Ch 13: Public Health

Question 1

What are factors that may affect disease transmission?

Answer 1

• Characteristics of the disease itself e.g. mechanisms of transmission
• Environmental factors e.g. climate
• Characteristics of the population e.g. levels of immunization
• These factors may be influenced by humans unintentionally, or intentionally as disease control measures

Question 2

What are the patterns of disease transmission?

Answer 2

• sporadic disease: is uncommon and occurs irregularly in a population.
• endemic diseases: occurs at a relatively constant rate.
• epidemics (also known as outbreaks): occur when there is an increase in the disease rate above what is considered normal within a population

pandemic: epidemics that spread across continents

Question 3

Describe pathogen factors

Answer 3

• distribution of a vector: can restrict the spread of disease due to geographic locations where vectors reside. Also affected by behaviours/risk factors that can promote the spread of infection, may mean that infections are more likely to spread in certain groups where these behaviours are more common than in other members of the population
  i. e. malaria is only found in areas near the equator because it is restricted to geographic areas where the Anopheles mosquito can live
• a pathogen’s virulence: the capacity of a pathogen to cause severe disease within its hosts
• a pathogen’s infectivity: the ability of a pathogen to spread from one host to another. Diseases with a high infectivity are readily able to spread through a population i.e. influenza
• persistence of a pathogen in asymptomatic carriers: may still be capable of transmitting infection to others i.e. tuberculosis. Can allow the spread of a pathogen within a population over longer periods of time or into new populations as individuals move due to being undetectable
• persistence of a pathogen within its definitive host or intermediate hosts
• a pathogen’s natural history: the different stages of infection, varies between pathogens and can influence how an infection spreads within a population

Question 4

Describe environment factors

Answer 4

• infrastructure
  
  • sewerage systems
  • water supply
  • roads
  • maintaining coordinated treatment programs
  • new environmental niches in urban environments i.e. water tanks for mosquito larvae, building dams or irrigation networks in malaria-prone regions may promote infection by modifying mosquito breeding areas. This may be through the creation of new breeding sites, modification of the local environment (e.g. humidity) and downstream impacts on the ecosystem
• climate: with global climate changes, the geographic distribution of pathogens and vectors and the diseases they carry can change. Increases in temperature and changes in rainfall are likely to result in the spread of disease-carrying vectors i.e. mosquitos into previously uninhabitable regions
  
  • rainfall
  • temperature
  • drought: the lack of fresh water can lead to breakdown in hygiene practices and promote disease spread
  • extreme events: displacement of populations and/or breakdown of usual sanitation, food and health infrastructures are major contributing factors to the promotion of the spread of disease i.e. tsunamis, floods – water supplies can become contaminated with human waste containing disease-carrying vectors

Question 5

Describe host factors affecting transmission of disease

Answer 5

• risk of exposure due to behaviour
• age
• sex
• socioeconomic status
• host’s immune system i.e. the elderly and chronically ill may be more susceptible
• population density
• proportion of the population with existing immunity to the disease
• movement of individuals

Question 6

Describe how movement of individuals can affect disease transmission

Answer 6

• the movement of individuals can facilitate the spread of disease, carriers or infected individuals travelling between populations can spread disease. This is because individuals carrying disease are able to infect other individuals in the areas they are travelling to allowing disease to spread faster and over larger geographical areas
  e. g. the arrival of Europeans in Australia was associated with the introduction of several new diseases into populations of Indigenous Australians including: chicken pox, measles, influenza, typhoid. These causes significant morbidity and mortality in the Indigenous population.
• this can also be intentional (i.e. quarantine and vaccination) or unintentional (i.e. agriculture, urbanisation or transportation)

Question 7

How can the spread of disease be prevented?

Answer 7

­Hand hygiene

• early surgeons did not wash their hands and contamination of surgeons’ clothes was considered a sign of experience. Post-operative death rates were incredibly high.
• today, regular handwashing and the use of sterile equipment are considered key elements in effective healthcare, preventing individuals from contracting infections (removing microbes), particularly those that are spread by faecal-oral or direct contact routes and then spreading it to other individuals

­Immunisation

• immunisation has substantially reduced worldwide morbidity and mortality. Immunisation programs may help to eradicate diseases by stopping their spread i.e. smallpox in 1980
• Australian children are routinely vaccinated against hepatitis B, pertussis, measles, tetanus and poliomyelitis
• herd immunity: the resistance of a group to invasion and spread of an infectious agent, based on the resistance to infection of a high proportion of individual members of the group. Disease spread cannot occur as there are too few susceptible individuals to sustain it. The resistance is a product of the number susceptible and the probability that that those who are susceptible will come into contact with an infected person

­Quarantine

• quarantine is a practice used to stop individuals who have (or may have) been exposed to infectious diseases from carrying that disease into healthy populations. Exposed individuals are kept from entering a healthy population until the incubation period of that disease has passed, proving they are not infected.
• quarantine is also used to prevent the entry of foreign plants, animals and their pathogens into Australia. We have particularly strict quarantine laws because of the potentially devastating impact of imported pathogens on our unique flora and fauna. As an island, protecting our borders from imported pathogens and pests is easier than in many other parts of the world

Question 8

How is disease monitored?

Answer 8

• to define and control disease outbreaks, public health authorities need to know when and where particular infections are occurring.
• in Australia, the list of notifiable diseases contains more than 70 conditions including chicken pox, syphilis, rabies and influenza. A doctor that diagnoses one of these conditions must report the case to the relevant state health authority. outbreaks or cases of unusual diseases can then be investigated.

àLimitations to data collected this way

• not all patients who are infected with a disease will seek health care.
• not all patients will receive a diagnosis.
• infections can be under-reported.
• there can be a delay between the onset of symptoms, diagnosis and reporting, limiting the ability of public health authorities to respond quickly to epidemics
• together this means that reported data are likely to be lower than actual number of cases

Current surveillance mechanisms

• digital disease surveillance: the widespread use of the Internet and social media provides a novel data source from which information about the frequency of diseases can be extracted
• Facebook, Twitter and mobile phones have all been used to monitor disease activity.*
• they have the advantage of providing information to public health authorities in real time. However, the quality of the data is limited by how effective algorithms are in determining whether or not a tweet or search is actually about an illness. Furthermore, high internet activity about an illness does not always correspond to high disease activity as it can be skewed by other events
• mathematical models that can predict the spread of disease are important tools in the control of an outbreak. Such tools can:
  
  • be used to explore the likely effects of newly emerging pathogens and changes in environmental conditions
  • design and predict the effects of potential public health interventions.
• to make these predictions, mathematical models include several assumptions about how different variables behave. The accuracy of these models is dependent on these assumptions being met.
• The use of mathematical models to predict disease spread involves close collaboration between mathematicians and biologists.

Question 9

How is an outbreak managed?

Answer 9

1. the first step in managing a disease outbreak is investigating cases to ensure they do meet the definition of an outbreak, this involves confirming he number and diagnosis of known cases and comparing this with background levels of disease
2. investigators can then formulate a case definition of which cases are considered part of the outbreak.

these include the type of illness, place and time

1. the next steps, finding cases, gathering information and forming hypotheses, are easiest if the mechanism of disease transmission is already known
2. investigators then have to find people affected by the outbreak. Tracking down infected individuals is achieved through contact tracing.

• the types of contacts sought will vary with the mechanisms of disease transmission. e.g. sexually transmitted diseases vs. airborne diseases.*
  1. as part of case finding, investigators may be able to track down the index case or the case that started the outbreak
• in the Soho cholera outbreak, the baby Frances Lewis was the likely index case.*
  1. investigators will then gather information from cases. this usually involves extensive interviews that include questions about usual activities, sick contacts, recent meals and travel.
• the aim of these interviews is to generate a hypothesis about how the outbreak is spreading.*
  1. once a hypothesis has been generated the interviewers can search for evidence to support or refute the hypothesis.
• this may require further interviews, site inspections and environmental sampling or analysis of collected data.*
  1. the finals steps of an outbreak investigation are to implement measures to control the spread and communicate the findings.
• these steps may take place while the other steps are ongoing*

Question 10

Discuss epidemiolgy and non-infectious disease

Answer 10

• infectious diseases are a major cause of death worldwide.
• in Australia, infectious disease cause a much smaller proportion of deaths in Australia than the world average. This may be due to:
• access to healthcare
• good sanitation
• clean drinking water
• good nutrition.
• a lack of access to these basic services is a substantial contributor to disease burden and a target for public health interventions
• the principles of epidemiology can also be applied to non-communicable diseases such as cancer or cardiovascular disease, to assess possible causes of disease or test the efficacy of public health interventions