Cholera

Cholera is not endemic in New Zealand, but occasional imported cases occur, mainly in travellers from Asia.

There are over 200 serogroups of Vibrio cholerae but only serogroups O1 or O139 that produce cholera toxin are associated with clinical cholera and have pandemic potential.

More detailed epidemiological information is available on the Institute of Environmental Science and Research (ESR) surveillance website.

An illness of variable severity characterised by watery diarrhoea and vomiting, which can lead to profound dehydration.

Laboratory definitive evidence for a confirmed case requires isolation of Vibrio cholerae serogroup O1 or O139 from a clinical specimen and confirmation that the organism is toxigenic (can produce the cholera toxin).

All specimens should be referred to the Enteric Reference laboratory at ESR for serotyping, toxin detection and confirmation.

All isolates of Vibrio cholerae are initially notifiable as suspected cholera until the strain has been determined. Unless the isolate is determined to be O1 or O139 and has the ability to produce cholera toxin, the case should be made ‘not a case’.

• Under investigation: A case that has been notified, but information is not yet available to classify it as probable or confirmed.
• Probable: A clinically compatible illness that is either a contact of a confirmed case of the same disease or has had contact with the same common source – that is, is part of a common-source outbreak.
• Confirmed: A clinically compatible illness accompanied by laboratory definitive evidence.
• Not a case: A case that has been investigated and subsequently found not to meet the case definition.

Note: Some strains of O1 and O139 do not possess the cholera toxin gene, and some strains of non-O1 non-O139 do possess the cholera toxin gene. Illness caused by these strains is not defined as ‘cholera’.

Contaminated water forms the reservoir with humans as the only host.

A few hours to 5 days, commonly 2–3 days.

Infection of humans occurs by ingestion of contaminated food (for example, rice, seafood, fresh vegetables and fruit) or water (for example, rivers, ponds, lakes, well water and even municipal water). Direct person-to-person transmission is probably rare because a large inoculum is necessary to transmit disease.

Usually from the onset of symptoms until a few days after recovery but occasionally persists for several months or years. Individuals with asymptomatic infections may shed the organism in their faeces for 1-10 days post-infection, though the risk to others in this situation is unclear.

V. cholerae persists indefinitely in aquatic environments and may survive up to 14 days in some foods.

Attending medical practitioners or laboratories must notify the local medical officer of health immediately about cases of cholera. The Ministry of Health assesses cases of cholera, and if necessary reports them to the World Health Organization (WHO), in accordance with the International Health Regulations (IHR) 2005.

While laboratories speciate, samples must be further typed at ESR for genes encoding heat-stable enterotoxin (NAG-ST), that is, NAG V. cholerae.

Obtain a history of travel, consumption of untreated water and possible contacts. Ensure laboratory confirmation by stool culture or rectal swab and further typing has been attempted. Ensure the laboratory is aware of any overseas travel history so that selective media for cholera can be used.

In health care facilities, only standard precautions are indicated in most cases. If the case is diapered or incontinent, apply contact precautions for the duration of illness.

Exclude from work those in high-risk groups, such as food handlers and caregivers (of patients, children and the elderly), until symptom free for 48 hours. In exceptional circumstances, where workplace hygiene or sanitation is uncertain, exclude until the case has submitted two consecutive negative stools, taken at least 24 hours apart.

Recommendations regarding restriction for cases and contacts of foodborne and waterborne illness are summarised in the exclusion and clearance criteria in Appendix 2: Enteric disease.

Advise the case and their caregivers of the nature of the infection and its mode of transmission.

Educate about hand and food hygiene.

Household members or those exposed to a possible common food source during the 5 days before onset of symptoms.

Obtain stool for vibrio culture from symptomatic contacts, especially if the infection was acquired in New Zealand. Inform the laboratory that cholera is suspected.

Nil if asymptomatic. If symptomatic, restrict as for case (while awaiting stool culture results).

Antimicrobial prophylaxis for contacts is not generally recommended because the rate of secondary spread is very low in Western countries.

Advise all contacts of the incubation period and typical symptoms of cholera, and to seek early medical attention if symptoms develop.

Educate about hand and food hygiene.

Check for other cases in the community. If the infection was acquired in New Zealand, undertake a thorough investigation to identify the source. This should include surveillance of contacts, stool testing of symptomatic contacts (see above) and assessment of possible food or water sources in association with the local territorial authority.

If indicated, check the water supply for microbiological contamination and compliance with the latest New Zealand drinking-water standards (Ministry of Health 2008). Liaise with the local territorial authority staff to investigate potential water sources of infection.

Clean and disinfect surfaces and articles soiled with stool or vomit. For more details, see Appendix 1: Disinfection.

In the event of a locally acquired case, consider a media release and direct communication with the population at risk and health professionals to encourage prompt reporting of symptoms. In communications with doctors, include recommendations regarding diagnosis, treatment and infection control.

If a water supply is involved, liaise with the local territorial authority to inform the public. Advise on the need to boil water.

Ensure complete case information is entered into EpiSurv.

Where food/food businesses are thought to be involved inform the Ministry for Primary Industries.

If an outbreak occurs, contact 0800GETMOH - CD option, and outbreak liaison staff at ESR, and complete the Outbreak Report Form.

Medical officers of health should also notify the Ministry of Health if even a single case of locally acquired cholera occurs. The IHR National Focal Point in the Ministry must use the IHR Decision Instrument for any event involving cholera, and then notify WHO if required.

• Ministry of Health. 2008. Drinking-water Standards for New Zealand 2005 (Revised 2008). Wellington: Ministry of Health.