Epidemiology & control of cholera
** Cholera is an acute, diarrheal illness caused by infection of the intestine with Vibrio cholerae. Infectious Agent: Vibrio.Cholerae Comma shaped , Gram negative , aerobic bacilli (may be straight after lab culture), Polar Flagellum
* Vibrio cholerae
* Classification of v.cholerae Vibrio .cholerae species can be classified according to: A. Biotypes: V. Cholerae has 2 major biotypes: classical and El Tor. Currently, El Tor is the predominant cholera pathogen worldwide.* B. Serotypes: V. Cholerae strains share H antigens; have different 0 antigens. The differentiation of the 0 antigen that allows for separation into pathogenic and nonpathogenic strains. V.Cholerae 01: This type is pathogenic and toxigenic Three types: Ogawa (A,B) Inaba (A,B) Hikojima (A,B,C)
V. Cholerae 0139 Bengal ( appeared first in India in 1992): V.cholerae 0139 differs from 01 strains in lipopolysacharide structure (LPS) and in producing capsular antigen. Toxigenic ( produces the same cholera toxin as V.cholerae 01).
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Atypical V.cholerae 01: non pathogenic and non toxigenic. V. Cholerae non 01: Vibrios that are biochemically indistinguishable from V.cholerae 01 but do not agglutinate in its antiserum. They are also known as Non-Agglutinable Vibrios (NAGs) or as Non-Cholera Vibrios (NCVs). Some are toxigenic
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* Determinants of survival of Vibrios Survive in water for 4-7 days Does not tolerate drying & acidic conditions Survives better in brackish water than in fresh water Reservoir of infection Man is the main reservoir of infection Aquatic reservoir: e.g seafood, plankton or water plants may exist Source of infection During the cholera season -Clinical cases , asymptomatic infection & carriers
* Since 1817, there have been 7 cholera pandemics. The first pandemic broke out in 1817. The last pandemic (the 7th ) broke out in 1961. The first 6 pandemics occurred from 1817-1923 and were caused by V. cholerae, the classical biotype. The pandemics originated in Asia with subsequent spread to other continents.
EPIDEMIOLOGY
The seventh pandemic broke out in 1961 . It began in Indonesia and affected more countries and continents than the previous 6 pandemics. It differed from previous pandemics by : a. It was caused by V. cholerae El Tor. b. Modern therapy markedly reduced case fatality rate.
In October 1992, an epidemic of cholera emerged from Madras, India as a result of a new serogroup (0139). Some experts regard this as an eighth pandemic. Currently: Cholera is endemic in areas of poor sanitation like India and Bangladesh Common in India, Sub-Saharan Africa, Southern Asia Very rare in industrialized countries
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Epidemiological types of Cholera
Epidemic type: Occurs in previously uninfected areas with sero-negative population Affects all age groups equally Associates with single mode of spread Associates with low rate of asymptomatic infection There is no environmental reservoir of infection*
Endemic type: Incidence is highest among 2-15 years of age. yearly seasonal outbreaks are prominent. Transmission is associated with environmental aquatic reservoir Multiple modes of spread Frequent asymptomatic infections leading to high antibody titer by 20 years of age
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* Mode of TRANSMISSION
Cholera is transmitted primarily through ingestion of contaminated water or food. Transmission of cholera is characterized by the following : High infective dose (108-1010 organisms) Person to person spread by direct contact is unlikely mode of transmission (since it is impossible to ingest such high inoculums by this mode)For the same reason fomites and flies are not important modes of transmission. Contamination of water or food provides the opportunity for multiplication and intake of the inoculums. In USA most sporadic cases of infection follow the ingestion of raw or under cooked sea food from polluted water(eating raw or undercooked shellfish) .
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PATHOGENESIS
V.Cholerae remains in lumen of small intestine (does not invade the intestinal wall). V. cholerae causes clinical disease by producing an enterotoxin Which acts locally & promotes the secretion of fluid and electrolytes into the lumen of the gut. Fluid loss originates in the duodenum and upper jejunum; the ileum is less affected.*
* Incubation period Few hours- 5 days ( usually 2-3 days) Clinical features The disease is characterized by a sudden onset of profuse painless watery diarrhea (rice water stool), occasional vomiting. Followed by rapid dehydration , acidosis and circulatory collapse. Fever is typically absent
It can be rapidly fatal (death may occur within few hours ). In untreated severe cases the case fatality rate may reach 30-50%, but with proper treatment the death rate is below 1%. Generally asymptomatic infection is much more than clinical illness , especially with organisms of the Eltor biotype. Ratio of infected cases: clinical cases For El Tor 36:1 Classical 4:1
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* Severity of the disease is also determined by Biotype of V.cholerae. % of severe cases (requiring hospitalization): -El Tor 2%, -Classical 11%
Risk Factors for Severe cholera
Close contact with a cholera patient Low gastric acidity (natural or acquired whether surgically induced or due to the use of antacids) Old Age ( because gastric acidity declines in old age) Absence or low titre of vibriocidal antibodies Those who are malnourished*
* Blood type : A person's susceptibility to cholera (and other diarrheas) is affected by their blood type. Those with type O blood are the most susceptible. Those with type AB are the most resistant. Between these two extremes are the A and B blood types, with type A being more resistant than type B. Absence of breast feeding
* Diagnosis Cholera should be suspected when patients present with watery diarrhea,& severe dehydration. Stool culture: diagnosis is confirmed by isolating V.cholerae of the serogroups 01 or 0139 from stool or rectal swabs of patients. For clinical purposes , a quick diagnosis can be made by dark field microscopic visualization of the characteristic vibrio motility , (moving like shooting stars), which is inhibited by specific antiserum.
* Serological by demonstrating a significant rise in titre of antitoxic and vibriocidal antibodies.
* Period of Communicability
Presumably for the duration of the stool positive stage, Usually for only few days after recovery. -By end of first week, 70% of patients non-infectious -By end of third week, 98% non-infectious Occasionally the carrier state may persist for several months. Very rarely chronic biliary infection lasting for years , has been observed in adults with intermittent shedding of vibrios in the stool. The carrier state in cholera El Tor may last for more than 12 years* Antibiotics shorten the period of communicability. The El Tor biotype produces a higher carrier state than the classical biotype Control of cholera Although cholera can be life-threatening, it is easily prevented and treated. In countries with advanced water and sanitation systems, cholera is not a major threat; however, even in such areas ,everyone especially travelers, should be aware of how the disease is transmitted and what can be done to prevent it.
* General preventive measures
Provision of safe water supply: Protection, purification, chlorination of public water supply. Provision of effective sewage disposal Protection of food from contamination and control of flies. Health education & personal hygiene* Health education & personal hygiene Health Education and public awareness regarding spread of disease, availability of treatment and precautions at domestic level are important to control and Prevent Cholera like: Use of boiled water if required. Avoid uncooked food unless it is peeled or washed and disinfected. Wash hands before preparing or eating food. Wash hands after using toilet or any contact with excreta. Dispose off human excreta promptly and safely.
* Specific preventive measures Control of patient, contacts & the environment. Measures for the Patients Notification is very important Isolation: hospitalization is desirable for acutely ill patient but strict isolation is not necessary. Concurrent disinfection of feces & vomitus & of articles used by the patient.
Treatment of patients Fluid therapy to correct the dehydration Antibiotics: Antibiotics should be started when cholera is suspected without waiting for lab confirmation. this includes the following choices Tetracycline , 500mgs 4 times daily for 3 days( for children 12.5mg /kg) Doxycycline ,200mg orally as a single dose for 3 days .
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* Other antibiotics that have been used include ciprofloxacin and azithromycin. Before hospital discharge, two negative stool cultures are required Antibiotics: eradicate infection (suppresses vibrio growth in the gut) reduce morbidity ( reduces volume of stool & shortens period of hospitalization). shorten the period of vibrio excretion (shorten period of communicability) prevent complications.
* Measures for the Contacts Surveillance of contacts for 5 days from last exposure. Investigation of contacts by stool culture for detection of unreported cases For household contacts, chemoprophylaxis with tetracycline, or doxycycline, for three days is recommended.
* Measures for the Environment Investigation of environmental source of infection investigation of possibilities of infection from polluted drinking water or from contaminated food.
* Vaccination Though vaccines are available they are not used for routine immunization and not used for prevention and control of epidemics. Vaccination against cholera is indicated for travelers from non-endemic countries to endemic countries .
* Three cholera vaccines are available Parental Killed whole cell vaccine - 2 doses, injected subcutaneously at an interval of 4 to 6 weeks -partial protection (50%) for 3 – 6 months - Not recommended.-
Oral killed whole cell / B subunit (Wc / Bs vaccine) Vaccine consisting of Formalin or heat killed V. cholerae 01 in combination with a recombinant B sub unit of cholera toxin. It is given orally in two-dose schedule, 10-14 days apart The vaccine confers 50-60% protection for at least 3 years.
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3) Oral Live attenuated vibrio cholera 01 vaccine - Single dose
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