Lesson 1: Introduction to Epidemiology
- Having identified a cluster of cases never before seen in the area, public health officials must seek additional information to assess the community’s health. Is the cluster limited to persons who have just returned from traveling where West Nile virus infection is common, or was the infection acquired locally, indicating that the community is truly at risk? Officials could check whether hospitals have seen more patients than usual for encephalitis. If so, officials could document when the increase in cases began, where the patients live or work or travel, and personal characteristics such as age. Mosquito traps could be placed to catch mosquitoes and test for presence of the West Nile virus. If warranted, officials could conduct a serosurvey of the community to document the extent of infection. Results of these efforts would help officials assess the community’s burden of disease and risk of infection.
- West Nile virus infection is spread by mosquitoes. Persons who spend time outdoors, particularly at times such as dusk when mosquitoes may be most active, can make personal decisions to reduce their own risk or not. Knowing that the risk is present but may be small, an avid gardener might or might not decide to curtail the time spent gardening in the evening, or use insect repellent containing DEET, or wear long pants and long-sleeve shirts even though it is August, or empty the bird bath where mosquitoes breed.
- What proportion of persons infected with West Nile virus actually develops encephalitis? Do some infected people have milder symptoms or no symptoms at all? Investigators could conduct a serosurvey to assess infection, and ask about symptoms and illness. In addition, what becomes of the persons who did develop encephalitis? What proportion survived? Did they recover completely or did some have continuing difficulties?
- Although the cause and mode of transmission were known (West Nile virus and mosquitoes, respectively), public health officials asked many questions regarding how the virus was introduced (mosquito on an airplane? wayward bird? bioterrorism?), whether the virus had a reservoir in the area (e.g., birds), what types of mosquitoes could transmit the virus, what were the host risk factors for infection or encephalitis, etc.
- Third criterion may be limiting because patient may not be aware of close contact
- Probably reasonable
- Criteria do not require sophisticated evaluation or testing, so can be used anywhere in the world
- Too broad. Most persons with cough and fever returning from Toronto, China, etc., are more likely to have upper respiratory infections than SARS.
The following tables can be created from the data in Tables 1.5 and 1.6:
|Persons at risk||143||107||212||179||173||499|
|Death rate (%)||6.3||12.1||62.3||67.0||85.5||88.4|
|Persons at risk||462||851||1,313|
|Death rate (%)||33.3||83.3||65.7|
|Persons at risk||83||1,230||1,313|
|Death rate (%)||37.3||67.6||65.7|
By reviewing the data in these tables, you can see that men (see Table B) and adults (see Table C) were more likely to die than were women and children. Death rates for both women and men declined as socioeconomic status increased (see Table A), but the men in even the highest socioeconomic class were more likely to die than the women in the lowest socioeconomic class. These data, which are consistent with the phrase “Women and children first,” represent the mortality experience of passengers on the Titanic.
Data Sources: Passengers on the Titanic [Internet]. StatSci.org; [updated 2002 Dec 29; cited 2005 April]. Available fromhttp://www.statsci.org/data/general/titanic.html.External .
Victims of the Titanic Disaster [Internet]. Encyclopedia Titanica; [cited 2005 April]. Available fromhttp://www.encyclopedia-titanica.orgExternal.
Note: the precise number of passengers, deaths, and class of service are disputed. The Encyclopedia Titanica website includes numerous discussions of these disputed numbers.
- Agent: Bacillus anthracis, a bacterium that can survive for years in spore form, is a necessary cause.
- Host: People are generally susceptible to anthrax. However, infection can be prevented by vaccination. Cuts or abrasions of the skin may permit entry of the bacteria.
- Environment: Persons at risk for naturally acquired infection are those who are likely to be exposed to infected animals or contaminated animal products, such as veterinarians, animal handlers, abattoir workers, and laboratorians. Persons who are potential targets of bioterrorism are also at increased risk.
- Component cause
- Necessary cause
- Component cause
- Sufficient cause
Reservoirs: humans and possibly monkeys
Portals of exit: skin (via mosquito bite)
Modes of transmission: indirect transmission to humans by mosquito vector
Portals of entry: through skin to blood (via mosquito bite)
Factors in host susceptibility: except for survivors of dengue infection who are immune to subsequent infection from the same serotype, susceptibility is universal