Chapter 11. Parasitology

Principles and Terms

• Parasitology is the study of parasites.  Obligate parasites must spend at least a part of their life cycle in a host. Facultative parasites are opportunistic parasites that are normally free-living but become parasites when chances arise.
• Many parasites depend on vectors for transmission. Biological vectors are those in which the parasite spends part of its life cycle (e.g., mosquitoes infected with malaria). Mechanic vectors are those the parasite does NOT depend on for completing their life cycle (e.g., flies carrying bacteria).
• Parasites use the resource of their host. Definitive hosts are those in which parasites reproduce sexually. Intermediate hosts are used by parasites for non-reproductive developmental stages.
• Some characteristics of obligate parasites
• Obligate parasites are often specialists in their vector and host range
• Obligate parasites tend to co-evolve with their host and become less injurious to the host over the course of evolution
• A wide variety of immune-evasion mechanisms have evolved in parasites, e.g., changing surface antigens (bacteria pathogens), attacking immune cells (HIV-1), and becoming intra-cellular parasites (viruses)
• Treatment toxicity: treatments of infection by eukaryotic infection often incur severe side effects to patients, because the shared eukaryotic nature of the pathogen and host.

Protists

• Protists are unicellular eukaryotes
• Classification of protists
• Plant-like protists (algae): single-celled photoautotrophs; e.g., euglenoids, diatoms, and dinoflagellates; Primary producers in fresh water and marine environments, but causing "red tides" that kill fish stock.
• Fungus-like protists (water molds and slime molds): single-celled slime molds can aggregate into a multicellular organism when food supply is depleted.
• Animal-like protists (protozoa): single-celled, heterotrophic, motile
• Mastigophorans:Human pathogens include Trypanosome (causing African sleeping sickness), Leishmania, Giardia, and Trichomonas.
• Amebozoa: Causing amoebic dysenteries
• Apicomplexans: Having complex life cycle, e.g., malaria parasite (see below)
• Ciliates: some cause dysentery
• Case Study: Plasmodium vivax
• Causative agent of malaria, the most prevalent vector-born disease in the world
• Obligate intracellular parasite.  It has a complex life cycle that involves both sexual and asexual reproduction: it reproduces asexually within human body ("intermediate host") and reproduces sexually within the Anopheles mosquito ("definitive host")
• Malaria life cycle: Malaria parasites exist as sporozoites in mosquito saliva. Sporozoites migrate to the live and become merozoites. Merozoites rupture the liver, enter the blood stream, and become trophozoites. Trophozoites reproduce asexually and release large quantities of merozoites by rupturing thee red blood cells. Some merozoites become gametocytes (male and female cells), which unite to form zygotes after entering the mosquito. Zygotes produce sporozoites, completing the cycle. 

Fungi 

• Mycology (the study of fungi); mycoses (fungal infections)
• Fungi are saprophytes, i.e., organisms releasing digestive enzymes to the outside environment and digesting dead organic matters and waste. As such fungi are important decomposers, able to decompose complex, woody substances and recycle organic compounds in the ecosystem.  Fungi are also producers of many  antibiotics.
• Morphology: Molds (multicellular, arranged as long filaments of cells joined together, called hyphae) and Yeasts (unicellular, non-filamentous fungi). A mass of hyphae is called mycelium.
• Reproduction: capable of both asexual and sexual modes of reproduction; produce "reproductive spores", which helps fungal reproduction and dispersal.  (Not to be confused with bacterial endospores, which help bacteria getting through harsh conditions and are not for reproductive purposes.) 
• Fungal diseases
• Candidiasis (yeast infection), caused by Candida albicans
• Pneumocystis pneumonia (in AIDS patients), caused by Pneumocystis jiroveci
• "Athlete's foot"
  
• Fungi are opportunistic pathogens, meaning fungi are generally harmless, free-living organisms, and become pathogenic only under special conditions.  These conditions include immune-compromised patients (e.g. AIDS patients), patients on immune-suppression drugs, and destruction of normal bacterial protective layer in our body resulting from heavy use of broad-spectrum antibiotics.

Helminthes

• Helminths are multicellular metazoa (animals), including Platyhelminthes (flatworms) and Nematoda (roundworms).
• Adaptive characteristics of parasitic helminthes (compared with non-parasitic, free-living helminthes):
• Lack of digestive system (obtain nutrients by absorption)
• Reduced nerve system (since no need searching for food)
• Reduced motility
• Complex reproductive system: Adult helminthes may be dioecious (male and female in separate individuals) or monoecious (one animal has both male and female reproductive organs, called "hermaphrodites")
• Case Study: Schistosoma (Blood flukes). The complex life cycle includes entering the body through skin, maturation and sexual reproduction in liver, eggs released in feces and hatching in water, and development in snails. 

Vector-borne Diseases

• Many arthropods serve as vectors (carriers, or transporters) of diseases.  Three types of arthropod vectors are:
• Arachnids (8-legged): mites, spiders, and ticks
• Insects (6-legged): bees, flies, fleas, and lice
• Crustacean: crabs, crayfish
• Roles (or mechanisms) of vectors in transmitting diseases:
• Mechanical transport of pathogens: Non-biological transfer, e.g., houseflies pick up pathogens on feces
• Vector as "intermediate host": e.g., Lyme disease spirochete grow in tick guts
• Vector as "definitive host": e.g., Plasmodium reproduce sexually in mosquito guts
• Control of vector-borne diseases: Vaccine and chemotherapy are not as effective as disease prevention through the elimination or control of vectors (control of mosquito, tick, and fly populations)