Epidemiology - Vibrio cholerae

Epidemiology

V. cholerae are found naturally in estuary and marine environ-ment worldwide.

◗ Geographical distribution

Cholera continues to be a major health problem in many parts of the world including Indian subcontinent and sub-Sahara in Africa. The condition is rare in the developed and industrious nations for the last many decades. A large num-ber of cases of cholera have been reported in the year 2004 from a total of 56 countries from different parts of the world. A total of 101,338 cases and 2345 deaths were reported from these countries. Majority of cases were reported from Africa and Asia. Cholera occurs as an endemic, epidemic, or a pan-demic disease.

Cholera pandemics: A total of six pandemics of choleraoccurred in the nineteenth century, affecting Europe and the United states, causing more than 115,000 deaths in 1832 and 50,000 deaths in 1856. The seventh pandemic of cholera appeared first in the twentieth century starting in 1961 and affected five continents by 1991. Unlike six earlier pandemics, which were caused by V. cholerae biotype Classical, the seventh pandemic was caused by V. cholerae biotype Eltor. The latter originated in Asia in the 1960s, reached Africa in the early 1970s, and was responsible for epidemic cholera. In 1991, the pandemic reached Peru and subsequently spread rapidly through many countries of South and Central America as well as in the United States and Canada. By mid-1995, more than 1 million cases and 10,000 deaths due to the disease were reported in the United States. The seventh pandemic of chol-era was different from previous pandemics of cholera in the following ways:

I.               It was the first pandemic to be originated from outside Indian subcontinent. It was originated from Sulawesi (Celebes), Indonesia. From here, it spread to Hong Kong and Philippines and entered India in 1964. By 1966, it entered and spread throughout the Indian subcontinent and West Asia. The pandemic then spread to Africa and parts of Southern Europe in 1970s.

II.               It was the first pandemic to be caused by V. cholerae bio-type Eltor in contrast to earlier six pandemics caused by V. choleraebiotype Classical.

III.               cholerae biotype Eltor, responsible for seventh pandemic,caused a less severe illness with a large proportion of mild and asymptomatic infections. Mortality rate was low, but characteristically carrier rate was high.

IV.               cholerae biotype Eltor has nearly replaced the V. cholerae biotype Classical strain following this pandemic spread. Hence, in countries like India, V. cholerae Classical strains are rarely encountered, but in Bangladesh, there has been a resurgence of cholera caused by V. cholerae Classical strains.

In 1992, a new epidemic strain V. cholerae O139 has emerged as a cause of epidemic cholera in India. This new epidemic strain was first reported from an outbreak of cholera in Chennai in the year 1992, followed by similar outbreaks of cholera from different parts of India. The new strain entered Bangladesh and spread across the country, causing epidemics of cholera by January 1993.

◗ Habitat

V. cholerae is a salt water bacterium. Marine ecosystem in associ-ation with plankton is the primary habitat of the bacteria. The bacteria can multiply freely in the water. The number of bacte-ria in contaminated water increases during the warm and hot months of the year. V. cholerae is never found in normal humans. In infected humans, V. cholerae inhabits the small intestine.

Reservoir, source, and transmission of the infection

Humans and water are the two main reservoirs of infection for cholera. Both chronic and convalescent carriers play an impor-tant role as reservoirs in the transmission of infection. These chronic carriers continue to excrete vibrios in their stool inter-mittently for several years.

Animals do not play any role in transmission of the disease. The infection is transmitted to humans by ingestion of con-taminated food or water. Hence, the infection rate is highest in the areas where community hygienic standards are low and potable water is not available. The infective dose of V. cholerae is usually high, more than 106 organisms/mL, because most organisms are killed by high acidity in the stomach. V. cholerae fails to survive in acidic environment. Therefore, any condi-tion (e.g., achlorhydria) that reduces production of gastric acid makes the person highly susceptible to infection. Transmission of infection by direct contact from person-to-person is rare. People of all ages are susceptible to infection by V. cholerae. Infants, however, are protected due to maternal antibodies transmitted during breast-feeding.