Cholera

Cholera in short

• an acute, extremely virulent diarrhoeal infection
• caused by the bacterium Vibrio cholerae
• infection is asymptomatic in 75–80% of people
• of those showing symptoms, the majority develop only mild disease
• among the ill, 10–20 % are severely affected
• without proper treatment, death can occur within hours due to severe dehydration
• a vaccine is available but is not yet routinely used in mass vaccination campaigns

Cholera in detail

Pathogenic agent
Vibrio cholerae is a gram negative, rod-shaped bacterium belonging to the Proteobacteria. In addition to humans, the natural reservoirs of the pathogen are brackish waters and estuaries. Two serogroups, O1 and O139, are responsible for epidemics among humans, with the former causing the majority of outbreaks; O139 is currently detected only in Southeast and East Asia. Other serogroups only cause mild disease and no epidemics.

Transmission/pathogenesis
The bacterium is transmitted mainly through contaminated water or food. Direct transmission from one person to another only rarely occurs. However, infected individuals - whether they show signs of disease or not - shed the pathogen with their faeces. Thus, V. cholerae can spread rapidly among people in areas with inadequate wastewater and drinking water treatment, such as slums or overcrowded camps.

After ingestion, V. cholerae that successfully pass the stomach colonize the gastrointestinal tract, adhering to villi and releasing the cholera enterotoxin. This leads to the activation of the enzyme adenylate cylase and results in disruption of the electrolyte metabolism.

Symptoms
Incubation time is extremely short in V. cholerae infections, sometimes lasting only hours. Abdominal pain, profuse watery diarrhoea and vomiting are the characteristic signs of disease. Rapid loss of body fluids leads to leg cramps, severe dehydration, kidney failure and shock. Without treatment, death can occur within hours.

Not every infected person develops symptoms; about 80% remain symptom-free but can still excrete the pathogen. Among the ill, approximately 20% develop severe symptoms and when left untreated, up to 50% of these may die. Treatment reduces the death rate to below 1%.

Treatment
Replacement of lost fluids and salts is the main therapy. Patients can be given oral rehydration solution – sugar and salts dissolved in water. Severe cases might require intravenous fluid replacement. Antibiotics can be given to shorten the duration of diarrhoea and excretion of the pathogen.
 
Prevention
The most important factor in cholera prevention is the adequate treatment of drinking and sewage water – still a critical determinant in many developing countries. Health education, e.g. the promotion of hand washing, and food hygiene are equally important in reducing the risk of infection.

Vaccination
Several oral cholera vaccines (OCV) are available. One (Dukoral®) has proven safe and effective and is internationally licensed. It has been used primarily by travellers and in certain emergency settings, but the World Health Organization (WHO) is currently evaluating its potential use in endemic settings. The vaccine, a mixture of recombinant B-subunit of cholera toxin and killed whole-cell V. cholerae (rBS-WC), is administered twice one week apart and needs some time to reach full protective efficacy. In addition, the costs for the drug and its distribution are high, and the WHO does not regularly recommend the vaccine once an outbreak has started.

Incidence and mortality
Throughout history, cholera outbreaks have repeatedly affected populations worldwide. The disease is endemic in the plains of the Indian Ganges River, where it has been known since antiquity. During the 19th and 20th centuries, it spread to other continents in seven pandemics, killing millions of people in Europe, Africa and the Americas. According to the WHO, the seventh pandemic started in 1961 in South Asia, reaching Africa in 1971 and the Americas in 1991. It is still ongoing – and experts state that there are currently no signs that it will end soon.

Cholera is now endemic to a large number of developing countries and a few years ago, public health authorities noted an apparent re-emergence of the disease: in 2006, the number of reported cholera cases increased 79% compared to 2005, with 236,896 cases in 52 countries and 6311 deaths. In 2007, the number of officially notified cases decreased by 25% compared to 2006, with 177,963 cases and 4031 deaths reported worldwide. The actual number, however, is believed to be much higher, as only a small portion (<10%) of cases are reported to the WHO.

More information:
- WHO: www.who.int/topics/cholera/en/
- The Global Task Force on Cholera Control: www.who.int/cholera/en/index.html
- The Centers for Disease Control and Prevention:
www.cdc.gov/nczved/dfbmd/disease_listing/cholera_gi.html

Literature:
- WHO fact sheet on cholera: www.who.int/mediacentre/factsheets/fs107/en/index.html
- The Centers for Disease Control and Prevention:
www.cdc.gov/nczved/dfbmd/disease_listing/cholera_gi.html
- Robert Koch Institute: www.rki.de/cln_100/nn_504462/DE/Content/InfAZ/C/Cholera/Cholera__node.html
- University of Florida, Medical Microbiology and Infectious Disease: www.snaption.net/med/cholerae.htm
- Cholera annual report 2007: www.who.int/wer/2008/wer8331.pdf