CIE AS LEVEL Biology : Infectious disease

What is a diseases

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

What is an infectious disease

Infectious diseases are caused by pathogens and are transmissible (can be spread between individuals within a population) e.g cholera, malaria, HIV/AIDS and tuberculosis

What are non infectious diseases

They are diseases that aren't caused by a pathogen and we transmissible e.g chronic obstructive pulinwry disease, lung cancer, sickle cell anaemia and cystic fibrosis.

Causative agents(pathogens) and the type of organism that cause infectious diseases

Cholera: Vibrio Chlora (pathogen)- bacterium
Malaria: Plasmodium falciparum, Plasmodium malaria, Plasmodium vivax, Plasmodium ovale- Protoctist
Tuberculosis : Mycobacterium Tuberculosis, Mycobacterium Bovis - bacterium
HIV/AIDS : human immunodeficiency virus - virus

Transmission of cholera

Cholera is caused by the bacterium Vibrio choleraeThe disease is water-borne (the bacterium lives in water) and food-borneThis means the disease occurs where people do not have access to proper sanitation (clean water supply) and uncontaminated foodCholera can be transmitted when people; bath or wash in contaminated water, drink contaminated water, or eat food exposed to contaminated waterInfected people egest large numbers of the bacteria in their faecesIf these faeces contaminate the water supply, or if infected people handle food or cooking utensils without washing their hands, then the bacteria are transmitted to uninfected people

Transmission of malaria

Malaria is caused by one of four species of the protoctist PlasmodiumThese 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 eggsIf the person they bite is infected with Plasmodium, the mosquito will take up some of the pathogen with the blood mealWhen 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-usedPlasmodium can also pass from mother to child across the placenta

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 liquidTB is transmitted when uninfected people then inhale these dropletsTB therefore spreads more quickly among people living in overcrowded conditionsThe form of TB caused by Mycobacterium bovis occurs in cattle but is spread to humans through contaminated meat and unpasteurised milkVery 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

Transmission of HIV/AIDS

Human Immunodeficiency Virus is a retrovirusThe HIV virus is not transmitted by a vector (unlike in malaria)The virus is unable to survive outside of the human bodyThe virus is spread by intimate human contact and can only be transmitted by direct exchange of body fluidsThis 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

Prevention and control of cholera

Cholera occurs when people do not have access to effective sanitation facilities and access to clean waterIt is difficult to prevent and control cholera because of:The fast-growing cities in developing countries not having the appropriate infrastructure. They have limited funds for large-scale projects such as the provision of drainage systems, sewage treatment facilities and clean water suppliesHumanitarian crises (eg. displacement of people due to wars or natural disasters), which can cause the destruction of sanitation infrastructure and/or the provision of poor sanitation facilities in overcrowded temporary housingThe use of raw human sewage to irrigate crops

Prevention of cholera can occur through:Providing adequate sewage treatment infrastructureThe provision of clean, piped water that has been chlorinated to kill bacteria (as this occurs in developed countries, cholera is very rare among them)vaccination programmes in areas where cholera is endemic

Cholera can be controlled by:Ready access to treatments such as oral rehydration therapy (a solution containing glucose, salts and water)Monitoring programmes by the World Health Organisation (WHO)Using antibiotics in severe cases (to reduce the risk of antibiotic resistance)

Prevention and control of malaria

The 3 main methods for reducing malaria are:Reducing the number of Anopheles mosquitoes in an area,Reducing the chance of being bitten by these mosquitoes,Using drugs to prevent Plasmodium infecting humans
As Anopheles mosquitoes (specifically female mosquitoes) are the vectors that transmit Plasmodium between human hosts, the transmission cycle of malaria can be broken (or at least reduced) by reducing the number of these mosquitoes. This can be achieved by:Spraying living areas with insecticides, such as DDTSpreading oil over the surfaces of water bodies that the mosquitoes breed in such as ponds and irrigation or drainage ditches (the mosquitoes lay their eggs in water but the larvae breathe air at the water surface - an oil layer makes this impossible and kills the larvae)Draining marshes and other unnecessary bodies of waterEnsuring ponds and irrigation or drainage ditches are stocked with fish that feed on mosquito larvaeSpraying these water bodies with a preparation containing the bacterium Bacillus thuringiensis, which kills mosquito larvae but is not toxic to other organisms
Unfortunately, mosquitoes lay eggs in even very small puddles and pools of water and therefore it is practically impossible to control all breeding sites using the methods listed above

Prophylactic (preventative) drugs (eg. chloroquine, mefloquine) are taken before, during and after a visit to a location where malaria is prevalent. However, the use of these drugs has resulted in drug-resistant strains of Plasmodium or the drugs are expensive and have disagreeable side-effectsOne of the best ways to prevent malaria is to avoid being bitten in the first place. People in malarial zones should sleep under bed nets (which can also be soaked periodically in insecticide to increase effectiveness) and should try to avoid exposing their skin at dusk when mosquitoes are most active

How should governments control malaria

To control malaria governments, WHO and institutions (eg. universities) are focusing on:Working within health systems to improve diagnosisImproving the supply of effective drugsUsing drugs in combination to reduce drug resistancePromoting appropriate methods to prevent transmission (eg. the use of biological controls to target the larvae and insecticide-treated bednets)

Recent scientific advances regarding the control of malaria are:Simple dipstick tests for diagnosing malaria - this means a diagnosis can be made much faster and does not require a laboratoryThe entire Plasmodium genome has been sequenced, which will help in the development of vaccines

Prevention and control of tuberculosis

TB is spread quickly from person to person when droplets released by the coughing or sneezing of an infected person with the active form of the illness are inhaled by an uninfected person (the droplets contain the TB-causing bacterium Mycobacterium tuberculosis)The process of contact tracing (and the subsequent testing of those contacts for the bacterium) is an important method of controlling the spread of TBContacts are screened for symptoms of TB infection, although the diagnosis can take up to two weeks

Prevention for TB occurs through the use of the BCG vaccine (the only vaccine for TB)The vaccine protects up to 70-80% of those who receive it, although its effectiveness decreases with age unless the person is exposed to TB

The form of TB that can be transmitted between cattle and humans (caused by Mycobacterium bovis) can be prevented by:Routinely testing cattle for TB and destroying those that test positivePasteurising milk (kills any TB-causing bacteria present in the milk)Ensuring meat is cooked properly

Prevention and control of HIV/AIDS

Preventing the spread of HIV is very difficult, as the virus has a long latent stage, which results in it being transmitted by people who have the virus but show no symptoms and do not know they are infectedThis occurs because the virus can change its surface proteins, making it difficult for the human immune system to recognise it and for a vaccine to be developed

To prevent the transmission of HIV the following measures can occur:Blood donations can be screened for HIV and heat-treated to kill any virusesHIV-positive mothers and their babies can be treated with drugs (as HIV can be transmitted across the placenta, during birth and through breast milkCondoms, femidoms and dental dams can be used to decrease the infection risk during sexual intercourse and oral sex by forming a physical barrier between body and fluidsEducation programmes about how the virus is transmitted can be released into the community to encourage people to change their behaviours in order to protect themselves and othersIntravenous drug users encouraged not to share needles

Controlling HIV can occur by:Contact tracing (and the subsequent testing of those contacts for the virus)Screening blood donationsPublic health measures widespread HIV testing of the population and education programmesNeedle-exchange schemes have been set up in some places to exchange used needles for new, sterile onesEncouraging high-risk groups (eg. male homosexuals, prostitutes, injecting drug users) to be testedUsing anti-retroviral drugs

The socio-economic status of a person or country with HIV can determine how it is controlled. For example, HIV-positive mothers are advised not to breastfeed in high-income countries, however, in low- and middle-income countries breastfeeding offers protection against other diseases (eg. cholera)

What we antibiotics and how do they work

Antibiotics are drugs that kill or stop the growth of bacteria (prokaryotes) but do not harm the cells of the infected organism. Antibiotics work by interfering with the growth or metabolism of the target bacterium. Antibiotics target a variety of processes including:synthesis of bacterial cell walls
activity of proteins in bacterial cell surface membranes
bacterial enzyme action
bacterial DNA synthesis
bacterial protein synthesis

How penicillin affects bacteria

Bacterial cell walls are composed of peptidoglycans (long molecules of peptides and sugars)
These peptidoglycan molecules are held together by cross-links that form between them
When a new bacterial cell is growing, it secretes enzymes known as autolysins that create small holes in the bacterial cell wall
These holes allow the bacterial cell wall to stretch, with new peptidoglycan molecules then joining up via the cross-links described above
Penicillin stops these cross-links forming by inhibiting the enzymes that catalyse their formation
However, the autolysins keep creating holes in the bacterial cell wall, making the walls weaker and weaker
As bacteria live in watery environments and take up water by osmosis, their weakened cell walls eventually burst as they can no longer withstand the pressure exerted on them from within the cell
This means penicillin is only effective against bacteria that are still growing (autolysins no longer create holes and no more cross-links between peptidoglycan molecules are formed once the growth of a bacterium is complete, as the bacterial cell wall no longer needs to expand)

Why can't penicillin (antibiotics) affect viruses

Penicillin (and other antibiotics) do not affect viruses as they do not have cells (or cell walls) and therefore cannot be targeted in any of the ways that an antibiotic targets a bacterial cellWhen a virus replicates, it uses the host cell's mechanisms for transcription and translation, so not even these processes can be targeted as antibiotics do not bind to the proteins that host cells use in these processesPenicillin is not effective against all bacteria (eg. tuberculosis) because the bacteria may have:Thick cell walls which reduce permeability,Enzymes which breakdown penicillin

Resistance to antibiotics

Within a bacterial population, there is variation caused by mutations (as occurs in populations of all species)
A chance mutation might cause some bacteria to become resistant to an antibiotic (eg. penicillin)
When the population is treated with this antibiotic, the resistant bacteria do not die,For example, a mutation may change an existing gene within the bacterial genome, causing it to give rise to a nucleotide sequence that codes for a slightly different protein that is not affected by the antibiotic being used
This means the resistant bacteria can continue to reproduce with less competition from the non-resistant bacteria, which are now dead
Therefore the genes for antibiotic resistance are passed on with a much greater frequency to the next generation
As bacteria only have one copy of each gene, a mutant gene will have an immediate effect on any bacterium possessing it
Over time, the whole population of bacteria becomes antibiotic-resistant because the antibiotic-resistant bacteria are best suited to their environment,This is an example of evolution by natural selection
Some pathogenic bacteria have become resistant to penicillin as they have acquired genes that code for the production of the enzyme β-lactamase (also known as penicillinase), which breaks down penicillin

Vertical trasnmission

Bacteria reproduce asexually by binary fission (the DNA of the bacterial chromosome is replicated and the bacterial cell divides in two, with each daughter cell receiving a copy of the chromosome)
Bacteria reproduce like this very rapidly (on average, every 20 minutes)
If one bacterium contains a mutant gene that gives it antibiotic resistance, all of its descendants (millions of which can be produced in a matter of hours) will also have the antibiotic resistance
This method of spreading antibiotic resistance within a bacterial population is known as vertical transmission

Horizontal transmission

Plasmids (the small rings of DNA present in bacterial cells) often contain antibiotic-resistant genes
These plasmids are frequently transferred between bacteria (even from one species to another)
This occurs during conjugation (when a thin tube forms between two bacteria to allow the exchange of DNA) - DNA from the bacterial chromosome can also be transferred in this way
In this way, a bacterium containing a mutant gene that gives it antibiotic resistance could pass this gene on to other bacteria (even those from a different species). This is how 'superbugs' with multiple resistance have developed (e.g. methicillin-resistant Staphylococcus aureus - MRSA)
This method of spreading antibiotic resistance within or between bacterial populations is known as horizontal transmission

What is antibiotic resistance in bacteria due to

Antibiotic resistance in bacteria is an example of natural selection that humans have helped to develop. This is due to the overuse of antibiotics in situations where they were not really necessary or the incorrect use of antibiotics, for example:For treatment of non-serious infections
Routine treatment to animals in agriculture
Failure to finish the prescribed course of antibiotics

Consequences of antibiotic resistance

Commonly prescribed antibiotics are becoming less effective for many reasons, the main being:Overuse of antibiotics and antibiotics being prescribed when not necessary, Large scale use of antibiotics in farming to prevent disease when livestock are kept in close quarters, even when animals are not sick, Patients failing to complete the full course of antibiotics prescribed by doctors

These factors have led to a reduction in the effectiveness of antibiotics, and an increase in the incidence of antibiotic resistanceThese bacteria are commonly known as superbugsThe most common example is a strain of Staphylococcus aureus that has developed resistance to a powerful antibiotic methicillin and is now known as MRSA (Methicillin-resistant Staphylococcus aureus) as well as other antibiotics (eg. penicillin)Bacteria living where there is a widespread use of many different antibiotics may have plasmids containing resistance genes for several different antibiotics, giving them multiple resistance and presenting a significant problem for doctorsIn addition, resistance may first appear in a non-pathogenic bacterium, but then be passed on to a pathogenic species by horizontal transmissionThere is a constant race to find new antibiotics as resistant strains are continuously evolving

Reducing antibiotic resistance and its impact

Ways to prevent the incidence of antibiotic resistance increasing include:Tighter controls in countries in which antibiotics are sold without a doctor's prescription
Doctors avoiding the overuse of antibiotics, prescribing them only when needed (patients must only be given antibiotics when absolutely essential) - doctors should test the bacteria first to make sure that they prescribe the correct antibiotic
Antibiotics not being used in non-serious infections that the immune system will 'clear up' (patients must not keep unused antibiotics for self-medication of such non-serious infections in the future)
When prescribed a course of antibiotics, the patient finishing the entire course (even if they feel better after a few days) so that all the bacteria are killed, and none are left to mutate to become resistant strains
Antibiotics not being used for viral infections (antibiotics have no effect on viruses anyway, and this just provides an unnecessary chance for bacteria to develop resistance)
The use of 'wide-spectrum' antibiotics being reduced and instead those antibiotics that are highly specific to the infection ('narrow-spectrum' antibiotics) being used
The type of antibiotics prescribed being changed so that the same antibiotic is not always prescribed for the same infections and diseases (this reduces the chance of a resistant strain developing)
The use of antibiotics being reduced and more tightly controlled in industries such as agriculture - controls are now in place to limit their use in farming, where antibiotics are used to prevent, rather than cure, bacterial infections

How can the spread of already resistant strains be limited

The spread of already-resistant strains can be limited by:Ensuring good hygiene practices such as handwashing and the use of hand sanitisers (this has reduced the rates of resistant strains of bacteria, such as MRSA, in hospitals)
Isolating infected patients to prevent the spread of resistant strains, in particular in surgical wards where MRSA can infect surgical wounds