Population biology of parasites of minnows
Introduction
This lab provides an introduction to the population biology of parasites. You have all had some
prior exposure to the basic concepts of population biology in your introductory classes. You
therefore have some familiarity with the fundamental principles (birth rate, mortality,
immigration, emigration, age distribution, and density) that underlie this vast field. Populations
can be defined as a group of organisms of the same species occupying a given space and time and
comprising a single gene pool.
Virtually all of the concepts in traditional population biology were developed in the context of
free-living organisms. But there are some problems when these concepts are applied to parasites.
Are all members of a single species in a host a population? Or do all members of a parasite
species in all of its hosts (intermediate and final) comprise a population? Another difficulty with
the concept of population as applied to parasites is that populations of free-living organisms
increase in number through birth or immigration (or both), whereas most parasites can only
increase through immigration (i.e. recruitment).
Parasite ecologists therefore recognize a hierarchical nesting of parasite populations. All of the
parasites of a single species in one host comprise an infrapopulation. All of the infrapopulations
of a given stage of species of parasite in all of its hosts are a component population. A
suprapopulation includes all of the parasites of a given species, in all stages of development,
within all hosts in an ecosystem. Not surprisingly, there are only a handful of studies at the
suprapopulation level.
These concepts are at the core of many of the most pressing questions in parasite ecology,
evolution, and especially human epidemiology. Why are some hosts more heavily infected than
others (why do some have larger infrapopulations than others)? What factors restrict (or
regulate?) the sizes of parasite populations in an individual host (immune status, gender, exposure
rates, size, diet, etc.)? Which individuals should be treated (vaccinated?) within a host
population? Such questions are clearly central to many of the most urgent parasite problems that
humans face.
This lab will use a standardized ‘kill and count’ approach to introduce basic principles of parasite
population biology. In so doing, you will get an impression of 1) the different kinds of parasites
that are typical in a natural host, 2) the complex web of interactions (involving up to 20-30
different host species) that exist even in this simple system and 3) the factors leading to the
tremendous variation in infection levels between hosts.
The System
Minnows are well-known to parasitologists as a ‘garbage-can’ of the parasite world, especially in
temperate ecosystems. They are infected with up to15 different types of helminth parasites, most
being larval trematodes. All of these have complex life-cycles, usually involving fish-eating birds
as final hosts. Some of these you will have seen before, others we will discuss as we observe
them in class.
One problem with obtaining accurate parasite census data, is the actual process of dissecting out
the important organs. It is important to do your dissections very carefully, so that we can obtain
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accurate census data. We will collect these data as a class. The quality of the information we get,
is dependent on the quality of each dissection.
The fish were purchased from a local bait shop from wild populations. Before dissection,
measure the maximum length (tip of snout to base of fork) of each fish and its weight. Do your
dissections at low power under the dissecting microscope. We will do a play-by-play on your
first dissections. Aim to necropsy up to 5-6 complete fish if possible.
Questions and data analyses
1. Population descriptors
A standardized starting point is to describe general patterns of infection. By convention, there are
2 general descriptors. Prevalence is the proportion of hosts in a sample that are infected with a
given species, usually expressed as a percentage. Mean intensity is the average number of
parasites, for each species, within each infected host (i.e. not including uninfected hosts). The
latter is always provided with some indication of error, usually standard deviation. We will go
over the format for producing a summary table in class. The ‘Quantitative Parasitology’ website
will calculate each of these metrics for you, or you can use Excel.
2. Frequency distributions
In this lab, we are especially interested in how parasite individuals of a given species are
distributed within a host population. One way to envision this is to imagine a population of 100
hosts that come into contact (at a local restaurant?) with say, 1000 parasite larvae. How will those
1000 parasites be distributed among available hosts? The simplest prediction is that they will
infect the hosts at random. However, for the vast majority of parasite populations, this never
happens. Instead, we tend to see clumped or aggregated (also known as overdispersed)
distributions whereby about 80% of the worms are often found in only about 20% of hosts.
Understanding why this might be so is at the heart of parasite population dynamics, especially in
the context of humans as hosts.
Therefore, an understanding of parasite frequency distributions is essential to an understanding of
parasite population biology. The precise form of the frequency distribution, and in particular the
proportion of uninfected and heavily infected hosts, in large measure determines the effectiveness
and relative importance of the factors capable of regulating parasite population sizes. Most
attention is focused on the two ends of the frequency distribution since uninfected hosts may well
be those that are genetically resistant, and heavily infected ones those are genetically susceptible
(this forms part of the ‘predisposition to infection’ controversy). Moreover, it is only in the
heavily infected hosts that regulatory mechanisms such as parasite-induced host mortality,
immunoresponsiveness and intra- and inter-specific competition are likely to operate. Remember
however, that frequency distributions are not static but change from place to place and from time
to time. The form of the distribution that one sees at any one time or place represents a balance
between factors which promote underdispersion and those that promote overdispersion.
At one time, the study of frequency distributions was primarily concerned with describing the
form of the distribution precisely by attempting to examine its ‘goodness of fit’ to a particular
mathematical model (e.g. Poisson, negative binomial, log-normal). More recently, their interest
lies in determining why a particular form of distribution is observed and why it changes in time
and/or space or between host species or between different types of parasites. One of the major
areas of research in parasite ecology and epidemiology today focuses on understanding the
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causation and significance of non random distributions in parasite-host systems. To do so, most
studies use parameters such as the variance:mean ratio to describe the degree of overdispersion,
rather than describing its fit to any particular model.
To look at frequency distributions, we will express the data in the form of number of hosts
containing 0-50, 51-100, 101-150 etc. parasites (for each species). We will go over the
methodology and analysis at the end of the lab period.
3. Factors affecting parasite population sizes
We will also evaluate whether variation in certain factors leads to variation in parasite population
sizes. Factors such as host species, host size and host sex are some obvious examples. We will
use simple t-tests to compare mean parasite intensity between sexes, and will use simple
correlation methods to determine if the intensities of the individual species are correlated with
host size and/or body condition.
4. Covariation in parasite intensities in individual hosts.
Parasites almost always co-occur with other parasites in the same host individual. You will see
this in the fish dissections. We will assess the pattern of co-occurrence in minnows. If species
intensities are negatively correlated, then as the intensity of species A increases, the intensities of
species B decreases. In free-living communities this is generally viewed as preliminary evidence
for competitive interactions. If the two are positively correlated, this is generally viewed as due to
similar exposure conditions, or to similar susceptibility-resistance mechanisms.
5. References
The literature on parasite population biology is enormous. Literature searches involving
descriptors such as ‘parasite population biology’, ‘metacercariae population biology’, ‘population
dynamics of parasite x in host y’, and so on will lead you to countless examples of studies that
have taken a similar approach as you have taken in the lab.
Anderson, R. M. & Gordon, D. M. (1982) Processes influencing the distribution of parasite
numbers within host populations with special emphasis on parasite-induced mortalities.
Parasitology, 85, 373-398.
Sandland, GJ, Goater, CP & Danylchuk AJ. (2001) Population dynamics of Ornithodiplostomum
ptychocheilus metacercariae in fathead minnows (Pimephales promelas) from 4
northern-Alberta lakes. Journal of Parasitology 87: 744-748.
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Assignment Description
The format convention we will use for this paper is the Journal of Parasitology
Grading rubric (total of 40 points)
Title: Informative, brief (1)
Abstract: A single paragraph the briefly captures each section of the full paper. (3)
Introduction: (8)
- Broad introductory statement to introduce the general phenomenon under study: over-dispersion
of parasites in hosts (2)
- Construction of broad framework of the research question through citation of previous research in
the primary literature. Cite at least two other peer-reviewed studies of minnow-parasite
interactions. (2)
- Describe the life cycles of the parasites we collect data on (2)
- Explicit statement of the specific research question tested in this study. (2)
Materials and methods: (4)
- Source of minnows under study (1)
- Description of dissection technique (1)
- Description of the statistical methods used to analyze the data (2)
Results: (12)
- Written narrative precedes presentation of the data (2)
- Conclusions supported by statistical inference (2)
- Frequency distribution of intensity of Op, and BS as separate graphs (2+2)
- Figure regressing intensity of Op and BS versus fish total length (2)
- Figure regressing intensity of Op v BS (2)
Discussion: (9)
- A restatement of the main findings of the study (1)
- Interpretation of the general patterns of over-dispersion observed in the data and correlation of
intensities within individual hosts (6)
- Comparison of these findings with at least three other published works in the primary literature (2)
References: (3)
- At least three references from the primary literature (2)
- Correct formatting of J Parasitology style (1)
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