Parasites and Parasitism Dr Jennifer Coetzee CMID / Ampath

So, naturalist observe, a flea Hath smaller fleas that on him prey, And these have smaller fleas to bite ‘em. And so proceed ad infinitum. J. Swift

Objectives Definition of the term “parasite” –How does it compare to other relationships between organisms Describe the life cycles that parasites employ Be familiar with the basic classification of the medically important parasites

Definition Parasitism: relationship in which one of the participants, the parasite, either harms its host or in some sense lives at the expense of the host. –Vital factors from the host –Food and/or other factors –Host suffers Ectoparasites: live on the surface of the host, endoparasites live internally

Other Relationships Between Organisms Phoresis: Physical carriage Mutualism: Relationship in which both partners benefit from the association. Usually obligatory. Commensalism: One partner benefits from the assocation, but the host is neither helped nor harmed.

Life Cycles of Parasites Helps us to understand the infective, invasive and diagnostic forms of each parasite. 3 major groups based on life cycles: –Those having no intermediate host –Those using one intermediate host –Those for which two intermediate hosts are necessary

Hosts Definitive host: one in which the parasite reaches sexual maturity. Intermediate host: one that is required for parasite development, but in which the parasite does not reach sexual maturity Host-parasite specificity: some parasites are very host specific, e.g. beef tapeworm. Others are less so, e.g. Toxoplasma

Direct Life Cycles No existence outside of the host, e.g. Trichomonas vaginalis Existence outside host as egg or cyst, e.g. amoebae, intestinal roundworms Free-living existence, e.g. Strongyloides stercoralis

Indirect Life Cycles One intermediate host, e.g. bef and pork tapeworms More than one intermediate host, e.g. Diphyllobothrium latum, some flukes

Harm to the Host Mechanical injury: boring a hole into the hot, or digging into skin / other tissues Direct mechanisms: blood loss with hookworm Stimulation of a damaging inflammatory or immune response Robbing the host of nutrition

Co-evolution of Hosts and Parasites Disease reflects a lack of co-adaptation Parasites need to balance the amount of harm to the host against its ability to transmit itself “Natural selection may favour high levels of exploitation of the host if negative effects on the host have relatively small effects on parasite transmission”

Equal Rights for Parasites! Parasites are among the most diverse of all species Main purpose for preserving free-living organisms… Integral part of our biosphere Role of parasites in evolution

Classification Protozoa Sarcomastigophora Apicomplexa Ciliophora Microspora Metazoa Nematoda Platyhelminthes –Cestodes –Trematodes Arthropoda

Bibliography Schmidt G.D., Roberts L.S. Foundations of Parasitology, Fifth Edition. Chapters 1-3. Windsor D.A Equal rights for parasites. Conservation Biology 9(1): 1-2. Brown P New Scientist (Nov 16).