3.1 Communicable diseases Flashcards
Communicable diseases
Disease caused by pathogens that can be spread through contact with contaminated surfaces, bodily fluids, blood products, insect bites or air and water. Examples include cholera, HIV and malaria.
Pathogens
Pathogens are microorganisms that can cause infectious diseases.
Viruses
Viruses are tiny, non-living (do not have cells) particles that can reproduce rapidly within the body.
To survive and replicate, they invade host cells (animals or plant cells).
The virus particle inserts its own DNA into the host cell’s DNA, which is then copied and used to make more viruses.
Once the new virus particles have been synthesised, they cause the cell to burst.
This releases the new viruses to invade more cells.
Measles
Measles is a highly contagious and a potentially serious viral infectious disease that can be fatal if complications arise.
Symptoms include fever and red skin rash.
The virus is spread by the inhalation of droplets from coughs or sneezes.
There is no treatment for measles but there is a vaccine for prevention.
HIV
Human immunodeficiency virus can eventually lead to AIDS (acquired immunodeficiency syndrome.
HIV starts with a flu-like illness. If untreated it can travel to the lymph nodes and attack cells of the immune system. It can stay hidden for a long time until the immune system is so badly damaged that it can no longer deal with other infections. This late stage HIV is known as aids.
The virus can be spread by direct sexual contact, the exchange of bodily fluids or from mother to child at birth or in breast milk.
There is no cure for HIV, although the use of antiretroviral drugs used early in the infection can now effectively control the disease to slow or halt the progress to AIDS.
TMV
Tobacco mosaic virus is a widespread plant pathogen that infects about 150 species of plants including tomato and cucumber.
The symptoms are distinctive mosaic pattern of discolouration of the leaves as the virus infects the chloroplasts. The plant will not grow as much due to the lack of photosynthesis.
The virus is spread by plants including tomato direct contact with an infected plant. The virus can stay in the soil for about 50 years.
There is no treatment for TMV and the best method of control is good field hygiene to prevent the spread of the virus.
Disease
A disease is an illness or disorder of the body or mind that leads to poor health.
Bacteria
Bacterial pathogens are cells that can infect plants and animals, causing disease. Not all bacteria is harmful - both skin and large intestine are home to hundreds of different bacterial species.
Bacterial pathogens produce toxins that cause damage to cells and tissue directly. In certain conditions bacteria can reproduce rapidly.
Salmonella
Salmonella food poisoning is spread by bacteria ingested in food, or on food prepared in unhygienic conditions. It is found in the gut of many animals.
The symptoms of a salmonella infection start between 8 - 72 hours of eating infected food. The bacteria secrete toxins that cause the body to react with; fever, abdominal cramps, vomiting and diarrhoea. These symptoms are the body trying to remove the infection from the gut. Generally the infection does not last very long but is more common in children.
Contaminated food that has not been cooked properly is the main cause of salmonella food poisoning.
It can be prevented with proper hygiene and cooking food thoroughly.
Gonorrhoea
Gonorrhoea is a sexually transmitted disease STD.
In the early stages symptoms include a thick, yellow or green discharge from the vagina or penis and pain when urinating. Some infectious people can be asymptomatic. If left untreated, it can cause infertility, ectopic pregnancy and pelvic pain. Babies born to women with the disease can get eye infections and be born blind.
It is spread by unprotected sexual contact.
Barrier methods of contraception such as a condom can prevent it spreading. It can be treated with antibiotics, but now many strains are becoming resistant to the most used antibiotics penicillin. Tracing of all sexual partners is essential so that all individuals can be tested and treated.
Fungi
Fungi are unicellular or multicellular organisms that each have bodies made up of hyphae (thread-like structures). The hyphae are able to grow and penetrate tissues, such as human skin or the surface of plants. The hyphae produce spores that can spread to other plants and animals
Rose black spot
Rose black spot is a fungal disease of plants where purple or black spots develop on the leaves, which often turn yellow and drop early. It affects the growth of plants as photosynthesis is reduced. It is spread in the environment by water or wind. They can be treated by using fungicides or removing and destroy the affected leaves.
Symptoms include purple or black spots on the leaves which can turn yellow and drop off. The reduction in photosynthesis means the plant has less energy to make flowers.
It is spread when spores from the fungus are carried in the wind or water.
The plant can be treated with fungicide chemicals. The leaves can be removed and burned to prevent the spread of infection. Breeding of varieties of plants that have resistance to the disease.
Protists
Protists are single-celled eukaryotic organisms (they have a nucleus). Lots of protists are parasites. This means that they live on or inside other organisms. They are often transferred to the host organism by a vector, such as an insect or mosquito.
Malaria
The pathogens that cause malaria are protists from the plasmodium family and can infect red blood cells in humans.
Malaria is spread by the female anopheles mosquito (a vector).
If a mosquito is carrying malaria, then it transfers the infection into a person’s bloodstream when it bites them.
Malaria leads to recurring and severe fevers, which can cause death.
If diagnosed early malaria can be treated with a combination of drugs however the protists that cause malaria have become resistant to most of the drugs. The spread can be reduced by controlling the mosquitos population - using insecticide, mosquito nets and prevent them from breading.
Non specific defence
The human body has non specific defences that are designed to prevent any pathogen from entering the body and causing harm.
Skin
Skin is a waterproof barrier that pathogens cannot pass through.
Good bacteria live permanently on our skin, covering the surface and preventing pathogens from growing on our skin.
Stomach acid
Glands in the stomach wall produce hydrochloric acid, which destroys any pathogens that enter the stomach.
Tears
Tears contain enzymes that destroy any pathogens that make contact with our eye.
Mucus
Mucus is produced in the respiratory system and traps the particles and bacteria entering our airways.
Immune system
Once a pathogen has entered the body they role of the immune system is to prevent the infectious organism from reproducing and to destroy it.
White blood cells help to defend against pathogens by phagocytosis, production of antibodies and antitoxins.
Phagocytosis
A white blood cell finds the pathogen and engulfs it by changing shape.
The white blood cell ingests (absorbs) and digests the pathogen, destroying it.
Production of antibodies
Lymphocytes produce antibodies.
Antibodies are y shaped proteins - each individual has the potential to make millions of different types of antibodies, each with a slightly different shape.
White blood cells recognise foreign antigens (proteins) on the surface of pathogens and produce protein molecules called antibodies.
Antibodies are specific to particular pathogens.
It may take a few days to make the antibodies that are specific to a pathogen and this may give the pathogen enough time to make you feel unwell.
Production of antitoxins
Some bacteria produce toxins that can damage cells and tissues.
The immune system responds to this by producing antitoxins, which neutralise the toxins released by the bacteria and prevent them from causing harm.
Immunity
There are two types of immunity:
Active immunity that comes from the body creating antibodies to a disease either by exposure to the disease (natural) or by vaccination (artificial)
Passive immunity that comes from antibodies given to you from another organism, for example in breast milk
Antibiotics
Medicines that treat the cause of the disease. Antibiotics such as penicillin are medicines that help to cure bacterial diseases by killing infective bacteria inside the body. Only certain antibiotics will work on certain diseases, so a doctor will prescribe different antibiotics depending on the type of infection. They work by stopping bacteria cellular processes such as the production of the cell wall.
Antibiotics will not work against viruses, as viruses reproduce inside cells. It is difficult to develop drugs that kill viruses without damaging the body’s tissues.
Antibiotic resistance
Since the discovery of antibiotics, it has saved millions of lives however antibiotics have been overused and antibiotics resistance has developed in many different types of bacterial diseases.
Bacteria like all organisms have random mutations in their DNA. One of these mutations gives them resistance to antibiotics. A strain of staphylococcus aureus has developed resistance to a powerful antibiotic methicillin. This is known as MRSA (methicillin resistance staphylococcus aureus. MRSA can infect wounds and is difficult to treat without antibiotics.
Preventing resistant bacteria
Doctors need to avoid overuse of antibiotics, prescribing them only when needed.
Antibiotics shouldn’t be used in non-serious infection that the immune system will clear up.
Antibiotics shouldn’t be used for viral infections.
Patients need to finish the whole course of antibiotics so that all the bacteria are killed and none are left behind to mutate to resistant strains.
Vaccinations
Vaccination will prevent illness in an individual by providing artificial immunity
Vaccination involves exposing an individual to the antigens of a pathogen in some form, triggering an immune response which results in the formation of memory cells which can make antibodies against it
If a vaccinated individual is infected with the pathogen, they can destroy it before they become infectious
How do vaccines work
Vaccination involves introducing small quantities of dead or inactive forms of a pathogen into the body to stimulate the white blood cells to produce antibodies
If the same pathogen re-enters the body the white blood cells respond quickly to produce the correct antibodies, preventing infection.
Painkillers
Medicines that treat the symptoms of the disease. For example ibuprofen can reduce pain and inflammation but does not kill the pathogens.
Discovering new drugs
Traditionally drugs were extracted from plants and microorganisms. New drugs are being developed all the time by scientists at universities and drug companies around the world.
Lots of the medicine that we use today are based on chemicals extracted from plants.
The heart drug digitalis originated from foxgloves.
The painkiller aspirin originates from willow.
Penicillin was discovered by Alexander Fleming from the penicillium mould.
Most new drugs are synthesised by chemists however the starting point may still be a chemical extracted from a plant.
Discovery of penicillin
Penicillin was first discovered by Alexander Fleming in 1928. He left some Petri dishes that had been contaminated with mould and found bacteria would not grow near the mould.
He discovered that the mould (penicillium notatum) was releasing a chemical (penicillin) that killed the bacteria surrounding it.
How do pathogens cause infection
Pathogens cause illness to their hosts through a variety of ways. The most obvious means is through direct damage of tissues or cells during replication, generally through the production of toxins, which allows the pathogen to reach new tissues or exit the cells inside which it replicated.
Primary response
During a primary infection levels of antibodies slowly increase, peak at around ten days and then gradually decrease. This is what happens when someone is vaccinated with a dead or inactive pathogen or when someone catches a disease for the very first time. It is called the primary immune response.
Secondary response
A second exposure to the antigen that was in the vaccine, or to the same pathogen that made the person ill before, causes the white blood cells to respond much more quickly and in larger quantities this time. This is the secondary immune response. The antibodies are produced so quickly by the memory cells that the pathogen is killed off before it can make the person ill. This is called being immune to a disease or having immunity.
Preventing infection
By being hygienic, we can destroy pathogens or wash them away so that they are not spread.
Using antibacterial products to clean surfaces, washing our hands and sneezing into a handkerchief are all hygienic measures to prevent the spread of infection.
By isolating infected individuals, we prevent contact with other organisms. This stops the infection from spreading.
Some protist diseases are carried by vectors.
A vector is an organism that carries pathogens without being infected.
For example, we can protect against mosquitos carrying malaria using mosquito nets.
Vaccination can make people immune to (resistant to) infection.
The more individuals that are vaccinated, the less likely the disease is to spread.
Spreading of disease
Some pathogens live on the surface of other organisms, such as the skin. If infected skin touches a surface, some of the pathogens can transfer from the infected skin to the surface. The pathogen then spreads to other people who also touch that surface. Athlete’s foot is spread in this way.
Some pathogens contaminate water supplies. Organisms drinking this contaminated water can be infected by pathogens.
Cholera is a disease spread in this way.
Some pathogens are released into the air inside droplets when an organism coughs or sneezes. These droplets can then be inhaled (breathed in) by other organisms.
The flu and common colds are spread in this way.
Testing new drugs
All new drugs need to be tested and trialed before they can be used in patients. They are tested for:
Toxicity - any harmful side effects
Efficacy - does the drug work
Dose - lowest dosage that can be used and still have an effect.
Developing new drugs
Preclinical testing is done in a laboratory using cells, tissues and live animals.
Clinical trials use healthy volunteers and patients. Very low doses of the drug are given at the start of the clinical trials.
If the drug is found to be safe, further clinical trials are carried out to find the optimum dose for the drug.
In double blind trials, some patients are given a placebo.
Preclinical testing
The drug is tested on cells in the lab
Computer models may also be used to simulate the metabolic pathways that may be taken by the drug
Efficacy and toxicity are tested at this stage
Whole organism testing
The drug is tested on animals to see the effect in a whole organism - all new medicines in the UK have to have tests on 2 different animals by law
Efficacy toxicity and dosage are tested at this stage.
Clinical trials
The drug is tested on human volunteers first, generally with a very low dose then increased. This is to make sure it is safe in a body that is working normally.
The next stage is to test on patients with the condition.
The patients are often split into two groups; one given the drug the other given a placebo. This is called a double-blind study - neither the doctor nor the patient knows if the patient is getting the placebo or the active drug.
Once the drug is found to be safe then the lowest effective dose is tested at this stage.
