Parasite Systematics and Ecology

Systematics

The study of the biological diversity and classification within an evolutionary context

Taxonomy

Science of classification that includes the following Taxons (groups): Domains, Kingdoms, Phyla (For plants Divisions), Classes, Orders, Families, Genera, Species.

Some include Super- and Sub- Taxons as well and some include “Group” as a Taxon

Taxonomic criteria vary depending on the parasitic group.  For example:

Protozoans –  Cyst morphology and flagella characteristics.

Platyhelminths – Reproductive organs.

Nematodes –  reproductive organs and sensory papillae.

Arthropods – Skeletal morphology

Hyperparasitism

parasites are hosts to other parasites

Isozymes

Enzymes that catalyze the same reaction but whose genes encoding them

  occur at different loci

Allozymes

Enzymes coded for by alleles

RFLP’s

Restriction fragment length polymorphisms, or banding patterns produced by DNA fragments on an electrophoretic gel (DNA fingerprint)

Ecology

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

For the parasite, the host is the desiredenvironment:

however, transmission stages (spores, eggs, and sometimes juveniles) must survive outside of the host.

Site-Specific

Parasites encounter several microenvironments even within the definitive host and most occupy a specific niche

Parasite Community

A single host individual is infected with more than one parasite species

Incidence

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

Prevalence

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

Abundance

average number of parasites per host in a sample of hosts.

Aggregated Population

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

Intrapopulation

number of parasites in an individual host.

Macroparasites

Large parasites that do not multiply (in the life-cycle stage of interest) in or on a host.  For example: Adult nematodes and tapeworms

Microparasites

Small parasites that multiply within a host. For example:

◦ bacteria and most protozoan infections

Population Structure

Described as the density, variance, and curve of best fit of a population of parasites. 

The purpose is to generate a theoretical frequency distribution of the parasites among hosts.

Parasites invest a lot of energy in the production of reproductive cells to counteract the low probability of individual offspring success.

Several methods are used:

   1. Asexual reproduction (many forms)

   2. Hermaphroditism

   3. Egg production (1,000-250,000 per day)

Some parasites appear to influence intermediate hosts and make them more susceptible to predation by the definitive host. 

For example:

One trematode Dicrocoelium (hosts are large herbivores) uses the ant as the 2^ndintermediste host. The infective stage lodges in the ant’s brain and makes it move to the top of the grass so sheep will eat them.

Epidemiology

The study of all ecological aspects of a disease to explain its transmission, distribution, prevalence, and incidence in a population.

Mathematical Models

Used to predict parasite population and community dynamics and behavior.