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Mikrocytosis_ USA 07_30_02

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Mikrocytosis, United States

Impact Worksheet, July 30, 2002

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Summary:

The first identification of Mikrocytos mackini in the US has been made in feral Pacific oysters in waters of the Dungeness Bay in the western part of the state of Washington. Mikrocytosis is a disease of 4 species of oysters caused by the parasite, Mikrocytos mackini. Mikrocytosis is known to occur along Canada’s west coast. The impact of mikrocytosis on the domestic oyster industry is not likely to be significant because: it typically occurs in oysters older than the age at commercial harvest; it has been found only in feral oysters and not farmed oysters; it is not contagious to humans; and movement or harvest of shellfish from waters of the Dungeness Bay was already prohibited because of high fecal coliforms and paralytic shellfish poisoning.

The total sales value of oysters in the US in 1998 was $29 million. Washington accounted for the highest sales value, at $14.5 million, of any state in the US. In 2001, US exports of oysters were valued at $8.6 million and went primarily to Canada and Hong Kong.

How extensive is the situation?

Mikrocytosis USThe first identification of Mikrocytos mackini (Mikrocytosis) in the US has been made in feral Pacific oysters in waters of the Dungeness Bay in the northwestern part of the state of Washington. Mikrocytosis was diagnosed on May 8, 2002, when 2 of 50 feral oysters examined as part of routine surveillance were identified as positive by laboratory diagnosis in the state of Washington. A Canadian laboratory confirmed the diagnosis. The oysters were estimated to be at least five years of age and there has been no report of mortality or morbidity following examination of oysters in the bay.

Oysters from a nearby farm grown on longlines (ropes spread over the sand) have previously tested negative for Mikrocytos mackini. Movement or harvest of shellfish from waters of the Dungeness Bay was already prohibited because of high fecal coliforms and paralytic shellfish poisoning.

Although not previously reported in the United States, Mikrocytosis is probably ubiquitous in feral oysters throughout the Strait of Georgia and is found in specific localities around Vancouver Island, British Columbia, Canada.

Source: USDA, OIE; Washington Department of Fish and Wildlife; Fisheries and Oceans, Canada @ www.pac.dfo-mpo.gc.ca/sci/sealane/aquac/pages/midmacoy.htm, last accessed on 7/26/02.

What Species are Susceptible to Mikrocytos mackini?

According to the OIE, susceptible host species of Mikrocytos mackini include Crassostrea gigas (Pacific oyster), Crassostrea virginica (Eastern oyster), Ostrea edulis (European flat oyster), and Ostrea conchaphila (Olympia oyster). Mikrocytosis is on the OIE list of ‘Diseases notifiable to the OIE’ and it is also included on OIE’s List B.

Source: OIE Diagnostic Manual of Aquatic Animal Diseases, Chapter 3.1.4

What is Mikrocytosis?

Mikrocytosis is caused by a parasite, Mikrocytos mackini. The disease is also known as Denman Island Disease and as Microcell Disease of Pacific Oysters. Mickrocytos mackini produces infection resulting in green pustules up to 5mm in diameter within the body wall or mantle and brown scars on the adjacent shell, but lesions visible to the eye are not always present. Severe infections seem to occur in oysters older than 2 years and are most common in oysters more than 5 years old. Clinical infections are seldom detected in farmed oysters that are harvested at 2 to 3 years of age. The mortality can be up to 40% in older oysters at low tide levels. The disease is more common in April and May after 3 to 4 months of temperatures of less than 10ºC. The parasite prefers high salinity. The disease is not known to be infectious to humans. A different form of Mikrocytosis is found in New South Wales, Australia. The Australian form is caused by a related parasite, Mikrocytos roughleyi, which has not been detected in the United States or Canada.

Source: OIE; USDA, Washington State Department of Fish and Wildlife; Canadian Department of Fisheries and Oceans, P.M. Hine in: Diseases of Aquatic Organisms, Vol 45: 215-217, 2001.

What is the size of the oyster industry in the US and in Washington?

In 1998, the US had 172 farms that produced oysters for food, with a total sales value of $27 million, and 21 farms that produced oysters for seed stock, with a total sales value of $2 million (Table 1). Some of the farms that produce seed stock may be the same farms that produce food oysters. Washington had 44 farms that produced $14 million worth of food oysters, the highest sales value of any state in the US. Washington had 6 farms that produced seed stock oysters, with sales valued at $0.4 million.

Four types of oysters are produced in Washington: the Pacific, Olympia, Kumamoto, and European flat oyster. With the possible exception of Kumamoto (Crassostrea sikamea), all appear to be susceptible to Mikrocytos mackini. Susceptibility of the Kumamoto oyster is unknown because although the Kumamoto was originally considered to be a type of Pacific oyster, it is now classified into its own species through genetic studies and susceptibility information is not available. Pacific oysters are by far the dominant type of oyster produced in Washington, accounting for 94% of the total value of oysters in the state. Oysters are farmed by setting seed oysters onto small pieces of shell in a hatchery. The pieces of shell are then spread directly onto the beach in an intertidal area (the area between low and high tide) or onto longlines, which are ropes that are spread over the sand to keep the oysters off of the ground.

Table 1: Number of farms and value of sales for oysters in the US and Washington, 1998

Type of oysters

Number of farms

Value of sales ($ million)

US total

WA

US total

WA

Food oysters

172

44

27.0

14.1

Seed stock oysters

21

6

2.0

0.4

Sources: USDA, NASS, 1998 Census of Aquaculture; Washington Agricultural Statistics Service; Western Regional Aquaculture Center, Industry Situation and Outlook Report, Nov 2001

What is the USA’s place in the international market for oysters?

The US produced 177,740 metric tons of oysters in 1999, accounting for nearly 5% of world production.

Source: United Nations FAO

What are the U.S. exports of oysters?

The US exported oysters worth $8.6 million and $3.5 million in 2001 and January through May 2002, respectively (Table 2). The oysters were exported primarily to Canada and Hong Kong. Lesser amounts went to a host of other countries including Japan and several EU countries. The value of oyster exports from Washington was $3.8 million and $1.6 million in 2001 and January through May 2002, respectively.1

Table 2: US exports of oysters, 2001 and January-May 2002

$ value (million)

2001

Jan-May 2002

US total

WA

US total

WA

Oysters

8.6

3.8

3.5

1.6

Source: World Trade Atlas

CEI’s plans for follow up:

CEI will continue to monitor the situation but has no plans at this time to issue additional reports. If you need more information or wish to comment on this worksheet, please contact Robert Harris at (970) 494-7327 or Christine Kopral at (970) 494-7325.

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The state-specific export data report the state from which the export began its export journey. This is not necessarily the state in which the merchandise is produced.

Mikrocytosis, United States, July 2002 4

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