BIO 6.2-6.5 - Disease challenges and strategies

Disease

A condition that impairs the normal functioning of an organ, part, structure or system of an organism

Infectious diseases

Caused by the spread of pathogenic agents between organisms

Non-infectious diseases

Not caused by pathogens and cannot spread between organisms. Instead, they are caused by factors such as genetics, the environment and lifestyle.

Contagious disease

An infectious disease that is readily spread between organisms

Emerging diseases include:

• Diseases caused by newly identified, previously unknown agents

• Diseases with increased incidence, virulence or geographic range over the past 20 years

Re-emerging disease

A disease that reappears after a significant decline in its incidence

Virulence

The ability of a pathogen to infect a host and cause harm

Endemic

The disease is regularly found in a particular area and the number of infections remains constant

Epidemic

The rapid spread of a disease within a restricted geographical area

Pandemic

The spread of disease across several continents

Outbreak

The sudden appearance of a disease in a restricted population

Factors that affect the emergence and spread of disease (10)

• New pathogen emerges

• Mutations form new strains

• Antigenic drift and shift

• Pathogen is easily spread

• Transport and migration

• Lack of herd immunity

• Human behaviour

• Farming practices and food production

• Overuse of antibiotics

• Lack of sanitation and hygiene

Antigenic drift

Minor changes in the pathogen. Small mutations to antigens on the pathogen’s surface accumulate, leading to reinfection in the same species because memory cells cannot recognise the mutated antigens.

Antigenic shift

Major changes to a virus. Two or more different strains of a virus combine to form a subtype with a mixture of surface antigens from the original viruses. This can lead to entirely new viruses which cause infection because organisms lack immunity to it.

Factors that contributed to Indigenous Australians being susceptible to diseases brought by Europeans

• Lack of immunity (no previous exposure)

• Lack of knowledge and experience with treating European diseases

• Disruption in living conditions (forced into close proximity with restricted food and water)

Physical methods of identifying viruses

• X-ray crystallography - determines the structure of viruses

• Electron microscopy - microscopic images of the virus

Immunological methods of identifying viruses

• Direct ELISA test (looks for an antigen) - a primary antibody with an enzyme indicator binds to an antigen on a surface and causes a colour change if a complex is formed

• Indirect ELISA test (looks for an antibody, not an antigen) - A primary antibody attached to a secondary antibody with an enzyme indicator binds to an antigen on a surface

Molecular methods of identifying viruses

• PCR amplifies DNA or RNA and the sequences are analysed

• Gene probes (DNA strands complementary to the target sequence) can be added to detect specific viral genetic sequences

• DNA sequencing

Phenotypic methods of identifying bacteria

• Microscopes can identify shape, size, stain (gram positive or negative) and physical features

• Use of different nutrient media to differentiate bacterial type by variations in growth patterns

• Use of biochemical tests to elicit different responses

Immunological methods of identifying bacteria

• Monoclonal antibodies specifically bind to the bacterial antigen to identify the bacteria

• Serology - diagnosis of a disease based on the presence of antigens or antibodies in a person’s serum (ELISA test)

• Immunofluorescence uses an antibody with an attached fluorescent marker. Antibody-antigen complexes can be identified using UV light.

Genotypic and molecular methods of identifying bacteria

• Radioactively tagged gene probes identify specific bacterial gene sequences

• DNA sequencing

• Plasmid fingerprinting

Resevoir

The habitat in which a pathogen lives, grows and multiplies (e.g. humans, animals, plants, soil and water)

Modes of transmission (5)

1. Direct physical contact - touching an infected host

2. Indirect physical contact - spread through food, water, surfaces, vectors

3. Airborne transmission - pathogens are spread through the air by sneezing, coughing, talking and breathing

4. Droplet transmission - droplets fall onto surfaces which are touched, then the person touches mucosal surfaces of their body

5. Faecal-oral transmission - contamination of food or water by faecal matter

Methods of controlling disease transmission

1. Prevention - practicing good hygiene, hand washing, disinfection, wearing gloves and masks, public education

2. Isolation and quarantine - separating infected people from others and restricting their movement

3. Screening - routine testing for the presence of a disease in a population

4. Vaccinations

5. Eradication of vectors

Scientific strategies to control disease transmission

Interventions that act to reduce transmission (e.g. vaccinations, sterilisation, quarantine)

Social strategies to control disease transmission

The policies put in place to SUPPORT the effectiveness of scientific strategies (e.g. advertisement campaigns, education programs, passing laws, providing government funding)

Herd immunity

A form of immunity that occurs when the vaccination of a significant portion of a population provides protection for individuals who have not developed immunity

• If more people are vaccinated, there is less chance of infection spreading throughout a population

Treatment for diseases

Viruses - antivirals inhibit the reproduction of viruses

Bacterial disease - antibiotics kill bacteria or inhibit their growth

Fungal diseases - fungicides

Prion diseases have no treatment