Viruses and Parasites

• Viruses


  Infectious agents that are too small to be seen with a light microscope and that are not cells
• Whereas prokaryotic and eukaryotic cells contain both DNA and RNA


  Individual virus particles contain only one kind of nucleic acid—either DNA and RNA but never both
• Cells grow and divide


  Viruses do neither
• Viral replication


  Virus particle infect a cell and program the host cell's machinery to synthesize the components required for the assembly of new virus particles
• Components of viruses


  • Nucleic acid core
  • Capsid
  • Envelope
  • Virion
• Nucleic acid


  The "bad news" because viruses use their genome, their genetic information to replicate themselves in host cells
• Sizes and shapes of viruses

  • Helical
  • Polyhedral
  • Complex – capsid is a combination of helical and icosahedral shapes
  • Bullet-shaped
  • Threadlike
• Host range of a virus


  The spectrum of hosts that a virus can infect
• Viral specificity


  The specific kinds of cells a virus can infect
• DNA Viruses


  • Adenoviridae
  • Poxviridae
  • Herpesviridae
  • Papovaviridae
  • Hepadnaviridae
  • Parvoviridae
• Parasite


  An organism that lives at the expense of another organism, called the host
• Pathogens


  Parasites that cause disease
• Parasitology


  The study of parasites
• Less than half the world's cultivable land is under cultivation, primarily because endemic to (always present in) those lands prevent humans and domesticated animals from inhabiting some of them
• Parasitic infections in wild and domestic animals provide sources of human infection and cause debilitation and death among the animals, thus preventing the raising of cattle and other animals or food
• Organisms considered as parasites

  • Protists
  • Fungi
  • Helminths
  • Arthropods
• Protists


  • Unicellular (though sometimes colonial), eukaryotic organisms with cells that have true nuclei and membrane-enclosed organelles
  • Most protists are microscopic, they vary in diameter from 5 µm to 5 mm
• Protists are a key part of food chains. Autotrophic protists capture energy from sunlight
• Some heterotrophic protists ingest autotrophs and other heterotrophs
• Others decompose, or digest, dead organic matter, which then can be recycled to living organisms
• Some protists are parasitic. They cause debilitation in large numbers of people and sometimes death, especially in poor countries that lack the resources to eradicate those protists
• Intestinal and urogenital protozoa


  • Entamoeba histolytica (Sarcodina)
  • Giardia lamblia (Mastigophora)
  • Balantidium coli (Ciliophora)
  • Trichomonas vaginalis (Mastigophora)
• Blood and tissue protozoa

  • Acanthamoeba castellani (Sarcodina)
  • Naegleria (Sarcodina)
  • Trypanosoma spp.
• Iliophora


  Main source of infection is water contaminated by pig feces and the mode of transmission is through the fecal oral route. Person to person transmission via food handlers has been implicated in outbreaks
• Trichomonas vaginalis (Mastigophora)


  Causes urogenital infections and the main mode of transmission is through sexual intercourse
• BLOOD AND TISSUE PROTOZOA

  • Acanthamoeba castellani (Sarcodina)
  • Naegleria (Sarcodina)
  • Trypanosoma spp.
  • Plasmodium spp.
• Acanthamoeba castellani (Sarcodina)


  Free living amoeba that causes inflammation of the brain substance and its meningeal coverings (meningoencephalitis). People acquire the infection usually while swimming in contaminated water. Eye infection occurs primarily in patients who wear contact lenses
• Naegleria (Sarcodina)


  Usually acquired transnasally when swimming in contaminated water. Penetrates the nasal mucosa and cribriform plate, enters, the central nervous system, and produces a rapidly fatal meningitis and encephalitis (primaryamoebic meningoencephalitis)
• Trypanosoma spp.


  • Trypanosoma cruzi
  • Trypanosoma brucei rhodesiense
  • Trypanosoma brucei gambiense
• Trypanosoma cruzi


  Found primarily in South and Central America and is transmitted by the bite of the reduviid or triatomid bud. Cause Chargas Disease (American Trypanosomiasis)
• Trypanosoma brucei rhodesiense & Trypanosoma brucei gambiense


  Involve the tsetse fly (Glossina) as the vector. T. gambiense infection (West African or Gambian Sleeping Sickness) is chronic. T. rhodesiense infection (East African or Rhodesian Sleeping Sickness) is more rapidly fatal.
• Plasmodium spp.


  • Plasmodium vivax
  • Plasmodium malariae
  • Plasmodium ovale
  • Plasmodium knowlesi
  • Plasmodium falciparum
• Plasmodium spp.


  The vector and definitive host is the female Anopheles mosquito. Five plasmodia species that caused Malaria
• Mycology is the scientific study of fungi
• Characteristics of Fungi


  • Many are saprophytes that digest dead organic matter and organic wastes
  • Some are parasites that obtain nutrients from tissues of other organisms
  • Most fungi, such as molds and mushrooms, are multicellular, but yeasts are unicellular
  • Many fungi reach their hosts by producing spores that are carried by wind or water
  • Many fungi reproduce both sexually and asexually, but a few have only asexual reproduction
• Importance of Fungi


  • In ecosystems, fungi are important decomposers
  • In the health sciences, they are important as facultative parasites—they can obtain nutrients from nonliving organic matter or from living organisms
  • Some fungi produce antibiotics that inhibit the growth of or kill bacteria
  • Fungi such as those that cause athlete's foot are nearly always present on the skin and rarely cause severe damage
  • Fungal parasites in plants cause diseases such as wilts, mildews, blights, rusts, and smuts and thereby produce extensive crop damage and economic losses
• Classification of Mycoses (Human Fungal Diseases)

  • Superficial diseases affect only keratinized tissue in the skin, hair, and nails
  • Subcutaneous diseases affect skin layers beneath keratinized tissue and can spread to lymph vessels
  • Systemic diseases invade internal organs and cause significant dectruction
• Phyla of Fungi


  • Zygomycota (Bread Molds)
  • Ascomycota (Sac Fungi)
  • Basidiomycota (Club Fungi)
  • Deuteromycota (Fungi Imperfecti)
• Helminths are bilaterally symmetrical, have a head and tail end, and their tissues are differentiated into three distinct tissue layers: ectoderm, mesoderm, and endoderm
• Types of Helminths that parasitize humans

  • Flatworms
  • Roundworms