PATHOGENS

Microorganisms that cause disease

WHAT IS A PATHOGEN?

A disease is not a single thing

It is a description of certain symptoms

Either physical, mental or both

Disease suggests a malfunction of body or mind

Has mental, physical and social aspects

WHAT IS A DISEASE?

For a microorganism to be considered a pathogen, it must;

Gain entry to the host

Colonise the tissues of the host

Resist the defences of the host

Cause damage to the host tissues

MICROORGANISMS AS PATHOGENS

Pathogens include the following;

Bacteria – eg pathogen responsible for food poisoning

Viruses – eg pathogen responsible for HIV

Fungi – eg pathogen responsible for athletes foot

Infection – when a pathogen gains entry into the host and colonises its tissue

Disease – occurs when an infection leads to recognisable symptoms in the host

Transmission – the transfer of a pathogen from one individual to another

HOW MICROORGANISMS GAIN ENTRY INTO THE HOST

Penetration of one of the organism’s interfaces with the environment

Such an interface is the skin

This forms a thick, continuous layer, which is an effective barrier to infection

Invasion usually occurs when the skin is broken by cuts/abrasions/bites

Some interfaces of the body have evolved to allow exchange of material between the internal and external environments

Therefore, the body linings at such surfaces have the following characteristics;

thin

moist (and therefore sticky)

large surface area

well supplied with blood vessels

Just as these characteristics allow for easy entry of molecules, they also make for easy entry of pathogenic microorganisms

Interfaces of the body are common points of entry and include the following;

The gas exchange system – pathogens that cause influenza, tuberculosis and bronchitis infect through the gas exchange surfaces.

The digestive system – food and water may carry pathogens into the stomach and intestines via the mouth. Cholera, typhoid and dysentery pathogens enter the body by this means.

To help prevent the entry of pathogens the body has a number of natural defences. These include;

A mucous layer – this covers exchange surfaces and forms a thick sticky barrier that is difficult to penetrate

The production of enzymes – breaks down the pathogens

The production of stomach acid – kills microorganism

HOW DO PATHOGENS CAUSE DISEASE?

Pathogens affect the body in two main ways;

By damaging host tissues

Sometimes, the number of pathogen causes damage, eg by preventing tissues functioning properly

Viruses prevent the synthesis of DNA, RNA and proteins by the host cell

Some diseases, such as malaria, have a single cause

Others like heart disease, have a number of causes

Pathogens, lifestyle and genetic factors can all cause disease

By producing toxin

Most bacterial pathogens produce toxins

Eg, the cholera bacterium produces a toxin which leads to excessive water loss from the lining of the intestines

How quickly a pathogen causes damage (and hence the onset of symptoms) is related to how rapidly the pathogen divides.

Pathogens like those causing gastroenteritis divide about every 30 minutes – so symptoms of diarrhoea and vomiting become apparent within 24 hours of infection.

The gastroenteritis pathogen also causes damage only if present in very large numbers.

Pathogens such as the typhoid bacterium, cause harm even in relatively small numbers

DATA AND DISEASE

EPIDEMIOLOGY

Study of the incidence (number of cases) and pattern of a disease

Used to find the means of preventing and controlling disease

What pattern/relationship does this graph show you?

All the lines are more or less at zero at the age of 30

All the lines follow approximately the same pattern – what does this show?

What about the differences between the four coloured lines?

What is the overall interpretation?

ANALYSING AND INTERPRETING DATA ON DISEASE

CORRELATONS AND CAUSAL RELATIONSHIPS

A correlation occurs when a change in one of the two variables is reflected by a change in the other.

Graph shows that there is a correlation between drinking alcohol and breast cancer.

Cannot conclude that drinking alcohol is the cause of breast cancer

LOOKING CRITICALLY AT DATA

Has the right factor been measured and have the correct questions been asked?

How were the data gathered? Was the method reliable?

Do those collecting the data have a genuine interest in the outcome of the research?

Has the study been repeated and the same results found?

Are there still unanswered questions?

LIFESTYLE AND HEALTH

Number of disorders that result from lifestyle choices

In some cases, harmful consequences are known at outset

In some cases, damage may only become apparent later

A measure of the probability that damage to health will occur as a result of a given hazard

WHAT IS RISK?

Concept of risk has two elements;

The probability that a hazardous event will occur The consequences of that hazardous event

This affects how we view risks

We become concerned when probability is high and consequences severe

MEASUREMENT OF RISK

Can be measured as a value that ranges from 0% (no harm will occur) to 100% (harm will definitely occur)

HEALTH RISKS NEED A TIMESCALE

To tell someone their risk of dying is 100% is meaningless

To tell someone that their risk of dying in the next month is 100% has a different meaning altogether

RISK IS OFTEN RELATIVE

Risk is measured by comparing the likelihood of harm occurring in those exposed to a hazard with those who are not exposed to it.

Even when a risk is quantified, there are loads of factors to consider that it is difficult to understand the risk.

For example

smokers may be 15 times more likely to develop lung cancer than non smokers

Over what time period does this occur?

How does the number of cigarettes smoked a day affect the figure?

do stress levels, alcohol intake, occupation, gender, pollution or other factors have an influence?

Does it change depending on where the smokers live?

RISK FACTORS AND CANCER

Cancer is not a single disease and does not have a single cause

Some causal factors are beyond individuals control, e.g. age and genetic factors

Others are lifestyle factors

LIFESTYLE FACTORS AND CANCER

Our lifestyle can expose us to environmental factors and carcinogenic factors that put us at risk of contracting cancer.

Thought that half the people who are diagnosed with cancer in the UK could have avoided getting the disease by altering their lifestyle.

Smoking – smokers and passive smokers at risk of getting cancer.

Diet – evidence suggests that a low-fat, high-fibre diet, rich in fruit and vegetables, reduces the risk

obesity – being overweight increases the risk of cancer.

Physical activity – people who take regular exercise are at lower risk from some cancers.

Sunlight – the greater the exposure to sunlight or light from sunbeds, the greater the risk of skin cancer

LIFESTYLE CHOICES AND CORONARY HEART DISEASE (CHD)

CHD is the largest cause of death in the UK.

There are a number of factors that increase the risk of an individual suffering from CHD.

When combined together, four or five such factors produce a greater risk.

Factors such as genes, sex and age are beyond our control, but there are others we can do something about.

LIFESTYLE CHOICES AND CHD

Smoking –smokers are between two and six times more likely to suffer from CHD. Giving up smoking is the single most effective way to increase life expectancy.

High blood pressure – prolonged stress, certain diets and lack of exercise all increase blood pressure, therefore the risk of CHD.

Blood cholesterol levels – can be kept lower by including fewer saturated fatty acids in the diet.

Obesity – a BMI of more than 25 brings an increased risk of CHD.

Diet – high levels of salt in the diet increase blood pressure, whilst high levels of saturated fatty acids increase blood cholesterol concentration. Both increase the risk of CHD. Foods such as fibre reduce the risk of CHD by lowering blood cholesterol levels.

Physical activity – aerobic exercise can lower blood pressure and blood cholesterol as well as helping to avoid obesity – all of which reduce the risk of CHD.

REDUCING THE RISK OF CHD AND CANCER

Avoid doing all the things that increase the risk!

Giving up or not taking up smoking.

Avoid becoming overweight.

Reducing salt intake in the diet.

Reducing intake of cholesterol and saturated fats in the diet.

Taking regular aerobic exercise.

Keeping alcohol consumption within safe limits.

Increasing the intake of fibre and antioxidants in the diet.