Module 7: Infectious Diseases

Module 7: Infectious Disease

Louis Pasteur – proved the ‘germ theory’.

He boiled broth in 1 straight necked flask and 1 swan necked flask, to remove any microbes in the broth at first.

• The straight necked flask was exposed to the air, which resulted in microbial growth. The swan necked flask was not exposed directly to the air and resulted in no microbial growth. This proved the germ theory.

Koch’s Postulates

• The microorganism must be found in all organisms with the disease.
• You must be able to isolate the microbe from the diseased organism and grow it in culture.
• The cultured microbe must cause the same disease once introduced to a healthy animal.
• Once again, the microbe from the introduced now diseased animal will be isolated to determine it is the same organism.
• Robert Koch discovered the bacterium that cause the diseases cholera and Tuberculosis.

The Innate Immune System

The innate immune response is the first and second line of defence.

First line of defence is physical and chemical barriers preventing any pathogens from entering the body. For example:

• Mucus membranes. Mucus membranes are a chemical barrier and can be located in the nose and eyes. Mucus is sticky and traps microbes; it also contains enzymes that kill pathogens.
• Small hairs in the nose. These hairs are a physical barrier and filter the microbes and dust attempting to enter the body, and are then expelled by sneezing.

The second line of defence causes an inflammatory response that releases histamines which causes the increase of white blood cells. Phagocytes, a type of white blood cell will engulf microbes and try to remove any foreign invaders.

Temperature in the body will also rise to a level that pathogens cannot cope in.

• Increase in temperature will also increase enzyme activity.

The Adaptive Immune System

The adaptive immune system is the third line of defence and will remember previously encountered pathogens.

• The Adaptive Immune System is consistent with lymphocytes which are white blood cells named B-cells, and T-cells.

B cells can either differentiate into memory B-cells, or plasma cells.

• Memory B-cells retain a specific antibody on its surface
• Plasma B-cells create antibodies to flag a specific antigen, upon which killer T-cells will destroy it.

Primary Exposure

1. The third line of defence is activated once the immune system detects a foreign antigen (a substance on the surface of a pathogen) by a phagocyte engulfing it.
2. Foreign antigen is incorporated into major histocompatibility complex on phagocyte.
3. Phagocyte will bind to Helper T-cell.
4. Helper T-cell will release cytokines which will result in three things.

• Production of Killer T cells.
• Production of Memory T cells and B cells.
• Differentiation of B-cells to plasma cells (will produce antibodies).

Secondary Exposure

Antibody production will be much faster, resulting in faster elimination of the pathogen.

A secondary exposure means the pathogen will quickly encounter a memory B-cell.

1. The memory B cell will bind to the foreign antigen.
2. The memory B cell will then differentiate into a plasma cell.
3. The plasma cell will release antibodies.
4. T-cells will release cytokines and killer T cells will eliminate that specific foreign antigen.

Australian Plant with Fungal Disease: Sooty mould is a fungal disease that infects bottlebrushes and causes a black, powdery fungus that reduces the plant’s ability to photosynthesise.

As a response to pathogens plants have physical, chemical and specific response.

Physical barriers include:

• Bark
• Waxy cuticles.

Chemical barriers include:

• Glucosides, antibacterial.

If their physical barriers become breached, they have a recognition system that recognises the chemical and physical signals from pathogens. As a result, the plant will change the permeability of the cell wall and trigger cell apoptosis to minimise tissue damage. It is essential for clearance of certain pathogens.

• Caused by lack of Vitamin C in the diet. Vitamin C is essential to making collagen, without it bleeding gums as new blood vessels are unable to be made.

Malaria:

How it facilitates entry:

Malaria facilitates its entry using a vector, that being a mosquito.

• Once an infected mosquito feeds on an individual, the protozoan (plasmodium) in the saliva of the mosquito will enter the blood stream.

How it transmits between hosts:

• The protozoan that causes the disease lives in the red blood cells of humans, so if a mosquito feeds on an individual infected with malaria, the protozoan is transmitted to the mosquito.
• The pathogen will then live in the mosquito’s saliva, and once that mosquito feeds on another individual (uninfected) they will transmit the pathogen.

How does hygiene prevent disease?

In 1846 it was proven there was a link between women dying in childbirth and lack of washing hands. As a result, the following was introduced:

• Washing hands regularly.
• Gloves.
• Sterile instruments.

After these procedures were introduced, women dying in childbirth dropped. This is because pathogens could not travel into the surgery room due to the sterile environment.

How does quarantine prevent disease?

Quarantine is the method of isolating a diseased organism from the rest of the population. Quarantine can last for any period of time, until that organism is not diseased or infectious. This prevents the disease from spreading.

• For example, in the COVID19 epidemic, people travelling into Australia had to quarantine for 14 days, the incubation period for the disease.
• Also, you cannot bring certain foods into Australia as they may possess disease causing microbes. Same for soil, organic items.

How does vaccination prevent disease?

Passive Immunisation:

Natural:

Antibodies acquired from the mother via breastfeeding/placental transfer. Lasts usually 6 months upon birth.

Artificial:

The introduction of antibiotics to an organism which provides short-term immunity to bacteria.

• Antibiotics will inhibit the formation of the bacterial cell wall, as a result the bacteria cannot divide. First antibiotic was penicillin.

Active Immunisation:

Natural:

When the body becomes naturally infected with a pathogen and the immune system creates a response against it. B-cells will produce specific antibodies for that pathogen, and memory cells will make it so upon second exposure to the pathogen, the immune systems response is faster.

Artificial:

Vaccination is when some pieces of DNA of a pathogen will be injected into your body to trigger an immune response. As a result, your body will produce memory T and B cells so upon exposure to that pathogen, the immune system can create a rapid response. The memory cells will ensure that many antibodies will be produced to remove the pathogen from your body.

• There are also live vaccines, where a weakened pathogen is injected into the body to produce an immune response. This response will generally be much greater than a non-live vaccine.

How do pesticides prevent disease?

There are many types of pesticides, each effective against a specific type of pest. Such as, insecticides (insects).

• Pesticides can kill these pests that carry certain diseases. In a malaria prone area insecticide will help control the disease spread, as mosquitos are used as a vector for the disease.

Historical ways of controlling spread of disease

When the plague was dominant in Europe before the 1700’s, they believed disease was caused by invisible vapours ‘miasmas’. As a way of controlling the spread of disease, a large suit was worn from head to toe with herbs inside of it. These herbs would ‘neutralise the miasmas’. Was not effective because the plague was a bacteria, transmitted by flea bites.

Current ways of controlling spread of disease

Quarantine – Isolation of diseased individuals until either disease is treated or has become non-infectious. Australia, for COVID-19 epidemic, poses a mandatory 14-day quarantine for new arrivals, 14 days is the incubation period for the disease.

• Technological Applications – COVIDSafe. COVIDSafe is an app that can determine if you have been in contact/near a recently registered person with COVID. This way you can begin isolating immediately without spreading disease.

Antiviral: Rapivab

This antiviral is used for the treatment of serious cases of Influenza.

The drug is effective, but costly. One dose is equivalent to around 1000 dollars.

Incidence: What is the risk of getting this disease?

Incidence = new cases of disease during time period

Prevalence: Proportion of people that have the disease at a time period.

Prevalence = Cases in a period of time

Protection of Indigenous Culture:

Western Australia’s government gave away Aboriginal rights for smoke bush. No royalties given to the aboriginal people.