population of hosts can be described as:

○ N = S + I + R (population size doesn’t change)

Spread of a disease

A pathogen can regularly invade a population with a high density of susceptible individuals

As susceptible individuals become infected, the number of susceptible individuals decrease

SIR model equation

Susceptible = dS/dt = -β(IS) → The change in the susceptible individuals over time is a function of how frequently they contact the infected individuals

infected = dI/dt = -βIS )- vI → The change in the infected individuals over time is a function of the production of new infected individuals minus those that recover

removed = dR/dt = vI → The change in the number of removed individuals are those that recover (and are immune to secondary infection

β = contact rate

v = recovery rate

1/ v = average period of infectivity

β/ v = fraction of population who comes into contact with an infected individual during period of infectivity

Basic reproductive rate of the disease, Ro

number of infectious individuals produced from one infected individual

○ Depends on the fraction of population who comes into contact with an infected individual during the infectious period

R0 < 1

infection will eventually die out

R0 > 1

infection will sprea

Spread of microparasites equation

Direct transmission, R0 will increase with:

○ Average time over which infected host is infectious (L)

An efficient parasite therefore keeps its host alive for a long time.

○ Number of susceptible indvls in population (S)

○ Transmission coefficient/contact rate (β)

R0 = S(0)βL

Critical Community Size

To figure out transmission threshold, need a critical population size (ST) where R0 = 1

S(T) = 1/βL

Can use this to determine how many individuals need to be immunized!

Aim to reduce S below ST can keep R0 < 1!

p(c) = 1 – ST/S0 , where:

p(c)=critical portion of population to be immunized

ST = S not immunized during program

S0 = S prior to any immunization program

pc = 1 – (1/R0)

Infection cycles

Immunity, combined with death from infection, reduces S, R0 and decreases incidence.

But eventually S will increase again (birth, immigration), increase R0 and increases incidence

Peaks

○ Measles: 1 or 2 years

○ Whooping cough: 3 – 4 years

○ Diptheria: 4 – 6 years

Epidemic Curves

Time series of new cases of infection following introduction

If ST large enough, spread with rate R0

As S declines (recovers or removed), R0 decline

If S falls below ST, then epidemic disappear

How will the epidemic curve change if R0 is higher?

○ Rapid rise of curve

○ Rapid removal of S

○ Earlier end of epidemic

Whether or not an epidemic ends depends on…

○ The rate at which new S are born or immigrate in