Identification of suitable areas for pathogen introduction and establishment to inform the design of risk-based surveillance for oyster diseases: the example of Mikrocytos mackini in the Charente-Maritime bay, France
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1
Ifremer, RBE-SG2M-LGPMM, France
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2
Ifremer, ODE-UL-LERPC, France
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3
RBE-PFOM-LPI, France
As classical control measures are of limited use in marine open systems, successful control of diseases is very unlikely once established in shellfish populations. Thus, early detection of exotic or emerging diseases and rapid mitigation response are crucial. However, exotic disease introduction or emerging disease appearance are rare events, thus surveillance should account for maximizing the chances of detection. Risk-based surveillance (RSB) is an interesting approach to focus surveillance on increased risk areas or time periods for such diseases introduction or establishment. In addition, RBS can address the issue of resource allocation. The aim of the developed approach was to inform the design of a general RBS system for shellfish diseases in France, using spatial risk assessment tools for a spatial decision support system.
Therefore this study aimed at designing an example approach on an EU exotic pathogen, Mikrocytos mackini, and further assessing its generalization to other pathogens of interest for molluscs. Mikrocytos mackini infection was chosen because this parasite of the Pacific oyster Crassostrea gigas represents a hazard to global oyster production in northern temperate regions, notably in the Pacific coast of Canada. In France, despite massive importations of oysters from Canada in the 70s to replace the native oyster populations, this parasite has never been detected. However, several detections of members of the genus Mikrocytos were recently reported in Europe including a new species Mikrocytos mimicus in Pacific oysters Crassostrea gigas in UK, suggesting that conditions in European waters might be suitable for the development of related species like M. mackini.
The first aim of the present study was to identify suitable areas for the introduction and the establishment of Mikrocytos mackini infection in the main oyster production area in France (Charente-Maritime bay). The second aim of this study was to assess the feasibility of such a risk-based surveillance.
GIS-based multi-criteria decision analysis was used to define a set of weighted rules based on existing published or expert knowledge. The risk factors were identified through systematic review of the scientific literature and expert knowledge. The weights of the risk factors were based on expert opinion elicitation, using an analytical hierarchy process. Suitability maps were generated by combining the risk factors to obtain a final weighted estimate of suitability for each space-time unit in the study area. A sensitivity analysis was implemented to account for the uncertainty of sparse data notably.
A low risk of introduction of Mikrocytos mackini in the whole Charente-Maritime bay, during the whole year, was assessed. In March and April, most of the Charente-Maritime bay was highly suitable for establishment of Mikrocytos mackini (Figure 1). Model results, both for introduction and establishment, were considered to be robust and not sensitive to changes by 25% in the relative weights of the input risk factors.
In March and April 2015 and 2016, sentinel oysters were placed on a surveillance site, located in one of the high risk space units containing oyster farming area (Loix-en-Ré). Weekly, oyster mortality was monitored and oyster samples were sent to the laboratory to look for infection by Mikrocytos mackini and other listed pathogens.
No mortality of sentinel oysters was observed during this period. Laboratory tests did not detect Mikrocytos mackini but interestingly revealed the presence of the protozoan parasite Marteilia refringens (OIE and EU listed pathogen) over the monitoring period. Although this parasite is considered endemic in France, it has never been detected in this farming area. Moreover, Marteilia refringens generally infects flat oysters Ostrea edulis and mussels whereas the Pacific oyster Crassostrea gigas is occasionally found infected and is considered as an epidemiological dead end (the parasite enters, starts its development but does not seem to be released). Further analyses are required to elucidate the risk factors for Marteilia refringens establishment in Pacific oysters. However, the detection of a parasite that has never been detected in the studied site and in a species rarely detected infected suggests that our approach is able to identify risky sites regarding the introduction of exotic pathogens.
At mid-term, methods and results of this study may provide a template to inform RBS and control strategies in France.
Figure 1: Risk map for Mikrocytos mackini establishement in oyster populations in France, Charente-Maritime, overlaid with the location of surveillance site (black pin)
Keywords:
Risk-based surveillance,
spatial risk,
mollusc disease,
Multi Criteria Decision Analysis,
Suitability maps
Conference:
AquaEpi I - 2016, Oslo, Norway, 20 Sep - 22 Sep, 2016.
Presentation Type:
Oral
Topic:
Aquatic Animal Epidemiology
Citation:
LUPO
C,
DORANT
Y,
LE MOINE
O,
GEAIRON
P,
GRIZON
J,
PEPIN
J,
FLEURY
E,
GARCIA
C and
ARZUL
I
(2016). Identification of suitable areas for pathogen introduction and establishment to inform the design of risk-based surveillance for oyster diseases: the example of Mikrocytos mackini in the Charente-Maritime bay, France.
Front. Vet. Sci.
Conference Abstract:
AquaEpi I - 2016.
doi: 10.3389/conf.FVETS.2016.02.00043
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Received:
29 Apr 2016;
Published Online:
14 Sep 2016.
*
Correspondence:
DVM, PhD. Coralie LUPO, Ifremer, RBE-SG2M-LGPMM, La Tremblade, 17390, France, coraliedora.lupo@gmail.com