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ECOLOGY OF PARASITES

Part 1: Introduction to ecology of parasites

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Introduction to ecology of parasites Problems and obstacles Parasite adaptations

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THE HOST AS AN ENVIRONMENT

Ecology is the study of relationships between organisms and their environments, with a focus on those factors that regulate numbers and distributions of organisms.

The host is, of course, a parasite’s environment in both ecological and evolutionary senses.

Most parasites encounter a wide variety of environmental conditions during their life cycles.

Although a parasite’s environment is primarily the host, transmission stages such as spores, eggs, and often juveniles must also survive abiotic conditions.

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A host usually represents a rich and highly regulated supply of nutrients.

Most body fluids of animals have a wide array of dissolved proteins, amino acids, carbohydrates, and nucleic acid precursors, and virtually all animals have mechanisms for maintaining the chemical makeup and osmotic balance of their body fluids.

We should expect parasites to exhibit traits that allow them to exploit such living environments, and we should expect evolutionary changes in hosts to be accompanied by parallel, perhaps adaptive, changes in their parasites

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INFECTION SITES

Host species include virtually the full spectrum of organisms, from humans to protozoans.

When viewed from a parasite’s perspective, all organisms are complex environments with many separate habitats.

Even the smallest insects and crustaceans offer many places, both internally and externally, that can be colonized by parasites.

And larger animals, such as rodents, birds, and human beings, provide dozens of microenvironments capable of supporting parasites.

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Although most endoparasites of vertebrates live in the digestive system, adult parasites are found in and on virtually all parts of the body, and

juvenile stages often undergo elaborate migrations through the body before arriving at their definitive sites.

Parasites are generally adapted to and restricted to particular sites within or upon a host.

Examples of this phenomenon are: - malarial parasites living inside red blood cells, - filarial nematodes that congregate in the heart or beneath the

skin, - bird mites that occur only on flight feathers, and - Monogeneans found in the urinary bladders of frogs.

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Site specificity is actually evidence of parasite adaptation to a particular habitat within a host

Parasites that inhabit the lumen of the intestine or other hollow organs are said to be coelozoic, while those living within tissues are called histozoic.

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PARASITE POPULATIONS

1) Quantitative Descriptors Parasitologists have adopted a number of terms

for describing parasite populations and communities of different parasite species.

Can be calculated from the observed data on the number of parasites in individual hosts.

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ECOLOGICAL TERMS AS APPLIED TO PARASITE POPULATIONS AND COMMUNITIES

Ecological term

Definition

Population structure A frequency distribution graph in which numbers of hosts (dependent variable) are plotted against parasite/host classes (independent variable), plus the calculated quantitative descriptors of the frequency distribution

Quantitative descriptors Numbers such as mean, prevalence, etc., that can be calculated from the observed data on the number of parasites in individual hosts.

Sampling unit One individual host animal in a collection of such hosts.

Infrapopulation Number of parasites in an individual host (can take the value of zero).

Density Average number of parasites per host in a sample of hosts, equal to the arithmetic mean.

Intensity Number of parasites in an infected host (cannot be zero).

Mean intensity Average number of parasites in infected hosts of a sample of hosts.

Metapopulation All the infrapopulations in a single host species in an ecosystem.

Suprapopulation All the parasites of a species regardless of developmental stage, in an ecosystem.

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Ecological term Definition

Infracommunity All the parasites of all species in an individual host.

Compound community All the parasites of all species in a sample of hosts of a single species in an ecosystem.

Prevalence Fraction or percentage of a single host species infected at a given time.

Incidence Number of new infections per unit time divided by the number of uninfected hosts at the beginning of the measured time.

Abundance Another term sometimes used as synonymous with density or mean.

Aggregated A situation in which most of the parasites occur in a relative minority of hosts and most hostindividuals are either uninfected or lightly infected.

Overdispersed A term sometimes used as a synonym for aggregated.

Variance/mean ratio Quotient of the variable (square of standard deviation of a frequency distribution) divided by the mean; sometimes used as a measure of aggregation.

k The value of a parameter of the negative binomial distribution; usually k must be calculated to describe an aggregated parasite population by use of mathematical models

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Example: Consider a sample of 10 mice with a total of 75 pinworms.

Density? Mean? Abundance? Prevalence?

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ANSWER

This sample would have a density (mean, abundance) of 7.5 worms per host.

However, these 75 worms could all be in one mouse

- in which case the prevalence would be 0.10

or

distributed among all the mice - the prevalence would equal 1.00

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2) Macro- and Microparasites Macroparasite Large parasites that do not multiply (in the life-cycle

stage of interest) in or on a host. Examples of macroparasites are adult tapeworms,

adult trematodes, most nematodes, acanthocephalans, and arthropods such as ticks and fleas.

Macroparasites often, if not typically, occur in aggregated or clumped populations.

That is, most of the parasites are in relatively few hosts of a species, while the majority of host species individuals are either uninfected or lightly infected

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Microparasites Small parasites that multiply within a host and these include bacteria, rickettsia, and

protozoan infections such as those that cause malaria (genus Plasmodium), trypanosomes, and amebas.

The measurement of the number of parasites within an individual host is usually difficult.

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POPULATION STRUCTURE

Parasite population structure is a critical piece of information for those seeking to control infections

Population structure is often described by the density (mean, abundance), variance (a statistical parameter whose value is related to the shape of a frequency distribution), and curve of best fit.

A graph can be constructed by plotting parasite per host classes along the X-axis and numbers of hosts that fall into these classes on the Y-axis.

The result is a frequency distribution that describes the parasite’s population structure.

16Population “structure” of the trematodeUvulifer ambloplitis (larvae) in bluegill sunfish in North Carolina over a three-year period.

Most of the host individualsare uninfected or only lightly infected, while most of the parasites are in a few host individuals. These frequency distributions match those predicted by the mathematical model (equation) known as the negativebinomial.

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EPIDEMIOLOGY

Epidemiology is the study of all ecological aspects of a disease to explain its

transmission distribution prevalence and incidence in a population. 2 types: Macroepidemiology Microepidemiology

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1) Macroepidemiology - concerns large-scale problems of disease

distribution, demographic and cultural factors that affect transmission, illness and death rates, and economic impacts.

- Collection of macroepidemiological data requires substantial funding, institutions such as hospitals or universities, trained personnel, and government policies that allow or even promote such data collection.

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2) Microepidemiology - concerns small-scale problems, for example,

the effect of individual host-parasite interactions, parasite strains, host genetic variation, and immunity on disease distribution.

- A complete understanding of disease transmission, especially when human behavioral factors are involved (as they typically are), requires study at both levels.

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The distribution of parasitism in a population may be influenced by a number of factors including:

1. The host age and sex, 2. Social and economic status, 3. Diet4. Ecological conditions that favor completion of

parasite life cycles

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THE HOST AGE, SEX

Pinworms are a good example of parasites whose distribution tends to be influenced by age, at least in developed countries, where children may serve as a source of parasites for the entire family.

Acute Toxoplasmosis is usually associated with young animals.

Trichomonas vaginalis lives in the vagina and urethra of women and in the prostate, seminal vesicles, and urethra of men.

It is transmitted primarily by sexual intercourse.

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SOCIAL AND ECONOMIC STATUS

Parasite infections poor country diseases. Poor maintenance of sanitation system. Level of education low Limited supply of clean drinking water Increase the risks of parasite infections E.g Leishmania mexicana infections often occur in

agricultural workers, thus illustrating the influence of occupation on health

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DIET

In many cultures certain food are considered best eaten raw and this can increase the risk of contracting parasitic diseases.

For example: The popularity of Japanese sushi and sashimi cuisine which includes raw fish – poses a risk of becoming infected with the number of infectious diseases including the nematode Anisakis spp.

In Europe countries, raw beef and pork are very popular risks of contracting Trichinella spiralis and tapeworm infection.

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ECOLOGICAL CONDITIONS THAT FAVOR COMPLETION OF PARASITE LIFE CYCLES

Some parasite simple or direct life cycle Some required intermediate host or the vector

2 factors1. Climatic2. The present of vectors

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CLIMATIC FACTORS

Climatic changes have an important influence on the epidemiology of most infectious diseases of humans.

Environmental factors such as temperature and rainfall vary seasonally in the majority of habitats, tending to induce regular cyclic fluctuations in the prevalence and intensity of parasitic infection.

The action of climate on host and parasite, however, is independent of population abundance.

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Climatic factors influence the population biology of human/animal disease agents in the following principal ways.

1. Host behaviour.2. Intermediate host abundance.3. Infective stage longevity4. Infectivity.5. Parasite development

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HOST BEHAVIOUR

Several changes in host behaviour, induced by the prevailing climatic conditions, often generate cyclic fluctuations in disease incidence.

E.g Such changes may be the result of differing work patterns associated with agricultural practices (the planting and harvesting of crops at different times of the year), or may result from social patterns influencing the behaviour of children

Agricultural practices are imponant to the transmission of helminth infections such as Ascaris and schistosomiasis

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INTERMEDIATE HOST ABUNDANCE

Seasonal changes in the prevalence of many indirectly transmitted parasites are in pan determined by the influence of climatic factors on the abundance of intermediate host populations.

Seasonal fluctuations in the transmission of malaria and schistosomiasis, for example, are to a large extent the result of changes in the abundance of mosquitoes and snails respectively

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INFECTIVE STAGE LONGEVITY

Climate has an important influence on the longevity of parasite transmission stages such as helminth eggs and larvae, the cysts of protozoa and free viral particles.

Temperature, for example, is a major determinant of the survival of the miracidia and cercariae of schistosome flukes and the L3 infective larvae of hookworms

The longevity of transmission stages which live in terrestrial habitats, such as the eggs of Ascaris and larvae of hookworms are also markedly influenced by soil moisture.

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INFECTIVITY

In addition to their influence on infective stage longevity, factors such as temperature and humidity have an impact on the infectivity of both transmission stages and infectious intermediate hosts.

Temperature, for instance controls the activity of schistosome miracidia and thus influences their ability to contact and penetrate the molluscan host.

ln addition, this factor also affects the rate at which infected snails produce cercariae.

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Climate may play a role in determining the activity and infectiousness of arthropod vectors.

For example, the optimum air temperature for the transmission of a filarial worm (Dirofilaria immitis) from dog to mosquito (Aedes trivittatus) is roughly 23°C, the biting efficiency of the vector decreases at lower or higher temperatures.

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PARASITE DEVELOPMENT

Temperature is an important determinant of the rate of parasite development either in the external habitat or within poikilothermic intermediate hosts such as snails or mosquitoes.

The rate of development of human hookworms, from egg to infective larva is most rapid at around 25-30C in moist conditions (roughly 5 days).

If temperatures are below 17-20C, the ova and larvae of Necator cease development and death rapidly follows.

Ancylostoma is able to develop at slightly lower temperatures than Necator and is thus found in certain temperate regions of the world.

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The influence of water temperature on a) The survival of Schistosoma mansoni miracidiab) The infectivity of S. mansoni miracidia to Biomphalaria c) The prepatent period prior to cercarial release of S. mansoni

in Biomphalaria.

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VECTOR

Among the most important epidemiological factors in parasitic infections are vectors which are often snails or blood-sucking arthropods.

Some of the most medically important vectors are anopheline mosquitoes, which transmit malarial parasites and snails of certain genera, which carry infective larval blood flukes, or schistosomes.

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Bulinus globosus is an important intermediate host for the trematode parasiteSchistosoma haematobium

Aedes egypti

Aedes albopictus

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TRANSMISSION BETWEEN HOSTS

Parasites may complete their life cycles by passing from one host to the next either directly or indirectly via one or more intermediate host species.

2 types

1) Direct transmission

2) Indirect transmission

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DIRECT TRANSMISSION

May be by contact between hosts (for example venereal diseases) or

By specialized or unspecialized transmission stages of the parasite that are picked up by inhalation (respiratory viruses),

ingestion (such as pinworm) or

penetration of the skin (such as hookworm).

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INDIRECT TRANSMISSION

Can involve biting by vectors (flies, mosquitoes, ticks and others) that serve as intermediate hosts (the parasite undergoing obligatory development within the vector).

In other cases, the parasite is ingested when an infected intermediate host is eaten by the predatory or scavenging final host.

A special case of direct transmission arises when the infection is conveyed by a parent to its unborn offspring (egg or embryo).

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TRANSMISSION BETWEEN HOSTS

Transmission by contact between hosts Transmission by an infective agent Transmission by ingestion Transmission by biting arthropod

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TRANSMISSION BY CONTACT BETWEEN HOSTS

Many direct transmitted viral and protozoan diseases, infection results from physical contact between hosts or by means of a very short-lived infective agent.

There are an agreement between observation and theory supports the assumption that transmission of many direct life cycle microparasites is directly proportional to the rate of encounter between hosts.

'Who mixes with whom' is an important determinant of the pattern of infection observed for directly transmitted infectious agents.

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TRANSMISSION BY AN INFECTIVE AGENT

Many directly and indirectly transmitted parasites produce transmission stages with a not insignificant lifespan outside of the host.

Examples: - The miracidia and cercariae of schistosomes, - The infective larvae of hookworms and - The eggs of Ascaris

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TRANSMISSION BY INGESTION

Transmission of a parasite which gains entry to the host by ingestion is influenced by the feeding behaviour of the host.

Ingestion may occur as a result of: - the host actively preying on infective stages (fish

predating digenean cercaria), - consuming food contaminated with infective agents

(human consumption of vegetables contaminated with Ascaris eggs) or

- consuming an intermediate host which is infected with larval parasites (human consumption of fish infected with Diphyllobothrium)- a predator-prey association existing between final and intermediate hosts.

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TRANSMISSION BY BITING ARTHROPOD

Many microparasites and macroparasites have indirect life cycles where transmission between hosts is achieved by a biting arthropod,

For example yellow fever, malaria, sleeping sickness and filariasis.

Transmission of a vector-bome disease is also influenced by the developmental period of the parasite in the vector, a period during which the host is infected but not infectious

This development delay is called the latent period and may often be significant in relation to the expected lifespan of the intermediate host.

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Mosquitoes Black flies Biting midges Sand fly Tick, mite and fleas