9F PP - Evolving Pathogens

U4 AOS2

Evolving Pathogens

Pathogens constantly evolve and may become more contagious, more virulent, or able to infect humans. If they spread quickly, they can cause an outbreak, epidemic or pandemic.

Antibiotic Resistance

Antibiotic resistance occurs when antibiotics are no longer effective against bacteria. Widespread antibiotic use creates a positive selection pressure, allowing resistant bacteria to survive and reproduce.

Natural Selection

Antibiotic resistance occurs through variation, selection pressure, selective advantage and heritability. Mutations create resistance alleles, antibiotics select for resistant bacteria, and resistant bacteria pass on the allele, increasing its frequency. Antibiotics do not cause resistance because the variation already exists.

Bacterial Conjugation

Bacteria can transfer resistance genes between each other through bacterial conjugation, where plasmids are transferred through cell-to-cell contact. This can rapidly spread resistance, even between different species

Causes of Resistance

Resistance is encouraged by incomplete treatment, inappropriate antibiotic use and widespread use. These increase the survival and selection of resistant bacteria.

Treating resistant bacteria

Resistance is encouraged by incomplete treatment, inappropriate antibiotic use and widespread use. These increase the survival and selection of resistant bacteria.

Antigenic Drift

Antigenic drift involves small, gradual mutations in genes coding for viral surface antigens. As mutations accumulate, memory cells may no longer recognise the virus, which is why flu vaccines are updated annually.

Antigenic Shift

Antigenic shift involves sudden, significant changes to viral surface antigens, often when different viral strains infect the same host and recombine. This can create a completely new subtype with little existing immunity, potentially causing an epidemic or pandemic.

Swine Flu

The 2009 Swine Flu pandemic resulted from antigenic shift, involving recombination of classic swine, human and avian influenza strains.

Treating Viruses

Antigenic drift and shift make viruses difficult to prevent and treat because their surface antigens continually change. Vaccines cannot be designed against a new virus caused by antigenic shift until it appears.

Antiviral Drugs

Antiviral drugs interfere with the viral life cycle and are mainly recommended for people who are very sick or at high risk. Their use is limited to reduce the chance of viruses developing resistance

Key Differences

Drift = gradual changes to existing viral antigens.
Shift = sudden major changes that can create a new viral subtype.
Antibiotic resistance = changes to the genome of an existing bacterial pathogen.

Challenges

Bacterial resistance and viral evolution make infections harder to treat. Scientific challenges include developing new treatments, while social challenges include drug misuse/overuse and disease spread through global movement, leading to longer hospital stays, higher medical costs and increased potential deaths.