2001–2002 - California Sea Grant - UC San Diego

Aquatic Invader Appears Free of Dangerous Human Parasite … Future Outbreak Possible
In 1992, shrimp trawlers in South
San Francisco Bay hauled up the
first mitten crabs on the West Coast.
Since then, the crabs have spread like
wildfire throughout the Bay–Delta,
clogging fish screens at water intake
stations and injuring fish in salvage
tanks.
States like Oregon and Alaska are
now on high alert, worried mitten
crabs will burrow into the Northwest’s
economy next. So far, however,
only one mitten crab has been
identified outside the Bay–Delta
area. In 1998, a male Japanese mitten
crab, a close relative of the Chinese
mitten crab, was pulled from the
Columbia River in Oregon, fanning
concerns of an alien invasion.
The Chinese mitten crab is native to estuaries and
creeks in the Yellow Sea in China and Korea.
Besides its furry pinchers, which earn it its sobriquet,
it is a nondescript brown crab—about 3 inches in
diameter. Photo: Lee Mecum, California
Department of Fish and Game
Although notorious for clogging pumping stations, eroding levees and stealing
bait, the Chinese mitten crab poses a perhaps equally significant, albeit less
recognized, human-health threat as a host for a group of parasites known as lung
flukes. In this project, completed in the spring of 2002, California Sea Grant funded marine
researchers to search for evidence of these potentially dangerous parasites in the San Francisco
Bay–Delta. A second part of the scientists’ research looked at whether all requisite hosts
for the flukes are present in sufficient abundance and distribution to sustain a future infestation.
Flukes come in Asian and North American varieties and can be acquired by eating raw or
undercooked infected mitten crabs (Eriocheir sinensis). People infected with flukes may
develop tuberculosis-like symptoms, resulting in permanent lung damage. The parasites can
also migrate into the brain. Fluke infections are very common in Asia, where fresh mitten
crabs, eaten raw, are a delicacy.
None of this would be such a serious concern—and wildlife officials could focus on dealing
with the crabs’ ecological consequences in the area—but for the fact that these furry-clawed
crustaceans are a prized culinary treat within immigrant communities in San Francisco and
Los Angeles. Despite prohibitions on their transport and sale, there is a flourishing underground
market in live mitten crabs. In addition, it is legal, with a fishing permit, to
recreationally trap and eat mitten crabs.
Because of this, health officials have expressed concern about the possibility of mitten crabs
introducing Asian lung flukes or spreading existing North American flukes. This concern was
heightened when fishermen began suggesting that the California Department of Fish and
Game open a commercial fishery for the lucrative, and abundant, mitten crab.
The purpose of this Sea Grant project was to search for lung flukes in Bay–Delta mitten
crabs and crayfish, the primary intermediate hosts for both Asian and North American lung
flukes. Because freshwater gastropods are the first host, a second emphasis was to sample the
region’s snail populations. Mammals, including humans, are the final host for flukes.
To evaluate the health threat, Sea Grant marine researchers, led by Dr. Jenifer Dugan of the
Marine Science Institute at University of California, Santa Barbara, collected more than 900
mitten crabs during the crabs’ breeding season from the North and South bays, the Tracy Fish
Collection Facility and several South Bay creeks. They also collected specimens from Coyote
Creek during the crab’s downstream breeding migration. Adults were preferentially collected
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