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Summary 10 Infectious Diseases
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EVERYTHING you need to know about "Infectious Diseases" for A-level Biology CIE
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Infectious Diseases21 March 2021 17:15
What Are Infectious Diseases?
• They are diseases caused by pathogens
• They are sometimes called communicable diseases as they are passed from infected to non-infected organisms
• Some diseases only affect the organism for a short time (colds, measles)
• Others are much longer-lasting (TB, HIV/AIDS)
• Some are spread by direct contact
• Some pathogens survive in water, food or animals so may be transmitted indirectly
• Some people may not show symptoms, but still pass on the disease (these people are carriers)
The Worldwide Importance of Infectious Disease
• Cholera
• Malaria
• HIV/AIDS
• Tuberculosis
• Measles
• The number of people, especially young people, infected with these diseases worldwide poses a serious public health problem for the foreseeable
future
Definitions
• Endemic - when a disease exists permanently in a particular region or population. Malaria is endemic to parts of Africa
• Epidemic – when a disease appears suddenly and spreads rapidly in a specific area or within a particular population. Eg Food poisoning
• Pandemic – an epidemic that spreads widely – through a whole country, continent or even the world. Eg – Spanish flu in 1918 which killed 10 million
people
• Incidence – the number of people who are diagnosed over a certain time period (a week, month or year)
• Prevalence – the number of people with a particular disease at any one time
• Mortality – the number of people who die from a particular disease
HIV and AIDS
• Acquired Immune Deficiency Syndrome (AIDS)
• AIDS is caused by the Human Immunodeficiency Virus (HIV)
• HIV is a retrovirus, which means that its genetic material is RNA, rather than DNA
• Inside a host cell, RNA is ‘reverse transcribed’ to form DNA, which is then incorporated into the human chromosomes
• HIV specifically targets T-helper cells, which are the white blood cells produced by the body to activate the immune system in response to a pathogen
• If the immune system is unable to switch on, the body is open to many opportunistic infections
• So AIDS is no single disease, but a collection of these opportunistic diseases caused by an inability of the body to fight in response to infection
Features:
• HIV has no vector, it has to be spread by direct exchange of body fluids
• The virus is actually quite weak outside its host. Unlike cholera and malaria, and many viral pathogens, HIV has a very limited lifespan outside the
human body
• It is a slow acting virus and after infection there may not be any symptoms until years later. At this stage the person is HIV positive, but does not have
AIDS
• Two of the early opportunistic diseases that infect sufferers of AIDS are both fungal, Candida albicans (thrush) and Pneumocystis jiroveci (an unusual
form of pneumonia)
Social, Economic, and Biological Factors:
• The most common treatment is a combination of drugs (AZT + two protease inhibitors) which prevent reverse transcription. The use of combined drugs
also helps fight the development of resistance in the virus. It also helps (to some extent) minimise the side effects of the individual drugs
• But there is no cure
• A vaccine would be the best solution but the virus keeps changing its antigens, so hides from the immune system
• Other attempts to prevent the disease spreading so rapidly is to encourage male circumcision – circumcised males are 7X less likely to pass on or
contract the disease
Tuberculosis (TB)
• A long-standing major world wide health problem
• Caused by either of two bacteria
• Most commonly Mycobacterium tuberculosis
• But also Mycobacterium bovis, which is present in cattle and can be transmitted to humans through unpasteurised milk or uncooked meat
• These pathogens live particularly in the lungs, but can invade the rest of the body from this first site of infection, even spreading to the bone marrow in
later stages
Features:
• Around one in three of us carry the TB bacteria around in our lungs, but as long as we are healthy, it causes us no problems and we can’t pass it on to
others
• However, should our immune system be compromised, through poor diet or illnesses such as HIV, then the bacterium takes the opportunity to
overwhelm the body. TB is the leading cause of death amongst HIV+ people (opportunistic infection)
, • Symptoms include a persistent cough, fever, appetite suppression and consequent weight loss, to the point of emaciation
Transmission:
• It is most commonly spread by droplet infection
• Sufferers have a persistent cough and the droplets they emit carry live Mycobacterium
• The bacteria have lipid-rich walls, which prevents them from drying out, so they may survive for several months in the air or the dust around the house
• Overcrowding and ill-ventilated conditions favour the transmission of the bacterium, but it’s still actually quite hard to catch because the bacterium
itself is not strongly infectious
• The form of TB caused by M. bovis is spread to humans in meat and milk
• It is thought to be transmitted to cattle in the first place by badgers, so occasionally there are badger culls to reduce the risk of infection
• About 800,000 people in Britain died between 1850 and 1950 from this form of TB, but very few people catch this in Britain now due to vaccination of
cattle
• Improvements in living standards in Britain reduced the incidence of TB before the 1950s and the rate fell further still after the introduction of a
nationwide vaccine
• Streptomycin proved an effective antibiotic against the disease and TB was practically eradicated
• High rates of TB are now present in cities with immigrant populations from countries where TB is more prevalent
• Some strains have developed resistance to the drugs used to treat TB
• The HIV/AIDS pandemic has caused a large increase in sufferers
• Poor housing in inner cities and homelessness has increased rates
• The breakdown of the TB control programme; partial treatment for TB increases the chance of resistant strains developing and many people fail to
complete the 6 month course of drugs prescribed to eliminate the disease completely from their system
Treatment:
• TB is treatable, but will still kill if not diagnosed early
• Identification of the bacteria is easy from a sample sputum from the lungs
• Because the disease is infectious, patients should be isolated (especially during the most infectious stage of two to four weeks) and contacts need to be
traced
• Treatment with specific antibiotics is given during isolation until the patient is seen to no longer be coughing up viable bacteria
• Then they are allowed back into the community with more antibiotics designed to eradicate the infection entirely from their body
• This may take six months or more
• If not killed the bacteria may well develop drug resistance, so a combination of drugs are given and the patient is told clearly that they need to
complete the course of drugs
• However, many people start to feel better sooner than six months and stop their treatment
• These people may be harbouring drug-resistant bacteria, which they can pass on to others
Drug-resistant TB:
• Resistance was seen early after the first antibiotic treatments were available in the 1950s
• Drug resistance occurs as a result of mutation when DNA replicates
• This is a random event and occurs in about one in every thousand bacteria
• If three drugs are used in the treatment process, the chance of a mutation arising against all three drugs is one in a thousand million
• If four drugs are used, the chance of mutation against all four becomes one in a billion
• If the person stops taking the drugs or goes untreated altogether, the remaining bacteria spread throughout the body, increasing the chances that
mutations will arise as bacteria survive for a long time and multiply, albeit slowly
• There are now strains of TB that are resistant to all the drugs we have available to treat the disease as a result of this behaviour
Prevention of TB:
• A vaccine, the Bacillus Calmette-Guerin (BCG) vaccine is available
• It is made from a weakened form of the live bovine version of the disease M. bovis
• It is used on children and immunity lasts about 15 years, or longer if the person is exposed to the TB bacterium
• It is only effective in 70-80% of people
• It is no longer given routinely to children in this country as TB is considered less of a threat due to our generally high standards of living
• Cattle are routinely tested for the bovine form of the disease and destroyed if found to be positive
• Pasteurising the milk kills the bacteria
• This programme is so effective that in the UK, less than 1% of the 9000 new cases of TB each year are due to M. Bovis
Malaria
• Malaria is the most important of all insect-borne diseases, present in 240 countries and threatening 2400 million people (40% of the world population)
• About 80% of these cases are in sub-Saharan Africa and this is where 90% of the fatalities occur
• Most of the people who die are children under 5
• Malaria kills more people than any other infectious disease, with the exception of TB
Transmission:
• Most malaria cases are caused by one of four species of a protoctist called Plasmodium
• Some species of Plasmodium can cause malaria in both humans and monkeys
• The parasite is transmitted from person to person via a vector, the female Anopheles mosquito, which takes a blood meal from humans in order to
provide sufficient protein to develop their eggs
• If the person they bite is infected with Plasmodium, they take up some of the pathogen's gametes as they drink the blood
• The male and female gametes fuse in the mosquito's gut and develop to form infective stages, which move to the mosquito's salivary glands
• When the mosquito feeds again, she injects an anticoagulant from her salivary glands to prevent the blood from clotting as she drinks it
• The infective stages pass out with this anticoagulant into the human's blood where the parasites enter the red blood cell and multiply
• Malaria may also be transmitted during blood transfusions, unsterile needles being shared, and it can also pass from mother to foetus across the
placenta
Features:
• If people are continuously re-infected by different strains of malaria, they become immune
• But this only happens if they survive the first bout of infections (basically the first five years of life)
• Mortality rate in children from malaria is very high
• Epidemics in places where malaria is not endemic is very dangerous as no-one has immunity
• It may also only occur after a rainy season (when larvae have developed in the rain water). Again, the population is not generally immune and it also
coincides with the maximum agricultural activity so the disease has a disastrous effect on the economy as sick people can't work
Treatment:
• Anti-malarial drugs include quinine and chloroquine
• These are also used as prophylactics, stopping an infection in advance if someone is bitten
• Chloroquine inhibits protein synthesis, so prevents the parasite spreading in the body
• Proguanil, another prophylactic used also inhibits the sexual reproduction of the Plasmodium within the mosquito
Resistance:
• Anti-malarial drugs have been used widely in some parts of the world and this has caused the appearance of drug-resistant strains of Plasmodium
• Chloroquine resistance is high in South America, Africa, and New Guinea, where newer drugs, such as mefloquine are used
• But mefloquine is expensive and can have nasty side effects, such as dizziness, vomiting, and disturbed sleep
• Resistance to mefloquine has already been seen in Thailand
• An antibiotic called doxycycline is also used prophylactically and artesunte (derived from a plant compound artemisin) can be used in combination with
mefloquine to treat infections by P. falciparum (a species of Plasmodium)
Prevention of Malaria:
• There are three main ways to control malaria:
- Reduce the number of mosquitos
- Avoid being bitten by mosquitos
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