Ch 23 bb - Infectious Diseases: Viral Diseases

virus

• a small infectious agent that can multiply only in living cells of animals, plants or bacteria; cannot reproduce or conduct metabolic processes without a host cell

  • parasites

  • vector-borne viruses multiply in both the invertebrate vector and the vertebrate host

virion

• a complete virus particle that is the extracellular infective form of the virus; it includes the capsid (outer protein shell), the RNA or DNA core, and sometimes external envelopes

virus classification

• viral genome can consist of:

  • DNA

  • RNA 

    • RNA viruses include retroviruses such as HIV

    • RNA viruses, especially retroviruses, are prone to mutate

  • can be single- or double-standard

  • can be linear or circular

  • can vary in length

• baltimore classification system classifies viruses based on replication method and type of nucleic acid genome

viral interaction with host

• viruses typically affect one type of cell, e.g., common cold viruses affect cells of upper respiratory tract

• viruses penetrate the host cell membrane or cell wall & inject its genome into host cell, force the host cell machinery to replicate the viral genome, the new viruses emerge from host cell (this kills the host cell)

latent infections

• viral DNA or RNA remains in host cells but doesn’t replicate or cause disease for an extended time

  • latent viral infections can be transmissible during the asymptomatic period so that person-to-person spread still occurs, ex: herpes viruses

chronic viral infections

• continuous viral shedding, e.g., with persistent hepatitis B or C

cancer

• some viruses don’t kill the cells they infect, but instead change cell function 

  • if this includes changing cell division, this can cause cancer

common viral infections include infections of

• respiratory tract (common cold, flu, pneumonia

• gastrointestinal tract (norovirus)

• liver (hepatitis A,B,C)

• nervous system (rabies, west nile, polio)

• skin (warts, chickenpox)

• placenta & fetus (zika virus, rubella, cytomegalovirus)

• multiple body systems (enteroviruses)

how viruses are transmitted

• inhaled

• swallowed (in food)

• insect bites

• sexual activity

• exposure to blood

viral disease management

• viral diseases are not treatable with antibiotics

• vaccines for viral infections include:

  • hepatitis B

  • influenza

  • polio

  • rabies

  • shingles

  • covid19

use of fetal cells to produce vaccines against viruses

• a few vaccines are prepared by growing the viruses in fetal embryo fibroblast cells:

  • varicella (chickenpox)

  • rubella 

  • hepatitis A

  • one version each of shingles and rabies

• these cells are descended from cells from 2 fetuses whose mothers voluntarily decided to have legal abortions approx. 40 years ago, one in England, one in Sweden

• these descendent cells were never part of the aborted fetus

influenza risk factors for serious complications

• age (under 2 years & over 65 years)

• living in nursing home/long term care facility

• underlying health conditions:

  • asthma 

  • chronic lung disease

  • diabetes

  • heart disease

  • kidney disorders

  • liver disorders 

  • BMI <40

  • weakened immune system

• during pregnancy & up to 2 weeks after end of pregnancy

types of influenza viruses

• influenza A and B viruses - cause seasonal flu

• a new influenza virus A such as can cause a pandemic when it

  • infects humans and causes illness

  • spreads easily and sustainably (continues without interruption) among humans

  • is different enough so people have little to no immunity to it

  • example: H5N1 is an avian influenza A virus

• influenza C virus - generally mild illness

• influenza D viruses - mostly affect cattle; not known to cause illness in humans

pandemic flu risk with influenza A viruses

• broad host range including birds, humans, other mammalian hosts

  • zoonotic disease

  • difficult to control spread of diseases in wild animals

• reassortment strains produced the influenza pandemics of 1957

reassortment

• when two influenza viruses infect the same host cell at the same time and exchange genetic material-this can create a novel influenza virus

• example: avian (bird) influenza a reassortment

  • hard to control because it can be carried long distances & transmitted by wild birds

pandemic influenza

• conditions that initiate a flu pandemic:

  • new influenza virus emerges (little/no human immunity)

  • it infects humans and causes illness

  • it spreads easily and sustainably (continues without interruption) among humans