Ecology CHAPTER 14: Parasitism and Infectious Diseases

Emerging infectious disease

A disease that is newly discovered or has been rare and then suddenly increases in occurrence.

Endoparasite

A parasite that lives inside an organism.

Ectoparasite

A parasite that lives on the outside of an organism.

t-test

A statistical test that determines if the distributions of data from two groups are significantly different.

Vector

An organism that a parasite uses to disperse from one host to another.

Cyclic population fluctuations of forest tent caterpillars.

At three sites in the province of Ontario, researchers quantified the population sizes of tent caterpillars by measuring the intensity of leaf removal on the trees. Over a 60-year period, the caterpillars exhibited large population fluctuations every 10 to 15 years. The rapidly growing populations ultimately succumb to an outbreak of infection by a virus, and the populations quickly die back.

________ infection in animals can also be quite harmful to the host.

Bacterial

....... can cause a wide variety of plant and animal diseases

Bacterial parasites

Testing North American versus European strains of white-nose fungus.

Bats exposed to the control treatment did not die. In contrast, bats exposed to the North American fungus strain or the European fungus strain died in large numbers. However, bats died faster when exposed to the European strain than the American strain.

The cyclical occurrence of measles in a human population.

Before vaccinations became available in 1968, the human population in London, England, experienced cycles of measles every 2 years. Once vaccination became available and the number of people vaccinated increased, the number of measles cases declined and the fluctuations no longer occurred.

Bacterial disease of plants.

Common plant diseases that are caused by bacteria include (a) shot-hole such as this Pseudomonas syringae mors-prunorum on a cherry tree and (b) Crown gall (Rhizobium radiobacter) shown on a birch tree in England.

Fungal control of the perching behavior in the yellow dungfly.

Compared to uninfected dungflies, infected dungflies perch higher on plants. Error bars are 1 standard deviation. (b) Uninfected dungflies never perch on the underside of a leaf, whereas infected flies do so 91 percent of the time.

Chronic wasting disease.

Deer infected with prions that cause chronic wasting disease lose weight and eventually die.

Ectoparasites.

Ectoparasites are parasites that live attached to the outside of their host. Common ectoparasites include ticks, fleas, lice, mites, mistletoes, and nematodes.

Human infections and deaths from West Nile virus in the United States.

Following the introduction of the virus to the United States in 1999, the number of human infections and deaths rose rapidly through 2003. Efforts to control mosquito populations, which carry the virus, caused a sharp decline in these effects on humans through 2011. In 2012, however, unusually warm temperatures with high precipitation in some regions of the country resulted in large mosquito populations and a new surge in West Nile infections and deaths. After 2012, infections and deaths declined substantially. Note that the two y axes have different scales.

________ parasites have large ecological impacts on a wide range of plants and animals.

Fungal

_________ include several groups of roundworms and flatworms that can cause serious diseases.

Helminths

An infected carpenter ant.

In Thailand, carpenter ants that become infected by a fungus crawl down from the canopy and attach themselves to the underside of a leaf by biting the leaf vein and then dying. After death, a spore-producing stalk grows out of the ant's head and releases its spores into the environment.

Controlling the spread of bird flu.

In an attempt to stop the spread of bird flu, millions of domesticated birds in Asia were killed and their bodies burned.

The dynamics of an infection over time.

In the basic S-I-R model, all individuals in a population are initially susceptible. When the infection is introduced at the beginning of the time period, there is an initial rapid growth in the number of infected individuals. As some infected individuals recover and become resistant, there are fewer susceptible individuals left to infect, so the number of infected individuals declines.

Transmission of fungal spores in the yellow dungfly.

Infected flies climb up a plant, position themselves upside down on the underside of a leaf, and die as the spores erupt from their abdomen. In contrast, uninfected flies position themselves lower on plants and sit upright on the top sides of leaves.

Reservoir species.

Many species of birds such as this 'Apapane (Himatione sanguinea, a species of Hawaiian honeycreeper) can survive infection and subsequently serve as a reservoir of avian malaria that can be spread to other birds through the bites of mosquitoes.

The rate at which the pathogen spreads through the population depends on two opposing factors.

One factor is the rate of transmission between individuals The other factor is the rate of recovery

______ typically have specific habitat needs and, as a result, often live in particular places on a host organism

Parasites

Mechanisms of parasite transmission.

Parasites can be transmitted vertically or horizontally. When transmission is horizontal, the parasite can be transmitted through a vector such as a mosquito, transmitted directly between two conspecifics, or transmitted to other species. When transmission is vertical, a parent host transmits the parasite to its offspring, such as when a mother bird transmits lice to its hatchlings at the nest. The illustration shows a water body surrounded by long grasses followed by trees. Two cranes, a frog, and a snail are in the water body while a mosquito and another crane fly above. An inset image pointing to a treetop shows a crane offspring in a nest. An arrow pointing from one to another standing crane, represents horizontal transmission directly between conspecifics (example, bird flu). A series of arrows pointing from a crane to the snail to the frog and back to the crane, represents horizontal transmission through multiple hosts (example, some helminths). Another series of arrows pointing from a crane to the mosquito to the flying crane, represents horizontal transmission through a vector (example, West Nile virus). An arrow pointing from a crane to the crane offspring, represents vertical transmission from parent to offspring (example, Chlamydia).

Preferred habitats.

Parasites have preferred habitats on their host. The human body, for example, offers a wide range of habitats for parasites.

However, the parasite-host model differs from the predator-prey model in two key ways.

Parasites—unlike predators—do not always remove host individuals from a population, and hosts—unlike prey—may develop immune responses that make some individuals resistant to the pathogen.

Tobacco mosaic virus.

Plants infected with the tobacco mosaic virus, such as this tobacco plant, develop blisters and light areas on their leaves. These effects can cause substantial reductions in plant growth.

All prions begin as a beneficial protein in the brain of an animal, but occasionally a protein folds into an incorrect shape and becomes pathogenic...

Prions do not contain any RNA or DNA; instead, they replicate by coming into contact with normal proteins and causing the normal proteins to fold incorrectly, with the first prion serving as a template.

__________ are a group of parasites that can cause a variety of diseases

Protozoans

Reservoir species

Species that can carry a parasite but do not succumb to the disease that the parasite causes in other species.

Infection tolerance

The ability of a host to minimize the harm once an infection has occurred.

Infection resistance

The ability of a host to prevent an infection from occurring.

Declining tree populations in North America.

The introduction of a fungus that causes Dutch elm disease killed many American elms, which were once common along North American streets. (a) A street in Detroit, Michigan, in 1974 before Dutch elm disease. (b) The same street in 1981 after Dutch Elm disease killed all the elm trees.

Parasite load

The number of parasites of a given species that an individual host can harbor.

Susceptible-Infected-Resistant (S-I-R) model

The simplest model of infectious disease transmission that incorporates immunity.

A parasitized amber snail.

The snail on the right has one normal eye stalk that is pale and slender and another that is infected by a parasitic flatworm, which causes the eye stalk to become enlarged and colorful. It also pulsates in a way that is attractive to predatory birds.

White-nose fungus in bats.

This species, known as the little brown bat, is infected by the fungus Geomyces destructans. One symptom of the infection is that the bat's nose turns white. When a bat colony becomes infected, a large proportion of the bats die.

Vertical transmission

When a parasite is transmitted from a parent to its offspring.

Horizontal transmission

When a parasite moves between individuals other than parents and their offspring.

Self-medicating chimpanzees.

When chimpanzees become ill from intestinal parasites, they swallow whole leaves and chew bitter twigs of plants not normally in their diet. These leaves and twigs reduce the number of parasites in their body and help them recover.

Parasitic worm emerging from a cricket.

When crickets (Nemobius sylvestris) are parasitized by horsehair worms (Paragordius tricuspidatus), the worm grows and develops until it is ready to reproduce. At that time, the worm causes the cricket to jump into the water and then the worm can emerge and reproduce in an aquatic environment.

Parasite-induced changes in isopod behavior.

When isopods are not infected by parasites, they generally stay under cover whether there are fish present or not. When they are infected, the isopods move out from under the cover, which makes them susceptible to predatory fish. Error bars are standard errors

Host-parasite population cycles.

When weevils are raised in the lab with parasitic wasps, the populations of the host and parasite experience cycles over time that resemble predator-prey cycles.

Because endoparasites often cause fatal diseases, they can .....

alter the abundance of host species and change the composition of ecological communities.

Most ectoparasites that attack animals are __________, including two groups of arachnids—ticks and mites—and two groups of insects—lice and fleas.

arthropods

Hosts experience higher fitness if they ....

avoid being parasitized

Parasites can control the ________ of many different animals

behavior

In this case, the parasite must evolve in such a way that it does not ....... of its host until after the host has reproduced and passed the parasite to its offspring.

cause the death

the American chestnut rapidly succumbed to the fungal disease known as ....

chestnut blight

Animals can also be infected by fungi. One high-profile animal fungal disease is caused by a species of ....... that infects amphibians, such as frogs and salamanders

chytrid fungus (Batrachochytrium dendrobatidis)

We have already mentioned several other emerging infectious diseases. These include the...

chytrid fungus that has decimated amphibians around the world; the H5N1 bird flu that jumped from birds to humans; and mad cow disease that infected cattle, sheep, and humans

A parasite typically infects only one or a few species of hosts, whereas a given host species can contain _______ of species of parasites.

dozens

Today, the chytrid fungus has spread around the world and it is suspected to have caused the extinction of ...

dozens of species of amphibians.

Parasites gain fitness when they ...

find a suitable host and reproduce

A disease will spread whenever the number of newly infected individuals is ______ than the number of recovered individuals

greater

Because bacteria must enter a plant through a wound in the plant's outer surface, they commonly require the assistance of _______ that pierce a plant's tissues.

herbivores

parasites generally have a _______ reproductive rate than their hosts and often do not kill their hosts

higher

Parasites can move between hosts through ...

horizontal transmission or vertical transmission.

For most hosts, the risk of becoming infected by a parasite generally increases with ........ because higher densities mean that individuals are likely to come into contact more often with the parasites or with infected individuals.

host population density

Given that the performance of most organisms is affected by abiotic conditions such as temperature and precipitation, it is perhaps no surprise that changing climates have the potential to alter ...... in wild animals, important crop plants, and humans.

host-parasite interactions

Fluctuations in parasite and host populations can also be caused by changes in the proportion of the host population that has achieved ____.

immunity

Endoparasites can be categorized as either

intracellular or intercellular.

__________ parasites live inside the cells of a host, whereas ________ parasites live in the spaces between the cells that include the cavities of a host's body.

intracellular; intercellular

New diseases typically emerge when a mutation allows a pathogen to...

jump to a new host species

One of the best-known diseases caused by prions is bovine spongiform encephalopathy, commonly known as ....

mad cow disease

In some cases, animal hosts have evolved both ...... defenses to combat parasites

mechanical and biochemical

An important assumption of the t-test is that the values from both groups are ...

normally distributed

Infection by a pathogen does ____ always result in a disease

not

reservoir species and immune individuals favor the persistence of the ....... over time.

parasite population

both chimps and humans use plants to medicate themselves against ....

parasitic infections

Those parasites that can cause an infectious disease are called ______

pathogens

When a pathogen is not restricted to a single host species, it has the ability to ..... even after it causes one of its hosts to go extinct.

persist and spread

parasite and host populations can cycle in a way that is similar to ...

predators and prey

The existence of vertical transmission and various methods of horizontal transmission can make it very challenging to ........ the spread of infectious diseases in humans, crops, domesticated animals, and wild organisms.

predict and control

In a population of hosts, the first individual to be infected by a pathogen is known as the ....

primary case of the disease

Chronic wasting disease is a more common ______ disease

prion

The percentages of S, I, and R individuals in a population are determined by

rates of transmission of the disease and acquisition of immunity, as well as the birth of new, susceptible individuals.

When we consider the evolutionary forces that affect adaptations of parasites and hosts, we often find that as one species in the interaction adapts, the other species _________ by adapting as well.

responds

If a parasite specializes on only one host species and is able to cause a lethal disease in only that host species, then it might eventually ...

run out of hosts and face extinction.

Some fungal parasites also damage crop plants. One of the best-known examples is a group of fungi that causes a disease known as _____

rust

Any individuals infected from this first individual are known as .....

secondary cases

Because our calculated t-value exceeds the critical t-value, we can conclude that the two means differ ______.

significantly

The rate of infection and the rate of recovery can be used to determine whether an infectious disease will

spread through a population.

The 1-mm-long worms attach to a plant root, inject digestive enzymes that break down root cells, and then consume the resulting slurry. This parasitic behavior can reduce ....

the growth, reproduction, and survival of the plant.

To calculate the t-test, we need to know three things from each group:

the mean, the variance, and the sample size.

The probability that a host will become infected by a parasite depends on numerous factors that include:

the parasite's mechanism of transmission, its mode of entry into a host's body, its ability to jump between species, the existence of reservoir species, and the response of the host's immune system.

he density of the host population can affect how easily parasites are ______ from one host to another.

transmitted

some parasites have evolved the ability to trick the immune system by making themselves _______

undetectable

Many sexually transmitted diseases can be passed by ......

vertical transmission

Although the caterpillar is susceptible to both predators and parasitoids, ______ have the greatest ability to reduce its abundance

viruses

degrees of freedom

which is defined as the sum of the two sample sizes minus 2

The Life of Zombies

"The ability of parasites to act as puppet masters over the behavior of their victims is just one way that parasites have evolved to improve their fitness."

The deadly chytrid fungus in Central America.

(a) Surveys of amphibians in Central America demonstrated that the fungus was spreading from the northwest to the southeast. (b) Surveys of amphibians at the El Copé site detected a sharp decline in amphibians, both in species that are active during the day and species that are active at night.