Viruses must first breach host barriers. What are the portals of entry?
- Skin
- Respiratory tract
- Gastrointestinal tract
- Genitorurinary tract
- Conjunctiva
How could it breach the skin?
- Mechanical trauma/ injection (HIV) where there is a break of the skin
- Bit of an infected mosquito (dengue virus)
- Bite of an infected animal (dog bite e.g. rabies)
How could it breach the respiratory tract?
- Local respiration infection (influenza): inhalation of aerosol droplets
- Asymptomatic initial infection then spreads e.g. chickenpox
- Host defences : mucous lining and ciliated cells + alveolar macrophages
How could it breach the gastrointestinal tract?
- Physical host defences of GIT
- Two types of infections
Systemic infection: enteroviruses, polio virus
Localised infection: rotavirus
Hostile env.
Physical host defences of GIT:
- stomach is acidic
- intestine is alkaline
- digestive enzymes
- bile detergents
- mucous lining epithelium
How could it breach the genitourinary tract?
- host defences
- Infection via sexual activity
- Abrasion in lining HIV
- Infect epithelium
- Infect underlying tissues such as lymphatic and sensory neurons
Host defences:
- Vagina is acidic
- mucous layer, macrophages
Conjunctiva
- host defences
Abrasion
localised infections e.g. conjunctivitis
Spread to other sites
Host defences:
- Tears
- mucous
- Proteases
Localised infection
- replicate at primary site of infection
- spread to adjacent cells
- they multiply at epithelial cells at site of entry (spread virus and shed to exterior)
- E.g. influenza
Disseminated infection
- Must breach physical and immune barriers
- Via circulatory system (move through blood stream e.g. HIV- T lymphocytes and monocytes)
- Via nervous system (entry via sensory {dorsal root ganglia} or motor {motor neurons} nerve ending
eg. poliovirus HSV
Systemic infection
- many organs are affected
- need to be in an organ in order to replicate
Types of infection
- Acute
- Persistant
Acute infection
- Rapid production of infections virons
- Rapid resolution and elimination of infection
- E.g. influenza
- Virus is eliminated and cleared by the immune response
- Uses an immunocompetent host
Persistant infection
- not cleared effectively by immune system
- virus particles are produced for a long time
Types of persistant infection
- Chronic: long periods of time with slow production of virus (e.g. HIV or Measles SSPE)
- Latent: after the initial acute response, it is cleared by immune system. Viral genome without any detectable infectious virus production. Virus re-awakes from latency + is reactivated.
E.g. herpesviruses
Spread to other?
- Aerosols
- Faecal to oral transmission
- Skin lesions
- Blood and body fluids
Transmission via aerosols
- spread via droplets
- Influenza virus
Faceal - oral transmission
- Virus can be shed in watery diarrhoea + vomiting
- extremely contagious
- Shellfish, when harvested in contaminated sewerage waters. When people come into contact with contaminated they can be infected.
- Poliovirus or Hep A
Skin lesions
HSV presenting core sores or genital herpes will be shed in the lesion and thus transmit to another person
Body and blood fluids
Blood- HIV
Blood can be transmitted via biting insects, childbirth
What determines the successfulness of transmission
- no. of viruses shed
- Stability of virus in env.
- No. of virus particles required to infect a new hsot
Routes of Transmission
- horizontal transmission
(respiratory, faecal-oral, sexual contact) - vertical transmission (Prenatal - HIV, perinatal (infected at birth canal) - Hep B and postnatal - milk/ direct contact - CMV)
- Zoonoses
Fate of host
Damage to cells due to virus replication:
- cell death by rupture of cell during virus release
- cell commits suicide in response to infection (Cell can’t function)
- infected cell loses function
- infected cell transformed by virus - tumours
Damage due to the host response to infection
- immunopathology - antibodies and immune cells destroy infected cells - tissue damage
- Inflammation - symptoms of immune cells infiltrating the area where the virus has occurred
