communicable disease Flashcards
what is disease
A disease is an illness or disorder of the body or mind that leads to poor health
Each disease is associated with a set of signs and symptoms
Communicable/infectious diseases are caused by pathogens and are transmissible (can be spread between individuals within a population)
Both plants and animals can be affected by pathogens
infectious and non infectious diseases
infectious disease- these are diseases cause by organisms know as pathogens. they are sometimes called communicable diseases as they are passed from infected to uninfected people (transmittable) some also affect animals and are passed from animal to humans
examples: cholera, malaria, HIV/AIDs
non-infectious disease- these are long term degenerative diseases that are not caused by pathogens. examples include diseases of the gas exchange and cardiovascular systems, inherited or genetic diseases, deficiency diseases caused by malnutrition, and mental diseases
examples: lung cancer, cystic fibrosis, sickle cell anaemia
types of pathogens
Bacteria
Viruses
Fungi
Protoctists
To control disease, it is very important to know what pathogen is causing it
bacteria
Bacteria are a diverse range of prokaryotic organisms
Some bacteria are non-pathogenic (they do not cause any disease or damage) while others are pathogenic
Pathogenic bacteria do not always infect the hosts of cells, they can remain within body cavities or spaces
tuberculosis
M. tuberculosis causes tuberculosis (TB) in humans
The bacteria infect the lungs, causing a chronic cough and bloody mucus
It is a disease often associated with poor hygiene and sanitation
M. bovine in cows can also transmit to humans to cause TB
bacterial meningitis
N. meningitidis causes bacterial meningitis in humans
Very few bacteria can cross the barrier created by the meninges (the tissue that surrounds the brain and spinal cord) however N. meningitidis crosses this barrier to cause acute inflammation
Inflammation of the meninges causes symptoms such as fever, headache, neck stiffness and a characteristic rash
ring rot
Ring rot diseases in potato plants are caused by bacterial pathogens
The bacteria infect the vascular tissue and prevent the transport of water, causing the plant to wilt and die
The infection spreads into the potato tubers where the vascular tissue is arranged in a ring, producing the characteristic black ring of rot
viruses
Viruses do not have a cellular structure
This means they can’t respire, produce ATP, replicate genetic material or synthesise protein
They infect host cells and hijack their machinery to replicate their own genetic material and proteins
tobacco mosaic virus
The first virus ever discovered was the Tobacco Mosaic Virus (TMV)
TMV infects several plant species
It causes a distinct yellowing of the leaves which produces a mosaic pattern
influenza viruses
Three different influenza viruses infect humans to cause the flu
Influenza A, influenza B and influenza C infect the cells that line the airways
They cause a high temperature, body aches and fatigue
Influenza A is the virus that causes the most cases of flu globally
It has a capsid that surrounds 8 single-stranded molecules of RNA
HIV
The human immunodeficiency virus (HIV) infects specific cells of the immune system
It is an enveloped retrovirus
The viral enzyme reverse transcriptase produces single-stranded DNA from its viral RNA
DNA polymerase synthesises double-stranded DNA from this single-stranded DNA
The double-stranded DNA is inserted into the host DNA and can remain inactive for many years
Once activated the DNA provirus is used to synthesise new viruses
protoctista (protists)
Protists are unicellular eukaryotes
malaria
Plasmodium falciparum is a protist that causes severe forms of malaria in humans
The parasite is spread by mosquitoes
Infected individuals experience fever, chills and fatigue
potato blight
P. infestans causes the infamous potato blight
The pathogen is unusual as it has some fungal characteristics
It is transmitted via spores
The first signs of potato blight are small, dark brown marks on the leaves which quickly increase in size and number
The protist destroys potato and tomato crops leaving them completely inedible
fungi
Fungi have a similar structure to plants
Their eukaryotic cells have cell walls and large central vacuoles
However, instead of being made of separate cells, their bodies consist of filaments known as hyphae
These hyphae form a network and spread throughout a host/soil
Fungal diseases are much more common in plants than animals
cattle ringworm and athletes foot
Cattle ringworm and athletes foot are fungal diseases that exist on the surface of the skin
black sigatoka
Fungal diseases in plants tend to be much more serious and can threaten entire crops
Black Sigatoka is a fungal disease in bananas
It spreads through the leaves of the plant, reducing its ability to photosynthesise
The lack of photosynthesis causes parts of the leaf to die; producing black streaks
Eventually, the whole leaf dies
disease transmission
In order for a population of pathogens to survive, they must be able to successfully transfer from host to host
If pathogens are unable to find new hosts then they will go extinct
Disease transmission is defined as the transfer of pathogens from an infected host to an uninfected host
Transmission can be very risky for pathogens
During the infective stages, pathogens produce a large number of individuals to increase the likelihood that some will find a new host and survive
There are two types of disease transmission:
Direct - from one host to another host
Indirect - a second organism (vector) that is unaffected by the pathogen transfers it to a new host
direct transmission
The direct transmission of a pathogen can involve physical contact between individuals
If the leaves of plants infected with Tobacco Mosaic Virus (TMV) touch the leaves of another uninfected plant, particles of the virus are transmitted
Sometimes individuals being within close proximity to each other is sufficient for direct transmission
The influenza viruses are spread in the air via tiny droplets of water. An infected individual breathes out droplets containing the virus and they are breathed in by an uninfected individual
Spores can also be involved in the direct transmission of pathogens
Spores are very small reproductive structures that are released into the environment. They are dispersed via wind or water
Once they reach a food source (host) they begin growing
Depending on the organism, spores can be produced via mitosis or meiosis so they can be haploid or diploid
P. infestans which causes potato blight produces specialised spores called sporangia. These structures are adapted for wind dispersal
transmission of HIV/AIDS
Human Immunodeficiency Virus is a retrovirus
The HIV virus is not transmitted by a vector (unlike in malaria)
The virus is unable to survive outside of the human body
HIV is spread by intimate human contact and can only be transmitted by direct exchange of body fluids
This means HIV can be transmitted in the following ways:
-sexual intercourse
-blood donation
-sharing of needles used by intravenous drug users
-from mother to child across the placenta
mixing of blood between mother and child during birth
-from mother to child through breast milk
transmission of tuberculosis
When infected people with the active form of the disease cough or sneeze, the Mycobacterium tuberculosis bacteria enter the air in tiny droplets of liquid
TB is transmitted when uninfected people then inhale these droplets
TB, therefore, spreads more quickly among people living in overcrowded conditions
The form of TB caused by Mycobacterium bovis occurs in cattle but is spread to humans through contaminated meat and unpasteurised milk
Very few people in developed countries now acquire TB in this way, although meat and milk can still be a source of infection in some developing countries
indirect transmission
Vectors are involved in the indirect transmission of pathogens
A vector is any organism that transfers a pathogen from an infected individual to an uninfected individual
The vector themselves usually aren’t harmed by the pathogen
A lot of disease vectors tend to be insects
Insects are ideal vectors as they reproduce in large numbers which increases the likelihood of pathogen transmission
transmission of malaria
Malaria is caused by one of four species of the protoctist Plasmodium
These protoctists are transmitted to humans by an insect vector:
Female Anopheles mosquitoes feed on human blood to obtain the protein they need to develop their eggs
If the person they bite is infected with Plasmodium, the mosquito will take up some of the pathogen with the blood meal
When feeding on the next human, Plasmodium pass from the mosquito to the new human’s blood
Malaria may also be transmitted during blood transfusion and when unsterile needles are re-used
Plasmodium can also pass from mother to child across the placenta
factors that affect disease transmission
The transmission of disease ultimately depends on:
The presence of the pathogens
If the pathogen is not present in the population then it cannot spread
The presence of susceptible individuals
A high number of immune or resistant individuals in a population will reduce the likelihood of transmission
resistance to disease
Animals and plants can be resistant to some diseases
Individuals who are resistant have mechanisms that prevent the infection or spread of pathogens within their body. They are not susceptible to disease.
These mechanisms are coded for by their genes
Individuals who are heterozygous for the sickle cell allele have resistance to malaria
Some humans are even resistant to HIV
resistance vs immunity
When resistant individuals are exposed to the pathogen for the first time they do not develop the disease and suffer no symptoms
An immune individual has been previously infected with the pathogen, suffered symptoms of the disease and recovered.
They are highly unlikely to develop symptoms of the disease when exposed to the pathogen again
The proportion of resistant or immune individuals in a population influences the potential for transmission (as they are not susceptible to disease)
The higher the proportion, the lower the probability of transmission
Different types of disease transmission are affected by different factors
Human pathogens are affected by specific factors dictated by human behaviour and population size
factors affecting direct transmission
Pathogens that spread through direct contact or by droplet infection need potential hosts to be within close proximity to each other
Places or areas with high population densities are more likely to have high infection rates
E.g. cities and schools
Tuberculosis (TB) transmission is very high in places where many people have to sleep in confined quarters
E.g. poor housing and homeless shelters
Farmers who use monocultures to maximise yield and profit can experience large disease outbreaks
Farmers grow a large number of crop plants in a small area
As the crops grow the leaves of different plants touch each other, making the transmission of pathogens such as tobacco mosaic virus (TMV) very easy
factors affecting indirect transmission
The indirect transmission of a pathogen can be affected by the biology of the vectors involved
Common disease vectors include mosquitoes and aphids
The population of vectors (usually insects) is influenced by weather and climate
factors affecting the distribution of malaria
Malaria is caused by one of four species of the protoctist Plasmodium but these protoctists are transmitted to humans by an insect vector (female Anopheles mosquitoes)
The Anopheles mosquitoes favour habitats that have high rainfall, high temperatures and high humidity
This means malaria can occur where these mosquitoes are present and, as a result, is found throughout the tropics and sub-tropics (about 80% of cases are in Africa)
The Anopheles mosquitoes found in Africa also have longer lifespans and prefer biting humans than animals
research regarding malaria
In the 1950s, the World Health Organisation (WHO) coordinated a worldwide eradication programme.
Whilst malaria was eradicated from some countries, the programme was mainly unsuccessful because:
Plasmodium became resistant to the drugs being used to try and control it
Anopheles mosquitoes became resistant to DDT and other insecticides being used against them
There is evidence that there are an increasing number of malaria epidemics due to climatic and environmental changes that favour the spread of the Anopheles mosquitoes
social factor of the spread of malaria
A social factor that has caused the number of cases of malaria in Africa, in particular, to increase in recent years is the increased migration of people due to war (when migration happens due to war the parasite can be transferred from areas that have the infection to new regions, and, if the Anopheles mosquito is breeding in the new region, then the mosquito vector will transfer the disease from one human to the next)
The degree or level of poverty in an area effecting transmission of disease in humans
Water-borne disease like typhoid, cholera and polio spread when human faecal matter enters and contaminates drinking water
Those below the poverty line usually live in areas with crowded housing with no sewage systems, sanitation facilities or water treatment facilities. In addition, many people in these areas have limited access to hygiene products
In the last 200 years, humans have spread across the globe, bringing their diseases and pathogens with them
the level of human movement and migrations effect on transmission of human disease
The level of human movement and migration that currently exists means that populations are more connected than ever
In the past, the ocean and bodies of water would have acted as natural geographic barriers to prevent the spread of pathogens
The first flu pandemic in 1918 took one year to spread around the globe. The flu pandemic in 2009 only took 3 months to reach West Africa from North America
An individual can become infected in one country (where the disease exists) and get a flight to another country thousands of miles away. They may not show any symptoms until they have already arrived in the new country
The historical danger of human migration is well known
When colonisers arrived in the Americas they brought many European diseases with them, such as smallpox
The Native Americans had no immunity or resistance as they had never been exposed to these pathogens before (they were a fully susceptible population)
The invasive pathogens rapidly spread through the population causing a large number of deaths
the behaviour or cultural practices of humans on the spread of disease
The behaviour or cultural practices of humans can also affect the transmission rate of diseases
For example, in parts of Africa, it is a religious and cultural tradition to touch and kiss the dead. This was a major problem during Ebola outbreaks there and scientists had to work with the public to try and inform them that this tradition was increasing the spread of the virus
endemic, epidemic, pandemic
Endemic - a disease that is always present in a population (even if very low numbers)
Epidemic - there is a large increase in the number of cases in a population (an outbreak)
Pandemic - an epidemic occurs on a large scale and crosses international boundaries
passive defences in plants
Passive defence mechanisms are always present
Some of these mechanisms are physical barriers that prevent pathogens from entering
Some are chemicals that reduce or prevent the growth of pathogens
active defences in plants
Active defence mechanisms in plants are activated when pathogens invade
Hypersensitivity deprives pathogens of resources
The formation of physical barriers by callose plays a major role in limiting the spread of pathogens
Cell signalling plays an important role in coordinating the active defence mechanisms
passive defence mechanisms
Physical barriers make it harder for pathogens to gain entry into plants
Examples of physical barriers:
Waxy cuticle
The only way that viruses and bacteria can penetrate the waxy cuticle of a leaf is if there is a wound on the leaf surface or stem.
Wounds are commonly caused by grazing herbivores
Cellulose cell wall
Closed stomata
Bark
Casparian strip
Some fungi species can invade a plant all the way to the endodermis but they are unable to push past the Casparian strip
Chemical defences prevent pathogens from growing on the surface of the plant by creating acidic conditions
Examples of chemical defences:
Toxic compounds
E.g. Catechol
Sticky resin found in the bark
This traps the pathogens so they can’t spread
Compounds that encourage the growth of competing microorganisms
Microorganisms such as yeast found on the leaf surface are completely harmless to plants. They are strong competitors against harmful pathogens
Enzyme inhibitors
E.g. Tannins
Receptor molecules
They detect the presence of pathogens and trigger other defence mechanisms
active defence mechanisms
Unlike animal cells, plant cells have cell walls. This means that substances can not freely move around the entire plant as the immune cells do in some animals, making cell signalling vital for plant defence
The active defence mechanisms of a plant are activated once a pathogen has invaded
Hypersensitivity is the rapid death of tissue surrounding the infection site
Although quite an extreme response, it is very effective as it deprives the pathogens of host tissue, nutrients and energy
Plants also create physical barriers to reduce the spread of a pathogen
Reinforced cell walls are formed when fungi and bacteria invade
-The invasion of pathogens stimulates the release of compounds callose and lignin
-These molecules are deposited between the cell surface membrane and the cell wall
-Callose is a polysaccharide that forms a matrix shape. Antimicrobial compounds that kill pathogens (hydrogen peroxide and phenols) can be deposited in this shape
Narrowing of the plasmodesmata
-Callose helps to reduce the size of the channels that connect neighbouring plant cells
Ingrowths into the xylem vessels (tyloses)
-The cytoplasm of nearby cells grows into the xylem to create a wall made of callose
Blockage of the phloem
-The sieve pores are filled with callose which prevents phloem sap from being transported
the importance of cell signalling in plant defences
Pathogens possess cellulase enzymes that digest the cellulose in plant cell walls
The molecules produced from this breakdown of cellulose act as signals to cell surface receptors
By stimulating these receptors they cause the release of defence chemicals called phytoalexins
Phytoalexins have several modes of action
-Disrupting pathogen metabolism
-Delaying pathogen reproduction
-Disrupting bacterial cell surface membranes
-Stimulating the release of chitinases (enzymes that break down the chitin cell walls in fungi)
chemicals involves in cell signalling
Salicylic acid is another important signalling molecule involved in plant defence
It migrates through the plant to uninfected areas.
Once there it activates defence mechanisms that protect the plant against pathogens for a period of time
This long-term protection is called systemic acquired resistance
Ethylene is a signalling compound that allows plants to communicate
Plants under attack from pathogens secrete ethylene onto their leaves. The ethylene vaporises, stimulating other leaves on the same plant to react (as well as other plants)
non specific immune response
Physical: body tissues act as barriers, preventing the entry of pathogens
E.g. skin, mucous membrane of the alimentary canal
Cellular: cells detect and signal the presence of pathogens. Protective substances are secreted and the pathogens are ingested and digested
Chemical: secreted substances generate an inhospitable environment for the growth of pathogens. These substances can trap pathogens, cause them to burst, or prevent them from entering cells and reproducing
Commensal organisms: the harmless bacteria and fungi present on and in the body compete with pathogens for nutrients
first line of defence
a human has three lines of defence
The first line of defence prevents the entry of pathogens and is comprised of the following:
-Skin
-Mucous membranes
-Expulsive reflexes
-Chemical secretions
skin
Skin posses an outer layer of dry, dead, hardened cells filled with keratin
Keratin is a tough fibrous protein
This layer of cells acts as a physical barrier to pathogens
There are secretions of sebum that contain fatty acids which have antimicrobial properties
Evaporation of sweat from the skin leaves behind a salt residue
The lack of moisture, low pH and high salinity creates an inhospitable environment for the growth of microorganisms
mucous membranes
Mucous membranes line the gut, airways and reproductive system
The mucous membrane consists of epithelial cells and mucus-secreting cells like goblet cells
Mucus contains lots of glycoproteins with long carbohydrate chains. These chains are what make mucus sticky
Viruses, bacteria, pollen and dust float about in the air that we breathe in
Mucus in the airways (trachea, bronchi and bronchioles) can trap these particles
The particles are then moved towards the back of the throat by cilia
Cilia are small hair-like structures on the surface of cells. Some ciliated epithelial cells have motile cilia that beat and move in a wave-like manner to move mucus along the airway
exclusive reflexes
When a pathogen irritates the lining of an airway it can trigger an expulsive reflex; a cough or sneeze
Both a cough and sneeze result in a sudden expulsion of air. This expelled air contains secretions from the respiratory tract along with the foreign particles that have entered
chemical secretions
Lysozymes are antimicrobial enzymes that breakdown the cell wall of bacteria
These special enzymes are found in body fluids such as blood, tears, sweat, and breast milk
Hydrochloric acid is produced by the cells that line the stomach
The acid creates a low pH inside the stomach which helps to kill any bacteria that has been ingested alongside food
The cells of the gut secrete mucus to prevent being damaged by hydrochloric acid
commensalism microorganisms
On average roughly 1kg of a human’s weight is made up of the bacteria on or inside their body
Candida albicans and E. coli are examples of bacteria commonly found on and in humans
These microorganisms grow on the skin, in the mouth and intestines however they do not cause disease
Their growth is limited by the defence mechanisms
Hosting these microorganisms can have a major benefit for humans
They compete with pathogenic microorganisms and prevent them from invading host tissue
Antibiotics often kill friendly gut bacteria which can allow for opportunistic pathogens to grow
second line of defence
When a pathogen manages to evade the first line of defence then the second line of defence will respond
The second line of defence involves phagocytic cells and antimicrobial proteins responding to the invading pathogens
Second-line responses include:
-Blood clotting
-Inflammation
-Wound repair
-Phagocytosis
blood clotting
When the body is wounded it responds rapidly
A break in the mucous membranes or skin membranes causes the release of molecules that trigger a chemical cascade which results in blood clotting
Blood clotting prevents excess blood loss, the entry of pathogens and provides a barrier (scab) for wound healing to occur
The chemical cascade involves a large number of steps and several plasma proteins
A small initial stimulus is amplified to produce a large amount of fibrin so that the wound is quickly sealed
inflammation
The surrounding area of a wound can sometimes become swollen, warm and painful to touch; this is described as inflammation
Inflammation is a local response to infection and tissue damage. It occurs via chemical signalling molecules which cause the migration of phagocytes into the tissue and increased blood flow
Body cells called mast cells respond to tissue damage by secreting the cell signalling molecule, histamine
