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Minimising the risk of norovirus contamination in Australian commercial oysters

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Brake, F (2016) Minimising the risk of norovirus contamination in Australian commercial oysters. PhD thesis, University of Tasmania.

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Abstract

Outbreaks of human gastroenteritis caused by norovirus (NoV)-contaminated oysters occur
worldwide and have periodically been linked to the consumption of Australian oysters.
Contamination with NoV can occur when human excrement (faeces and vomit) containing the
virus flows into oyster growing areas. Minimal data about the occurrence of NoV in Australian
oysters was available at the commencement of this thesis due to the expense involved in testing
oysters for viruses because no Australian commercial laboratories had the capacity to perform
the test. To that time, all Australian samples were tested in New Zealand.
The aim of this Thesis was to establish an informed strategy for minimising the risk of NoV in
commercial Australian oysters. To collect the NoV data, a sensitive and accurate method to
detect NoV was adopted in this study and validated for Australian oysters and conditions.
Epidemiological evidence suggests that the occurrence of NoV in Australian oysters is low;
therefore a sampling regime for the detection of NoV at low occurrence and concentration in
oysters was designed. Using this sampling programme, NoV was detected in oysters (8.3%, n =
163) from a harvest area that had been associated with 2 NoV illness outbreaks. The results
showed the need for a comprehensive sampling regime to ensure the detection of NoV.
To address the lack of systematically collected data on the occurrence of NoV in Australian
growing areas and due to the impending imposition of international standards for NoV in
Australia, a survey was conducted. This involved the collection of oysters from two
geographically distinct oyster growing areas on four occasions from each of the three main
oyster producing States in Australia and testing samples for NoV and E. coli (n = 120). The
growing areas selected were considered by Australian shellfish authorities to be the most
compromised in those States with respect to the potential for human faecal contamination, as
identified by shoreline surveys.
NoV GII was detected in two samples (1.7%) but NoV GI was not detected. Some of the
samples were found to have more than the guidance concentration of 230 E. coli per 100 g of
shellfish flesh but these samples did not contain detectable concentrations of NoV. These results
reinforce epidemiological data suggesting that NoV contamination of commercially produced
Australian oysters is rare.
There is minimal data about NoV dispersal in waterways following sewage overflows.
The persistence of NoV in Sydney Rock oysters (SRO) following sewage overflows was also
unknown. These knowledge gaps were addressed by strategically placing SRO in an estuary
downstream from a sewage pump station known to overflow periodically and initiating sampling
after an overflow event. After the event NoV GII was detected up to 5.29 km downstream and
persisted in SRO closest to the pump station outfall for 42 days. NoV GII concentrations
decreased significantly over time; a reduction rate of 8.5% per day was observed in oysters
located at two sites near the outfall (p<0.001). Five days after the overflow, NoV GII
concentrations were found to decrease significantly as a function of distance at a rate of 5.8% per
km (p<0.001). The decline in E. coli concentration with distance was 20.1% per km (p<0.001).
NoV GI and Hepatitis A virus were not detected. A comparison of NoV GII reduction rates from
oysters over time, derived from this study and other, published, research collectively suggest that
GII reduction rates from oysters may be broadly similar, regardless of environmental conditions,
oyster species and genotype.
In the final phase of the project, an investigation of risk management options for Australian
commercial oyster harvest areas to protect oyster consumers from NoV was conducted. The
result, a culmination of data and research, is a combination of strategies recommended for
minimising the risk of NoV prevalence in Australian oysters. It was found that the risk is real, as
demonstrated by the occurrence of 1-2 outbreaks of NoV illness associated with oysters
annually. This was supported by the findings of a NoV contamination rate of 1.7% of oysters
from Australian growing areas. Recommendations were made for risk management strategies to
prevent contamination of oyster growing areas and to enable rapid detection and notification of
contamination events when they occur.
It was found that communication between local councils, water utility operators and shellfish
authorities regarding reporting sewage spills and the condition of potential sources of sewage
spills (i.e. on-site sewage management systems (OSMS)) was lacking. A theoretical NoV
contamination event in oyster growing areas was considered, showing that it was possible for a
small human faecal mishap to cause illness in oyster consumers. Recommendations for a riskbased
virus monitoring programme are proposed, including: regulation of OSMS by local
councils; workshops on water quality to stimulate increased communication between
Environmental Health Officers and shellfish authorities; upgrading sewage treatment plants
where the effluent flows into oyster growing areas; mandatory dye studies of STP effluent flows
and potential sources of sewage overflows for each growing area to enable more effective closure times and delineation of the impacted area closed for harvesting; ASQAP to include
guidance to put local management committees in place for each growing area, ASQAP to include
guidance for portable toilets to be on oyster harvesting boats, and the development of an
alternative indicator for NoV other than faecal coliforms i.e. bacteriophage testing.
The results of this Thesis will improve NoV risk management strategies used by shellfish
authorities to protect the Australian oyster consumer and help to ensure that future control
measures are commensurate with the risk associated with the growing area rather than requiring
mandatory end product testing regimes for all areas.

Item Type: Thesis (PhD)
Keywords: Oyster, shellfish, safety, norovirus, foodborne,
Copyright Information:

Copyright 2015 the author

Additional Information:

Chapter 4 appears to be the equivalent of a post print version of an article published as: Brake F.A., Ross T., Holds G., Kiermeier A., & McLeod C. 2014, A survey of Australian oysters for the presence of human noroviruses, Food microbiology 44, 264 – 270

Date Deposited: 13 Oct 2016 23:41
Last Modified: 14 Oct 2016 00:05
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