Activities of pathogens Flashcards
Questions to ask yourself when presented with illness
- What has caused her illness (viral, bacterial, parasitic?)
- How did her symptoms develop (pathological processes, direct damage vs immune side effects?)
- Where did it come from (school, food)?
- How is this illness diagnosed (lab tests, clinical examination)?
- How is this illness treated (antibiotics etc)?
- How is this illness prevented (hygiene, exercise, vaccination)?
Cold symptoms - fever, loss of apatite, lethargy, coughing, sneezing, mucus production
Whilst some of these like nasal and throat pain are caused by a combination of viral activity and immune response, many of these are solely or at least predominantly caused by the immune systems response to the pathogen, and not the pathogen itself.
Fever: release of IL-6 from activated macrophages that interacts with the hypothalamus and causes fever and behavioural changes.
Cough is a reflex caused by stimulation of nocioceptors in the throat. On the skin thsi would elicit a pain response.
Sneezing and Rhinorrhoea is caused by stimulation of the trigeminal nerve by the surrounding inflammation. Trigeminal nerves are sensory nerves that innervate the nasal epithelium, and its thought that stimultaion of these nerves (perhaps by histamine) will cause the sneezing and nose run reflex.
Cytokine release also causes aches and pains, loss of appetite, and lethargy.
In addition to this, viruses can cause damage to cells when they leave cells to go an infect other cells. Bacteria can damage cells by the release of toxins or over crowding.
Causes of common colds and flu
the most common cause of colds is the rhinovirus (30-50% or 1/3) Although there are hundreds of different subtypes, and often the body requires more than one bout of exposure to become immune to one subtype.
the second most is coronaviruses at 10-15%
Most of these viruses have very very low death rates, but the influenza virus can cause serious illness, and they can lead to increased risk of pneumonia.
Transmission of colds
Direct vs indirect transmission
Direct
Aerosol/droplets: When we sneeze dropltes of savlia and mucus are ejected from our moths. These can contain viruse particles, with the mucus aiding its transmission since it will seemlessly fuse with another persons mucus, allowing the virus access to the epithelial cells underneath. Direct contact also occurs when a person will sneeze into their hand, shake someone elses hand, and that person will then proceed to touch a mucus membrane (the eye or mouth or nose) causing infection.
Indirect
When the droplets with viral particles lands on an object, that is then touched by another person. However, this is unlikely as they cannt survive for long on other objects.
Nuclei droplets: Dried up droplets with 1 or 2 particles of virus can remain suspended in the air for a short time.
Diagnosis of common cold
Done through clinical observation only. There are no specific laboratory tests for it.
Only big risk associated is pneumonia, which can often be picked up through checking the throat.
Treatment of common cold
Almost everyone gets better on their own. There are symptomatic treatemnts like nasal decongestants and sedating anti-histamines for coughs, but most other proposed treatments don’t actually work.
There is no evidence for vitamin C, zinc, ginseng, probiotics - there is possible evidence that vitamin C helps to prevent the sickness.
Antibiotics are absoltuely not useful, they can only work against bacteria, and common colds are caused by viruses only.
Structure of a virus
Viruses are very basic. They are a strand of nucleic acid (whether that be DNA, mRNA, double stranded, single stranded, or partially double stranded) with a few proteins enzymes, surrounded by a protein caspid. This protein caspid may contain surface viral proteins or if the virus is an enveloped virus then most of these surface viral proteins will be in the envolope not the caspid.
Classification of viruses
Structural classification: Used academically, but often not very clinically relevant or memorable.
Disease: This is when the name of the virus is afte the type of disease it causes. For example “hepatitis” allows you to instantly know that it is a disease of the liver. However, this is not an all-inclusive categorization because often these viruses will affect more than one organ.
Tranmission: Also clinically meaningful. Such as ‘respiratory disease’, since usually the site of transmission is also where the infection culminates. However, this isn’t always the case, for example gasointestinally transmitted viruses like meningitis and polio don’t cause grastric diseases.
Rhinovirus and HIV tropism
Rhinovirus is able to infect the throat and nasal epithelium because it can bind to ICAM1 receptors and LDL receptors.
HIV has glycoprotein 120 on its surface which can similarly bind with high affinity to CD4 (clustered on T helper cells).
How does the immune system recognize viruses
- Surface viral proteins: on the outside of the protein caspid on viruses, of the envolope that they reside in, there are proteins characteristic of the viruses that can be detected by the immune system.
- Cell death and injury: When viruses use a cell to replicate, they distrub normal cell function and often when they leave the cell to go and infect other cells they end up damaging the cell. The cell will release intracellular contents (DAMPs) but also cytokines that will activate nearby immune cells and stimulate and inflammatory response.
- Infected cells antigens: Infected cells may have put foeign viral proteins in their MHC molecules, or because of the stress they are under may retract normal surface antigens. As a result, immune cells that recognize either the lack of self-antigens when there should be or the presence of foreign antigens will become activated.
Epidemiology of cold viruses
It is common for healthy children to get up to 7 colds a year, and 10-15% perfectly healthy children can even have up to 12 colds a year. This is because a childs immune system is not yet as fully developed as an adults.
For adults, 1-2 a year is also common.
