Cholera Case Study Questions and Answers

"As the epidemic advanced, the disproportion between the number of cases in houses supplied by the Southwark and Vauxhall Company and those supplied by the Lambeth Company, became not quite so great. . . ." From an epidemiological point of view, why was this so? Give detailed epidemiological reasoning and discussion addressing person, place, and time issues.

The epidemic was the result of contaminated water from the Southwark and Vauxhall Company. Only rarely is cholera spread from person to person, but some of this may have occurred as the epidemic progressed. The most likely explanation is that people serviced by the Lambeth Company could have been exposed to water in the homes of people serviced by Southwark and Vauxhall Company.

Several instances of causal association were presented or alluded to in the examples presented in the case. List the various instances of association you can identify from the examples or situations presented in the case. What role did social class, poverty, and housing arrangements play in association? What role did water play in association?

Association was found in the following cases: food infected by those who had cholera; water infected by feces or cholera evacuations that was then used for cooking and drinking; involuntary passage of evacuations from infected individuals; poverty was found to be a major risk-factor for the spread of disease because poor people lived in cramped conditions where cooking and eating took place in the same room as the sick person; and lack of sanitary conditions including hand washing.

Snow suggested several reasons for the failure of contaminated water to produce disease in all who consume it. Why is it that not everyone who consumes cholera pathogens gets ill or dies?

Boiling water kills cholera germs. The amount of cholera in the water at any given time will influence the chance of ingestion. The pathogen will sink to the bottom of the water if left to settle.

What are the various aspects of the person presented in the above water company case concerning the epidemic of cholera, especially as the person concept relates to the households which were recipients of the supplies of the water as compared to those who used different water supplies?

Cases reflected persons who died from cholera during the first seven weeks of the cholera epidemic and who lived in districts that received water from Southwark and Vauxhall Company or Lambeth Company. Cases were compared to the number of households in these districts in order to calculate deaths per 10,000 houses.

What are the time factors and implications for this case? Explain the time lag from Attacks in Figure I.2 to Deaths in Figure I.3.

Cholera spreads quickly and the incubation period is relatively short, generally 24-48 hours. This is illustrated in Figures 1.2 and 1.3 by the lag between attacks and deaths. The attacks generally occur 1-2 days before the death, as noted on the figures.

What epidemiological phenomenon can be observed in Locksbrook, near Bath, example?

Cholera was not necessarily contracted through person-to-person contact, but could be contracted through a point source (ie, contaminated water).

Describe the disease cholera as presented in the Snow case. Describe cholera as it is known today.

Cholera, as described by Snow, was a disease that was first introduced at a sea port, and was then spread though human interaction. Its transmission could occur through communication from a sick to a healthy person. Snow hypothesized that a cholera agent was ingested and then reproduced in the stomach and bowels. The agent served as an irritant to the stomach, or else it withdrew fluid from the blood circulating in the capillaries. It was characterized by a short incubation period and fast spreading. Cholera is an acute, diarrheal illness caused by infection of the intestine with the bacterium Vibrio cholerae. The infection can be mild or without symptoms. The infection can also be severe (approximately one in 20 infected persons). Severe disease is characterized by profuse watery diarrhea, vomiting, and leg cramps, with often rapid loss of body fluids leads to dehydration and shock. Death can occur within hours if treatment is not received.

What are the epidemiological implications of Snow's observations about sizes of cities and length of epidemics?

Duration of cholera in a place is in direct proportion to the number of the population.

Concerning the water supply system and structure from the two different water companies, explain the soundness of the research approaches. Is this a descriptive or analytic epidemiologic research design?

Given that the same districts were serviced by both water companies in a somewhat random fashion, Snow was allowed to compare cholera deaths according to where they got their water supply without the threat of confounding factors explaining the difference. This is an analytic epidemiologic study because a comparison group is involved.

What were some of the practical problems and barriers that Snow faced which slowed his inquiry of the cholera epidemic of 1854? How would this relate to modern day epidemiological investigations?

Identifying the water supply company that serviced households where a case was present was complicated by the fact that residents seldom remembered the name of the water company. Receipts and information from landlords were pursued in order to identify the household's water source. When this information was not attainable, Snow performed a chemical test on the water to identify the water supply. Obtaining accurate exposure and disease status must always be a primary aim in order to avoid biased results.

Concerning John Barnes, how was cholera communicated? What were the modes of disease transmission? What is the correct epidemiological term for the modes of transmission that were identified?

John Barnes contracted cholera through his infected sister. Her infected clothing was sent to Barnes without being washed; he contracted cholera upon opening the box and touching the clothing. Fomite transmission was involved in the actual transfer.

In the examples or circumstances presented, what were the various modes of transmission stated or at least alluded to? Which were correct? Which were incorrect and why? What role did personal hygiene and sanitation (including food preparation and hand washing) play in the transmission of the disease and continuation of an outbreak?

Primary mode of transmission was person-to-person contact. Effluvia given off from a sick person was also mentioned as a mode of transmission. Snow hypothesized that affection began first with the alimentary canal. Thus, personal hygiene and sanitation played a role in transmission because people were in contact with infected individuals and did not wash their hands properly. When they prepared or ate food, cholera was transmitted to the stomach and bowels. Bed linens that hosted cholera "evacuations" also were a mode of transmission. Infected water was seen as a transmitter.

What are the place factors and implications for this case? Compare the workhouse to the brewery to the pub. Discuss migration and its effect on the epidemic. What place issues are important to this case?

Proximity to the pump affected cholera contraction. Those who lived close to the pump were more likely to get their water from the Broad Street pump and thus to contract cholera. The few areas not affected were the brewery and the workhouse. The brewery served workers beer and used a deeper well for water, so the workers did not contract cholera. The workhouse had a pump well on site and Grand Junction Water Works supplied their water so that only a few contracted the disease. Migration also affected the spread of the disease; when people were threatened by the disease, they moved away decreasing their likelihood of contracting and spreading the disease.

What hypotheses were developed by John Snow about the cause (etiology), signs and symptoms, spread and course of the cholera disease? How do the observations and hypotheses of Snow conform with modern understanding and knowledge of cholera?

Snow believed that cholera was caused by morbid material entering the alimentary canal, which then moved to the stomach and bowels where the material reproduced and acted as an irritant. Alternatively, the poison withdraws fluids from the blood circulating in the capillaries. He believed that cholera could be transmitted through person-to-person contact and infected water.

What were Snow's initial and basic hypotheses concerning the epidemic? What processes and procedures did Snow use to establish his hypotheses (prove or disprove them) about the outbreak?

Snow hypothesized that the source of the cholera epidemic was the water source. To form his hypothesis he talked with patients, family members, and friends to reveal their water habits. He observed the water content for bacteria or "poison." He also plotted the cases to observe the place factors in the epidemic.

What evidence did Snow use to establish the fact that a cholera epidemic was occurring? Did Snow clearly demonstrate the cause and source of the outbreak of cholera in the Golden Square area? Explain.

Snow plotted the cases of cholera on a map to show that cholera was especially prevalent in areas directly surrounding the pump on Broad Street. He talked with survivors, relatives of deceased, and those who were surprisingly unaffected by the epidemic to determine what was causing it. He asked if those that contracted cholera had drunk water from the Broad Street pump. He also observed samples of water from the pump and found that some people had noted the foul smell of the water or the film that formed on it as the epidemic was occurring.

Several different theories and hypotheses have been presented. Which of these are consistent with known scientific and biomedical knowledge and common sense?

Some alternative theories associated with cholera were: --"elevation of the ground" --"communication by effluvia given off from the patient into the surrounding air, and inhaled by others into the lungs" --"an unknown something in the atmosphere which becomes localized, and has its effects increased by the gases given off from decomposing animal and vegetable matters" --cholera poison believed to "exist in the air about the sick, requiring the existence of calcareous or magnesian salts in the drinking water to give it effect." None of these alternative theories were grounded in scientific or biomedical knowledge and few were derived through a common sense approach! The existence of the contaminated water and the proximity to the water from specific pumps (of those who died and consumed/bathed/used) showed an association between the disease, death and the source of the water. The true causative nature was not determined until the water was more thoroughly inspected and the cholera bacterium was identified as the pathogen or infectious agent.

From Figure I.2, what is the index case? What date is the beginning of the epidemic? What other time factors are discerned from this chart? How did Snow establish the time of onset?

The index case is on August 31. The date of the beginning of the epidemic is the night between August 31 and September 1. There is also a visible time lag between attacks and deaths. It is also evident that the epidemic came and went quickly, partially because of migration and protective measures such as taking the handle off the pump.

What principles were correct and what were incorrect in Snow's observations about time and season and the effects on epidemics? Why?

The primary source of cholera is by drinking water or eating food contaminated by the cholera bacterium.

The obvious control measure applied was the removal of the handle from the Broad Street pump. What role did the removal of the pump handle play in the decline of the epidemic? Did the removal of the pump handle have an effect on the control epidemic? What other social and political roles did removing the handle from the pump play?

The removal of the pump handle was an effective strategy in reducing the prevalence of cholera. The flight of the population also reduced the cholera epidemic. By the time the pump handle was removed, it may have been that the microorganism leading to the cholera was no longer in the well water. Removing the handle showed that the political leaders were responsive to epidemiologic information and concerned about the public's health.

What did the well people do differently than the ill—those who got the disease (for example the inmates in the workhouse)?

The well (not sick) people did not drink water from the Broad Street pump; they drank from other pumps. They also fled to areas that were not infected with cholera to protect themselves.

How long is a fortnight?

Two weeks

What is different about those who develop cholera compared with those who do not?

Those who get the disease are more likely to be poor and/or live in cramped housing. They are less likely to have adequate sanitary conditions and less likely to follow optimal hygiene practices.

What accurate observations were made about wells, cesspools, ground water, purity of the water, And its contamination? What inaccurate and misunderstood observations were made about the water, its flow, and its contamination?

Wells may have cracks or holes which allow the entrance of impurities, contaminating the water source. Ground water or streams could run alongside sewage and become contaminated that way. Cesspools, if not emptied, can leak and penetrate springs or other water sources. Snow observed animalcules that organic matter could live on. He also observed a large amount of chlorides, indicating an impure source.