Norovirus

(endorsed 2011)

Guideline

No guideline value has been set for Norovirus and its inclusion in routine monitoring programs is not recommended.

A multiple barrier approach from catchment to tap is recommended to minimise the risk of contamination. Protecting catchments from human and animal wastes is a priority. Operation of barriers should be monitored to ensure effectiveness and that microbial health-based targets are being met.

General description

Noroviruses are single-stranded RNA non-enveloped viruses from the family of Caliciviridae. They were previously described as Norwalk-like viruses, after the original strain that caused an outbreak of illness in a school in Norwalk, Ohio in 1968. The viruses were discovered by electron microscopy and were referred to as “small round structured viruses” due to their appearance. Morphologically similar viruses known as Hawaii, Wollan, Ditchling, Parramatta, Snow Mountain and Montgomery County agents were subsequently identified.

Noroviruses are a major cause of acute viral gastroenteritis in all age groups and are typically more prevalent in winter. The major route of transmission is person-to-person by the faecal-oral route although transmission by fomites and via contact with contaminated surfaces has also been suggested. Environmental transmission from drinking water, recreational water and food has been reported.

Investigations of Norovirus occurrence have been hampered by the lack of a culture-based assay. They can be detected using electron microscopy, enzyme immunoassays and PCR-based methods.

Australian significance

There is little information on the occurrence of Norovirus and there have been no reported outbreaks associated with Australian drinking water supplies.

Internationally, numerous outbreaks have been attributed to contaminated drinking water and recreational water (Lodder and de Roda Husman 2005). Noroviruses have been detected in high concentrations in surface water and sewage (Lodder and de Roda Husman, 2005).

Method of identification and detection

Detection of viruses in water typically requires concentration from large volumes of water (10–1000 litres depending on the source). Assays for Norovirus in water are typically performed using PCR-based methods (Lodder and de Roda Husman 2005). A limitation of PCR-based methods is that they do not measure infectivity.

Preventing contamination of drinking water

A multiple barrier approach operating from catchment to tap should be used to minimise the risk of contamination. Human faecal waste is the source of infectious Norovirus spp. in water supplies, and protection of water catchments from contamination by human wastes is a priority. Water from catchments receiving human waste is likely to be susceptible to contamination, and treatment, including effective filtration and disinfection, will be required to ensure a safe supply.

The lower the quality of source water, the greater the reliance on water treatment processes. Sanitary surveys of water catchments should be undertaken to identify potential sources of human waste, assess risk factors for contamination, provide a basis for catchment management to reduce these risks, and determine the level of water treatment required.

Groundwater from confined aquifers or from depth should be free from contamination by Norovirus. However, bores need to be well maintained and protected from intrusion of surface and subsurface contamination. Integrity should be monitored using traditional indicators of faecal contamination.

Where Norovirus are suspected or known to be present in the raw water, treatment will be required. Norovirus spp. are sensitive to disinfection using agents such as chlorine (Shin and Sobsey 2008) and UV light. Media filtration (with coagulation) and membrane filtration can reduce concentrations by 90% or more depending on membrane pore size and effectiveness of operation. Filtration plants should be operated by trained and skilled personnel.

The integrity of distribution systems should be maintained. Backflow prevention policies should be applied and faults and burst mains should be repaired in a way that will prevent contamination.

Health considerations

Norovirus is the most common cause of acute viral gastroenteritis in developed countries (Lopman et al. 2003). The incubation period is usually 24-48 hours, but cases can occur within 12 hours of exposure. Symptoms include nausea, vomiting (more common in children), abdominal cramps and diarrhoea. Low grade fever can occur. As infections can lead to vomiting and no diarrhoea, the condition is also known as “winter vomiting disease.” Symptoms are usually mild and last for 24-60 hrs. High attack rates in outbreaks indicate that the infecting dose is very low. This has been confirmed by risk assessment (Teunis et al. 2008)

Derivation of guideline

The infectious dose for many viruses is very low (1-10 particles) and risk assessments have indicated that safe drinking water should contain less than 1 virus per 1000 litres of water (Regli et al. 1991, WHO, 2004). No guideline value is proposed and inclusion in routine verification monitoring programs is not recommended. The focus should be on monitoring of control measures including prevention of source water contamination by human waste, effective disinfection, and protection of distribution systems from ingress of faecal material.

References

Lodder WJ, de Roda Husman AM (2005). Presence of Noroviruses and other enteric viruses in sewage and surface waters in The Netherlands. Applied and Environmental Microbiology, 71:1453–1461.

Lopman BA, Reacher MH, van Duijnhoven Y, Hanon FX, Brown D, Koopmans M (2003). Viral gastroenteritis outbreaks in Europe, 1995-2000. Emerging Infectious Diseases, 9:90-96.

Regli S, Rose JB, Haas CN, Gerba CP (1991). Modelling the risk from Giardia and viruses in drinking water. Journal of the American Water Works Association, 83:76-84.

Shin G-A, Sobsey M (2008). Inactivation of Norovirus by chlorine disinfection in water. Water Research, 42:4562-4568.

Teunis PFM, Moe CL, Liu P, Miller SE, Lindesmith L, Baric RS, Pendu JL, Calderon R (2008). Norwalk virus: How infectious is it? Journal of Medical Virology, 80:1468-1476.

WHO (World Health Organization) (2004). Guidelines for Drinking-water Quality, Sections 7.2 Health-based target setting. World Health Organization, Geneva, Switzerland.