pathology Flashcards
(4 cards)
What is infectious disease and what is the primary impact of infectious disease on human populations?
Infectious disease arises from pathogen-host interaction, such that the host acts as a resource for pathogen life-history strategies. These diseases (e.g. pneumonia, HIV/AIDS and rotavirus) remain major killers in LEDCs, with vaccination efforts often hampered by logistical challenges (e.g. issues with cold chain transport in sub-saharan Africa). However, hosts may evolve in response to pathogen-mediated selective pressure, as indicated by the prevalence of sickle cell anemia in countries where malaria is prevalent (as the mutation in heterozygous form confers resistance to malaria).
What are some examples of differing pathogen life-history strategies impacting host exploitation?
The host serves as a resource for the pathogen, used to facilitate growth and reproduction. Measles infection, for example, is characterised by a rapid increase in viremia (bloodstream viral load), rapid symptom manifestation (fever, rash) and rapid cross-host transmission before host immune response can resolve viremia to an undetectable volume of viral RNA (approx 14 weeks). The initial viremia peak of HIV infection is partially controlled by the immune system, such that viremia decreases but is not eliminated. HIV infections enter an eight-year period of clinical latency during which HIV virions preferentially attack CD4 T cells, resulting in a point in which immune control is lost and viremia peaks once more (AIDS).
How can SI models be used to model disease spread?
Within the simple susceptible-infected (SI) model, compartments represent host states, with the proportion of infected individuals represented (y). Each time a host is colonised, that host becomes unavailable for further infection for a particular strain (susceptible hosts are becoming consumed by the pathogen, leading to a population-level reduction in host resources - analogous to host immunity). These models code for the probability of infection for an uninfected individual. Without accounting for recovery or death, these models predict infection spread throughout the entire susceptible population over a given time-course (y=1).
dy/dt = λ(1-y)
λ = per capita rate of transition between states
How do SI models vary with respect to pathogen life-history strategy?
If the pathogen is directly transmitted (e.g. influenza), population size impacts the rate/probability of transmission (as contact with conspecifics increases the likelihood of infection).
