1. ECOLOGY OF PARASITES
1 Part 1: Introduction to ecology of parasites

2.  Introduction to ecology of parasites
 Problems and obstacles
 Parasite adaptations
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3. 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. 3

4.  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,
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perhaps adaptive, changes in their parasites

5. 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. 5

6.  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. 6

7.  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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8. 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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9. 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.
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Suprapopulation All the parasites of a species regardless of developmental stage, in an ecosystem.

10. 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
host
individuals 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 10

11.  Example: Consider a sample of 10 mice with a total of 75
pinworms.
 Density?
 Mean?
 Abundance?
 Prevalence?
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12. 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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13. 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
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are either uninfected or lightly infected

14. 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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15. 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.
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16. Most of the host individuals
are 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 negative
binomial.
Population “structure” of the trematode
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Uvulifer ambloplitis (larvae) in bluegill sunfish in
North Carolina over a three-year period.

17. 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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18. 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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19. 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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20.  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. Diet
4. Ecological conditions that favor completion of parasite
life cycles
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21. 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.
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 It is transmitted primarily by sexual intercourse.

22. 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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23. 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
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infection.

24. ECOLOGICAL CONDITIONS THAT FAVOR
COMPLETION OF PARASITE LIFE CYCLES
 Some parasite  simple or direct life cycle
 Some required  intermediate host or the vector
 2 factors
1. Climatic
2. The present of vectors
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25. 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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26.  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 longevity
4. Infectivity.
5. Parasite development
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27. 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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28. 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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29. 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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30. 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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31.  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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32. 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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33. The influence of water temperature on
a) The survival of Schistosoma mansoni miracidia
b) The infectivity of S. mansoni miracidia to Biomphalaria
c) The prepatent period prior to cercarial release of S. mansoni in 33
Biomphalaria.

34. 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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35. Bulinus globosus is an important intermediate
host for the trematode parasiteSchistosoma
haematobium
Aedes egypti
Aedes albopictus
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36. 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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37. 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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38. 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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39. TRANSMISSION BETWEEN HOSTS
 Transmission by contact between hosts
 Transmission by an infective agent
 Transmission by ingestion
 Transmission by biting arthropod
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40. 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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41. 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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42. 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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43. 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
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lifespan of the intermediate host.

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