Chapter 12 Parasitism © 2002 by Prentice Hall, Inc. Upper Saddle River, NJ 07458

Chapter 12Parasitism

Upper Saddle River, NJ 07458

Outline• Parasites feed on a host, but generally

do not kill it• Hosts have evolved many defenses

(e.g., immune responses) against parasites

• Models show that the rate of spread of diseases is govern by the density of susceptibles in the population, the transmission rate of the disease and the length of life of the infected host

Outline• Parasites can substantially

decrease host population size• Parasites can affect the structure

of host communities• Parasitoids help in biological

control by reducing the density of pests

Defining Parasites• Parasite: a predatory organism

that feeds off another but generally does not kill it

• Host: prey of a parasite• Parasitoid: Cases where the host

does not survive but one host is insufficient for the development of the parasitoid

Defining Parasites• Some parasites live with their

host most of their lives (e.g., tapeworms)

• Some parasites drop off after prolonged periods of feeding (e.g., ticks, leeches)

Defining Parasites• Are mosquitoes and Wilde beasts

parasites?• Some parasites are parasitic on other

plants– Holoparasites: lack chlorophyll, and are

totally dependent on another plant for water and nutrients

– Hemiparasites: photosynthesize, but do not have a root system, so they rely on the host for this function

• Ex. mistletoe

Defining Parasites• Monophagous: parasites that feed off one to

three closely related species• Polyphagous: parasites that feed off many

host species

Defining Parasites• Ectoparasites: live on the outside

of the host's body (e.g., fleas and ticks)

• Endoparasites: live inside the host's body (e.g., tapeworms and bacteria)

Defining Parasites• Haustoria: plant parasite outgrowths that

penetrate inside a host plant to tap into it's nutrient supply

• Use of multiple hosts: fluke 肝蛭 (Figure 12.3)

Adult flukes produce eggs inside a cow.The eggs are passed in the cow’s feces.

Adult lancet fluke

Life cycle of lancet fluke,Dicrocoelium Dendriticum

Ants eat the “slime balls.”Some of the flukes migrate into

the ant’s brain, causing it to climbto the tip of a blade of grass where

it can be eaten by a cow.

Snails eat the fluke eggs; later theeggs hatch in the snail’s intestine.

The eggs hatch and asexuallyproduce offspring.

The offspring are passed from thesnail in “slime balls.”

Defining Parasites• Parasites outnumber free-living species 4 to 1 (Figure

0 2 161412104 6 8

Mammals

True bugs

Beetles

Butterflies and moths

Trees (95)

Average number of parasite species per host

Defense Against Parasites• Cellular defense reactions

– Eggs of parasatoids are rendered inviable by encapsulating them

• Immune responses in vertebrates– Phagocytes may engulf and digest small

alien bodies, and encapsulate and isolate larger ones

– Hosts may develop a "'memory,"' that may make then immune to reinfection

Defense Against Parasites• Defensive displays or maneuvers

– Actions intended to deter parasites

• Grooming and preening behavior– Behavior intended to remove

parasites

Modeling Parasitism• Differ from models of predation

and herbivory– Life cycles of many parasites involves

intermediate hosts– Models of parasite population

dynamics generally describe the population growth rate by the average number of new disease cases

Modeling Parasitism• For microparasites, the number of infected

hosts is the most important factor• Rp = NBL

– Rp = number of infected hosts, with p for parasite and R for net reproductive rate

• Transmission threshold; Rp = 1• For disease to spread; Rp > 1• For disease to die out; Rp < 1• Microparasites are transferred from host to host

– N = density of susceptible hosts in population– B = transmission rate of disease– L = average period over which the infected host

remains infectious

Modeling Parasitism– Generalizations (cont.).

•As L increases so does Rp• If diseases are highly infectious, Rp

increases •Large populations of susceptible hosts

promotes the spread of diseases

Modeling Parasitism•Critical threshold NT where Rp = 1

– NT is an estimate of the number of susceptible hosts needed to maintain the parasite population at a constant size

– NT = 1 / BL» If B is large, N is small» If B or L are small, the disease can only

persist only in a large population

(thousa

1948 1952 1956 1960 1964 1968 1972 1976 1980

Modeling Parasitism• Many diseases undergo periodic cycles

– Ex. Measles 麻疹 in England (Figure 12.5)• Peaks occur because host immunity is developed• New births lead to new susceptible hosts, and cycle repeats

Modeling Parasitism• Parasites spread by a vector

– Lifecycle of both parasite and vector become important in controlling diseases

– Ex. Farmers use insecticides to kill aphids, which transmit viral diseases to crops (rather than chemicals to kill the parasite)

– Ex. Yellow fever was eradicated in the US by inoculation rather than eradication of all mosquitoes

Parasites Affect Host Populations

• Using biological control to study the effects of parasites on hosts– Ex. Hawkins 1999: Biological control of pests,

especially by parasitoids, was greater in exotic, simplified, managed habitats than in natural habitats

– Control is most often exerted by a single parasitoid species, in contrast to natural systems, which require a suite of generalized enemies

– Thus, biological control projects can not providerigorous evidence of the importance of parasites in natural systems

Parasites Affect Host Populations• Effects of introduced parasites on

natural systems– Chestnut blight in the Appalachian

Mountains of North America• Virtually eliminated chestnut tree (Figure 12.6)

1934 1941 1953

– Chestnut blight in the Appalachian Mountains of North America• Introduced in New York in 1904• In Britain, 25 million elm trees (out of 30

million) were wiped out by the disease between 1960s and the 1990s (Figure 12.7)

– Rinderpest, •A virus with at least 47 natural

artiodactyls hosts, most of which occur in Africa

•The virus belongs to a class known asmorbilliviruses, which includes measles and distemper

•Spread by food and water contaminated by dung of sick animals

•Can be fatal to certain animals (buffalo, eland, kudu, and warthogs)

– Rinderpest, (cont.). •Major epidemic swept through Africa in

the 1890s, leaving vast areas uninhabited by certain species

•80% of hoofed stock died. Disease traveled 5,000 km in eight years

•Brought under control in the 1960s, through the use of cattle vaccinations

– Endangered species

– Endangered species• 1. Many endangered species are threatened by

diseases from domestic animals (Table 12.2)

– Endangered species (cont.).•Ex. The demise of the marsupial wolf in

Tasmania was because of a distemper-like disease obtained from domestic dogs

•Some endangered species have been given vaccinations to protect them from disease

– Mountain gorillas were vaccinated for measles

• Natural systems– Massive mortality of big horn sheep

from infection by lungworms (Protostrongylus stilesi and P. rushi )•Predisposes animals to pathogens,

which cause pneumonia

– Massive mortality of big horn sheep from infection by lungworms (Protostrongylus stilesi and P. rushi ) (cont.).• Infection rates of 91% and mortalities of

50-75% have been reported

– Colorado pine tree plantations and mistletoe. Mistletoe can cause 30% loss in extractable timber

– Saline marshes in North America and the plant parasite, Cuscuta salina (Figure 12.8a)

Parasites Affect Host Populations– Saline marshes in North America and the plant

parasite, Cuscuta salina (cont.)• Infects the most common plant in California

marshes, Salicorniavirginica thus promoting the growth of two other species, Limonium and Frankenia (Figure 12.8b)

Uninfected Infected With

Cuscuta

Salicornia Limonium Frankenia

– Parasite removal experiments•Fuller and Blaustein (1996) compared

the survivorship of parasite infected and uninfected free-living deer mice

– Conducted in large outdoor enclosures– Decreased over-winter survivorship for those

deer mice infected with the protozoan Eimeria arizonensis

– Contamination spread through the digestion of contaminated feces

•Hurtrez-Bousses et al. (1997) reduced the number of blowfly larvae parasites in young blue tits in Corsica

– Blowfly larvae suck blood from chicks, causing anemia and high mortality

– Removal was accomplished by removing nests from nest boxes, and microwaving the nests to kill the parasites, and then returning the nests and chicks

Parasites Affect Host Populations• Hurtrez-Bousses et al. (1997) reduced the number of

blowfly larvae parasites in young blue tits in Corsica (cont.).

– Chicks from microwaved nests were found to have greater body weight at fledging (Figure 12.9)

fledgin

• Stiling and Rossi (1997) manipulated parasitic infection levels of a gall-making fly on a coastal plant, Borrichia frutescens, on isolated islands off the coast of Florida

– Low rates of parasitism treatment» Allowed potted plants on one island to be

colonized by gallflies

– Low rates of parasitism treatment (cont.).» Allowed potted plants on one island to be

colonized by gallflies» Plants were removed before parasitoids

could find them– High rates of parasitism treatment

» They left plants on the island longer, to allow the parasites to colonize the galls

– Results: High degree of parasitism of gallflies resulted in a significant reduction in the number of new galls (Figure 12.10)

May June July Aug Sept Oct Nov1995

High parasitoidsLow parasitoids

101520253035

2030405060708090

Parasites Affect Communities

• Parasites affect the presence or absence of various species in a community

• The meningeal brainworm Parelaphostrongylus tenuis– Usual host is the white-tailed deer,

which is tolerant of the infection

• The meningeal brainworm Parelaphostrongylus tenuis (cont.).– All other cervids and the pronghorn

antelope are potential hosts

– All other cervids and the pronghorn antelope are potential hosts (cont.).• Worm causes severe neurological

damage

– The worm makes the white-tailed deer a potential competitor with other cervids, because they can not survive in the same area as the white-tailed deer. This phenomenon is known as apparent competition

Parasites and Biological Control

• Not all parasites are detrimental to humans

• Many are used to protect crops from pests: Biological control

• Many are used to protect crops from pests: Biological control (cont.).– Only 16% of classical biological

control would qualify as economic successes

• Many are used to protect crops(cont.).– Organisms used in biological control,

are released in a 'hit or miss' technique: Just release a bunch of parasites and predators, and hope that one of them does the job.

• Many are used to protect crops(cont.).– New techniques: Ex. novel parasite-

host associations

• Many are used to protect crops(cont.).– Review of 548 control projects: the

more parasites that were releaseds, the lower the rate of establishment

• Necessary attributes of a good agent of biological control (Huffaker and Kennett 1969)– General adaptability to the

environment and host– High search capacity

• Necessary attributes of a good agent of biological control cont.)– High rate of increase relative to the

host's– General mobility adequate for

dispersal

• Necessary attributes of a good agent of biological control cont.)– Minimal time lag effects in

responding to changes in host numbers

• Methods affecting the success of biological control (Stiling 1990)– Factor of greatest importance:

climatic match between the control agent's locality of origin and the region in which it will be released

• Methods affecting the success of biological control (Stiling 1990)(cont.).– Importance of climatic variation was

underscored by another review of biological failures (Stiling 1993)

Parasites and Biological Control– Importance of climatic variation was underscored by another

review of biological failures (Stiling 1993)(cont.).• Climate accounted for 34.5 % of the failures (Figure 12.11)

40 30 20 10 0

Death by natural enemies

Lack of alternative hosts

Prey has refuge

Wrong strain of host

Wrong climate

Other reasons

% reasons in failure in biocontrol

• Risks of biological control– Reduction in native Hawaiian

lepidopterans was partly due to wasp species introduced for biological control of lepidopteran crop pests

• Risks of biological control– Reduction in native Hawaiian

lepidopterans (cont.)•Wasps attacked target exotic species

but also non-target native species

– Reduction in native Hawaiian lepidopterans (cont.)•Stresses the importance of more

narrowly focused release, rather than the traditional "hit and miss" technique

• Risks of biological control– Problems with crops

•While it is of interest to insure that the agent does not adversely affect the crop, however, non-crop plants are not as vigorously tested

Applied Ecology• Importance of parasites to plants

and animals• Five main categories of disease-

causing organisms that most plants and animals are susceptible to (Table 1)

Applied Ecology• Leading causes of human death

by disease, worldwide (Table 2)

Summary• The true definition of parasite is

problematic. Parasites may include many species that feed on plants, plus more traditional parasites such as tapeworms, leeches, bacteria, viruses and parasitoids that attack animals. 80% of all life forms are considered parasitic

Summary• The presence of various parasite

defense mechanisms is testament to the importance of parasitism in nature

Summary• Mathematical models suggest that

effective parasites will keep their hosts alive as long as possible, thereby facilitating the transmission of parasites to additional hosts.

Summary• The huge influence of introduced

diseases, such as chestnut blight and Dutch elm disease, provide evidence for the severe effects that parasites can have on a host's population and host density

Summary• Parasites of insects can often be used

as control mechanisms against crop and forest pests. This technique is termed biological control. Finding the attributes of successful biological control agents is valuable. Unfortunately, biological control agents can have a significant impact on non-target natural populations

Discussion Question #1• Discuss some of the main

differences between microparasites, macroparasites, parasitoids, predators, and herbivores in terms of their life history strategies.

Discussion Question #2• Why can we eradicate some

diseases, such as yellow fever, through vaccinations, while we have not been able to eradicate diseases such as malaria?

Discussion Question #3• Can we ever expect chemical

pesticides to be replaced entirely by biological control agents? Explain.

Discussion Question #4• Parasites are usually very small

organisms, capable of passing through screen enclosures. With this in mind, how might you design an experiment to remove parasites from a population and compare that population's survival rates with those of an unmanipulated control population?

Discussion Question #5• Do you think that biological

control would be more successful on islands or on mainland continental areas? Against native or exotic pests?

Chapter 12 Parasitism © 2002 by Prentice Hall, Inc. Upper Saddle River, NJ 07458

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