BIO220H1 Lecture 11: Final Topics in Evoluntionary Medicine

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12 Feb 2019
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Final Topics in Evolutionary Medicine
Pathogen evolution in response to vaccination
Myopia: a consequence of mismatch with the current environment?
Fever: disease or defense?
Vaccines
Vaccines are biological treatments meant to improve immune
responses to future exposures to specific diseases.
In essence, antigens in the vaccine prime the immune system so that it responds
quickly to future assaults by those same (or very similar) antigens.
Making vaccines is an evolutionary process.
Smallpox
Smallpox is an orthopox virus that was once widespread and led to
sometimes fatal infectious disease.
Spread by inhalation of airborne virus particles or direct contact
with contaminated material.
A worldwide vaccination program eliminated the disease.
Last case in Canada was in 1946, and in the world 1977 (Somalia).
The smallpox vaccine
In the 1700’s it was common knowledge that milkmaids did not get
smallpox.
Milkmaids did get a much less virulent disease, cowpox (a related
orthopox virus).
In 1796 Jenner began exposing patients to the puss from blisters of
infected milkmaids.
He subsequently determined those patients were immune to smallpox.
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Vaccine preventable diseases
Anthrax
Cervical Cancer
Diphtheria
Hepatitis A
Hepatitis B
Haemophilus influenzae b
Human Papillomavirus (HPV)
Influenza (Seasonal Flu)
Japanese Encephalitis (JE)
Lyme Disease
Measles
Meningococcal
Monkeypox
Mumps
Pertussis (Whooping Cough)
Pneumococcal
Poliomyelitis (Polio)
Rabies
Rotavirus
Rubella (German Measles)
Shingles (Herpes Zoster)
Smallpox
Tetanus (Lockjaw)
Tuberculosis
Typhoid Fever
Varicella (Chickenpox)
Yellow Fever
Deaths in children under 5 from vaccine preventable diseases
Access to, and delivery of, vaccines remain public health priorities.
Evolution in a vaccinated world
Antibiotics are failing with increasing frequency, due to
evolutionary responses. What about vaccines?
Immune systems are sources of massive selection on pathogens (e.g., influenza).
By activating an immune response, vaccination can be
viewed as a potential source of selection on pathogens.
Given widespread vaccination, should we be worried about
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its evolutionary effects?
Evolution in a vaccinated world?
Evolutionary response in the pathogen resistant
Two case studies
Hepatitis B
Malaria
Hepatitis B virus
Globally significant cause of hepatitis, liver cirrhosis, and liver cancer.
Vaccine targets the ‘a’ determinant (a major surface antigen).
In the 90s, there was concern of vaccine resistance: a large clinical
trial found a number of vaccinated individuals ultimately acquired infections.
These infections were caused by a vaccine resistant mutant.
Key mutation in the viruses single nucleotide mutation
GlyArg
Could not be seen by the immune system
Mutant allele increased in frequency in vaccinated people.
Thus, the HBV populations are evolving in response to vaccine.
BUT! These ‘vaccine resistant’ strains have not become a major public
health problem. Health benefits of vaccination have continued.
Possible explanations
Mutant strains are outcompeted in unvaccinated hosts?
(i.e., there is a cost to resistance)
Vaccine still protects against severe disease?
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