7 The Epidemiology of Cholera - Springer Link

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Since the first pandemic of cholera in 1817 spread through the Middle East to Europe, cholera has been among the most feared of the classic epidemic ...
7 The Epidemiology of Cholera Roger I. Glass and Robert E. Black

1. INTRODUCTION 1.1. Epidemiologic Lessons from the Early Pandemics and Epidemics Since the first pandemic of cholera in 1817 spread through the Middle East to Europe, cholera has been among the most feared of the classic epidemic diseases. l Cholera was highly virulent, decimating entire communities within weeks of its introduction. The disease had a high case-fatality ratio that approached 50% in some areas and spread relentlessly in WOrldwide pandemics from endemic foci in Asia to the Middle East, Europe, East Africa, and the Americas. While cholera epidemics have been extensively described and studied, epidemiologic understanding of the transmission of V. cholerae 01 is still too inadequate to permit effective control measures that would contain the disease and prevent its emergence and spread. 2 Proper and timely rehydration therapy can reduce mortality to less than 1%, and antibiotic treatment can decrease shedding of vibrios, but neither of these treatment measures has significantly altered the spread of disease. Public health measures to control the spread of cholera have been based, until recently, on epidemiologic insights drawn from observations made in the nineteenth century. Preventive measures included the provision of good, safe drinking water, proper disposal of human waste, education and attention to personal hygiene, the quarantine of goods and travelers from infected countries, and vaccination. These measures may be useful at times but have been inadequate to prevent the spread of cholera. These inadequacies underscore the need to understand the epidemiology of cholera more completely. During the early pandemics, the spread of cholera from Asia to Europe followed the routes of travelers and merchants, which suggested that man was an important reservoir of infection. International spread of cholera by ships led to International Sanitary Conventions Cholera, edited by Dhiman Barua and William B. Greenough III. Plenum Medical Book Company, New York, 1992. 129

inconvenience and economic losses. They were mostly ineffective, perhaps because either many individuals were mild cases or asymptomatic ally infected or other modes of transmission were involved. The fecal-oral route of transmission of cholera was identified by early pioneers working with cholera including John Snow 3 and Robert Koch. 4 Each recognized that some agent or poison in human feces was the etiologic cause of disease. Snow demonstrated that cholera was spread by drinking water that was contaminated with fecal wastes. Koch identified the cholera vibrio in intestinal contents of victims who died during outbreaks in Egypt and in India. Rudolph Emmerich, a student of Pettenkoffer, attempted to disprove the etiologic role of cholera vibrios by swallowing a pure culture of the organism. He came down with a severe case of cholera, thereby fulfilling Koch's postulates and confirming both the causative role of the organism and its oral route of inoculation. 5 Early observers noted that when cholera returned to areas that had previously been infected, the population suffered illnesses of lesser severity and decreased fatality. These observations supported concepts of immunity to disease, which were new in the late nineteenth century, and experiments conducted in Spain by Jaime Ferran in 1892.6

1.2. New Concepts in the Epidemiology of Cholera In the past 30 years, results of laboratory and epidemiologic studies have led to a major evolution in thinking about the epidemiology of cholera. First, while humans were long believed to be the only reservoir of V. cholerae 01 , the organism now appears to have a freeliving cycle with a natural reservoir in the environment. 7,8 This means that the control of cholera will not be achieved merely by containing the movement of infected individuals, but will require either altering man's exposure to this previously undetected reservoir of infection or placing more emphasis on control of the secondary spread of disease. Second, cholera was long felt to be spread primarily by drinking contaminated water. Investigations of recent outbreaks have identified raw bivalves (clams, oysters, and mussels) and undercooked shellfish (shrimp, crabs) to be important vehicles of transmission. 9 - 14 Some of these creatures have been harvested at some distance offshore, suggesting that the vibrios maintain a lifecycle that does not require continuous inoculation with human feces. Furthermore, in arid and inland areas of Africa that should be inhospitable to the marine vibrios, the disease has also taken hold, indicating that the organisms may survive under a much broader range of environmental conditions 15, 16 and that transmission may occur by other routes such as person-to-person contact, 17,18,19 nosocomial spread, 18,20 or consumption of foods other than seafoods that are contaminated. 21 Third, numerous studies of family contacts of cholera patients have documented the high rates of asymptomatic infection in many cholera-endemic areas. 5 This makes it nearly impossible to identify the index case responsible for beginning an epidemic or introducing the organism into a new environment. Finally, although much has been learned about cholera transmission from studies of epidemics, similar studies of traditional endemic disease have failed to identify a single predominant mode of spread. This may be due to the presence of multiple routes of transmission, confounding factors such as immunity and asymptomatic infections and the occurrence of transient inocula (e.g., sporadic fecal contamination of food and running water) that are difficult to identify and quantify

2. CHOLERA IN THE SEVENTH PANDEMIC The seventh pandemic of cholera that was caused by the EI Tor biotype of V. cholerae 01 began in Sulawesi in 19612 1,22 (see Figure 1). It extended in yearly waves to the Pacific Islands and Southeast Asia, the Middle East, and the U.S.S.R. In the 1970s, the disease continued to spread through Africa, outbreaks occurred in Europe,IO,13 and isolated cases were identified in coastal areas of the United States bordering on the Gulf of Mexic09,23,24 and in Mexico in 1983. 25 During this pandemic, more than 100 countries have reported cholera and no sign of remission is visible26 (see Figure 2). (See Chapter 1.) As the seventh pandemic of cholera has spread, the EI Tor biotype of Vibrio cholerae 01 has completely replaced its predecessor, the classical biotype. The complete replacement of one strain by another led to much speculation about the biologic advantages of the EI Tor vibrio and the higher rates of asymptomatic infections of the new strain. In 1979, 6 years after EI Tor cholera had completely replaced classical cholera in Bangladesh, the classical strain reemerged. 27,28 The reappearance of the classical strain after years of absence suggests its long-term survival in the country at levels below the threshold of detection. Reasons for its reappearance and current coexistence with EI Tor strains are as poorly understood as reasons for its original displacement. The World Health Organization (WHO) monitors countries reporting cholera in order to follow trends in the disease over time. 26 This surveillance is quite incomplete since many countries with major cholera problems do not report their cases for political, economic, or other reasons including the lack of facilities for surveillance. Even in countries with nationwide surveillance, reporting is incomplete both because of problems of reporting and difficulties in field diagnosis of a disease with a clinical spectrum that ranges from mild, nonspecific diarrhea to severe purging with dehydration. In 1991, WHO was notified of more than 400,000 cases of cholera from more than 45 countries, with 386,805 cases being reported from Latin America (see Figure 3). In 1985, an alternative estimate of the disease burden of cholera was made by a review panel of experts brought together by the National Academy of Sciences of the United States. 29 This panel, using a Delphi method, estimated from their own experience and from a review of the literature that about 5.5 million cases of cholera occur annually in Asia and Africa, 8% of these cases are sufficiently severe to require hospitalization, and 20% of the severe cases would result in deaths totaling approximately 120,000 per year. In endemic settings, the prevalence of severe, dehydrating cholera may appear to be relatively low, as in Bangladesh where the incidence of hospitalization has been 1.0-3.0 cases per thousand people per year for the past 20 years. 30 Such figures must be interpreted with care. First, this incidence occurs for the entire popUlation from the age of 2 years to midadult life, so a person's cumulative risk of severe cholera in the first 20 years of life is about 6%. Given a 20% case-fatality rate, roughly 1% of people living in Bangladesh and exposed to cholera might die if left untreated. Second, studies of family contacts of diarrhea cases demonstrate the broad spectrum of infection and disease. For every individual with severe disease, more than ten will have mild to moderate diarrhea and an equal number will have an asymptomatic infection. The rates of severe disease reported do not properly reflect the more

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prevalent milder cases. Finally, epidemics of cholera in developing countries without endemic cholera may be associated with higher case-fatality rates, both because of the lack of pre-existing immunity in the population and inexperience in administering rehydration therapy. Since the epidemics occur sporadically and unpredictably, their contribution to estimates of disease burden have not been addressed. The case-fatality rates for cholera must be interpreted carefully in view of this known spectrum of disease and the effectiveness of proper rehydration therapy. Case-fatality rates have often been reported as the death rate among patients who were hospitalized (Table 1).31

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