You are contacted by the
Consultant Microbiologist of a District General Hospital for
advice and help. An outbreak of diarrhoea and vomiting affecting
five adult wards has occurred in the past three days. Over 30
cases have been identified so far. Describe the investigations,
control of infection measures, and other steps you would
recommend to establish the cause, mode of spread of infection and
control of the outbreak.
Small outbreaks of viral gastroenteritis are common in hospitals, especially among paediatric wards where rotaviruses and adenoviruses are usually involved. Large outbreaks involving adult wards are rare and small round structured viruses (noroviruses) are more likely to be involved. In such an outbreak situation, steps should be taken at the same time to establish the cause, mode of spread and control of the outbreak simultaneously.
Detailed information about the outbreak should be obtained. In particular, any factors common to those affected should be determined, such as whether they have consumed the same type of food, are the five wards served by the same medical teams or other personnel, are any health workers involved. Fresh faecal specimens should be collected from an affected patient as soon as possible to ensure the maximum chance for detecting the pathogen. Faecal specimens should be sent for bacteriological and parasitical examination as well as virological examination.
As a quick screen, the specimens could first be examined for the presence of rotaviruses or adenoviruses by EIA or other techniques although they are likely to be negative. The specimens should then be prepared for electron microscopy. Routine electron microscopy should be able to detect most of the diarrhoeal viruses. However in the case of noroviruses which is by far the chief suspect, the sensitivity is very poor given the small size of the particles and the fact that they are usually present in small numbers. A more sensitive technique would be immune electron microscopy (IEM). Both conventional IEM or solid phase IEM (SPIEM) may be used for the detection of noroviruses. However, even with SPIEM, one would expect only a small proportion of the specimens processed to yield a positive result. However, this is sufficient evidence to implicate SRSV as the agent. RT-PCR is being increasingly used to diagnose Norwalk-like virus infection. It is considerably more sensitive than EM and could potentially be used to investigate implicated foods such as shellfish. Where resources permit, DNA sequencing could be carried out on the PCR product to verify that the outbreak is from a single source.
Other diarrhoeal viruses are occasionally involved in outbreaks in adults including caliciviruses (sappoviruses), astroviruses, rotaviruses and adenoviruses. Normal electron microscopy should have little difficulty in detecting these viruses.
Mode of Spread
Diarrhoeal viruses are mainly spread by the faecal-oral route, either through infected foods such as shellfish or through an infected food handler. However, other modes of transmission are thought to be possible, in particular through aerosol sprays set up by vomitus. Food could be contaminated by this means or the viruses could be inhaled by other persons. noroviruses have in particular been associated with airborne transmission.
It is essential to determine whether any particular food consumed within last 5 days may be responsible. Whether any food handlers had been taken ill with gastroenteritis before or during the onset of the outbreak, whether any health workers working in any of the 5 wards were taken ill as they may play a role in the transmission of the virus to other wards. The plotting of an epidemic curve would determine whether the outbreak is a single source outbreak or involve person to person transmission.
The first aspect of control must be to interrupt person to person transmission. This may be very difficult in view of the information emerging about airborne transmission. Personal hygiene for both staff and patients should be emphasized, particularly hand washing. Prompt removal of vomitus and diarrhoea is important in order to reduce the chance of environmental contamination. The area should be disinfected by chlorox after cleaning. Affected persons should be isolated or cohorted together. This may be difficult in such a large outbreak. Any staff taken ill should refrain from coming to work. Transfer of staff from ward to ward should be avoided since this may spread infection to other wards. Since noroviruses may be spread through aerosols, it may be prudent to use respiratory precautions such as the use of facemasks for staff and visitors. If the outbreak cannot be controlled, then it may prove to be necessary to close down affected wards, services, or even the whole hospital.
High risk foods such as oysters and other shellfish are most unlikely to form part of the hospital menu. Other implicated foods are likely to have been contaminated by the food handler. Food handlers who developed gastrointestinal symptoms should not be allowed to work. A scrupulously high standard of personal hygiene should be enforced for food handlers. Adequate toilet and hand washing facilities should be available which are regularly serviced. Work surfaces should be thoroughly cleaned to remove any environmental contamination. An effective disinfectant against Norwalk-like viruses such as clorox should be used. Attempts should be made to rethink food handling practices to try to reduce the amount of handling of cooked or uncooked foods. It may be wise to take high risk foods such as salads and sandwiches until the outbreak is clearly at an end.
There are clearly a lot of problems involved in controlling outbreaks of viral gastroenteritis, in particularly those associated with noroviruses. Fortunately, such outbreaks usually terminate spontaneously in 7 - 14 days and rarely lead to severe illness, even in those who are elderly or immunocompromised. Norovirus outbreaks occur commonly in hospital and it is recommend that each hospital has a infection control plan drawn up against such an occurrence.