week 10

• Infectious and vector borne disease
• Infection


  The invasion of an organism's body tissues by disease-causing agents
• Infectious diseases


  Illnesses resulting from an infection, also called communicable diseases
• Infectious diseases often involve damage caused by the host reaction to the organisms and their toxins
• Infectious agents


  • Viruses
  • Bacteria
  • Parasitic worms
  • Fungi
  • Protozoa
• Until quite recently, infectious disease was the biggest killer of man
• In the United States in 1900, infectious disease caused 53% of deaths, while all other causes were 47%
• In the United States today, infectious disease causes 2.7% of deaths, while all other causes are 97%
• Infectious disease is still a major burden elsewhere in the world
• Forms of infectious disease causing most mortality

  • Respiratory infections
  • Diarrhoea
  • HIV/AIDS
  • Tuberculosis
  • Malaria
  • Meningitis
  • Pertussis
  • Measles
  • Hepatitis B
• A small number of microbes cause the majority of infections
• Pathogens causing major infectious diseases

  • Pertussis - Bordetella pertussis
  • Tuberculosis - Mycobacterium tuberculosis
  • Typhoid fever - Salmonella enterica
  • Influenza - Influenza virus
  • AIDS - HIV
  • Malaria - Plasmodium falciparum
  • Sleeping sickness - Trypanosoma brucei
  • Bilharzia - Schistosoma mansoni
  • Filariasis - Brugia malayi
• Influenza virus


  There are three types that infect humans: Type A, B and C
• The influenza virus is typically spread through coughs and sneezes, or through physical contact
• The influenza virus also infects some other animals, such as pigs and birds
• In animal reservoirs, the influenza virus is prone to mutation and recombination, resulting in the rapid evolution of new strains
• Influenza virus


  • Contained within a capsid of protein and phospholipid bilayer
  • The viral surface is decorated with Neuraminidase (N) and Haemagglutinin (H) antigens, enabling cell binding and entry
  • The genome comprises 7 or 8 negative sense RNA strands
• Major influenza pandemics


  • Russian flu (1889)
  • Spanish flu (1918)
  • Asian flu (1957)
  • Hong Kong flu (1968)
  • Swine flu (2009)
• Every year, new strains of influenza spread around the world, causing about 3 - 5 million cases of severe illness
• Larger outbreaks (called pandemics) are less frequent, and are caused by recently evolved more virulent strains
• Of the 16 H and 9 N subtypes, only H 1, 2 & 3, and N 1 & 2 are common in humans
• How the influenza virus infects host cells


  Refer to video
• HIV


  A retrovirus that infects human immune cells called macrophages and helper T-lymphocytes
• As T-lymphocytes are depleted, this can eventually cause the acquired immunodeficiency syndrome (AIDS)
• HIV may be spread through blood, semen or breast milk
• Without treatment, average survival time after infection with HIV is estimated to be 9 to 11 years
• HIV genome and structure

  • The genome comprises two copies of positive-sense single-stranded RNA, coding for only 9 genes
  • Retroviruses, including HIV, can convert their RNA genomes into DNA to integrate into the host chromosomes
• Mycobacterium tuberculosis


  An obligate human pathogen with an unusual, waxy coating of mycolic acid on its cell surface, which makes it extremely tough
• M. tuberculosis grows very slowly, doubling ~ once per day, so visible colonies require several weeks to grow on agar plates
• M. tuberculosis is spread via air droplets (coughing, sneezing etc)
• M. tuberculosis infects macrophages in the lung, which are unable to kill this tough bacteria
• Symptoms of tuberculosis


  • The body tries to wall off the infection by creating granulomas, which are barriers formed from highly active immune cells
  • These accumulate in the lung, and can be seen by chest X-ray
  • Most infections are asymptomatic - this is called latent infection
  • About 10% of latent infections progress to active disease
  • The classic symptoms of active TB are a chronic cough with blood-containing sputum, fever, night sweats, and weight loss
• Today, ~1/3 of the world's population carries latent Mtb
• About 10% of latent infections progress to active disease which can spread to other organs and, if left untreated, kills about half of those infected
• Co-infection with HIV is a major risk factor for progression to active tuberculosis disease
• Historically, tuberculosis killed >1 billion people
• The BCG vaccine and use of antibiotics has reduced tuberculosis rates considerably
• Cholera


  An infection of the epithelial cells of the small intestine by the bacterium Vibrio cholerae
• Cholera is spread by faecal contamination of food and water
• Cholera is rare in the developed world due to good sanitation