Module 7 Infectious Disease Summary Notes PDF

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Module 7 Infectious Disease In this module, you will:     

Examine the treatment, preventions and control of infectious disease both locally and globally. Study the human immune system and its response to an infectious disease. Understand the value or learning about infectious disease and its cost to humans through losses in productivity and impact on overall health. Consider the medical and agricultural applications that draw on the work of a variety of scientists. Engage with all the Working Scientifically skills for practical investigations involving the focus content to collect, process, and analyse data and identify trends, patterns and relationships related to infectious disease.

Infectious Disease 

A disease is any condition that impairs or interferes with the normal functioning of the body.

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Infectious diseases are caused by the invasion of a pathogen.  These diseases can be transmitted from one host to another.

Infectious diseases are also called communicable diseases or transmissible diseases

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Infectious diseases have dramatically affected life on Earth.  They have acted as a selection pressure in the Evolution of different populations.

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During the middle ages, in the 14th Century, nearly one-third of the world’s population died from Bubonic Plague.  This infectious disease was also referred to as ‘The Black Death’ and was caused by the bacteria Yersinia pestis.

Bacteria Prions

Fungi

Virus

Infectious Pathogens

Protist

Parasite

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New molecular technologies and computer-assisted analysis are now speeding up the diagnosis process for infectious diseases.  This means specific treatments can be started earlier which hastens recovery times.

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New electronic tools and computer processing also assist in drug dosing and prescribing to improve the quality and efficiency of treatment procedures.

Describes a variety of infectious diseases caused by pathogens, including microorganisms, macro-organisms and noncellular pathogens, and collect primary and secondary data and information relating to disease transmission, including:    

Classifying different pathogens that cause disease in plants and animals. Investigate the transmission of a disease during an epidemic Design and conduct a practical investigation relating to the microbial testing of water or food samples. Investigate modes of transmission of infectious diseases, including direct contact, indirect contact and vector transmission.

Prion Pathogens 

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Prions or ‘proteinaceous infectious particle’ are infectious, self-reproducing pathogens, that are made of protein and do not have a nucleic acid genome.  They do not contain any genetic material (DNA or RNA). The normal form of the protein is called the cellular prion protein (PrPC).  The infectious prion protein is a misfolded version of the normal cellular protein. The prions increase in number by converting correctly folded versions of the protein to more prions with the altered abnormal shape. Normal prion proteins consist of about 40% alpha helices Infectious prion proteins consist are about 55% beta pleated sheets and 20% alpha helices.

It is believed that the pathogenic features of the misfolded prion proteins relates to the high amounts of beta pleated sheets.

It is a CHAIN REACTION

Prion Replication Process

Transmissible Spongiform Encephalopathies (TSEs) 

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TSEs are a group of progressive conditions that affect the brain and nervous systems of many animals.  When prions accumulate in tissues, they cause large vacuoles so that the tissue begins to resemble a sponge.  Tissues commonly affected are the cortex and cerebellum of the brain with degeneration of brain tissue. Diseases caused by prions are called Spongiform Diseases.  This is because the brain tissue of an infected person is full of holes, like that of a sponge. All prion diseases show a severe loss of neurons.  This causes loss of full control of bodily movements and changes in behaviour. In humans, the symptoms can involve:  Personality Changes  Psychiatric Problems  Insomnia, Progressive Loss of Intellectual Capacity  Loss of the Ability to Move or Speak.

 TSEs in Sheep and Goats cause ‘Scrapie’  TSEs in Humans causes  CJD (Creutzfeldt-Jakob Disease)  FFI (Fatal Familial Insomnia)  Kuru  TSEs in cattle causes BSE (Bovine Spongiform Encephalopathy or Mad Cow Disease)  TSEs in Cats, Pumas and Cheetahs causes FSE (Feline Spongiform Encephalopathy)

Infection 

Humans can be infected in three ways:  Acquired Infection – For example, through food or medical procedures  Hereditary Transmission  Sporadically – Spontaneous formation of an infectious prion, which then proceeds to replicate continuously triggering Apoptosis (programmed cell death).

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Many prion diseases have a very long incubation period, e.g. at least 10 years.  Symptoms do not show for a long time due to the long incubation period.  After the long incubation period, there is a rapid progression of the disease. This causes fatal loss of brain and nervous functionality. Many prions are not affected by normal methods of destroying pathogens.  They are not destroyed or deactivated by normal cooking at high temperatures  They are not destroyed or deactivated by normal sterilisation techniques such boiling or irradiation (the process of applying radiation to matter).

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Treatment 

There is currently no treatment for prion diseases.

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Prion diseases are always fatal.  When symptoms show, the disease progresses rapidly.  This leads to death occurring within a few months to a few years.

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Most treatments currently are aimed at alleviating the pain of the patient and making them as comfortable as possible. There are several research projects, e.g.  Some researchers are developing therapeutic strategies against the diseased prion protein  Others are trialling Stem Cell Transplants to restore lost tissue and recolonise damaged areas.

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Virus Pathogens   

Viruses are very small (~ 30 – 300 nm). They reproduce by taking over host cells. They are infectious particles with either the DNA or RNA genome surrounded by a protein coat called a capsid.

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A phage (bacteriophage) is a virus that infects bacteria.  Phage’s are used as antibacterial agents.  As they infect bacteria, using them as antibacterial agents will help in killing the bacteria inside the host, which is causing disease.

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A viroid is a plant pathogen consisting of a molecule of naked circular RNA with only a few hundred nucleotides.

Living or Non-Living There has been debate amongst scientists as to whether viruses are living or non-living.  

They are not cellular and hence do not comply with the cell theory which states that “All living things consist of cells”. They also cannot metabolise or reproduce on their own.

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However, viruses contain nucleic acid (either DNA or RNA) which is a requirement for life.

 Viruses can be crystallised (form or cause to form crystals).

Types of Viruses There are four main types of viruses: 1) 2) 3) 4)

Icosahedral – Capsid has 20 flat sides, giving it a special shape Helical – Capsid is rod shaped Enveloped – Capsid is encased in a baggy membrane Complex – No Capsid

Features of Viruses  Genome can be DNA or RNA as single linear or circular molecule  Capsid is a protein shell with the shape dependent on the type of virus, e.g. rod shape, polyhedral, or more complex  Viral envelope occurs in some viruses  It is derived from the host cell phospholipids and membrane proteins.  Viruses can be crystallised and remain viable (of a plant, animal, or cell) capable of surviving or living successfully, especially under particular environmental conditions) and infectious.  They lack metabolic enzymes and ribosomes for making proteins  However, they can express their genes and do chemical synthesis when they are inside a living host cell  Using the host cell’s metabolic equipment and chemical pathways, they make more virus proteins and nucleic acid.  Host range is the limited number of host species that each particular virus can infect.  The host specificity of the virus is due to a ‘lock and key’ fit between viral surface proteins and surface receptor molecules on the outside of host cells.

Viral Diseases 

Currently, there are no cures for viral diseases, but vaccination reduces their incidence

The incidence of a disease is the rate of new cases of the disease and gives the probability of contracting the disease. The prevalence is the proportion of the population that has a disease at a point in time.

Viral diseases include:        

Measles Mumps Poliomyelitis Chickenpox AIDS Influenza Glandular Fever

Some viruses are used to control pest populations. E.g., Myxomatosis is used in the control of rabbits.

Viral Vaccines

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The first viral vaccines were based on attenuated or weaker viruses. E.g., Smallpox vaccine used Cowpox virus. Vaccines using live attenuated viruses include measles, mumps, rubella, and chickenpox. Vaccines using inactivated or killed viruses include Polio and Hepatitis A. A vaccine that uses segments or conjugates include hepatitis B and the influenza vaccine.

Some viruses keep mutating from one person to the next o This causes vaccination for these viruses to become difficult, as the virus has already changed its format by the time the vaccine is developed.

An example of this is the Common Cold or Influenza, which is caused by the Influenza virus.

Virus Replication  

Viruses are unable to reproduce on their own. They can only replicate inside host cells. The viral protein coat contains chemicals that allow the virus to attach itself to the surface of the host’s cell. o Once the virus attaches itself to the cell, it then enters and takes over the cell’s reproductive mechanisms to make many copies of itself.  After a while, there is an accumulation of viruses inside the host cell, which causes it to swell.  Eventually, the cell becomes so full of copies of the virus, that it bursts and dies, releasing the viruses.  The viruses then travel to other host cells to repeat the replication process.

Virus Replication

Bacterial Pathogens

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Bacteria are single-celled prokaryotic organisms  They have a cell wall, but no membrane bound organelles or nucleus.  Their genetic material is a single large chromosome  A circular thread of DNA double helix  They are larger than viruses, but smaller than protozoans.  They vary in size from 0.5 - 100 μm (1mm = 1000 μm).  Bacteria are classified on the basis of their shape;  Coccus – Spherical Shape  Bacillus – Rod Shape  Spirillum – Spiral Shape  Vibrio – Comma Shape  Rickettsiae – Oval Shape  Bacteria are found everywhere and many are beneficial to humans.  There are estimated to be 1000 trillion bacteria in the human body and most of these are beneficial.  Those bacteria that do cause diseases do so by producing toxins or chemicals that are harmful to the host’s body.  Bacteria cause half of all human diseases

Bacterial Reproduction     

Bacteria reproduce by asexual reproduction using the process of binary fission. The time it takes for the number of bacteria to double is known as the Generation Time. The generation time varies for different species and is between 10 minutes and 24 hours. This means that thousands of bacteria can be produced in a very short time. Bacteria can also directly transfer DNA to another bacterium by conjugation when two cells are temporarily joined.  Conjugation – The process by which one bacterium transfers genetic material to another bacterium through direct contact.

Generalised Bacteria Structure

Structural Features of Bacteria include:

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Cell Membrane - Biological membranes have three primary functions: 1. They keep toxic substances out of the cell 2. They contain receptors and channels that allow specific molecules, such as ions, nutrients, wastes, and metabolic products, that mediate cellular and extracellular activities to pass between organelles and between the cell and the outside environment. 3. They separate vital but incompatible metabolic processes conducted within organelles. Cell Wall – Rigid structure. Is a protective layer outside Cell Membrane Capsule – Sticky layer of polysaccharide or protein. Is a virulence factor as it aids diseasecausing ability and protects against phagocytosis against macrophages. Fimbriae – Hair-like appendages. Help bacteria stick to their substrate or to one-another. Nucleoid – region where the bacterial double stranded circular DNA circular chromosome is located. Plasmids – Small rings of independently replicating DNA with only a few genes. The genes in the plasmids often provide genetic advantages e.g. Drug Resistance.

Endospores – In harsh conditions, the bacterium surrounds a copy of its chromosome with a tough multi-layered structure forming an endospore.  The original cell lyses and releases the endospores 

Endospores can survive boiling and high pressure and can be dormant for long periods of time.

Bacteria Classification 

Bacteria can be classified and named by shape, e.g. Cocci, Bacilli, Spirilla, Vibrio, Rickettsiae and others as shown in the diagram on the right.

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Bacteria are also subdivided through Gram Staining Bacteria have mesh-like cell walls that are made of a polymer called peptidoglycan (also known as Murein). Different species of bacteria have different cell wall characteristics and based on the structure of their cell walls, different species of bacteria can be classified as either Gram-Positive or Gram-Negative.

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Gram Staining 

Gram Staining involves adding a purple dye called crystal violet to bacterial cells.

Gram-Positive bacteria have a thicker layer of peptidoglycan  The thick layer absorbs and holds the stain and bacteria give off a Purple or ‘Positive’ result. Gram Negative bacteria have a thinner of peptidoglycan

 The thinner layer does not absorb and hold much of the stain and bacteria give off a Pink or ‘Negative’ result.

Bacteria and Antibiotics 

Antibiotics - Chemicals that kill or inhibit the growth of bacteria and are used to treat bacterial infections.  Antibiotics are produced by soil bacteria and fungus  Antibiotics can: o Inhibit cell wall synthesis o Inhibit protein synthesis o Alter the cell membrane and its permeability o Inhibit nucleic acid synthesis For example:  Tetracycline – Used to treat various infections. These include Acne, Cholera, Brucellosis, Plague, Malaria and Syphilis. This antibiotic is a protein synthesis inhibitor.  Penicillin – Used to treat bacterial infections. This antibiotic affects cell wall synthesis.

Bacterial Vaccines and Toxoids 

A vaccine is a biological preparation that improves immunity to a particular disease.  A vaccine typically contains an agent that resembles a disease-causing microorganism, and is often made from: o Weakened or killed forms of the microbe o Its toxins or o One of its surface proteins

Vaccines contain killed or attenuated (weakened) bacteria that activate the immune system.  Antibodies are then created against that particular bacteria  Antibodies – specific substances produced in the blood as a reaction to an antigen.  Antigen – Substance that causes immune response. E.g.  

Whooping cough vaccine has killed bacteria Measles vaccines has living, attenuated (weakened) bacteria.

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Toxoids are modified toxins that have the antigen but it cannot destroy tissues E.g. Toxoids for Tetanus and Diphtheria.

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Booster injections for vaccines and toxoids increase the number of antibodies and the number of circulating memory cells to sustain immunity.

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A bacterial exotoxin is a toxin (e.g. protein secreted by bacteria) that is released outside the cell.  They are produced by both Gram-Positive Bacteria and Gram-Negative Bacteria

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There are many types of exotoxins based on structure and function.

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Exotoxins may be secreted or like endotoxins, are released during lysis of the cell.

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Bacterial endotoxins are part of the outer membrane of the cell wall of Gram-Negative Bacteria. They are lipopolysaccharides and are only one type of endotoxin.

Antibiotics and Bacterial Infections  

Antibiotics are chemicals that inhibit or destroy microorganisms such as bacteria. They are drugs that have a toxic effect on microbial cells, inhibiting the growth or destroying pathogens.  They affect the microorganism but not the host

Discovery of Antibiotics 

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In 1928, Alexander Fleming (1881-1955) discovered the first antibiotic when he observed the effect of Penicillin mould on the growth of bacterial colonies on an agar plate.  He realised the mould produced a substance that killed nearby bacteria A decade later, an Australian, Howard Florey, and Ernest Chain developed methods to isolate and produce antibiotics on a large scale.

Many antibiotics have been discovered, but only a few are safe for human use. The first antibacterial drug used on humans was sulfonamide in 1935. During the 1940’s research produced a variety of drugs derived from Penicillin In the 1950’s, two tetracycline’s and chloramphenicol were also used for infectious diseases

How Antibiotics Work 

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Antibiotics work in several ways; o Penicillin disrupts the cell wall of bacteria o Amphotericin destroys cell membranes o Streptomycin disrupts protein synthesis Broad spectrum antibiotics act against a range of bacteria, including non-pathogenic forms Some patients have side effects from the use of antibiotics, ranging from: o Vomiting o Nausea o Diarrhoea o Skin rashes To more serious effects such as o Deadness o Loss of balance o Damage to bone marrow o Serious allergy

Resistance to Antibiotics  Natural selection has led to the evolution of some strains of bacteria resistant to antibiotics  In order to minimise the number of resistant strains, it is important that people take the complete course of their antibiotic and that antibiotics are not overprescribed

 The evolution of antibiotic resistant bacteria is accelerated by the following; o The overuse of antibiotics, e.g. in the treatment of many diseases such as viral infections – ‘coughs’ and ‘colds’ can lead to pathogens becoming resistant to antibiotics. o When symptoms disappear and people stop taking the antibiotics  Some resistant forms of the bacteria may survive and cause a future infection o The overuse of antibiotics for food-producing animals, e.g. chickens and pigs are fed antibiotics to prevent infection and promote growth o Producing cleaning products that contain antibacterial ingredients act as a selective pressure into the environment that favours the survival of antibiotic resistant strains of bacteria.  In most instances, the quantities of these products are not enough to kill all the bacteria.

Protist Pathogens   

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Protists are a...