Viral infections

• Polio virus: Rare due to success of vaccine. Can cause one of the following

  • Asymptomatic illness: 90% of infections
  • Abortive poliomyelitis: 5% of infections cause fever and headache.
  • Nonparalytic poliomyelitis: 1-2% of infections the virus goes to CNS causing back pain and muscle spasm.
  • Paralytic polio: 0.1-2% of infections infect the spinal cord causing paralysis. The virus spreads from the blood to the anterior horn cells of spinal cord and the motor cortex of brain.
• Small pox: Eradicated in 1980 and is the largest virus (230-300nm). Replication of virus takes place in host cells cytoplasm.

  
• Mumps: Respiratory infection caused by mumps virus. Spread through saliva droplets when talking, sneezing etc. Symptoms include swollen glands, fever and fatigue. Complications are rare but can lead to deafness, meningitis and sterility.
• Hepatitis A (picornavirus, naked capsid): causes acute infectious hepatitis via faecal oral route (contaminated water). Symptoms include jaundice, fever and abdominal pain. Treatment: vaccine.

  • HAV enters bloodstream when ingested via oropharynx and targets parenchymal liver cells. Virus replicates in these cells and is released into bile and from there into stool.
• Hepatitis B (enveloped DNA): Can be acute or chronic and symptomatic or asymptomatic. Transmitted through blood, saliva and genital discharge. Virus replicates in liver but symptoms show 45 days later. Chronic HBV can lead to cirrhosis and primary liver cancer.

  • Virus enters cell and removes envelope exposing viral capsid which is transported into nucleus. Transcription takes place where viral DNA is used to make viral RNA. Then viral RNA is translated to produce viral proteins which form capsid. Within the capsid reverse transcriptase turns the viral RNA into double stranded viral DNA.

  
• Hepatitis C (RNA genome): Can cause chronic hepatitis which can lead to cirrhosis and liver cancer. Transmitted via infected blood and sexually. Can be asymptomatic which can spread the virus further.

  
• Influenza virus is a single stranded enveloped negative RNA virus that is spread by droplets from coughs and sneezes. Only influenza A and B are pathogenic. Children and immunocompromised people are at higher risks of further complications such as pneumonia. Diagnostic vs common cold:

  
• Rubella: Also known as German Measles. Transmitted via airborne droplets. Symptoms include mild flu like symptoms such as sore throat, runny nose and fever. If pregnant it can cause birth defects.

  
• Measles: A respiratory infection caused by measles virus that replicates in the respiratory tract and spreads to lymph nodes then all over the body. It causes rash, coughing and high temperature. Can recover and gain lifelong immunity, or have no resolution which can be fatal (immunocompromised patients).

  
• Influenza virus structure: 8 segments of negative RNA. Contains NA (neuraminidase spikes) and HA (hemagglutinin) on surface. Both are used to differentiate between different influenza strains.

  
• Influenza virus infecting cell:

  1. Binding entering
  2. Uncoating of -RNA that enters nucleus
  3. Transcription: -RNA turned into +RNA and replicated. mRNA is also made.
  4. mRNA transported to cytoplasm and synthesis viral protein.
  5. Viral RNA in nucleus leaves and assembles with viral protein to make viral core.
  6. Budding exit- Viral core envelopes itself in plasma membrane that has NA and HA embedded on its surface.

  
• Influenza virus re-assortment: Happens during budding exit when viral RNA envelopes in plasma membrane. If the plasma membrane has different neuraminidase and hemagglutinin to the original strain then a new strain is formed, which can cause a pandemic.

  
• Common colds: 30% caused by rhinoviruses while 15% caused by coronavirus. Incubation period is the time between exposure and onset symptoms.

  • Rhinovirus is self limiting and cannot cause serious complications. Has an incubation period of 8 to 10 hours.
  • Coronavirus has a longer incubation period of 3 days. Spread by aerosols. Can cause complications such as SARS (severe acute respiratory syndrome) and atypical pneumonia.
• Zoonosis: a disease that is transmitted from animals to humans via bites/scratches (rabies), vectors carrying pathogens (mosquitos/ticks), contact (dermatophyte fungi) and faeces (hookworm disease).

  
• Ebola virus: filamentous, enveloped, negative strand RNA virus. Transmitted by direct contact with bodily fluids. Incubation period of 2 to 21 days. Symptoms include fever, headache, muscle pain, bloody stools and vomiting blood.
• Prions (proteinaceous infectious particle): Infectious agent made only of protein. Resistant to sterilisation. Can cause scrapie (disease of sheep), chronic wasting disease (in deer) and Creutzfeldt-Jakob disease (human).