MedMicroTest1ppt2

• Type of nucleuic acid present: DNA, RNA, ss/ds, +/-
• Mode of replication
• Symmetry of virus particle: icosahedral, helical, or complex
• Presence or absence of external envelope

• Metabolically inert: must parasitize host to replicate
• Infect every form of life (not other viruses)
• Cause some of the most common AND most serious diseases
• Difficult targets for antiviral agents due to cell infection (vaccines are more useful)
• Obligate intracellular parasites

• Attachment (highly specific)
• Penetration
• Uncoating
• Replication using host machinery
• Assembly of new virus particles
• Release of virus particles

• Cell lysis (lytic infection): release of new viral particles by cell lysis
• polio and influenza
• Persistant infection: host cell may remain alive and continue to release virus particles by budding
• Infected person may act symptomless
• hepatitis B
• Latent infection: virus remains silent by putting their genetic material without replicating....
• in cytoplasm (herpesvirus)
• in genome (retrovirus, hep B)
• Transform the host cell into a tumor cell: may become cancerous
• may be benign growth (papillomaviruses)

• Inhaled droplets (rhinovirus, influenza)
• Food or water (hepatitis A, noroviruses)
• Direct transfer from other infected hosts (HIV, hepatitis B)
• Bites of vector arthropods (yellow fever, West Nile virus)

• Viral attachment to a host cell depends on SPECIFIC INTERACTIONS between the virus and the receptor molecules of the host cell membrane
• molecules of nucleocapsid in naked viruses
• virus membrane in enveloped viruses
• eg. Influenza attaches by its hemagglutinin to sialic acid on cells of mucous membrane and on red blood cells.
• In this process the hemagglutinin binds to sialic acid, conformationally changes to bring the two membranes together, and then fuses them releasing the viral DNA into the cell.
• Only very specific animals (and often cells) are infected.

• Many transforming viruses carry "oncogenes" that cause transformation when incorporated into the host genome
• These have arisen from incorporation of HOST oncogenes into the viral genome during viral replication
• Many sequences are also found in normal human genes

• Eukaryotes that are distinct from plants and animals
• Multinecleate or multicellular organisms with a thick carbohydrate cell wall containing glucans and chitin
• May grow as thread-like hyphae or single cell
• Normal flora (C. albicans) or local infection (skin, hair)
• Pathogenic fungi from the external environment can digest material and take up nutrients directly from host tissue
• Mycology: study of fungi
• Mycoses: fungal infection (plural)
• Mycosis: fungal infection (singular)

• Filamentous: grows as the mass of hyphae (mycelium)
• Asexual reproduction results in spores which are a common cause of infection (after inhalation)
• vegetative mycelium - grows out
• aerial mycelium - grows up to release spores
• Yeast-like: grow as a single cell which reproduces by division
• Dimorphic: show both forms in their life cycle
• Many form hyphae at environmental temperatures, but occur as yeast in the host
• Candida is the reverse of the above (thrush is hyphae form)

• Superficial mycoses: fungus grows on body surfaces (skin, hair, nails, mouth, or vagina)
• spread by person-to-person contact or from animal-to-human contact
• eg. tinea pedis (athlete's foot) and vaginal candidiasis (thrush)
• Subcutaneous mycoses: fungus infects via skin and grows in nails and deeper layers of skin
• eg. mycetoma (Madura foot) and sporotrichosis
• Systemic or deep mycoses: infects internal organs
• often result from the opportunistic growth of fungi in individuals with impaired immune competence
• primarily acquired via the respiratory tract
• eg. histoplasmosis (breathing in the spores of Histoplasma capsulatum) and systemic candidiasis

An organism (animal) that lives in or on a host organism and gets food from or at the expense of its host

• Protozoa: microscopic, single-celled organisms that can be free-living or parasitic in nature
• Transmission is fecal-oral route (intestinal protozoa) or via arthropod vector (blood/tissue protozoa)
• Helminths: Large, multicellular, parasitic worms
• Microscopic at larval stage, macroscopic at adult stage
• Can be either free-living or parasitic in nature
• Arthropods (ectoparasites): important as vectors of many different pathoges
• blood-sucking arthropods (mosquitos, fleas, ticks, lice, mites) attach or burrow into the skin and remain there for long periods of time

• Single-celled animals, ranging in size from 2-100nm
• Many species are free-living, but others are important parasites of humans
• Protozoal infections are most prevalent in tropical and subtropical regions, but can occur in temperate regions (global warming)
• May cause disease directly (eg. rupture of red cells in malaria), but typically the pathology is caused by host-response
• Malaria presents the most severe global problem of all parasites (>1.5 million deaths each year)

• Infects a wide variety of cells (red blood cells, macrophages, epithelial cells, brain, muscle).
• Plasmodium (malaria) and Leishmania (Leishmaniasis) are insect-transmitted
• Toxoplasma (Toxoplasmosis) can be aquired by ingestion of feces from contaminated cat OR from mother in utero
• Antigens of intracellular parasites may be presented at the surface of the host cell, which can then be a target for cytotoxic T cells or T_H1 helper cells
• Survival within cells, especially macrophages (Leishmania, Toxoplasma), requires devices to evade/inactivate intracellular enzymes or reactive oxygen and nitrogen metabolites
• eg- Leishmania is covered in lipophosphoglycan to protect against digestion

• Lives in the blood, intestine, or genitourinary systemMany are transmitted by ingestion of contaminated food/water
• Trichonomas vaginalis is transmitted through sexual activity
• Trypanosomes are transmitted by insect vectors (T. brucei [sleeping sickness] by Tsetse flies)
• Try to survive by avoiding immune recognition of their plasma membrane by host
• eg - Trypanosomes undergo repeated antigenic variation of surface antigens
• eg - Malaria parasites show polymorphisms in dominant surface antigens
• eg - Amebae can consume complement at the cell surface

• Swallowing infective eggs or larvae via the fecal-oral route
• Swallowing infective larvae in the tissues of another host
• Active penetration of the skin by larval stages
• The bite of an infected blood-sucking insect vector

• Flatworms (platyhelminths): the trematodes (flukes) and cestodes (tapeworms)
• Thorny-headed worms (acanthocephalins): reside in the gastrointestinal tract
• Roundworms (nematodes): reside in the gastrointestinal tract, blood, lymphatic system, or subcutaneous tissues

• Greater frequency in tropical and subtropical regions that favor fecal-oral contact, poor practices in food preparation and consumptions, and the availability of suitable vectors
• Greater frequency in children and individuals closely associated with domestic animals
• Many helminths live in the intestine
• Some live in deeper tissues where they actively feed on host tissues or intestinal contents
• For majority sexual reproduction results in the production eggs, which are released from the host in fecal material

• Acquired by eating undercooked or raw meat containing larval stages
• Frequently infect humans
• Relatively harmless despite potential for great size

• AKA blood fluke
• Most important species of fluke
• Cause of schistosomiasis (disease in millions)
• Three main species - S. haematobium, S. japonicum, S. mansoni

• Directly by their feeding (blood and tissue fluids)
• Indirectly my transmitting infections

• feed on human blood and tissue fluids
• Blood feeders include mosquitoes, midges, biting flies, bugs, fleas, and ticks
• Contact may be temporary or permanent
• Mosquitoes are temporary ectoparasites, feeding for only a few minutes
• Scabies mite Sarcoptes scabiei lives permanently on humans, burrowing into superficial layers of skin to feed and lay eggs.  Heavy infections can build up causing a severe inflammatory condition.

• Transmit a wide variety of pathogens (from viruses to worms)
• The ability to transmit infections acquired from animals to humans poses a constant threat of acquiring zoonoses
• Zoonosis: any infectious disease that can be transmitted between species (from animals to humans)
• Reverse zoonosis: from humans to animals
• 61% of pathogens were found to be zoonotic
• Mosquito-transmitted West Nile Virus has become a significant threat in the USA

• Everyone has the normal form of the prion protein PrP
• It is found on the surface of the brain
• Begins to aggregate when it changes to its misfolded form, interrupt normal brain functions
• Infection occurs when a small amount of misfolded protein is accidentally ingested or introduced into the blood
• Papua New Guinea - devastation due to cannibalism
• Recent concern about the prions that cause mad cow disease infecting humans (cow and human PrP are very similar)

• Small, proteinaceous infectious particles (filterable)
• Extreme resistance to heat, disinfectants, and irradiation
• Susceptible to high concentrations of phenol, periodate, sodium hydroxide, sodium hypochlorite
• Diseases have long incubation period, appear late in life
• Cannot be cultured in vitro (protein)
• Does not elicit immune or inflammatory responses
• Resistant to Proteinase K and detergent insolubility

• Prion diseases that develop large vacuoles in the CNS
• humans: Kuru ("to shake") and Creutzfeldt-Jakob diseases (CJD)
• cattle: bovine spongiform encephalopathy (BSE)
• sheep: scrapie

• Host-derived glycoprotein termed PrP^sc
• derived from natural prion protein PRP^c
• Association results from conformational conversion from α-helices to β-sheets
• The PrP^sc aggregates into amyloid fibrils and plaques

• Origin in human prion diseases may arise from spontaneous mutations
• However, most require exposure to infective agent
• Eating contaminated food material
• Use of contaminated medical products (blood, hormone extracts, transplants)
• Use of contaminated instruments during surgical procedures (prions bind strongly to metal surfaces)
• Possibility of mother-fetus transmission during pregnancy (unconfirmed)
• Eating brains of dead, infected, humans (kuru)

• More effective between the same species, but CAN occur between different species
• eg - cattle with BSE to humans through consumption of meat resulted in vCJD outbreaks
• BSE is a result of transfer from sheep with scrapie to cows
• Human vCJD and BE were caused by the same prion strain
• The number of human infections "Likely" to arise from the BSE cattle in UK is controversial (long incubation time)

• Kuru is a disease (shaking, paralysis, death) which was identified with cannabalistic behavior (undercooked brains of relatives) in Papa Nuew Guinea
• More than 2700 infections between 1957-2004
• Incubation time estimated at 50+ years
• Fatality rate fell from 200+/year to 6/year after prohibition of cannibalistic ritual
• Most patients with kuru were heterozygous at codon 129

• Cannot be cultured
• No immune response
• Early stages CANNOT be diagnosed
• Clinical appearances (memory loss, personality changes, hallucinations, and speech impairment) indicate probability, but CANNOT be confirmed until post-mortem, histology
• A recent test, a conformation-dependent immunoassay, makes use of the abnormal prion RESISTANCE to protease digestion, is more accurate

• There is no treatment OR vaccine
• Understanding of the nature of the interactions between PrP^sc and PrP^c may eventually offer hope of regulating/mitigating PrP^sc