CIE A Level Biology复习笔记10.1.3 Prevention & Control of Disease

Prevention & control of cholera

• Cholera occurs when people do not have access to effective sanitation facilities and access to clean water
• It 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 supplies
  • Humanitarian 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 housing
  • The use of raw human sewage to irrigate crops
• Prevention of cholera can occur through:
  • Providing adequate sewage treatment infrastructure
  • The 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 & 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 DDT
  • Spreading 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 water
  • Ensuring ponds and irrigation or drainage ditches are stocked with fish that feed on mosquito larvae
  • Spraying 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-effects
• One 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
• 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
• To control malaria governments, WHO and institutions (eg. universities) are focusing on:
  • Working within health systems to improve diagnosis
  • Improving the supply of effective drugs
  • Using drugs in combination to reduce drug resistance
  • Promoting 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 laboratory
  • The entire Plasmodium genome has been sequenced, which will help in the development of vaccines

Prevention & control of tuberculosis (TB)

• 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 TB
  • Contacts 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 positive
  • Pasteurising milk (kills any TB-causing bacteria present in the milk)
  • Ensuring meat is cooked properly

Prevention & 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 infected
  • This 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 viruses
  • HIV-positive mothers and their babies can be treated with drugs (as HIV can be transmitted across the placenta, during birth and through breast milk
  • Condoms, 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 fluids
  • Education 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 others
  • Intravenous 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 donations
  • Public health measures widespread HIV testing of the population and education programmes
  • Needle-exchange schemes have been set up in some places to exchange used needles for new, sterile ones
  • Encouraging high-risk groups (eg. male homosexuals, prostitutes, injecting drug users) to be tested
  • Using 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)