BTE4610 Chapter 6: Epidemiology

Epidemiology

• the study of how often diseases occur in different groups of people and why, and the application of this study to the control of diseases and other health problems

• surveillance and descriptive studies—used to study distribution

• analytical studies—used to study determinants

• epidemiological information is used to plan and evaluate strategies to prevent illness and as a guide to the management of patients in whom disease has already developed

Endemic

• diseases that persist at a moderate or steady state level within a given geographic area

Epidemic

• an unusually high number of cases in excess of normal expectation of a similar illness in a population, community, or region

Pandemic

• a worldwide epidemic

Sporadic

• disease outbreaks that have no pattern of occurrence in a given time or location

Herd immunity

• based on the premise that if a majority of a population (herd) is mostly protected from a disease through immunization or genetic resistance, the chance of a major epidemic is unlikely

• susceptible populations are insulated by healthy populations

• for poliovirus vaccination, over 85% herd immunity prevented epidemic of the virus

Progress of infectious disease 

• illness is worse the more virus there is (higher virus titer)

• incubation period: time between infection with a virus and onset of symptoms; depends on amount of virus you receive; differs by virus and host 

• prodromal period: first appearance of mild or nonspecific signs and symptoms of an illness 

• communicable period: time period when an infected individual or animal is contagious and he/she can directly or indirectly infect another person, animal, or arthropod 

• convalescence: recovery period after an illness

Possible effects that animal viruses may have on cells

• transformation into tumor cell (~50% of cancers are caused by viral infections) 

• cell death and release of virus (lytic infection) 

• latent infection: viruses are inactive (present but not replicating); may revert to lytic infection 

• persistent infection: slow release of virus without cell death 

• cell fusion 

Patterns of diseases

• acute, non-persistent infections 

• acute infections followed by a persistent infection (release of virions from host cell does not result in cell lysis; infected cell remains alive and continues to produce virus indefinitely)

• transformation: conversion of normal cell into tumor cell 

• latent infection—there is a delay between infection by the virus and lytic events

  • HSV1: causative agent of oral herpes/cold sores—genome remains present in the form of an episome (circular DNA molecule independent of the host chromosome) in the nucleus

  • varicella zoster: chickenpox → latency → shingles

Pathogenesis of varicella zoster virus

• shingles are most commonly found in adults over the age of 60 who were diagnosed with chickenpox when they were under the age of 1

• infection via conjunctiva and upper respiratory tract → replication in primary lymph nodes → primary viremia

• → replication in liver, spleen, and other organs → secondary viremia; infection of skin and appearance of rash 

• → infection of sensory ganglia, establishment of latent infection → latency 

• → infection of CNS or re-infection of skin and reappearance of rash 

Transformation 

• immortalization of cells in culture 

• tumor formation in vivo can help develop causation/confirm oncogenicity of an agent 

• mouse NIH 3T3 cells: immortalized mouse fibroblasts which are sensitive to transformation (to cancer cells) by certain viruses → key model for studying oncogenesis 

• transform NIH 3T3 cells with viral proteins → grow in culture → transplant transformed NIH 3T3 cells into nude mice (lack a thymus → deficiency of mature T cells → immunocompromised, unable to reject implanted cells) → nude mice developed tumors that were excised and allowed to grow in a culture dish 

  • can see if this particular viral protein is directly causing cancer