Case 16- Virus Flashcards
Main steps of the Virus life cycle
Virus can only replicate inside other living cells.
• Attachment- can have spike proteins which interact with components on the cell
• Entry- can be through phagocytosis
• Replication and protein synthesis- the viral capsule tends to be broken down through an endosome, exposing the viral genome
• Assembly- reform the replicated viral components into a unit
• Release- sometimes destroys the cell
Types of viral transmission
Horizontal transmission- spreads from one person to another person in the same generation, how most viruses are spread.
Vertical transmission- between mother and fetus/baby. How some virus’s are spreed
Zoonotic transmission- not many viruses, the virus is spread from a non human animal to a human often via a biting arthropod vector i.e. an insect
Main types of viral transmission- how the virus gets into the body
Viruses enter humans through the skin or mucous membranes i.e. GI tract or direct inhalation. Can be through damage in the skin. The main routes of transmission are: Respiratory, Faecal-oral. Other types of transmission:
• Sexual= HIV-1, HPV, Herpes simplex virus (HSV), Hepatitis B (HBV)
• Mechanical- Parenteral route, Intravenous drug users and blood transfusion (HBV, HIV-1). Can be through abrasions in the skin (Herpes simplex virus, papillma viruses, some pox viruses). Biting insects
• Urine (rare)
• Via conjunctiva- can gain access to the respiratory tract when you rub your eyes
Examples of respiratory transmisiion
- Rhinovirus (common cold)- most common viral infective agent in humans, about 100 different immunologically distinct types. Adults tend to get it 4-6 times a year
- Influenza (flu)- 20,000 deaths a year in the UK
- Covid-19
- Chicken pox (Varicella Zoster virus) and Smallpox (variola virus)- cause more generalised infection, for example, in the skin
How infectious droplets enter the body in a respiratory infection
1) Inhaled directly into the respiratory tract
2) Enter the conjunctiva and move to the respiratory tract
3) Move from fingers / cold surfaces to the respiratory tract
Faecal-oral transmission
Ingesting food or water that is contaminated with faeal matter. Infections increase with poor sanitation and personal hygiene
Picornaviruses
Non-enveloped virus’s that are spread through the faecal oral route.
• Poliovirus- direct personal contact, sewage contaminated water, very rarely inadequately disinfected swimming pools
• Hepatitis A- sewage contaminated water, vegetables/fruit washed in sewage contaminated water
Polio
Effective vaccination bought down the polio cases. The majority of Polio is asymptomatic but 1:200 will lead to irreversible paralysis. Treated with iron lung. Can cause wasting of limbs and muscle atrophy. Polio remains endemic in three countries: Afghanistan, Nigeria and Pakistan, its on the rise in warzones. Spreads in areas with poor infrastructure and sanitation.
How is norovirus transmitted
Faecal-oral route
1) Contact with infected individual
2) Surface contact- doors, handles, tables
3) Contaminated food/water
4) Aerosols produced from vomit
Viruses that spread through vertical transmission
- Rubella (German measles)- can cause problems with hearing and developmental problems
- HIV- during birth or through breast milk. 15% of children with mothers who have HIV will catch the virus.
Arbovirus
A type of zoonotic virus. It is transmitted via arthropods (they gain entry via the skin). Includes Dengue fever, yellow fever and St Louis encephalitis. Dengue fever is common in the tropics and spread via mosquitos, most will have a mild disease but 10% will develop a severe disease and get haemorrhagic fever.
Zoonotic viruses
Most are Arbovirus, an exception is rabies where the virus spreads from a non-human animal directly via a bite, no vector. Humans are often dead end hosts i.e. it is transmitted no further but in some cases you get human to human transmission
Pathogenicity, Virulence and Pathogenicity factors
Pathogenicity- the severity of the disease caused by different viruses. The more pathogenic a virus the more severe the disease it will cause.
Pathogenicity factors- the characteristics that disease causing viruses possess that allow them to be pathogenic
Virulence- the severity of the disease caused by different strains of the same virus
The Pathogenicity factors of virus’s
- Entry
- Initial replication
- Immune evasion
- Dissemination- spreading
- Further replication
- Shedding- leaves the body in order to infect other hosts
Localized infections
Causes infections at the site of entry or contiguous with, typically epithelia and skin. Won’t have systemic symptoms. Examples; Influenza (Respiratory epithelium), Rhinoviruses (Respiratory epithelium) and Norovirus (GI epithelium).
Generalised infections
Causes infections that spread to target organs or tissues remote from the site of entry. Polio spreads through the faecal-oral route but can cause musculoskeletal issues. Examples- chickenpox, measles, Hepatitis (A,B,C,E) and Rabies
How virus’s spread in a generalised infection
- Initial replication in the respiratory epithelia
- The virus can spread to regional lymph nodes and then enter into the bloodstream (primary viraemia)
- The virus can then spread to other organs like the liver, spleen, bone marrow and blood vessel epithelium. You then get multiplication of the virus at these sites.
- You then get re-entry into the blood stream from these secondary sites (secondary viraemia)
- You may get further multiplication and shedding from the target organ
Incubation period
The time period between viral exposure and onset of illness (symptoms become apparent)
Diseases with different incubation periods
1) Diseases with a short incubation period (<7 days)- typically localised infections, influenza (1-3 days)
2) Diseases with medium incubation periods (7 to 21 days): typically, generalised infection, measles (13-14 days)
3) Long incubation periods (weeks to months)- typically generalised infections. Hepatitis A from sewage contaminated water (3-5 weeks).
4) Very long incubation period (years)- subacute sclerosing panencephalitis (SSPE) measles (rare complication). 2-6 years.
Generation time
The time period between viral exposure and the individual becoming infectious to other.
A lot of diseases spread because the person is infectious before the symptoms even appear
Reproductive number (R0)
The number of subsequent infections in a susceptible population caused by a viral illness in one individual during its infective period. The amount of people the person is likely to spread the disease to. The higher the reproductive number the more infectious a viral disease is.
The generation time and R0 allows predictions on the size and spread of viral disease outbreaks
Acute infections
Normally short lived, self limiting i.e. the flu, common cold and rabies. Tend to be localised in a particular area, no chronic consequences
